Monday, October 23, 2017

LIGO y Virgo detectan por primera vez ondas gravitacionales procedentes de una colisión de estrellas de neutrones

[Extraído de la nota de prensa realizada por la Universidad de la Islas Baleares, y el Ministerio de Educación de España, la versión completa se encuentra aquí]




Hace dos años, el 14 de Setiembre de 2015, la Colaboración Científica LIGO en EE.UU. y la Colaboración Virgo en Europa, iniciaron conjuntamente una nueva era para la astronomía con la primera observación directa de ondas gravitacionales, las perturbaciones del espacio-tiempo predichas por la teoría de la Relatividad General de Albert Einstein, procedentes de la fusión de dos agujeros negros.

Ahora, los mismos protagonistas, junto con otros 70 observatorios terrestres y espaciales anuncian otro descubrimiento histórico: la primera observación simultánea de ondas gravitacionales procedentes de la espectacular colisión de dos estrellas de neutrones, y de contrapartidas en todo el espectro electromagnético, un evento cósmico que marca el inicio de la astronomía de multi-mensajeros con ondas gravitacionales. Los resultados LIGO-Virgo se publican hoy en la revista Physical Review Letters; trabajos adicionales de las colaboraciones LIGO-Virgo y de la comunidad astronómica han sido enviados y algunos han sido ya aceptados para su publicación en diferentes revistas

En este descubrimiento han participado el Grupo de Relatividad y Gravitación de la Universidad de las Illes Balears, a través de su participación en la Colaboración Científica LIGO, así como el Grupo Virgo de la Universidad de Valencia, miembro de la Colaboración Virgo. Además, ha habido importantes aportaciones de los grupos españoles que forman parte de INTEGRAL, el equipo AGILE, la colaboración del Fermi-LAT, la colaboración Vinrouge, la colaboración Master, el experimento ePESSTO, la colaboración TOROS, la Red Global BOOTES, la colaboración HAWC, la colaboración Pierre Auger, la colaboración ANTARES, el equipo EURO VLBI, entre otros.

Los resultados del descubrimiento se han hecho públicos durante la rueda de prensa celebrada hoy lunes 16 de octubre de 2017, en la sede del Ministerio de Economía, Industria y Competitividad (Madrid). En esta rueda de prensa han intervenido D. Juan María Vázquez, Secretario General de Ciencia e Innovación, Dña. Alicia Sintes, investigadora principal de la Colaboración Científica LIGO en la Universidad de les Illes Balears, D. José Antonio Font, investigador principal de la Colaboración Virgo en la Universidad de Valencia, D. Eusebio Sánchez, del CIEMAT y representante del experimento DES en España, D. Alberto J. Castro-Tirado, del Instituto de Astrofísica de Andalucía (IAA-CSIC) e investigador principal de la Red Global BOOTES, D. Manuel Reina, representante de INTA y jefe de proyecto técnico de la participación española en INTEGRAL, y Dña. Josefa Becerra González, del Instituto de Astrofísica de Canarias (IAC) y colaboradora en estudios de multifrecuencia. El acto también ha contado con la presencia de representantes del Gobierno de la Comunidad de las Illes Balears y del Gobierno de la Comunidad Valenciana, representantes de ambas universidades, presidentes de la Real Sociedad Española de Física, la Sociedad Española de Gravitación y Relatividad y la Sociedad Española de Astronomía, coordinadores de las redes temáticas REDONGRA y RENATA y representantes de la Red Española de Supercomputación. Además, también ha habido representación de todos los experimentos asociados con el descubrimiento en los que ha habido participación española. Durante este acto también se ha hecho un breve seguimiento del comienzo del anuncio mundial desde el National Press Club en Washington, DC., EE.UU.

Una señal estelar
Las estrellas de neutrones son las estrellas más pequeñas y densas conocidas y se forman cuando estrellas más masivas explotan en forma de supernovas. A medida que la órbita de las dos estrellas de neutrones fue disminuyendo en forma de espiral, el sistema binario emitió ondas gravitacionales que fueron detectadas durante unos 100 segundos. Al colisionar, con una velocidad de aproximadamente la tercera parte de la velocidad de la luz, se emitió un destello de luz en forma de rayos gamma que fue observado en la Tierra unos dos segundos después de la detección de las propias ondas gravitacionales. En los días y semanas posteriores a la colisión, otras formas de luz o radiaciones electromagnéticas – incluyendo rayos X, ultravioleta, óptica, infrarroja y ondas de radio – fueron también detectadas.

Las observaciones han dado a los astrónomos una oportunidad sin precedentes para investigar la colisión de dos estrellas de neutrones. Por ejemplo, las observaciones realizadas por el observatorio Gemini de Estados Unidos, el European Very Large Telescope y el Hubble Space Telescope de la NASA revelan trazas de materiales recientemente sintetizados, incluyendo oro y platino, descifrando el misterio no resuelto durante décadas sobre dónde se producen aproximadamente la mitad de todos los elementos químicos más pesados que el hierro.

La señal gravitacional, conocida como GW170817, fue detectada el 17 de agosto a las 14:41 hora peninsular por los dos detectores americanos LIGO avanzado. Es la señal más intensa detectada por la red de interferómetros LIGO-Virgo hasta la fecha. La información proporcionada por el tercer detector, Virgo avanzado, situado cerca de Pisa, Italia, permitió mejorar la localización del evento cósmico.

LIGO avanzado es un detector de ondas gravitacionales de segunda generación que consiste en dos interferómetros idénticos situados en Hanford, Washington, y Livingston, Luisiana. Empezando las operaciones en septiembre de 2015, LIGO avanzado ha realizado dos periodos de observación. El segundo periodo de observación O2 comenzó el 30 de noviembre de 2016 y terminó el 25 de agosto de 2017. El detector Virgo avanzado también es un instrumento de segunda generación. El 1 de agosto de 2017, Virgo avanzado se unió a los detectores LIGO para trabajar conjuntamente durante las últimas cuatro semanas del periodo de observación O2.

La Colaboración Virgo está formada por más de 280 físicos e ingenieros pertenecientes a 20 grupos de investigación europeos diferentes. Alrededor de 1500 científicos de la Colaboración Científica LIGO y de la Colaboración Virgo trabajan conjuntamente para operar los detectores y para procesar y entender los datos de las ondas gravitacionales que capturan.

“La naturaleza ha vuelto a ser muy generosa con nosotros al situar este evento excepcional a una distancia notablemente cercana a la Tierra, justo antes de que acabase este segundo periodo de observación de LIGO-Virgo avanzado y en el momento en que había tres detectores de la red en operación permitiendo localizar la fuente de forma precisa”, dice Alicia Sintes, emocionada con este nuevo descubrimiento.

Font apunta que “la histórica detección de la primera señal gravitacional de la colisión de dos estrellas de neutrones, junto con la correspondiente emisión electromagnética, marca el inicio de una nueva era de descubrimiento que promete ofrecer respuestas a preguntas fundamentales en astrofísica relativista, cosmología, física nuclear, o la naturaleza de la gravitación. Es revolucionario.”

La detección

El 17 de agosto, el software de análisis de datos a tiempo real de LIGO captó una fuerte señal de ondas gravitacionales desde el espacio en uno de los dos detectores LIGO (Hanford). Casi al mismo tiempo, el Gamma-ray Burst Monitor del Fermi Gamma-ray Space Telescope de la NASA detectó una explosión de rayos gamma. El software de análisis LIGO-Virgo consideró ambas señales de manera conjunta y se observó que era improbable que fueran una coincidencia fortuita, mientras que otro análisis paralelo y automatizado de LIGO indicaba que había una señal de onda gravitacional coincidente en el otro detector LIGO (Livingston). La rápida detección de la onda gravitacional por el equipo de LIGO-Virgo, junto con la detección de los rayos gamma de Fermi, permitieron el lanzamiento del seguimiento por telescopios alrededor del mundo.

Los datos de LIGO indicaron que dos objetos astrofísicos situados a una distancia relativamente pequeña de la Tierra, alrededor de 130 millones de años-luz, habían estado aproximándose en órbitas espirales. Presumiblemente, los objetos no eran tan grandes como un sistema binario de agujeros negros – objetos que LIGO y Virgo ya habían detectado previamente. En su lugar, se estimó que los dos objetos en órbita espiral debían estar en un rango de entre 1,1 y 1,6 veces la masa del Sol, es decir, en el rango de masa de las estrellas de neutrones. Una estrella de neutrones es una estrella de unos 20 kilómetros de diámetro y de material tan denso que una cucharadita de su material equivaldría a una masa de alrededor de mil millones de toneladas.

Mientras que los sistemas binarios de agujeros negros producen un leve gorjeo de una fracción de segundo en la banda sensible del detector LIGO, el gorjeo del 17 de agosto duró aproximadamente 100 segundos y se pudo ver a través de toda la gama de frecuencias de LIGO – aproximadamente el mismo rango que los instrumentos musicales comunes. Los científicos pudieron identificar la fuente del gorjeo como objetos mucho menos masivos que los agujeros negros observados hasta la fecha. Los análisis mostraron que un evento de estas características sucede menos de una vez en 80.000 años por coincidencia aleatoria, por lo que se identificó de inmediato como una detección muy segura.

Sascha Husa, profesor de física de la UIB e investigador de la Colaboración Científica LIGO, comenta que “el anuncio de hoy marca la culminación de casi una década de trabajo en la UIB, desarrollando modelos de las formas de onda de la fusión de binarias compactas, que han ayudado a dirigir los telescopios en la dirección correcta.”

Los investigadores teóricos predicen que al colisionar estrellas de neutrones se deben emitir ondas gravitacionales y rayos gamma, junto con poderosos chorros que emiten luz a través de todo el espectro electromagnético. La explosión de rayos gamma detectada por Fermi es lo que se conoce como una ráfaga corta de rayos gamma (short gamma-ray burst); las nuevas observaciones confirman que al menos algunas de las ráfagas cortas de rayos gamma son generadas por la fusión de estrellas de neutrones – algo que se había teorizado anteriormente. Sin embargo, mientras este misterio parece resuelto, otros nuevos han surgido. La ráfaga corta de rayos gamma observada fue una de las más cercana a la Tierra vista hasta ahora, pero fue sorprendentemente débil para su distancia. Los científicos están empezando a proponer modelos para obtener respuestas y es probable que surjan nuevas ideas en los próximos años.

Una mancha en el cielo

Aunque la onda gravitacional fue captada en primer lugar por los detectores LIGO en EE.UU., Virgo, en Italia, jugó un papel clave en la historia. Debido a su orientación con respecto a la fuente en el momento de la detección, Virgo recuperó una pequeña señal que, combinada con el tamaño de la señal y los tiempos de detección en los detectores LIGO, permitió a los científicos triangular con precisión la posición en el cielo. Tras realizar una investigación minuciosa para asegurarse de que las señales no eran un artefacto de la instrumentación, los científicos concluyeron que la onda gravitacional provenía de una región relativamente pequeña (28 grados cuadrados) en el cielo del hemisferio sur.

Fermi fue capaz de dar una localización posteriormente confirmada y mejorada en gran medida gracias a las coordenadas proporcionadas por la detección combinada de los observatorios LIGO-Virgo. Con estas coordenadas, diferentes observatorios de todo el mundo fueron capaces, horas después, de comenzar a buscar en la región del cielo de donde debía proceder la señal. Así pues, un nuevo punto de luz parecido al de una nueva estrella, fue encontrado primero por diferentes telescopios ópticos. Seguidamente, alrededor de 70 observatorios terrestres y en el espacio observaron el evento en sus correspondientes longitudes de onda.

“Esta detección abre la ventana de una largamente esperada astronomía de multi-mensajeros,” dice David H. Reitze de Caltech, director ejecutivo del Laboratorio LIGO. “Es la primera vez que hemos observado un evento astrofísico catastrófico en forma ondas gravitacionales y ondas electromagnéticas – nuestros mensajeros cósmicos. La astronomía de ondas gravitacionales ofrece nuevas oportunidades para entender las propiedades de las estrellas de neutrones de maneras que simplemente no son posibles únicamente con la astronomía electromagnética”.

“Desde DES-Spain estamos muy emocionados con el descubrimiento de las contrapartidas ópticas de las detecciones de ondas gravitacionales. Es realmente un hito extraordinario para la ciencia, y abre una nueva ventana en la astrofísica y cosmología observacional”, dice Enrique Gaztañaga, investigador del Instituto de Ciencias del Espacio (IEEC-CSIC). Por su parte, Juan García-Bellido (Universidad Autónoma de Madrid) apunta que “el grupo de ondas gravitacionales del cartografiado DES trabaja desde hace tiempo para el seguimiento óptico de un evento como este. Horas después de la colisión, la cámara de DES descubrió de forma independiente la fuente en el visible e infrarrojo cercano, lo que ayudó a su localización en la galaxia NGC 4993. Al disponer del corrimiento al rojo de la galaxia, se ha podido determinar el ritmo de expansión del universo”. Además, Diego Torres, investigador del IEEC-CSIC y líder del único grupo en España de la colaboración Fermi-LAT, remarca también que “la detección de ondas gravitacionales proveniente de una fusión de estrellas de neutrones, y la búsqueda y el hallazgo de la contrapartida en todas las longitudes de onda denota el verdadero inicio de la astronomía de multi-mensajeros”.

J. Miguel Mas Hesse, director del Centro de Astrobiología (CSIC-INTA) relata como “el instrumento SPI a bordo de INTEGRAL detectó el flash de rayos gamma emitido en el momento de la fusión de las estrellas de neutrones, una emisión muy intensa durante sólo 2 segundos. La galaxia en la que se encontraban estas estrellas fue observada en los días siguientes mediante la cámara óptica de INTEGRAL, OMC. OMC es un instrumento liderado por investigadores del Instituto Nacional de Técnica Aeroespacial (INTA). La Universidad de Valencia fue la responsable de la fabricación de los sistemas de imagen de los demás instrumentos a borde del observatorio INTEGRAL.” Por otro lado, Mª Dolores Sabau Graziati, directora del Departamento de Cargas Útiles y Ciencias del Espacio del INTA, que ha participado tanto en el instrumento OMC como en las máscaras de codificación de los otros tres instrumentos principales de INTEGRAL añade que “están contentísimos de haber contribuido a este hito astrofísico”.

Una bola de fuego y un resplandor

Cada observatorio electromagnético publicará sus propias observaciones detalladas del citado evento astrofísico. Mientras tanto, la perspectiva general de todos los observatorios involucrados parece confirmar que la señal de la onda gravitacional realmente fue producida por un par estrellas de neutrones en órbita espiral.

Aproximadamente hace 130 millones de años, las dos estrellas de neutrones se encontraban en sus últimas órbitas espirales, separadas sólo por unos 300 kilómetros, incrementando su velocidad orbital mientras disminuía la distancia entre ellas. A medida que las estrellas giraban cada vez más rápido y más cerca la una de la otra, se deformaron y distorsionaron el espacio-tiempo circundante, emitiendo energía en forma de potentes ondas gravitacionales, antes de chocar entre sí.
En el momento de la colisión, la mayor parte de las dos estrellas de neutrones se fusionaron en un objeto ultradenso a la vez que se emitía una “bola de fuego” de rayos gamma. Las mediciones iniciales de rayos gamma, combinadas con la detección de las ondas gravitacionales, han proporcionado también una confirmación de la teoría de la Relatividad General de Einstein, que predice que las ondas gravitatorias deben viajar a la velocidad de la luz.

Las investigaciones teóricas han predicho que lo que sigue a la bola de fuego inicial es una “kilonova” – un fenómeno por el cual el material que queda tras la colisión de las estrellas de neutrones, que brilla con luz, es expulsado de la región circundante muy lejos en el espacio. Las nuevas observaciones basadas en el espectro electromagnético muestran que los elementos pesados, como el plomo y el oro, se crean en estas colisiones y posteriormente se distribuyen por todo el universo.

En las próximas semanas y meses, los telescopios de todo el mundo continuarán observando el resplandor de la fusión de estrellas de neutrones y reunirán más evidencias sobre las diversas etapas de la fusión, su interacción con su entorno y los procesos que producen los elementos más pesados del universo.

La significativa contribución española

En España, el seguimiento de contrapartidas electromagnéticas de GW170817 ha supuesto una actividad frenética. Por ejemplo, la red de telescopios robóticos MASTER de la Universidad de Moscú, en la que participa el IAC, consiguió una de las primeras detecciones en luz visible asociada al evento gravitacional GW170817. Los datos de MASTER junto con los de otros muchos telescopios ópticos, infrarrojos y de radio, contribuyeron a clasificar esta fuente extragaláctica como una kilonova.

“Los telescopios robóticos de reacción rápida y gran campo de visión en el visible e infrarrojo jugarán en los próximos años un papel fundamental en la identificación de contrapartidas electromagnéticas de los eventos de ondas gravitacionales producidos por la fusión de estrellas de neutrones y otros objetos compactos”, remarca Rafael Rebolo López, director del IAC.
Josefa Becerra, investigadora post-doctoral en el IAC y ponente en esta rueda de prensa, que ha contribuido con observaciones en rayos X con Chandra, y observaciones en radio y óptico-IR, nos cuenta como principalmente se ha encargado de la espectroscopia óptica con Gemini, cuyos resultados se publican simultáneamente hoy en la revista Nature.

Alberto J. Castro Tirado, Profesor de Investigación del Instituto de Astrofísica de Andalucía – CSIC comenta como “la observación de la contrapartida óptica en el Hemisferio Norte, horas después tras su descubrimiento desde el Hemisferio Sur, fue todo un reto. La única instalación española que lo logró fue el telescopio robótico Javier Gorosabel, que inauguramos en 2015 como parte de la estación astronómica BOOTES-5, en el Observatorio Nacional de San Pedro Mártir en Baja California (México) y con el que terminamos el despliegue de la Red BOOTES en el Hemisferio Norte. La observación se hizo apuntando casi al horizonte pocos minutos después de la puesta de Sol”. Además “el uso del VLT, el conjunto de 4 telescopios de 8,2m de diámetro en el Observatorio Austral Europeo en Cerro Paranal (Chile) por parte de nuestra colaboración, y con el que adquirimos espectros durante 15 días que cubren desde la zona del ultravioleta cercano hasta el infrarrojo cercano nos permitió identificar la “kilonova” asociada con la fuente emisora de ondas gravitaciones en la galaxia NGC 4993 a 130 millones de años-luz”.

Las contribuciones españolas para observar la contrapartida óptica han sido también cruciales para una nueva medida de la “constante de Hubble” – la cantidad que representa el ritmo de expansión local del universo, que determina la escala global del mismo y que es de importancia fundamental en cosmología. La nueva medida no precisa de ninguna “escalera de distancias cósmicas” y es independiente de medidas previas de esta cantidad fundamental. Por lo tanto, el nuevo descubrimiento también da inicio a la era de la cosmología de ondas gravitacionales.

Todas estas observaciones tienen el apoyo de “la Sociedad Española de Astronomía (SEA) , con sus 800 profesionales, y la RIA, como organismo que coordina las Infraestructuras Españolas Científico-Técnicas en Astronomía, que continuará aportando todo su conocimiento, experiencia e instrumentación de vanguardia al servicio de los dos desafíos que se nos plantean en los próximos años: incrementar la detección simultanea de contrapartidas electromagnéticas a estas ondas gravitacionales y avanzar en la comprensión de los fenómenos físicos que se vislumbran a través de esta nueva y fascinante ventana al universo”, como han enfatizado Francesca Figueras, presidenta de la SEA y Vicent Martínez, coordinador de la RIA.

Neutrinos, los otros mensajeros

Los rayos cósmicos de muy alta energía fueron descubiertos hace más de un siglo y aún hoy se desconoce qué “aceleradores cósmicos” puedan producirlos. Los objetos compactos (estrellas de neutrones, micro-quásares, etc.) podrían ser algunos de estos aceleradores, pero no se conocen muy bien los mecanismos que estarían actuando. La información combinada que pueden proporcionar las observaciones “multi-mensajero” son por tanto esenciales. En particular la observación de neutrinos de muy alta energía revelaría la aceleración de protones y núcleos cargados. El que los detectores de ondas gravitacionales pueden “avisar” de los cataclismos de objetos compactos a otros instrumentos, entre ellos a los telescopios de neutrinos, abre enormes posibilidades a la Física de Astropartículas. Esa búsqueda combinada ya se está produciendo: la astronomía de multi-mensajeros crece.

Enrique Zas, del Instituto Galego de Física de Altas Enerxías, y Juan José Hernández Rey, director del Instituto de Física Corpuscular (Universidad de Valencia/CSIC), indican que telescopios de neutrinos, entre ellos Pierre Auger y ANTARES, en el que trabajan científicos españoles, están buscando neutrinos asociados a esta fusión de estrellas de neutrones observada en ondas gravitacionales por LIGO-Virgo, “lo que demostraría que estos cataclismos son una de las (aún) misteriosas fuentes de rayos cósmicos”.


Enlaces relacionados
Artículo: “GW170817: Observation of gravitational waves from a binary neutron star merger.”

Monday, October 9, 2017

CTA Releases its Updated Science Case

This is the press release about it, which has been originally published by CTA and reproduced by the Institute of Space Sciences as well as by plenty of institutes and news agencies around the world.




 




The latest iteration of the Cherenkov Telescope Array’s (CTA’s) science case, Science with the Cherenkov Telescope Array, was made available today via the CTA website library and will be published in a special edition of International Journal of Modern Physics D in the coming weeks. The work includes more than 200 pages that introduce and elaborate on CTA’s major science themes and place CTA in the context of other major observatories.

“The release of this document represents a major milestone for CTA, and it details the breadth and the richness of the science that will be done with the observatory over the next decade,” says CTA Co-Spokesperson Prof. Rene Ong. “The document would not have been possible without the hard work of literally hundreds of CTA Consortium members over a period of many years.”

CTA will be the foremost global observatory for very high-energy gamma-ray astronomy over the next decade and beyond. The scientific potential of CTA is extremely broad: from understanding the role of relativistic cosmic particles to the search for dark matter. CTA will explore the extreme Universe, probing environments from the immediate neighbourhood of black holes to cosmic voids on the largest scales. With its ability to cover an enormous range in photon energy from 20 GeV to 300 TeV, CTA will improve on all aspects of performance with respect to current instruments. And its wider field of view and improved sensitivity will enable CTA to survey hundreds of times faster than previous TeV telescopes.

CTA will seek to address a wide range of questions in astrophysics and fundamental physics that fall under three major study themes: understanding the origin and role of relativistic cosmic particles, probing extreme environments and exploring frontiers in physics (Chapter 1).

“The Key Science Projects described in the document – surveys and deep observations of key objects – will provide legacy data sets of lasting value and will provide important input for the planning of CTA's user programme,” said CTA Spokesperson Prof. Werner Hofmann.

Some of the most promising discoveries will come from a survey of our Milky Way galaxy, which should discover more Galactic sources for improved population studies and for advancing our understanding the origin of cosmic rays (Chapter 6); the Ramon y Cajal researcher from the Institute for Space Sciences (IEEC-CSIC) explains: "we will observe our Galaxy with a sensitivity 10 times better that with the current instruments, allowing us to finally understand long-standing questions such the origin of the cosmic rays, which have been eluding us for 100 years!";  the search for the elusive dark matter with models not accessible by other experiments (Chapter 4); and the detection of transient phenomena like gamma-ray bursts and gravitational wave events associated with catastrophic events in the Universe (Chapter 9).

“For me, the most exciting aspect of CTA is the potential for truly unexpected discoveries,” says CTA Project Scientist, Prof. Jim Hinton. “CTA pushes to shorter timescales, higher energies and more distant objects. Pushing back the frontiers in astronomy always leads to something truly new and exciting, and now we’re all just itching to get started.”

It has been a decade since science planning for CTA started, resulting in a series of publications in a special edition of Astroparticle Physics in 2013. The current work began that same year with an organized effort by the CTA Consortium to develop CTA’s Key Science Projects (KSPs) in 2013. After three years of development and refinement that including internal and external reviews, the KSPs were incorporated into a single document: Science with the Cherenkov Telescope Array.

Notes for Editors:
CTA (http://www.cta-observatory.org) is a global initiative to build the world’s largest and most sensitive high-energy gamma-ray observatory. More than 1,350 scientists and engineers from 32 countries are engaged in the scientific and technical development of CTA. The Observatory will be constructed by the CTAO gGmbH, which is governed by Shareholders and Associate Members from a growing number of countries.

CTA will serve as an open observatory to the world-wide physics and astrophysics communities. The CTA Observatory will detect high-energy radiation with unprecedented accuracy and approximately 10 times better sensitivity than current instruments, providing novel insights into the most extreme events in the Universe.
CTA is included in the 2008 roadmap of the European Strategy Forum on Research Infrastructures (ESFRI). This project is receiving funding from the European Union’s Horizon 2020 research and innovation programs under agreement No 676134. This project has received funding from the European Union’s Seventh Framework Programme ([FP7/2007-2013] [FP7/2007-2011]) under Grant Agreement 262053.
Contact Information:
Prof. Rene Ong, CTA Co-Spokesperson
+1-3108253622; rene@astro.ucla.edu

Prof. Jim Hinton, CTA Project Scientist
+49-6221-1516201; jim.hinton@mpi-hd.mpg.de

Diego Torres,
dtorres@ice.csic.es

Prof. Werner Hofmann, CTA Spokesperson
+49-6221-516330; werner.hofmann@mpi-hd.mpg.de

Prof. Ulrich Straumann, CTAO gGmbH Managing Director
+49-6221-516471; strauman@physik.uzh.ch

Megan Grunewald, CTA Communications Officer
+49-6221-516471; mgrunewald@cta-observatory.org
 

Tuesday, September 5, 2017

ESA INTEGRAL Picture Of the Month in September 2017 comes from the EXO 1745-248 study



Low Mass X-ray Binaries (LMXBs), binary systems containing a compact object, are among the brightest and most extreme systems in the Universe. In these systems a neutron star (1.4-2 M) or black hole (5-15 M) accretes matter transferred by a low-mass (less than 1 M) companion star. This matter in-spirals toward the compact object usually forming an accretion disk in which a large amount of potential energy is dissipated reaching temperatures of tens to hundreds of millions of degrees Kelvin and making LMXBs powerful sources in the soft and hard X-ray band. The low magnetic field of the compact objects allows the disk to extend to small radii, experiencing strong gravity and reaching high velocities, thus making these systems ideal laboratories to study the behavior of the accretion flow in the relativistic regime.

With the aid of the ESA missions XMM-Newton and INTEGRAL, a transient neutron star LMXB, EXO 1745-248, hosted in the Globular Cluster Terzan 5, has been studied during an X-ray outburst. The high-quality broad-band spectra provided by INTEGRAL have helped to constrain the continuum, dominated by a high-temperature (40 keV) thermal Comptonization, allowing the high energy resolution, spectroscopic instruments onboard XMM-Newton to unveil a wealth of narrow and broad emission lines superimposed to the continuum.

Features at energies compatible with K-α transitions of ionized Sulfur, Argon, Calcium, and Iron were detected, with a broadness compatible with Doppler broadening in the inner part of an accretion disk truncated at about 40 km from the neutron star center. Strikingly, at least one narrow emission line ascribed to neutral or mildly ionized Iron is needed to model the prominent emission complex detected between 5.5 and 7.5 keV. The different ionization states and broadness suggest an origin in a region located farther from the neutron star than where the other emission lines are produced.

In the figure the light curve of the 2015 outburst displayed by EXO 1745-248 as observed by IBIS/ISGRI and JEM-X on board INTEGRAL is shown. For completeness, the light curve obtained from Swift/XRT (and published previously by Tetarenko, 2016) is also shown. The hard-to-soft spectral state transition of EXO 1745-248 around 57131 MJD is marked with a dashed vertical line in the plots. Around this date, the count-rate of the source in the IBIS/ISGRI decreases significantly, while it continues to increase in JEM-X. The times of the XMM-Newton observation are also marked by red dashed vertical lines. Broad-band spectra of the source during the outburst are also shown together with the best fit model (upper panel), and residuals in units of sigma with respect to the best fit model (bottom panel). The spectra from different instruments have been fitted simultaneously. These are XMM-Newton/RGS1 (red), XMM-Newton/RGS2 (green), XMM-Newton/EPIC-pn (black), INTEGRAL/JEMX1 (blue), INTEGRAL/JEMX2 (cyan), and INTEGRAL/ISGRI (magenta).

This study has been led by the University of Palermo (Italy) and the INAF - Astronomical Observatory of Rome (Italy), has been partially performed at the Institut de Ciéncies de l'Espai (IEEC-CSIC) in Barcelona (Spain), in collaboration with the ISDC - Data Centre for Astrophysics in Versoix (Switzerland), the University of Cagliari (Italy), and other European institutions.

Reference:
  • "XMM-Newton and INTEGRAL view of the hard state of EXO 1745-248 during its 2015 outburst",
    M. Matranga, A. Papitto, T. Di Salvo, E. Bozzo, D. F. Torres, R. Iaria, L. Burderi, N. Rea, D. de Martino, C. Sanchez-Fernandez, A. F. Gambino, C. Ferrigno, L. Stella,
    2017, A&A, 603, A39
Credit of the figure:

Thursday, August 24, 2017

EXO 1745-248 hard state observations


Transient low-mass X-ray binaries (LMXBs) often show outbursts lasting typically a few-weeks and characterized by a high X-ray luminosity (Lx ≈ 1036 − 1038 erg s−1), while for most of the time they are found in X-ray quiescence (LX ≈ 1031 − 1033 erg s−1). EXO 1745–248 is one of them.

The broad-band coverage, and the sensitivity of instrumet on board of XMM-Newton and INTEGRAL, offers the opportunity to characterize the hard X-ray spectrum during EXO 1745–248 outburst.

We have recently reported on quasi-simultaneous XMM-Newton and INTEGRAL observations of the X-ray transient EXO 1745– 248 located in the globular cluster Terzan 5, performed ten days after the beginning of the outburst (on 2015 March 16th) shown by the source between March and June 2015. The source was caught in a hard state, emitting a 0.8-100 keV luminosity of ≃ 1E37 erg s−1.

The spectral continuum was dominated by thermal Comptonization of seed photons with temperature kTin ≃ 1.3 keV, by a cloud with moderate optical depth τ ≃ 2 and electron temperature kTe ≃ 40 keV. A weaker soft thermal component at temperature kTth ≃ 0.6–0.7 keV and compatible with a fraction of the neutron star radius was also detected. A rich emission line spectrum was observed by the EPIC-pn on-board XMM-Newton; features at energies compatible with K-α transitions of ionized sulfur, argon, calcium and iron were detected, with a broadness compatible with either thermal Compton broadening or Doppler broadening in the inner parts of an accretion disk truncated at 20 ± 6 gravitational radii from the neutron star. Strikingly, at least one narrow emission line ascribed to neutral or mildly ionized iron is needed to model the prominent emission complex detected between 5.5 and 7.5 keV. The different ionization state and broadness suggest an origin in a region located farther from the neutron star than where the other emission lines are produced. Seven consecutive type-I bursts were detected during the XMM-Newton observation, none of which showed hints of photospheric radius expansion. A thorough search for coherent pulsations from the EPIC-pn light curve did not result in any significant detection. Upper limits ranging from a few to 15% on the signal amplitude were set, depending on the unknown spin and orbital parameters of the system.

Find the full paper (A&A) here.

Wednesday, August 2, 2017

Normal TeV emission from the Galactic Center during the G2 pericenter passage

The primary motivation behind this observing campaign was to search for any flaring emission that may occur due to the passage of the G2 object near to the supermassive black hole (SMBH) at the center of the Milky Way galaxy.



The proximity of the passage of the G2 object to the SMBH could have provided a unique opportunity to study the  process  of  accretion  of  an  Earth-mass  body  onto  a  black hole, as well as addressing several questions regarding particle-acceleration mechanisms near to a SMBH. However, the results of recent observations at other wavelengths suggest that the G2 object  has  not been  disrupted  by  its  proximity  to  the  SMBH, therefore it is perhaps not surprising that no evidence for an enhancement in the VHE flux of Sgr A* was found.
 
The GC region has been observed with the MAGIC telescopes between  2012  and  2015,  collecting 67  hours  of  good-quality data. No effect of the G2 object on the VHE gamma-ray emission from the GC was detected during the 4 year observation campaign.

The lack of variability from the direction of Sgr A*, as measured by MAGIC, makes it difficult to rule out single models describing particle acceleration and gamma-ray emission mechanisms at the source. These observations may still prove useful as an accurate measurement of the baseline emission from Sgr A* in the case of any possible flaring activity in the future. Along  with the variability  study,  the  large  exposure  of  67 hours allowed us to derive a precise energy spectrum of Sgr A*, which  agrees  with  previous  measurements  within  errors. Furthermore we were able to study the morphology of the GC region.

As a result of this study, we confirm the detection in the VHE gamma-ray band of the supernova remnant G0.9+0.1, and report the detection with MAGIC of a VHE source of unknown nature in the region of the GC Radio Arc.

Find more details in our paper, available in: A&A 601, A33 (2017)

Wednesday, July 19, 2017

New book: Modelling Pulsar Wind Nebulae

Later this year, Springer is publishing 'Modelling Pulsar Wind Nebuale', in its Astrophysics and Space Library Series. This is an edited book, containing reviews and discussions on the status of pulsar wind nebuale research from a variety of perspectives. 


The book assesses, among others, the following questions: What kind of models do we already have and what kinds of models are needed to reach a more profound understanding of nebulae? Can models be combined? Which are the most promising avenues for unifying model classes? Can they be made versatile enough to interpret observations of hundreds of sources? To what extent are the results from different radiative models comparable? What key features are they missing? Up to what extent time-dependent models without spatial information are reliable/useful? Are hybrid hadronic/leptonic models necessary for modelling nebulae in general? What is the best case for a hadronic-dominated nebula? How can we differentiate hadronic from leptonic nebulae at an observational level? What is the impact of hybrid models and how can they be observationally tested? How do we move forward: What features are the models missing to account for the forthcoming data? 

Monday, July 17, 2017

GeV detection of HESS J0632+057

HESS J0632+057 is the only gamma-ray binary that has been detected at TeV energies, but not at GeV energies yet. Based on nearly nine years of Fermi Large Area Telescope (LAT) Pass 8 data, we report here on a deep search for the gamma- ray emission from HESS J0632+057 in the 0.1–300 GeV energy range. We find a previously unknown gamma-ray source, FermiJ0632.6+0548, spatially coincident with HESS J0632+057. The measured flux of Fermi J0632.6+0548 is consistent with the previous flux upper limit on HESS J0632+057 and shows variability that can be related to the HESS J0632+057 orbital phase. We propose that Fermi J0632.6+0548 is the GeV counterpart of HESS J0632+057. Considering the Very High Energy (VHE) spectrum of HESS J0632+057, a possible spectral turnover above 10 GeV may exist in Fermi J0632.6+0548, as appears to be common in other established gamma-ray binaries. 



Discussion
 
Using nearly nine years of Fermi-LAT data, we have carried out a detailed search for gamma- ray emission from HESS J0632+057, leading to the discovery of a previously unknown gamma-ray source, Fermi J0632.6+0548. 

Fermi J0632.6+0548 is spatially coincident with HESS J0632+057, and has a flux level that is consistent with the upper limit previously reported by Caliandro et al. (2013). Based on the orbital phase definition of HESS J0632+057 (Aliu et al. 2014), we searched for orbital variability, finding a flux and spectral change in two broad phase intervals (0.0–0.5 and 0.5–1.0). This variability further hints at a physical association with HESS J0632+057. However, because of the low statistics, neither a significant flux variability in an orbital light curve built with smaller bins, nor the 315-days orbital period in the power spectrum could be detected, leaving the association as likely, but conservatively unconfirmed. 

Malyshev & Chernyakova (2016) recently reported a 200–600 GeV detection of HESS J0632+057 at the 5σ level during the orbital phases 0.2–0.4 and 0.6–0.8. For the sake of comparison, we carried out Fermi-LAT data analysis in the 10–600 GeV range without gating off PSR J0633+0632, similar to what was done by Malyshev & Chernyakova (2016). In the 200– 600 GeV range, we confirm that two photons at energies 223 GeV (arrived at mission elapsed time (MET) 301884864, MJD 55404.04) and 578 GeV (arrived at MET 347664434, MJD 55933.89) are spatially consistent with HESS J0632+057. However, no detection of HESS J0632+057 was made during orbital phase 0.2–0.4 and 0.6–0.8 in 200–600 GeV, which is inconsistent with Malyshev & Chernyakova (2016). The inconsistency may be due to the different orbital phase definition adopted: In Malyshev & Chernyakova’s work, the orbital phases for the above-mentioned two photons are reported as 0.70 (223 GeV photon) and 0.36 (578 GeV photon). In fact these authors are using the orbital phase definition from Bongiorno et al. (2011) (MJD0 = 54857, period P = 321 days). These two photons yields the detection of HESS J0632+057 at 5σ level during orbital phases 0.2–0.4 and 0.6–0.8, in the 200–600 GeV range. On the other hand, in our analysis we used the orbital phase definition from Aliu et al. (2014), which has the same MJD0 but a refined period (P = 315 days). Correspondingly, the orbital phase of these two photons are calculated as 0.74 (223 GeV photon) and 0.42 (578 GeV photon). Thus, there is only one photon located in these orbital phases, which may explain the non-detection. The different spatial-spectral models used may also lead to the inconsistency: The preliminary seven-year source list was adopted in our analysis together with additional extended templates accounting for gamma-ray contributions from the Rosette Nebula and Monoceros Loop, while Malyshev & Chernyakova (2016) used the second catalog of hard Fermi-LAT Sources (2FHL; Ackermann et al. 2016). 

For a constraint on the spectral turnover from the VHE to the HE range, Malyshev & Chernyakova (2016) modelled Fermi-LAT data with a broken power law during the orbital phase 0.2–0.4 and 0.6–0.8 over 10–600 GeV. A 2σ (3σ) limit on the break energy (Ebr) was put as Ebr=180–200 GeV (Ebr=140–200 GeV), with a corresponding photon index Γ <1.2 (Γ <1.6) below Ebr. In the orbital phases 0.2–0.4 and 0.6–0.8, our analysis yielded non-detection, neither in the 10–600 GeV range or in the sub energy ranges (10–200 GeV or 200-600 GeV). Thus, further spectral constrains are insignificant. 


Fermi J0632.6+0548 is spatially coincident with 3FHL J0632.7+0550, which is a gamma- ray source detected in the Third Catalog of Hard Fermi-LAT Sources (3FHL, Fermi-LAT Collaboration, 2017). 3FHL J0632.7+0550 is proposed to be associated with HESS J0632+057 and is located within the 95% error circle of FermiJ0632.6+0548. Without gating off PSR J0633+0632, Fermi J0632.6+0548 is detected in the range 10–600 GeV with TS=25 and a photon index of 1.74±0.41, which is consistent with the photon index of 3FHL J0632.7+0550, 1.86±0.37, hinting for a possible association. 

If the association between Fermi J0632.6+0548 and HESS J0632+057 posed in this paper is real, it will be the first detection of HESS J0632+057 in the high energy (HE) GeV range, completing its radiation spectrum from radio to TeV. Adopting a distance of 1.4 kpc (Aragona et al. 2010; Casares et al. 2012), the GeV luminosity of HESS J0632+057 is 2 × 1033 erg s1, about two orders of magnitude lower than those of known gamma-ray binaries (Caliandro et al. 2013, 2015; Hadasch et al. 2012; Ackerman et al. 2012a; Corbet et al. 2016). The radio, X-ray, and TeV luminosities of HESS J0632+057 are also dimmer than known galactic gamma-ray binaries (e.g., Paredes et al. 2007; Skilton et al. 2009; Aliu et al. 2014). Despite the different orbital parameters and multi-wavelength behavior, the companion stars in gamma-ray binaries HESS J0632+057 and LS I +61 303 are very similar. HESS J0632+057 has a B0Vpe star as companion (MWC 148; Aragona et al. 2012), whereas the spectral type of the companion star in LS I +61 303 is B0Ve (Zamanov et al. 2016). The lower GeV luminosity can be due to a much larger orbital separation (at periastron the system is twice the size of LS I +61 303, while at apastron it is about seven times bigger, Casares et al. 2012, Zamanov et al. 2016). MWC 148 has a similar radius and mass as LS I +61 303, but its circumstellar disc is about five times larger (Zamanov et al. 2016). The compact object in LS I +61 303 only passes through the outer part of the circumstellar disc at periastron. However, in HESS J0632+057 the compact object goes into the innermost parts and penetrates deeply in the disc during periastron passage (Zamanov et al. 2016), which may lead to large absorption/obscuration effects and explain the low GeV emission. 

Detection of HESSJ0632+057 with ground-based imaging atmospheric Cherenkov telescopes from hundreds of GeV to several TeV (Figure 3; Aliu et al 2014) indicates that the VHE spectrum is not a simple extrapolation of the LAT spectra we detected, but likely a different spectral component. Thus, a spectral turnover should exist in Fermi-LAT spectrum. The spectral turnover could arise due to pair production on stellar photons for gamma rays above 50 GeV (Dubus 2006; Sierpowska-Bartosik & Torres 2009), or distinct emission components for HE and VHE spectra. We modeled the HESS J0632+057 with a broken power law in the 0.1–300 GeV range. However, the likelihood ratio test indicates that a broken power law is not significantly preferred over a simple power law model. Thus, the spectral turnover in Fermi-LAT spectrum could not be explicitly determined because of the low statistics. Based on the SEDs of HESS J0632+057 (Figure 3), we propose the spectral turnover to be above 10 GeV, which is consistent with the estimation by Caliandro et al. (2013). In the well-studied gamma-ray binaries LS 5039 and LS I +61 303, the GeV spectra are best represented by a power law with an exponential cutoff. These spectra do not extrapolate to the VHE range either (Hadasch et al. 2012). Thus, despite its low GeV flux, HESS J0632+057 resembles known gamma-ray binaries and hints for the authenticity of this gamma-ray association. 
 
LS I +61 303 shows 1667-day multi-wavelength super-orbital modulation, which may be due to the quasi-periodic variation of the circumstellar disc (Chernyakova et al. 2012; Li et al. 2012, 2014; Ackermann et al. 2013; Ahnen et al. 2016; Saha et al. 2016). Hosting a similar companion, HESS J0632+057 may also have multi-wavelength super-orbital modulation. However, its much longer orbital period than LS I +61 303 (26.496 days, Gregory 2002) makes the detection difficult.


 Read the full paper at xxx.lanl.gov/abs/1707.04280. Soon in The Astrophysical Journal.



Friday, July 14, 2017

Dust Radiative Transfer Modeling of the Infrared Ring around the Magnetar SGR 1900+14



A peculiar infrared ring-like structure was discovered by Spitzer around the strongly magnetized neutron star SGR 1900+14. This infrared (IR) structure was suggested to be due to a dust-free cavity, produced by the Soft Gamma-ray Repeaters (SGRs) Giant Flare occurring in 1998, and kept illuminated by surrounding stars. Using a 3D dust radiative transfer code, we aimed to reproduce the emission morphology and the integrated emission flux of this structure assuming different spatial distributions and densities for the dust, and different positions for the illuminating stars. We found that a dust-free ellipsoidal cavity can reproduce the shape, flux, and spectrum of the ring-like IR emission, provided that the illuminating stars are inside the cavity and that the interstellar medium has high gas density (n H ~ 1000 cm−3). We further constrain the emitting region to have a sharp inner boundary and to be significantly extended in the radial direction, possibly even just a cavity in a smooth molecular cloud. We discuss possible scenarios for the formation of the dustless cavity and the particular geometry that allows it to be IR-bright.







From the discussion of our work:

We have performed several dust RT calculations as- suming elliptical dust shell/cavity geometries as well as a disrupted wind profile, and by positioning the two supergiants stars inside or outside the dust cavity. We have found that the dust ring morphology, similar to that found on the Spitzer data of SGR1900+14, is re- covered only in the cases where the stars are inside the cavity. Furthermore, we approximately reproduce the total integrated fluxes at 16 and 24 μm only by assuming a gas density of nH ∼ 1000 cm-3 for all the dust geometries we assumed. The corresponding mass of the dust responsible for the ring emission is Mdust ∼2M_sun.

Given these results, the first question to ask is whether or not the models that reproduce the observed dust emission morphology and total flux are realistic or not. In particular the gas density, implied by our modelling to explain the Spitzer infrared luminosity by dust illumination, appears to be very high compared to that of the diffuse galactic ISM. Before discussing the possible nature of this high density, we first clarify what assumptions/parameters in our modelling might have caused an artificial high gas density, not representative of the real ISM density around the magnetar. Firstly, we point out that the gas density is not measured directly from the gas emission but inferred from the dust density divided by a dust-to gas ratio of 0.00619 (which is characteristic of the assumed Milky Way dust model, see section 2.5). However, this dust-to-gas ratio, representative of the kpc scale ISM of the nearby Milky Way regions, presents sig- nificant local variations in the ISM (see e.g. Reach et al. 2015). Furthermore, since the assumed size distribution of the grains is also representative of the local Milky Way, this also has an effect on the derived dust density. In fact, the MIR emission in our modelling is mainly produced by small grains (sizes ∼ 10-3 -- 10-2 μm) which are stochastically heated. If the grain size dis- tribution is more skewed towards smaller grain sizes, compared to the one we are assuming, this would re- quire significantly less dust mass to reproduce the ob- served MIR emission. The grain size distribution is known to be affected by both dust destruction and formation processes, but it is not possible to constrain it further with our observations. On the other hand, we also note that if the cavity has been created by dust destruction, the grain size distribution there should in- stead favour the presence of large dust grains rather than small ones (Waxman & Draine 2000; Perna & Lazzati 2002). In fact, a number of studies (Fruchter et al. 2001; Perna & Lazzati 2002; Perna, Lazzati & Fiore 2003) have shown that the X-ray flux is more effective at destroying small grains than larger ones. The precise evolution of the dust grain distribution is dependent on both the spectral shape and overall intensity of the illuminating source, on the composition of the grains, as well as on the relative importance of the processes of X-ray Heating, Coulomb Explosion and Ion Field Emission, the last two of which being particularly uncertain (Fruchter et al. 2001; Perna & Lazzati 2002). How- ever, even within these uncertainties, all the models generally predict that smaller grains will be destroyed to larger distances than larger ones2. Therefore, there would be a region in which only selective destruction took place, leaving behind a dust distribution skewed towards large grains. At the inner edge the distribution would be skewed towards big grains, progressively changing into the undisturbed (pre-burst) distribution at larger distances. An attempt at modeling these effects would be worthwhile if the quality of the data were to allow a comparison with observations, but this is not possible with the current data. Finally, in our modelling we only assumed the two supergiant stars, the most luminous stars in the field, to be heating the dust shell/cavity. However, other sources of radiation might well play a role (e.g. other fainter stars within the cavity) and, in this case, the needed gas density to match the observed fluxes would be lower. On the other hand, note that the constant ∼ 1034 erg/s X-ray luminosity emitted by the magnetar is too low to power the dust emission. In fact, the wavelength– integrated dust emission luminosity for the models that fit the MIR fluxes is in the range 3.7-4 ×1035 erg/s. An additional mechanism to heat the dust is also collisional heating in hot plasma, where the dust is heated by the collisions with high energy electrons. This is expected if the dust is embedded in shocked gas with temperatures of order of 106 K. However, in this case we might expect to see an X-ray diffuse emission from the hot gas around the magnetar as well, which is not observed.



Vrba et al.(2000) argued that SGR 1900+14 is associated with a cluster of young stars (much fainter in apparent magnitude than the two M supergiants we considered in our modelling) which are probably embedded in a dense medium. This interpretation is qualitatively consistent with our results. As proposed by (Wachter et al. 2008), the 1998 Giant Flare could have produced the cavity by destroying the dust within it. Assuming a constant dust density within the cavity region, corresponding to nH=1000cm-3, we estimated a total dust mass of order of 3M_sun that was plausibly present be- fore being destroyed by the flare. An energy of about E ∼ 6 × 10^45erg would suffice to destroy this amount of dust, consistent with the estimates by (Wachter et al. 2008) based on Eq. 25 in Fruchter et al. (2001). The size of the region with destroyed dust would be larger for smaller grains, as discussed above. In this scenario, the high density we derived would be similar to the high density ISM around the magnetar. Furthermore, we note that high density of the ISM (nH=105–107 cm−3) has been found in the environment surrounding GRBs (Lazzati & Perna 2002), which should be similar to that where magnetars are located.

The wind model we considered was meant to be sim- ilar to the scenario where the dust distribution outside the cavity was mainly determined by the wind of the magnetar progenitor while internally disrupted by the Giant Flare. However, gas densities at 1pc distance in a typical stellar wind are expected to be several orders of magnitudes lower than those we found. Thus, this last scenario is unlikely if the ring density is indeed so high. Another possibility is that the dust emission ring is the infrared emission from the supernova remnant (SNR) of the magnetar progenitor. The dust mass associated with the shell model with nH =1000 cm-3 is Mdust=1.9 M_sun, which is a factor 3–4 higher than the measurement of dust mass around SN1987 by Matsuura et al. (2011, 0.4–0.7 M⊙). However, given the large uncertainties in the inferred dust masses, and that our value for the dust density might have been overestimated because of the reasons given above, it may well be that the amount of dust needed for the shell model is compatible with that of SNRs. The SNR scenario was also considered by (Wachter et al. 2008) but discarded because of the lack of observed radio and X-ray emission from the ring. However, if we consider i) the IR/X luminosity ratio of ∼ 10^-1 -- 10^2 measured by Koo et al. (2016) for many SNRs, ii) the total IR luminosity of the ring in our models (∼ 4 × 1035 erg/s), and the X-ray detection limit for the ring (∼ 2 × 1033erg/s in the 2--10 keV band Wachter et al. 2008), this structure is still compatible with being a SNR with high IR/X ratio.

On the other hand, we can also compare the 24μm luminosity with the expected X-ray luminosity, according to Figure 12 of Seok et al. (2013), which studied a sample of SNR in the Large Magellanic Cloud. If the magnetar was located at the distance of the LMC (50kpc), we would have νFν (24μm) = 1.5 × 10−10 erg/s/cm2, and the relative expected X-ray flux would be 2 × 10−10 erg/s/cm2. This would translate in an intrinsic X-ray luminosity of ∼ 6 × 1037 erg/s, which should have been clearly detected in the case of the SGR 1900+14. Hence, given the information we have at hand we can- not discard the SNR scenario on the base of the ob- served IR/X-ray luminosity ratio, although it would be a rather peculiar remnant compared with what we see around other Galactic pulsars or magnetars (Green 1984; Martin et al. 2014).

However, if we also consider the shape of the normal- ized average surface brightness profiles, shown in Fig.8, this provides a strong evidence that 1) there is very little amount of dust inside the cavity and 2) the emitting dust is much more extended than a simple thin shell. These findings are compatible with the scenario where the cavity has been produced by the Giant Flare within a high density medium. However, the SNR scenario would still be acceptable in the case the transition in density between the shell and the surrounding ISM is smoother than what we assumed in our modelling.

Regardless of the origin or the exact distribution of the illuminated dust, or the exact nature of the dust free cavity, our models show that we are able to ob- serve this illuminated dust structure only because of two favourable characteristics: 1) the high dust density in the local region, and 2) the illuminating stars coincidentally lay inside the shell. Similar dust structures might potentially be present around many other magnetars or pulsars but they would be invisible to us because of the lack of either one of the two above local properties of this particular object.

arXiv e-print (arXiv:1701.07442)
The Astrophysical Journal, Volume 837, Issue 1, article id. 9, 10 pp. (2017).

Wednesday, January 11, 2017

The puzzling case of the accreting millisecond X–ray pulsar IGR J00291+5934: flaring optical emission during quiescence



We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Although the source was in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). After subtracting the flares, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even when a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars, was detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing the new outburst of the source, which occurred a few months later, in 2015.

From the conclusions
We presented the results of optical gri photometry of the accret- ing millisecond X–ray pulsar IGR J00291+5934 during quies- cence. Observations were carried out with the GTC equipped with OSIRIS on 2014, August 31.
The system displays a strong flaring activity in all bands, above all in the g band. This flaring activity is comparable to the activitiy previously observed by Jonker et al. (2008) in in- tensity (∼ 1mag) and duration. When the flares were subtracted, we observed an indication of a sinusoidal modulation at the sys- tem orbital period. As in the case of the observations reported in Jonker et al. (2008), a phase shift of the light curves with respect to phase 0.5 is detected. All our optical light curves are con- sistent with an enhanced activity of the source, with a significant ∆I with respect to the observations during quiescence reported in D’Avanzo et al. (2007) of 0.7 ± 0.1 mag. Finally, the spectral en- ergy distribution built during a flare (at phase ∼ 0.2) was fitted by a power law with index α = 0.31 ± 0.32, which is consistent with what is predicted for a multi-colour black body of an accretion disc with T > 30, 000 K. All these results can be explained when we consider that the principal player in the quiescent emission of IGR J00291+5934 of our dataset is not the companion star, as expected for a quiescent LMXB, but the accretion disc. The disc, after it was emptied out during the 2008 outburst, started replen- ishing in preparation to the next outburst (which occurred in July 2015). In this way, both the observed brightening of the source and the phase shift of the light curves can be explained, since the main optical emitter in the system might be an asymmetry in the disc, like a hot spot or a superhump. The controversial re- sults reported in D’Avanzo et al. (2007) and Jonker et al. (2008) could also be accounted for in this scenario: in the first case, the system was observed after the end of an outburst, which proba- bly left the accretion disc almost empty, thus explaining why the typical modulation due to the irradiated companion star alone was observed. In the latter, instead, the system was preparing it- self for the 2008 outburst; thus the accretion disc was no longer empty and probably strongly contributed to the quiescent optical emission of the system (as in the case of the 2014 dataset).

According to this picture, we thus conclude that the observed 2014 flaring activity might indicate an accretion disc during quiescence. Magnetic reconnection events in the disc might be a likely possibility. Further multi–wavelength optical observa- tions during quiescence, possibly over longer timescales, could shed light on the true origin of the quiescent flares of IGR J00291+5934.

The full paper (A&A) can be accessed at here.

Thursday, December 22, 2016

Search for transitions between states in redbacks and black widows using seven years of Fermi-LAT observations



Considering about seven years of Fermi-Large Area Telescope (LAT) data, we present a systematic search for variability possibly related to transitions between states in redbacks and black widow systems. Transitions are characterized by sudden and significant changes in the gamma-ray flux that persist on a timescale much larger than the orbital period. This phenomenology was already detected in the case of two redback systems, PSR J1023+0038 and PSR J12274853, for which we present here a dedicated study. We show the existence of only one transition for each of these systems over the past seven years. We determine their spectra, establishing high-energy cutoffs at a few GeV for the high gamma-ray state of PSR J1023+0038 and for both states of PSR J12274853. The surveying capability of the Fermi-LAT allows studying whether similar phenomenology has occurred in other sources. Although we have not found any hint for a state transition for most of the studied pulsars, we note two black-widow systems, PSR J2234+0944 and PSR J14464701, whose apparent variability is reminiscent of the transitions in PSR J1023+0038 and PSR J12274853. For the other systems we set limits on potential transitions in their measured gamma-ray light curves.



Discussion

We temporally enlarged the analysis of both J1023+0038 and J12274853, the known transitional pulsars, by considering nearly seven years of Fermi-LAT data. Our results on the light curves of these systems confirmed previous reports. Only one transition is detected for each. We found that they transitioned from a low to a high state, and from a high to a low state, respectively. In addition, we determined their spectra, and confirmed the existence of high-energy cutoffs at a few GeV with the significance above 3σ for the high gamma-ray state of J1023+0038 and for both states of J12274853. 

We searched for state transitions in all known RBs and BWs by analyzing their long term light curves in different time binnings. Our analysis included a fixed 60-day time binning, used for detection of the already known transitional pulsars mentioned above. We have also performed simulations for each source in order to determine, assuming their average level of flux, the minimum integration time needed for a Fermi-LAT detection at a TS=25 level. This is a flux-motivated, source-by-source-determined binning, and we have used it to study the light curves as well. By analyzing the light curves we were able to determine whether a transition has happened and if not, what are the features of the transitions that can be ruled out. 

For most of the pulsars, we have not found any hint for a state transition in our search. In the light of negative results, trying to infer conclusions regarding e.g., rate of transitions, seems daunting. Transitions are inextricably linked to the local scenario, for instance, to the variations in mass accretion rate. A negative result cannot be directly used to imply that all RBs and BWs other than the swinging ones, have actually finished any swinging phase, and are all in a final -fully recycled- state. Future surveying may prove the opposite, and when this swinging will happen, if it does, can simply not be predicted. 

We found two particularly interesting cases in our search. J2234+0944 and J14464701 are, in contrast with the known transitional pulsars, BW systems. Both of these sources have very low companion masses. Both were discovered at Parkes as part of a radio search program for pulsars in coincidence with unidentified Fermi-LAT sources (see Ray et al. 2012). The radio detection of J2234+0944 was before the possible transition at MJD 55500. 

J2234+0944 has a period of 3.63 ms and is part of a system with a companion of at least 0.015 M, in an orbit of 0.42 days. J14464701 is in a system with a companion of at least 0.019 M, in an orbit of 0.27 days (Keith et al. 2012). The orbits are almost circular, which is consistent with the model in which the spin-up of the pulsar is associated with Roche lobe overflow from a nearby companion. Both orbital periods are much smaller than the timescale for the variability we have found. Thus the latter can hardly have an orbital origin. But are these indeed state transitions similar to those found in J1023+0038 and J12274853? 

The variability of J14464701 is not conclusive, although a possible back-and-forth flux jumps during the years spanned by Fermi observations is compatible with the data. Inconclusiveness arises from the fact of it being a very dim source in comparison to the known transitional pulsars (see Table 1), and from the (related) lack of a sufficient number of points in each of the putative states. We recommend further monitoring of this source in gamma rays and other frequencies. The variability of J2234+0944 is clearer, and a flux jump seems to have happened (see Table 2 and Fig. 3). Its brightness in gamma rays allows for a clear distinction of two states that can be deemed similar to those in J12274853, with an apparent transition from lower to higher gamma-ray fluxes. However, the low level of fluxes found in existing X-ray observations cast doubts that we are witnessing the same phenomenology. Future X-ray observations will tell whether this pulsar has a short timescale phenomenology as that found for J1023+0038 and J12274853, yet at a significantly lower level of flux. If not, we may be witnessing a gamma-ray state transition produced at the intra-binary shock and/or with a dim (if any) counterpart at lower frequencies. The latter would not be impossible within the propeller model used to investigate J1023+0038 and J12274853. If the propeller is strong enough to preclude any matter from reaching the surface and the disk component is significantly dimmer in X-rays in comparison with redback systems, it is in fact expected that the X-ray emission would be undetectable, smaller by even more than several orders of magnitude in comparison with that of J1023+0038 and J12274853 (see Fig. 1 in Papitto & Torres 2015). A proper model, together with deep X-ray observations would help test this setting. Alternatively, we can also entertain the possibility that the pulsar magnetosphere could globally vary (see, e.g., the study by Ng et al. 2016, even though it is a very different system). If this is the case, the variation in the two states would be explained by a closer-to-the-pulsar phenomenology, and could just be interpreted as being different outer gap-generated emission, as if the pulsar would be isolated. This would naturally encompass the fact that gamma-ray pulsations are found both before and after the flux jump. 

The full paper (in press in ApJ) can be obtained here