Search

We have performed the first sensitive X-ray observation of the low-mass X-ray binary SAX J1750.8-2900 in quiescence with XMM-Newton. The spectrum was fit to both a classical black body model, and a non-magnetized, pure hydrogen neutron star atmosphere model. A power law component was added to these models, but we found that it was not required by the fits. The distance to SAX J1750.8-2900 is known to be D = 6.79 kpc from a previous analysis of photospheric radius expansion bursts. This distance implies a bolometric luminosity (as given by the NS atmosphere model) of (1.05 +/- 0.12) x 10^34 (D/6.79 kpc)^2 erg s^-1, which is the highest known luminosity for a NS LMXB in quiescence. One simple explanation for this surprising result could be that the crust and core of the NS were not in thermal equilibrium during the observation. We argue that this was likely not the case, and that the core temperature of the NS in SAX J1750.8-2900 is unusually high.

We present a study of the two main peaks of the so-called cathedral QPO in XTE J1859+226. While looking at the temporal evolution of the two features we show that they do not manifest the same amplitude of variations of their power, and do not seem to follow the flux variations in the same way. We then present their RMS-spectra and show that they do not have the same shape, slope and cut-off energy. We discuss these different facts and try to answer the question regarding the genuineness of their harmonic relationship., Comment: 6 pages, 4 figures. Proceedings of the "HTRS 2011: Fast X-ray timing and spectroscopy at extreme count rates" conference held in Champery Switzerland

We present the first direct measurement of the spatial cross-correlation function of high-mass X-ray binaries (HMXBs) and active OB star-forming complexes in the Milky Way. This result relied on a sample containing 79 hard X-ray selected HMXBs and 458 OB associations. Clustering between the two populations is detected with a significance above 7-sigmas for distances < 1 kpc. Thus, HMXBs closely trace the underlying distribution of the massive star-forming regions that are expected to produce the progenitor stars of HMXBs. The average offset of 0.4+-0.2 kpc between HMXBs and OB associations is consistent with being due to natal kicks at velocities of the order of 100+-50 km/s. The characteristic scale of the correlation function suggests an average kinematical age (since the supernova phase) of ~4 Myr for the HMXB population. Despite being derived from a global view of our Galaxy, these signatures of HMXB evolution are consistent with theoretical expectations as well as observations of individual objects., Comment: 18 pages, 10 figures, 3 tables, accepted for publication in ApJ

We present the analysis of two long, quasi-uninterrupted RXTE observations of Cygnus X-1 that span several days within a 10 d interval. The spectral characteristics during this observation cover the region where previous observations have shown the source to be most dynamic. Despite that the source behavior on time scales of hours and days is remarkably similar to that on year time scales. This includes a variety of spectral/temporal correlations that previously had only been observed over Cyg X-1's long-term evolution. Furthermore, we observe a full transition from a hard to a soft spectral state that occurs within less than 2.5 hours - shorter than previously reported for any other similar Cyg X-1 transition. We describe the spectra with a phenomenological model dominated by a broken power law, and we fit the X-ray variability power spectra with a combination of a cutoff power law and Lorentzian components. The spectral and timing properties are correlated: the power spectrum Lorentzian components have an energy-dependent amplitude, and their peak frequencies increase with photon spectral index. Averaged over 3.2-10 Hz, the time lag between the variability in the 4.5-5.7 keV and 9.5-15 keV bands increases with decreasing hardness when the variability is dominated by the Lorentzian components during the hard state. The lag is small when there is a large power law noise contribution, shortly after the transition to the soft state. Interestingly, the soft state not only shows the shortest lags, but also the longest lags when the spectrum is at its softest and faintest. We discuss our results in terms of emission models for black hole binaries., Comment: 13 pages, 15 figures, accepted for publication in Astronomy and Astrophysics

Since the launch of INTEGRAL in 2002, about 300 new sources have been discovered. Understanding the nature of these objects is of prime importance for many aspects of astrophysics, such as the evolution of stars, population of sources (Galactic and extra-Galactic), and ultimately the physics powering them. However, their nature cannot be established from the soft gamma-ray observations. The first step towards unveiling the nature of those sources is to refine their X-ray position, in order to finally find counterparts at other wavelengths. X-ray spectra are also of prime importance to obtain clues on the nature of the objects. Since the discovery of the first IGR in 2003, our group has been active in several aspects of these studies. Here, we present the main results we have obtained through 7 years of multi-instrumental (Chandra, XMM, Swift, RXTE) campaigns., Comment: 6 pages, 4 figures, accepted for publication in PoS, proceedings of the 8th INTEGRAL Workshop "The Restless Gamma-ray Universe" (September 2010, Dublin, Ireland)

In 2009, Cygnus X-3 (Cyg X-3) became the first microquasar to be detected in the GeV {\gamma}-ray regime, via the satellites Fermi and AGILE. The addition of this new band to the observational toolbox holds promise for building a more detailed understanding of the relativistic jets of this and other systems. We present a rich dataset of radio, hard and soft X-ray, and {\gamma}-ray observations of Cyg X-3 made during a flaring episode in 2010 May. We detect a ~3-d softening and recovery of the X-ray emission, followed almost immediately by a ~1-Jy radio flare at 15 GHz, followed by a 4.3{\sigma} {\gamma}-ray flare (E > 100 MeV) ~1.5 d later. The radio sampling is sparse, but we use archival data to argue that it is unlikely the {\gamma}-ray flare was followed by any significant unobserved radio flares. In this case, the sequencing of the observed events is difficult to explain in a model in which the {\gamma}-ray emission is due to inverse Compton scattering of the companion star's radiation field. Our observations suggest that other mechanisms may also be responsible for {\gamma}-ray emission from Cyg X-3., Comment: 6 pages, 3 figures, accepted ApJL

Abridged: We present a spectral and temporal analysis of the Cathedral QPO detected in the power density spectra of the microquasar XTE J1859+226 obtained with RXTE. This type of QPO manifests as two peaks with similar amplitudes and harmonically related centroid frequencies (~3 and ~6 Hz). The amplitude of the ~3 Hz feature varies in anticorrelation with the count rate, by about ~50. The ~6 Hz feature shows a slight increase (~7%) of its amplitude with count rate. The RMS-spectra of the two peaks are quite different. The ~3 Hz feature is softer than the other one, and shows a cut-off at an energy of ~6 keV while the RMS of the 6 Hz increases up to at least 20 keV. We also study the bicoherence, $b^2(\mu,\nu)$. The values b^2(~3,~3) and b^2(~6,~6) are rather high and similar to the type C QPOs of GRS 1915+105. By comparison with the latter source the fact that the bicoherence of the ~3 Hz feature is higher than that of the other peak, would tend to indicate that the ~3 Hz is the fundamental QPO. The value of b^2(~3,~6)$ is, however, low and therefore indicates a behavior that is different than that seen in GRS 1915+105. We suggest that, rather than pure harmonics, we may see different modes of the same underlying phenomenon competing to produce QPOs at different frequencies., Comment: 7 pages, 5 figures, published in ApJ, vol 735, p79 Version corrected for small mistakes

We summarize the results of our long-running campaign to help understand the nature of high-mass X-ray binaries (HMXBs), emphasizing recent Suzaku observations of IGR J16207-5129 and IGR J17391-3021. Thanks to the expanding ranks of HMXBs in our Galaxy, we are able to perform more reliable statistical analyses on the three currently-known sub-classes of HMXB: those with supergiant companions (SGXBs); those with Be companions (BEXBs); and the enigmatic Supergiant Fast X-ray Transients (SFXTs). We discuss new diagnostic tools, akin to the "Corbet diagram," in which HMXBs tend to segregate based on their dominant accretion mechanism. We show how SFXTs span across the divided populations of BEXBs and SGXBs, bolstering the intriguing possibility that some SFXTs represent an evolutionary link. The use of HMXBs as tracers of recent massive star formation is revisited as we present the first ever spatial correlation function for HMXBs and OB star-forming complexes. Our results indicate that at distances less than a few kpc from a given HMXB, it is more likely to have neighbors that are known massive-star forming regions as opposed to objects drawn from random distributions. The characteristic scale of the correlation function holds valuable clues to HMXB evolutionary timescales., Comment: 6 pages, 5 figures, to appear in proceedings of the 4th International MAXI Workshop in Tokyo, "The First Year of MAXI: Monitoring Variable X-ray Sources"

The field containing the candidate High Mass X-ray Binary IGR J01363+6610 was observed by XMM-Newton on 2009 July 31 for 28 ks. A Be star was previously suggested as the possible counterpart of the INTEGRAL source, and although Chandra, during a 2007 observation, did not detect an X-ray source at the position of the Be star, we find a variable source (XMMU J013549.5+661243) with an average X-ray flux of 2e-13 ergs/cm2/s (0.2-12 keV, unabsorbed) at this position with XMM-Newton. The spectrum of this source is consistent with a hard power-law with a photon index of 1.4 +/- 0.3 and a column density of 1.5e22 cm^-2 (90% confidence errors). These results, along with our optical investigation of other X-ray sources in the field, makes the association with the Be star very likely, and the 2 kpc distance estimate for the Be star indicates an X-ray luminosity of 9.1e31 ergs/s. This is lower than typical for a Be X-ray binary, and the upper limit on the luminosity was even lower (<1.4e31 ergs/s assuming the same spectral model) during the Chandra observation. We discuss possible implications of the very low quiescent luminosity for the physical properties of IGR J01363+6610., Comment: 9 pages, accepted by ApJ

GRS1915+105 is a very peculiar black hole binary that exhibits accretion-related states that are not observed in any other stellar-mass black hole system. One of these states, however -- referred to as the plateau state -- may be related to the canonical hard state of black hole X-ray binaries. Both the plateau and hard state are associated with steady, relatively lower X-ray emission and flat/inverted radio emission, that is sometimes resolved into compact, self-absorbed jets. However, while generally black hole binaries quench their jets when the luminosity becomes too high, GRS1915+105 seems to sustain them despite the fact that it accretes at near- or super-Eddington rates. In order to investigate the relationship between the plateau and the hard state, we fit two multi-wavelength observations using a steady-state outflow-dominated model, developed for hard state black hole binaries. The data sets consist of quasi-simultaneous observations in radio, near-infrared and X-ray bands. Interestingly, we find both significant differences between the two plateau states, as well as between the best-fit model parameters and those representative of the hard state. We discuss our interpretation of these results, and the possible implications for GRS 1915+105's relationship to canonical black hole candidates., Comment: accepted for publication in MNRAS

An analysis of IGR J16207-5129 is presented based on observations taken with Suzaku. The data set represents ~80 ks of effective exposure time in a broad energy range between 0.5 and 60 keV, including unprecedented spectral sensitivity above 15 keV. The average source spectrum is well described by an absorbed power law in which we measured a large intrinsic absorption of nH = 16.2(-1.1/+0.9)x10^22 /cm2. This confirms that IGR J16207-5129 belongs to the class of absorbed HMXBs. We were able to constrain the cutoff energy at 19(-4/+8) keV which argues in favor of a neutron star as the primary. Our observation includes an epoch in which the source count rate is compatible with no flux suggesting a possible eclipse. We discuss the nature of this source in light of these and of other recent results., Comment: 12 pages, 6 figures, accepted for publication in ApJ

We analyzed radio and X-ray observations of GRS 1915+105, between May 1995 and June 2006, focusing on the times characterized by radio flares and cycles of hard dips-soft spikes in the X-ray lightcurve. Assuming these flares to be discrete ejections, we applied a plasmon model to the radio data, with good agreement with the lightcurves. We fitted a total of 687 radio flares with a standard model of a plasmon. We found that the distribution of width is t_0=1160 s with an rms deviation of 360 s, while that of the amplitude is S_{max}=59 mJy with an rms deviation of 28 mJy. The distribution of width is thus rather peaked, while that of the amplitude not. Regarding radio and X-ray links, this study confirms previous observations on smaller datasets, namely that X-ray cycles of hard dips-soft spikes are always followed by radio flares. A strong correlation is found between the length of X-ray "dips" in the X-ray lightcurves, and the amplitude and fluence of the subsequent radio oscillations. A model of an exponential rise of the form L_{15 GHz}(Delta t) = L_{max} (1-exp(-(Delta t - Delta t_{min})/tau) is in good agreement with the observations, with the maximum fluence L_{max} of the order 70 Jy.s, and the characteristic time tau of the order 200-500 s. We discuss possible physical interpretations of this correlation, regarding the nature of the ejected material, and the physical process responsible for the ejection., Comment: 10 pages, 10 figures, emulateapj, accepted by ApJ

Abridged : The various IBIS/ISGRI catalogues contain a large population of hard X-ray sources whose nature is still unknown. Even if the $>20$ keV positional uncertainty provided by ISGRI is unprecedented, it is still too large to pinpoint the counterpart at other wavelengths, which is the only secure way of obtaining a source identification. We continue the work of trying to reveal the nature of these hard X-ray sources, starting with analysis of X-ray data collected via focusing X-ray telescopes, in order to obtain arcsec accurate X-ray positions. We can then identify counterparts at infrared and optical wavelengths and try to unveil the nature of the sources. We analysed data from observations of 13 \integral\ sources made with the \swift\ satellite. The X-ray images obtained by the X-Ray Telescope instrument allowed us to find possible counterparts to the IGR sources with a positional accuracy of a few arcsec. We then browsed the online catalogues to search for counterparts at other wavelengths. We also made use of the X-ray spectral parameters in trying to identify the nature of those objects. For the 13 objects, we found possible counterparts at X-ray energies and identified the IR/optical and/or UV counterparts as seen with \swift/UVOT. We also discuss the likelihood of association of the X-ray and \integral\ source in each case., Comment: 11 pages, 2 figures, accepted for publication in A&A, version corrected after A&A language editing

We report on 0.3-10 keV X-ray observations by the Chandra X-ray Observatory of the fields of 22 sources that were discovered as hard X-ray (20-100 keV) sources by the INTEGRAL satellite (IGR sources). The purpose of the Chandra observations is to localize the sources and to measure their soft X-ray spectra in order to determine the nature of the sources. We find very likely Chandra counterparts for 18 of the 22 sources. We discuss the implications for each source, considering previous results and new optical or IR identifications, and we identify or suggest identifications for the nature of 16 of the sources. Two of the sources, IGR J14003-6326 and IGR J17448-3232, are extended on arcminute scales. We identify the former as a pulsar wind nebula (PWN) with a surrounding supernova remnant (SNR) and the latter as a SNR. In the group of 242 IGR sources, there is only one other source that has previously been identified as a SNR. We confirm a previous identification of IGR J14331-6112 as an High-Mass X-ray Binary (HMXB), and we suggest that IGR J17404-3655, IGR J16287-5021, IGR J17354-3255, IGR J17507-2647, IGR J17586-2129, and IGR J13186-6257 are candidate HMXBs. Our results indicate or confirm that IGR J19267+1325, IGR J18173-2509, and IGR J18308-1232 are Cataclysmic Variables (CVs), and we suggest that IGR J15529-5029 may also be a CV. We confirm that IGR J14471-6414 is an Active Galactic Nucleus (AGN), and we also suggest that IGR J19443+2117 and IGR J18485-0047 may be AGN. Finally, we found Chandra counterparts for IGR J11098-6457 and IGR J18134-1636, but more information is required to determine the nature of these two sources., Comment: 14 pages, Accepted by ApJ

It is widely believed that the low frequency quasi-periodic X-ray oscillations observed in microquasars are correlated to, but do not originate at, the physical radius of the inner edge of the accretion disk. Models relating the QPO frequency and color radius are hindered by observations showing contradicting trend correlations between the microquasars GRO 1655-40, XTE J1550-564 and GRS 1915+105. The first shows a negative correlation and the latter two a positive one. By taking into account relativistic rotation in the accretion disk, the Accretion-Ejection Instability (AEI) model predicts a turnover in the frequency-radius relationship, and has been successfully compared with observations of GRO J1655-40 and GRS 1915+105. We present further evidence supporting the AEI model prediction by using observations of the microquasar GRS 1915+105. By combining a data set including $\theta$-, $\beta$- and $\alpha$-class X-ray light curves, we observe positive, negative and null correlations in the frequency-radius relationship. This is the first time a single source has shown a possible inversion in the QPO frequency-color radius curve predicted by the AEI model.

The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is discovering hundreds of new hard X-ray sources, many of which remain unidentified. We report on optical spectroscopy of five such sources for which X-ray observations at lower energies (~0.5-10 keV) and higher angular resolutions than INTEGRAL have allowed for unique optical counterparts to be located. We find that IGR J16426+6536 and IGR J22292+6647 are Type 1 Seyfert active galactic nuclei (with IGR J16426+6536 further classified as a Seyfert 1.5) which have redshifts of z=0.323 and z=0.113, respectively. IGR J18308-1232 is identified as a cataclysmic variable (CV), and we confirm a previous identification of IGR J19267+1325 as a magnetic CV. IGR J18214-1318 is identified as an obscured high mass X-ray binary (HMXB), which are systems thought to have a compact object embedded in the stellar wind of a massive star. We combine Chandra fluxes with distances based on the optical observations to calculate X-ray luminosities of the HMXB and CVs, finding L_(0.3-10 keV)=5e36 erg s^-1 for IGR J18214-1318, L_(0.3-10 keV)=1.3e32 erg s^-1 for IGR J18308-1232, and L_(0.3-10 keV)=6.7e32 erg s^-1 for IGR J19267+1325., Comment: 8 pages, 5 figures, accepted for publication by ApJ

The onset of most microquasar outbursts is characterized by a state transition between a Low/Hard State (LHS) and a High/Soft State (HSS). Besides drastic spectral and timing changes, this transition often shows a discrete ejection event detectable in the radio range. However, the exact nature of the ejected material and the mechanisms that give birth to these phenomena are yet to be unraveled. Recent simultaneous radio and X-ray observations on several sources point to a coronal nature of the ejected material. In the cases of GRS 1915+105, XTE J1550-564, and the 2002 outburst of GX 339-4, the flux of the Compton component decreases sharply just before an ejection is detected in the radio range. Finally, in the case of H1743-322, drastic physical changes occurred in the corona just before the state transition, compatible with the disappearance of part of this medium. Thus, the behaviour of at least 4 microquasars points in the direction of an ejection of the corona at the state transition, feature that is yet to be confirmed (or infirmed) in the case of other available sources., Comment: 4 pages, 2 figures, Proceedings of the 2nd International Simbol-X symposium held in Paris, 2-5 December 2008

We report on a 12 hr XMM-Newton observation of the supergiant High-Mass X-ray Binary IGR J16207-5129. This is only the second soft X-ray (0.4-15 keV, in this case) study of the source since it was discovered by the INTEGRAL satellite. The average energy spectrum is very similar to those of neutron star HMXBs, being dominated by a highly absorbed power-law component with a photon index of 1.15. The spectrum also exhibits a soft excess below 2 keV and an iron Kalpha emission line at 6.39+/-0.03 keV. For the primary power-law component, the column density is 1.19E23 cm^-2, indicating local absorption, likely from the stellar wind, and placing IGR J16207-5129 in the category of obscured IGR HMXBs. The source exhibits a very high level of variability with an rms noise level of 64%+/-21% in the 0.0001 to 0.05 Hz frequency range. Although the energy spectrum suggests that the system may harbor a neutron star, no pulsations are detected with a 90% confidence upper limit of 2% in a frequency range from 0.0001 to 88 Hz. We discuss similarities between IGR J16207-5129 and other apparently non-pulsating HMXBs, including other IGR HMXBs as well as 4U 2206+54 (but see arXiv:0812.2365) and 4U 1700-377., Comment: 10 pages, 10 figures, Accepted by ApJ

We study the evolution of some microquasars during their outbursts as observed with the X-ray telescopes RXTE and INTEGRAL. We focus on the interplay between the accretion disc, and the medium responsible for the production of the hard X-rays (the so-called corona). By comparing the behaviour of two sources (XTE J1550-564 and GRS 1915+105) at X-ray energies and radio wavelengths, we propose a scenario in which the discrete ejections are triggered in coincidence with soft X-ray peaks during the outburst. We also suggest, in those two sources, that the ejected material is the corona that is seen to disappear in coincidence with the X-ray maxima. We then turn to two other sources, XTE J1748-248, and XTE J1859+226, and study whether the same conclusions can be drawn from the existing multi-wave length (radio+X-ray) data., Comment: 10 pages, 7 figures, Proceedings of the 7th INTEGRAL workshop held in Copenhagen 8-11 September 2008

We report on the results of observations of hard X-ray sources in the Galactic plane with the Chandra X-ray Observatory. The hard X-ray IGR sources were discovered by the INTEGRAL satellite, and the goals of the Chandra observations are to provide sub-arcsecond localizations to obtain optical and infrared counterparts and to provide constraints on their 0.3-10 keV spectra. We obtained relatively short, ~5 ks, observations for 20 IGR sources and find a bright Chandra source in INTEGRAL error circles in 12 cases. In 11 of these cases, a cross-correlation with optical and/or infrared source catalogs yields a counterpart, and the range of J-band magnitudes is 8.1-16.4. Also, in 4 cases, the Chandra X-ray spectra show evidence for absorbing material surrounding the compact object with a column density of local material in excess of 5x10^22 cm^-2. We confirm that IGR J00234+6141 is a Cataclysmic Variable and IGR J14515-5542 is an Active Galactic Nucleus (AGN). We also confirm that IGR J06074+2205, IGR J10101-5645, IGR J11305-6256, and IGR J17200-3116 are High Mass X-ray Binaries (HMXBs). Our results (along with follow-up optical spectroscopy reported elsewhere) indicate that IGR J11435-6109 is an HMXB and IGR J18259-0706 is an AGN. We find that IGR J09026-4812, IGR J18214-1318, and IGR J18325-0756 may be HMXBs. In cases where we do not find a Chandra counterpart, the flux upper limits place interesting constraints on the luminosities of black hole and neutron star X-ray transients in quiescence., Comment: Accepted by ApJ

Context: The INTEGRAL hard X-ray observatory has revealed an emerging population of highly obscured X-ray binary systems through multi-wavelength observations. Previous studies have shown that many of these sources are high-mass X-ray binaries hosting neutron stars orbiting around luminous and evolved companion stars. Aims: To better understand this newly-discovered population, we have selected a sample of sources for which an accurate localisation is available to identify the stellar counterpart and reveal the nature of the companion star and of the binary system. Methods: We performed an intensive study of a sample of thirteen INTEGRAL sources, through multi-wavelength optical to NIR photometric and spectroscopic observations, using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate astrometry and identified candidate counterparts for which we give the optical and NIR magnitudes. We detected many spectral lines allowing us to determine the spectral type of the companion star. We fitted with stellar black bodies the mid-infrared to optical spectral energy distributions of these sources. From the spectral analysis and SED fitting we identified the nature of the companion stars and of the binary systems. (abridged)., Comment: A&A in press; The official date of acceptance is 15/12/2007; 25 pages, 6 figures, 8 tables. New version with language editing required by editor

We have used the RXTE and INTEGRAL satellites simultaneously to observe the High Mass X-ray binary IGR J19140+0951. The spectra obtained in the 3-80 keV range have allowed us to perform a precise spectral analysis of the system along its binary orbit. The spectral evolution confirms the supergiant nature of the companion star and the neutron star nature of the compact object. Using a simple stellar wind model to describe the evolution of the photoelectric absorption, we were able to restrict the orbital inclination angle in the range 37-75 degrees. This analysis leads to a wind mass-loss rate from the companion star of ~ 10e-7 Msun/year, consistent with its expected spectral type. We have detected a soft excess in at least three observations, for the first time for this source. Such soft excesses have been reported in several HMXBs in the past. We discuss the possible origin of this excess, and suggest, based on its spectral properties and occurrences prior to the superior conjunction, that it may be explained as the reprocessing of the X-ray emission originating from the neutron star by the surrounding ionised gas., Comment: Proceedings of "A Population Explosion: The Nature and Evolution of X-ray Binaries in Diverse Environments", St. Petersburg Beach, Florida, USA, 28 October - 2 November, 2007, 3 pages, 1 figure

We collected the parameters (position, absorption, spin, orbital period, etc..), when known, of all Galactic sources detected by INTEGRAL during its four first years of activity. We use these parameters to test theoretical predictions. For example, it is clear that HMXBs tend to be found mostly in the tangential direction of the Galactic arms, while LMXBs tend to be clustered in the Galactic bulge. We then focus on HMXBs and present two possible new tools, in addition to the well-known ``Corbet-diagram'', to distinguish between Be-HMXBs and Sg-HMXBs, Comment: 5 pages, 3 figures proceedings of "A population explosion: the nature and evolution of X-ray binaries in diverse environments", conference held in St.Petersburg Beach, Florida; R.M.Bandyopadhyay, S.Wachter, D.Gelino, C.R.Gelino, eds

Based on spectral simulations, I show how focusing of the X-ray radiations above 10 keV will open a new window for the study of microquasars. With simulations of soft and hard state spectra of Galactic sources, I discuss how SIMBOL-X can help to precisely measure the spin of black holes. Spectral study on short ~1 s time scales will also allow the accretion-ejection connections to be accessed, and the formation of jets possibly witnessed in X-rays. I then turn to external galaxies, and demonstrate that spectral studies of hard sources will be possible up to at least ~1 Mpc. For such sources, subtle spectral signatures (e.g. a reflection bump) will clearly be detected. I finally discuss the implications that these original results will bring on the physics of microquasars and black holes., Comment: 6 pages, 4 figures, invited talk submitted to the proceedings of "SIMBOL-X: the hard X-ray Universe in focus", conference, held May 14-16, 2007, Bologna (Italy)

The HEGRA gamma-ray source TeV J2032+4130 is considered the prototypical 'dark accelerator', since it was the first TeV source detected with no firm counterparts at lower frequencies. The Whipple collaboration observed this source in 2003-5 and the emission hotspot appears displaced about 9 arcminutes to the northeast of the HEGRA position, though given the large positional uncertainties the HEGRA and Whipple positions are consistent. Here we report on Westerbork Synthesis Radio Telescope (WSRT), Very Large Array (VLA), Chandra and INTEGRAL data covering the locations of the Whipple and HEGRA hotspots. We confirm a dual-lobed radio source (also see Marti et al., 2007) coincident with the Whipple hotspot, as well as a weak, partially non-thermal shell-like object, with a location and morphology very similar to the HEGRA source, in our WSRT and mosaicked VLA datasets, respectively. Due to its extended nature, it is likely that the latter structure is a more plausible counterpart of the reported very high energy (VHE) gamma-ray emissions in this region. If so, TeV J2032+4130 may not be a 'dark accelerator' after all. Further observations with the new generation of imaging Cherenkov telescopes are needed to pin down the precise location and morphology of the TeV emission region and thus clear up the confusion over its possible lower frequency counterparts., Comment: accepted, MNRAS

We report the results of a series of RXTE and ATCA observations of the recently-discovered X-ray transient IGR J17497-2821. Our 3-200 keV PCA+HEXTE spectral analysis shows very little variations over a period of ~10 days around the maximum of the outburst. IGR J17497-2821 is found in a typical Low Hard State (LHS) of X-ray binaries (XRB), well represented by an absorbed Comptonized spectrum with an iron edge at about 7 keV. The high value of the absorption (~4 e22 cm-2) suggests that the source is located at a large distance, either close to the Galactic center or beyond. The timing analysis shows no particular features, while the shape of the power density spectra is also typical of LHS of XRBs, with ~36% RMS variability. No radio counterpart is found down to a limit of 0.21 mJy at 4.80 GHz and 8.64 GHz. Although the position of IGR J17497-2821 in the radio to X-ray flux diagram is well below the correlation usually observed in the LHS of black holes, the comparison of its X-ray properties with those of other sources leads us to suggest that it is a black hole candidate., Comment: 6 pages, 4 figures accepted for publication in the Astrophysical Journal Letters

We report the results of 3 observations of GRS 1915+105 during which the source is found to show the X-ray dips/spike sequences (cycles). These observations were performed simultaneously with \integral, RXTE, the Ryle and Nancay radio telescopes. They show the so-called $\nu$, $\lambda$ and $\beta$ classes of variability during which a high level of correlated X-ray and radio variability is observed. We study the connection between the accretion processes seen in the X-ray behaviour, and the ejections seen in radio. By observing ejection during class $\lambda$, we generalise the fact that the discrete ejections in GRS 1915+105 occur after the cycles seen at X-ray energies, and identify the most likely trigger of the ejection through a spectral approach to our INTEGRAL data. We show that each ejection is very probably the result of the ejection of a Comptonising medium responsible for the hard X-ray emission seen above 15 keV with INTEGRAL. We study the rapid variability of the source, and observe the presence of Low Frequency Quasi Periodic Oscillations during the X-ray dips. The ubiquity of the former behaviour, and the following ejection may suggest a link between the QPO and the mechanism responsible for the ejection., Comment: 10 pages, 7 figures, proceedings of the microquasar workshop held in Como, Italy, Sept. 18-22, 2006. Eds T. Belloni

We report high-energy results obtained with INTEGRAL and Rossi-XTE on two microquasars: the persistent high-mass system Cygnus X-1 and the transient low-mass binary SWIFT J1753.5-0127. INTEGRAL observed Cygnus X-1 from 2002 to 2004: the spectral (5-1000 keV) properties of the source, seen at least in three distinct spectral states, show disc and corona changes. In 2003 June, a high-energy tail at several hundred keV in excess of the thermal Comptonization model was observed, suggesting the presence of an additional non-thermal component. At that time, we detected an unusual correlation between radio data and high-energy hardness. We also report and compare the results obtained with simultaneous observations of the transient source SWIFT J1753.5-0127 performed with Rossi-XTE, INTEGRAL, VLA, REM and NTT on 2005 August 10-12 near its hard X-ray outburst. Broad-band spectra and fast time-variability properties are derived on this source (probably located in the galactic halo) together with radio, IR and optical data. We build a spectral energy distribution of the source and derive interesting multiwavelength constraints. Significantly detected up to 600 keV in a typical Low/Hard State, the transient does not seem to follow the usual radio/X-ray correlation., Comment: 5 figures, 2 in color

Hard X-ray imaging of the Galactic plane by the INTEGRAL satellite is uncovering large numbers of 20-100 keV "IGR" sources. We present results from Chandra, INTEGRAL, optical, and IR observations of 4 IGR sources: 3 sources in the Norma region of the Galaxy (IGR J16195-4945, IGR J16207-5129, and IGR J16167-4957) and one that is closer to the Galactic center (IGR J17195-4100). In all 4 cases, one relatively bright Chandra source is seen in the INTEGRAL error circle, and these are likely to be the soft X-ray counterparts of the IGR sources. They have hard 0.3-10 keV spectra with power-law photon indices of 0.5 to 1.1. While many previously studied IGR sources show high column densities, only IGR J16195-4945 has a column density that could be as high as 10^23 cm^-2. Using optical and IR sky survey catalogs and our own photometry, we have obtained identifications for all 4 sources. The J-band magnitudes are in the range 14.9-10.4, and we have used the optical/IR spectral energy distributions (SEDs) to constrain the nature of the sources. Blackbody components with temperature lower limits of >9400 K for IGR J16195-4945 and >18,000 K for IGR J16207-5129 indicate that these are very likely High-Mass X-ray Binaries (HMXBs). However, for IGR J16167-4957 and IGR J17195-4100, low extinction and the SEDs indicate later spectral types for the putative companions, indicating that these are not HMXBs., Comment: Accepted by the Astrophysical Journal after some reduction in the length of the paper and removal of one figure

We report on the spectral analysis of RXTE and INTEGRAL data of the 2005 April outburst of the transient Atoll source Aql X-1. Although this outburst is one of the faintest ever detected for this source in the soft X-rays (RXTE/ASM), one of our INTEGRAL observations, taken close to the soft X-ray peak, shows that the source flux was quite high, with a 20-200 keV flux of 2.05 x 10^-9 erg cm^-2 s^-1. On this occasion we detect the source up to 150 keV for the first time. We compare and discuss the similarity of the source behavior with that of black hole transients especially XTE J1550-564., Comment: 4 pages 2 figures, Accepted for publication in A&A as a RESEARCH NOTE

We report the results of extensive high-energy observations of the X-ray transient and black hole candidate XTE J1720-318 performed with INTEGRAL, XMM-Newton and RXTE. The source, which underwent an outburst in January 2003, was observed in February in a spectral state dominated by a soft component with a weak high-energy tail. The XMM-Newton data provided a high column density Nh of 1.2*e22 cm^{-2} which suggests that the source lies at the Galactic Center distance. The simultaneous RXTE and INTEGRAL Target of Opportunity observations allowed us to measure the weak and steep tail, typical of a black-hole binary in the so-called High/Soft State. We could follow the evolution of the source outburst over several months using the INTEGRAL Galactic Center survey observations. The source regained activity at the end of March: it showed a clear transition towards a much harder state, and then decayed to a quiescent state in summer. In the hard state, the source was detected up to 200 keV with a typical power law index of 1.8 and a peak luminosity of 7.5*e36 ergs s^{-1} in the 2-100 keV band, for an assumed distance of 8 kpc. We conclude that XTE J1720-318 is indeed representative of the class of the black hole X-ray novae which populate our Galactic bulge and we discuss its properties in the frame of the spectral models used for transient black hole binaries., Comment: avril 2004

IGR J19140+098 (SIMBAD corrected name IGR J19140+0951) is a new X-ray transient, discovered by INTEGRAL during an observation of GRS 1915+015. The source presents strong variations on timescales from seconds to days. We present results of multiwavelength observations, including spectral analysis of INTEGRAL observations, and propose that IGR J19140+098 is a Galactic X-ray binary. Further classification of the source is also discussed., Comment: 3 pages, 5 figures, To be published in the Proceedings of the 5th INTEGRAL Workshop: "The INTEGRAL Universe", February 16-20, 2004, Munich

We present a scenario for the variability of the microquasar GRS 1915+105. This starts from previous works, leading to the tentative identification of the Accretion-Ejection Instability as the source of the low-frequency Quasi-Periodic Oscillation of microquasars and other accreting sources. We follow the physics of this instability: its conditions (the magnetic field and geometry adapted to MHD jet models), its instability criterion, and its consequences (cooling down of the disk, heating and excitation of the corona). Comparing them to the observed properties of the source, in particular the detailed properties of its spectral states, we first derive a model for the $\sim$ 30 minutes cycles often exhibited by GRS 1915+105. In our model this is a limit cycle determined by the advection of poloidal magnetic flux to the inner region of the disk, and its destruction by reconnection (leading to relativistic ejections) with the magnetic flux trapped in the vicinity of the central source. We show how it leads to natural explanations for observed behaviors of GRS 1915+105, including the three basic states of Belloni et al (2000). We then discuss how this could be extrapolated further to understand the longer-term variability of this and other microquasars., Comment: 11 pages, 5 color figures (3 color, quality lowered), accepted for publication in ApJ. Full quality paper (ps) available at http://isdc.unige.ch/~rodrigue/FTP/Flood_Tagger.ps

In 2009, Cygnus X-3 (Cyg X-3) became the first microquasar to be detected in the GeV gamma-ray regime, via the satellites Fermi and AGILE. The addition of this new band to the observational toolbox holds promise for building a more detailed understanding of the relativistic jets of this and other systems. We present a rich dataset of radio, hard and soft X-ray, and gamma-ray observations of Cyg X-3 made during a flaring episode in 2010 May. We detect a approx.3-d softening and recovery of the X-ray emission, followed almost immediately by a approx.1-Jy radio flare at 15 GHz, followed by a 4.3sigma gamma-ray flare (E > 100 MeV) approx.1.5 d later. The radio sampling is sparse, but we use archival data to argue that it is unlikely the gamma-ray flare was followed by any significant unobserved radio flares. In this case, the sequencing of the observed events is difficult to explain in a model in which the gamma-ray emission is due to inverse Compton scattering of the companion star's radiation field. Our observations suggest that other mechanisms may also be responsible for gamma-ray emission from Cyg X-3.