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

Monday, December 12, 2016

A rotationally-powered magnetar nebula around Swift J1834.9–0846



A wind nebula, generating extended X-ray emission, was recently detected surrounding Swift J1834.9-0846. This is the first magnetar for which such a wind nebula was found. Here, we investigate whether there is a plausible scenario where the pulsar wind nebula (PWN) can be sustained without the need of advocating for additional sources of energy other than rotational. We do this by using a detailed radiative and dynamical code that studies the evolution of the nebula and its particle population in time. We find that such a scenario indeed exists: Swift J1834.9-0846's nebula can be explained as being rotationally-powered, as all other known PWNe are, if it is currently being compressed by the environment. The latter introduces several effects, the most important of which is the appearance of adiabatic heating, being increasingly dominant over the escape of particles as reverberation goes by. The need of reverberation naturally explains why this is the only magnetar nebula detected, and provides estimates for Swift 1834.9-0846's age. 




 

Discussion
Our simulations show that a rotationally-powered PWN can generate the extended emission observed around Swift J1834.9–0846 with no other source of energy beyond the pulsar’s spin-down power. The latter is always (by model construction) an upper limit to the injected energy in particles, which is given by (1 − η)Lsd(t) at any given time, with η < 1 being the instantaneous sharing, also known as magnetic fraction. Our results do not preclude nor rule out further injection of pairs. For instance, more particles could be injected by a yet-unclear transfer of the magnetar’s magnetic

field, twisted in the inner magnetosphere, into particle accel- eration, or by short bursts. Instead, these results show that an additional source of energy is not needed in order to understand the observations, under the assumption of a smooth spin-down braking. The latter is the usual assumption for all pulsars and PWNe, and is equally caveated: the pulsar evolution could be far from such a pleasant ride. How much a magnetar burst, for instance, may change the average properties of a surrounding PWN, are subjects for future studies.

Here, our study demonstrates the plausibility of a PWN ori- gin of the emission without invoking any special relationship with the high magnetic field at the surface of the neutron star. This is in line with what we have already seen in a systematic study of all TeV-detected PWNe: Nebula parameters such as field, magnetization, break energy, etc. neither correlate nor anti-correlate with the pulsar’s features, as determined from the spin measurements (Torres et al. 2014). We have also seen that high-B pulsars can maintain PWNe as well, and that ef- ficiencies are not a good tracer of PWN observability as soon as a complex time evolution for the particle injection and their burning is considered.

A detailed dynamical-radiative model was used to see that the size and the spectrum of Swift J1834.9–0846’s nebula can be matched within a rotationally-powered scenario. The requirement for this to happen is that the nebula should currently be compressed by the environment. We have found that this is possible for an age of around 8000 years, about 1.6 times the estimated characteristic age. Thus, Swift J1834.9–0846 is unrelated with W41 in this model, what seems likely given the existence of several other candidates for such a connection. The values of the magnetic field and the instantaneous sharing of energy in the nebula are similar to those found in simulations for other systems. Swift J1834.9–0846’s nebula would also be strongly particle-dominated, as all others. Perhaps the most notable deviation in resulting parameters appears for the break energy (10^7, with the typical value being about few 10^6). Physically, this difference may simply point to the influence of reverberation, through adiabatic heating.

We have shown that diffusion losses (i.e., particles escaping from the nebula) are actually more important at high energies than synchrotron losses. This is unusual, and is a result of the low magnetic field that we found in the nebula. This may

sound counter-intuitive taken into account the high field at the surface of the magnetar, but it is not: The low PWN field is a result of the low spin-down and period, and the large size of the nebula, and it is driven in our model in the same way we believe it is for all other PWNe. We have shown too that a sub-Bohm diffusion is not preferred, and that the absence of diffusion in freely-expanding nebulae cannot produce a good match to the observational data. In fact, we have found, that a rotationally-powered PWN can explain the observations only when simulations consider reverberation effects. In this case, simulations take into account that particles can gain energy via adiabatic heating, thus enhancing the population of pairs that can emit at X-ray energies. Particularly, we found that the larger is the age along the compression process, the larger is the energy at which the adiabatic (heating) timescale dom- inates over the escape one, and the faster the particles gain energy. For about a thousand years, then, a high luminosity can be maintained. As soon as the compression burn off all particles, and the pressure inside the PWN makes it bounce producing a subsequent reduction of the magnetic field, the luminosity will decrease. These results are in line (extrapolating several orders of magnitude, towards the magnetar realm) the study of Bandiera (2014). He used a simplified analyti- cal approach to show that middle age PWNe are more likely observable during the reverberation phase. In particular, the spin-down and X-ray luminosity of Swift J1834.9–0846 are roughly consistent with the extrapolated curve in figure 3 of Bandiera (2014) (i.e., Swift J1834.9–0846 is at or slightly above the extrapolated tc+ line of figure 3 in that paper).

The need of reverberation for fitting the spectral and size data can in fact explain why it would be difficult to find other magnetar nebulae (or in general, other nebulae of pulsars having a low spin-down power). Only those nebulae in the process of reverberating might shine enough to be- come detectable by our instruments: Before reverberation, the timescales for losses are just too long so that the rate of photon emission is minimal. Too much time after reverberation, all pairs are burnt, the magnetic field is low, and the nebulae may again be invisible. Since reverberation in such low spin-down power pulsars lasts for about a thousand years, it is reasonable to find few such nebulae. The additional effect found so that for a sufficiently low nebular magnetic field the catastrophic losses dominate over synchrotron adds to the dim character of the systems.

While this work was being prepared, two papers have been published with opposite conclusions to the ones obtained here, i.e., it is impossible to explain the observations of Swift J1834.9–0846 with a rotationally-powered nebula (Tong 2016, Granot et al. 2016). While these papers do not present an spectral analysis via simulations as we do, they do present interesting theoretical considerations that apparently imply the inability of Swift J1834.9–0846 to rotationally-power the nebula. Thus, it is important to discuss our results upon the light of these considerations, in an effort to understand the origin of the divergence in conclusions.

Tong (2016) proposes that the magnetar and the nebula can both be understood in the framework of a wind braking evolution. Tong’s (2016) claim to support the wind braking (and rule out the rotationally-powered) scenario for the nebula is that the magnetar’s rotational energy loss rate is not enough to power the particle luminosity. He is considering that only a small portion of the particle energy is converted to non-thermal X-rays, and thus that the particle luminosity of Swift J1834.9–0846 should be 1035 erg s-1 or higher, depending on the X-ray efficiency. Since this is beyond the spin-down power, the argument goes, the nebula cannot be rotationally-powered. This is true only in the case in which there is no accumulation of electrons in the nebula along the lifetime of the pulsar. If we allow for time evolution, and thus for accumulation of all electrons that are not burnt by losses, one can have an instantaneous income of electrons always limited by the spin-down power at the time of the injection, but many years for accumulating such electrons. The X-ray emission we see today should not be directly compared with the electron population injected today unless their burning is instantaneous, since the emission may not be dominated by the fresh electrons, but by the burning of the accumulated pairs.

Tong’s subsequent estimations relies in a number of radiative and dynamical approximations. For instance, in an esti- mate that would be equally applicable to essentially all nebu- lae of a few pc in size, Tong (2016) imposed a lower limit to the magnetic field at 240 μG. This magnetic field is so large for a nebula that is not considered being compressed that not even Crab reaches such value (e.g., Tanaka & Takahara 2010, Bucciantini et al. 2011, Torres et al. 2013b). He also consid- ered that the nebula is about a factor of 3 smaller than mea- sured. However, a size of 1 pc (∼50 arcsec) is missing about half the X-ray extended flux. This is contributing to over- estimating the field as well as the needed particle density to achieve the same nebula luminosity. He also assumed that the nebula is in equipartition, despite no known nebulae seems to be in such state e.g., see Tanaka & Takahara (2010), Buc- ciantini et al. (2011), Torres et al. (2014). This yields to a total pressure in particles that is > 200 times larger than the one we derived in our models.

Granot et al. (2016) proposed that the nebula is powered predominantly by outflows from the magnetar, whose main energy source is said to be the decay of the internal magnetic field. The conversion mechanism of this internal field into accelerated particles in a wind is not understood. They have also concluded that in the case Swift J1834.9–0846 is related to W41, the magnetar velocity should be at most a few 10 km s-1. They considered an age in the range 5 < t_age < 100 kyr for the complex, despite the lower end would be contradicting the estimations by Tian et al. referred to above. By compar- ing ours with Granot et al.’s work, and despite they seem to echo Tong’s argument at times, it would seem that some of the initial assumptions are very similar or the same than those adopted here. Among similarities in the approaches are those related to radiation, e.g., our finding of comparable magnetic fields (ours is ∼5 μG, theirs has a fiducial value around 4μG in a nebula of similar size), or the acceleration constraints considered to fix the maximum energy at injection (although we track this along the time evolution of the nebula since Lsd(t), B(t), etc. depend on time).4 Our obtained value of magnetiza- tion and their considered range also seems to be comparable. Differences –or at least an unclear direct comparison– rely on how similar the assumptions for the age and thus the dynam- ical evolution are. The approaches to deal with reverberation are also different, ours is relying in a direct, numerical solution of the dynamical set of equations. We do not find elec- trons cooling fast by synchrotron emission, but Granot et al. (2016) do not seem to include diffusion losses along most of their analysis. Without the latter, indeed synchrotron losses dominate at high enough energies. We do find a tS ynch smaller than the estimated age of the SNR W41 (which for us is larger than 50 kyr), and we do not believe Swift J1834.9–0846 and W41 are necessarily related. Other smaller differences may also intervene. For instance, we do not make any radiative approximations in our estimates of synchrotron emission, nor on the determination of the magnetic field along time, nor on the dynamical evolution, nor on the detailed balance (which for us is searched by a numerical solution of the full diffusion- loss equation). The concurrency of the impact of all of these approximations is hard to track.

To finish, we would like to note that Swift J1834.9– 0846 and the environment of W41, as well as that of XMMU J183435.3-084443 remains an exquisite case for further investigation with forthcoming powerful instruments such as the Square Kilometer (e.g., Taylor 2012) and the Cherenkov Telescope Arrays (e.g., Actis et al. 2013). The latter could provide observations sensitive enough to spatially and spectrally separate the contributions to the total TeV emis- sion, thus directly testing not only this work but models of all sources involved. The former could provide constraints to the lower-energy particle population in the nebulae which would help determine model parameters that were currently assumed. We look forward to doing these observations in the near future.


This paper is in press in ApJ and can be accessed here.

Tuesday, November 15, 2016

Multi-band study of a new asynchronous magnetic cataclysmic variable and a flaring X-ray source


 

In search for the counterpart to the Fermi-LAT source 3FGL J0838.8-2829, we report on 1) a new magnetic Cataclysmic Variable (mCV), RX J0838-2827, that we identify as an asynchronous system (therefore not associated with this Fermi-LAT source) and 2) on a new X-ray flaring source, XMM J083850.4-282759, that might be tentatively identified as new candidate Transitional Millisecond Pulsar, possibly associated with the gamma-ray source. We observed the field in the X-ray band with Swift, twice with XMM-Newton, as well as performed infrared, optical (with OAGH, ESO-NTT and IAC80) and radio (ATCA) observations, and we report on archival INTEGRAL observations. RX J0838-2827 is extremely variable in the X-ray and optical bands, and timing analysis reveals the presence of several periodicities modulating its X-ray and optical emission. The most evident modulations are interpreted as due to the binary system orbital period of ~1.64hr and the white dwarf spin period of ~1.47hr. Furthermore, a strong flux modulation at ~15.2hr is observed at all energy bands, consistent with the beat frequency between spin and orbital periods. Optical spectra show prominent Hbeta, HeI and HeII lines doppler modulated at the binary orbital period and at the long 15.2hr beat period. Therefore RX J0838-2827 accretes through a disc-less configuration and could be either a strongly asynchronous Polar or a rare example of a pre-polar system in its way to reach synchronism. Furthermore, we studied the two X-ray sources laying within the 3FGL J0838.8-2829 error circle. One of them, XMM J083850.4-282759, showed a variable X-ray emission and a ~600 s long flare in our second XMM-Newton observation, similar to what observed in Transitional Millisecond Pulsars during the sub-luminous state.


Friday, October 28, 2016

El IEEC celebra su 20o aniversario con un ciclo de conferencias “Descubriendo el Universo” en CosmoCaixa (nota de prensa del IEEC)

El Instituto de Estudios Espaciales de Cataluña (IEEC) cumple 20 años. Dos décadas dedicadas a conocer el Universo. Las cuatro unidades que conforman el IEEC: El Instituto de Ciencias del Cosmos (UB), el Instituto de Ciencias del Espacio (CSIC), el Centro de Estudios e Investigación Espaciales (UAB) y el grupo de investigación en Ciencia y Tecnología del Espacio (UPC) han participado durante estos 20 años en varias misiones espaciales de las distintas agencias espaciales y han contribuido de forma relevante al conocimiento de la astrofísica y la astronomía.

Para celebrar el 20 aniversario, el IEEC ha organizado conjuntamente con la Obra Social "la Caixa" el ciclo de conferencias divulgativas Descubriendo el Universo. El Instituto de Estudios Espaciales de Cataluña (IEEC), 20 años haciendo investigación en el espacio, con el fin de acercar la investigación espacial que se hace en Cataluña. Las charlas tendrán lugar el 16, el 22 y el 29 de noviembre a las 19h en el CosmoCaixa de Barcelona

Descubriendo el Universo. El Instituto de Estudios Espaciales de Cataluña (IEEC), 20 años haciendo investigación en el espacio

El 16 de noviembre Ignasi Ribas, Investigador del Instituto de Ciencias del Espacio (IEEC- CISC) y director del Observatorio Astronómico del Montsec (IEEC) hará la charla Buscando la Tierra 2.0.

¿Hay otros seres vivos en el Universo? ¿Cuánto tardaremos en encontrar otros planetas como el nuestro? ¿Cómo sabremos si están habitados? Éstas son preguntas que no dejan indiferente a nadie. Lo más alentador es que la ciencia está a un paso de poder dar respuesta. A fecha de hoy conocemos más de 2000 planetas que orbitan otras estrellas, y la Naturaleza nos ha revelado una riqueza de sistemas planetarios que nunca habríamos imaginado. Esto ha sido gracias al desarrollo de técnicas para lograr medir la velocidad, el brillo y la posición de las estrellas con una precisión exquisita. En esta charla se hará un repaso de los métodos que nos permiten descubrir exoplanetas y se describirán los últimos hallazgos más espectaculares. También se presentarán las nuevas misiones espaciales y proyectos, con especial énfasis en el instrumento CARMENES, que deben permitir continuar con esta gran revolución científica para llegar a descubrir planetas similares a nuestra Tierra, las Tierras 2.0.

El 22 de noviembre, Francesca Figueras, investigadora y vice-directora del Instituto de Ciencias del Cosmos (IEEC-UB) y co-directora del Instituto de Estudios Espaciales de Cataluña (IEEC) dará una charla sobre los primeros datos de la misión Gaia: la galaxia en un petabyte. Los primeros datos de la misión GAIA nos han permitido obtener el mapa en 3D más preciso de nuestra galaxia. Desde su lanzamiento en 2013 sus dos telescopios captan la luz de las estrellas y otros cuerpos celestes que encuentra en su órbita con una tecnología tan precisa como si desde la Tierra pudiéramos observar una moneda de un euro situada en la Luna. Con GAIA multiplicaremos por diez mil el conocimiento que hasta ahora tenemos de la Vía Láctea ya que el satélite se mueve de forma constante y cambia su ángulo respecto al Sol. Así, puede registrar no sólo las estrellas sino también otros cuerpos como quásares, planetas extrasolares o asteroides. Observará todos los objetos celestes hasta un brillo 400.000 veces menor que la que aprecia el ojo humano a simple vista. Cada objeto lo verá entre 75 y 100 veces para poder así crear una reconstrucción en 3D. Durante esta charla conoceremos los últimos datos recogidos por GAIA después de haber pasado el ecuador de la misión.

Y para cerrar el ciclo, el 29 de noviembre, Carlos F. Sopuerta, Investigador del Instituto de Ciencias del Espacio (IEEC-CSIC) hablará sobre El descubrimiento de las ondas gravitatorias y el comienzo de una nueva Astronomía. LIGO (Observatorio de Ondas Gravitatorias de Interferometría Láser) acaba de inaugurar la era de la Astronomía de Ondas Gravitatorias observando el primer sistema binario de agujeros negros. Al mismo tiempo, por un lado, la misión espacial LISA Pathfinder de ESA ha demostrado la tecnología para el futuro detector espacial de Ondas Gravitatorias, la misión L3 de ESA, y por otro lado, radiotelescopios monitorizando púlsares están muy cerca de detectar ondas gravitatorias de muy bajas frecuencias. Todos estos acontecimientos hacen que las ondas gravitatorias sean un ingrediente muy importante para entender mejor el universo, sobre todo los fenómenos más "oscuros" como aquellos que involucran agujeros negros. En esta charla hablaremos de cómo son estas ondas, como se pueden detectar y qué descubrimientos podemos esperar en los próximos años.

PS: Estaré presentando la conferencia de Ignasi Ribas el 16 de Noviembre (DFT).

Friday, September 23, 2016

Firmado el acuerdo para la instalación del observatorio del CTA del Hemisferio Norte en La Palma (nota de prensa del IEEC)


El 19 de septiembre, el Consorcio de la Red de Telescopios Cherenkov (CTA) ha firmado un acuerdo marco con el Instituto de Astrofísica de Canarias (IAC) para instalar el observatorio Cherenkov del hemisferio norte en el Roque de los Muchachos de la isla de La Palma.
El observatorio CTA-Norte se situará en el Roque de los Muchachos, en la isla de La Palma, la quinta isla más grande de las Islas Canarias. A 2.200 m de altitud y situado en una meseta por debajo del borde de un cráter volcánico extinto. Esta ubicación ofrece excelentes condiciones para la observación astronómica.
El acuerdo permite la construcción de la matriz norte de CTA en el Roque de los Muchachos y asegura el acceso a la infraestructura y los servicios comunes necesarios para el funcionamiento del Observatorio, incluyendo la conexión digital de la red CTA con el resto del mundo.
En virtud de este acuerdo, España recibirá el 10% del tiempo de observación, que podrá ser repartido entre la red del hemisferio Norte y la del Sur. La futura contribución de España a la construcción de los telescopios facilitará el acceso de los grupos españoles a tiempo de observación adicional como parte de los programas científicos clave del Observatorio y al tiempo que se ofrecerá en competición abierta a todos los países socios del mismo.  
Participación del IEEC
Este acuerdo implica un paso importante para los investigadores del Instituto de Estudios Espaciales de Cataluña (IEEC) que forman parte del consorcio CTA. El IEEC tiene una presencia relevante dentro del consorcio CTA y cuenta con varios grupos de investigadores que han sido miembros fundadores del proyecto.
Por un lado, el grupo del Instituto de Ciencias del Espacio (IEEC-CSIC) liderado por el profesor de investigación ICREA y director de la unidad, Diego F. Torres y la investigadora Ramón y Cajal Emma de Oña Wilhelmi, ha actuado como director científico y es responsable del desarrollo de uno de los módulos principales del software de control del observatorio.
Por otro, el grupo del Instituto de Ciencias del Cosmos (IEEC-UB), liderado por el catedrático y director científico del ICCUB, Josep Maria Paredes y el profesor agregado Marc Ribó, ha participado en la definición de los objetivos científicos del consorcio y están involucrados en el diseño y producción de la microelectrónica para las cámaras de CTA.
Finalmente, el grupo del Centro de Estudios e Investigación Espaciales (IEEC-UAB) liderado por Lluís Font, profesor titular del Departamento de Física de la UAB y Markus Gaug, investigador postdoctoral, coordina el grupo de instalaciones centrales de calibración, que incluyen la calibración del telescopios individuales, de la red de telescopios, de la atmósfera, y de la precisión en el apuntamiento a las fuentes, y participa junto con el Instituto de Física de Altas Energías (IFAE) en el desarrollo de instrumentación avanzada para la caracterización de la atmósfera.
Además del IEEC en el consorcio también participan diversos grupos de investigación españoles de del Instituto de Física de Altas Energías de Barcelona (IFAE), del Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), de la Universidad Complutense de Madrid, de la Universidad de Jaén y del Instituto de Astrofísica de Canarias.

Sobre el consorcio CTA
El consorcio CTA está formado por más de 1.200 científicos que trabajan en 200 centros de investigación de 32 países, siendo España y Japón los dos mayores contribuyentes a CTA-Norte. Su objetivo es la construcción de una infraestructura para la detección e investigación de rayos gamma de muy alta energía que proporcionarán información sobre los fenómenos más violentos y extremos que ocurren en el Universo.

El Ministerio de Economía y Competitividad ha sumado esfuerzos con otras entidades internacionales y nacionales para el proyecto CTA-Norte en España, aportando 40 millones de euros, lo que supone la mitad del coste total de su construcción.

La infraestructura CTA está formada por dos observatorios, uno en cada hemisferio del planeta. La red estará formada por unos 120 telescopios distribuidos entre los dos hemisferios. El Observatorio CTA- Norte se emplazará en el Observatorio del Roque de los Muchachos, de la isla de La Palma, mientras que CTA-Sur se ubicará en las instalaciones del Observatorio Europeo Austral (ESO) en el Paranal (Chile).