J. Rodriguez

A description of sources detected by INTEGRAL during the first 4 years of observations

Context. In its first 4 years of observing the sky above 20 keV, INTEGRAL-ISGRI has detected 500 sources, around half of which are new or unknown at these energies. Follow-up observations at other wavelengths revealed that some of these sources feature unusually large column densities, long pulsations, and other interes ting characteristics. Aims. We investigate where new and previously-known sources detected by ISGRI fit in the parameter space of high-energy object s, and we use the parameters to test correlations expected from theoretical predictions. For example, the influence of the l ocal absorbing matter on periodic modulations is studied for Galactic High-Mass X-ray Binaries (HMXBs) with OB supergiant and Be companions. We examine the spatial distribution of different types of sources in the Milky Way using various projections of the Galactic plane, in order to highlight signatures of stellar evolution and to sp eculate on the origin of the group of sources whose classifica tions are still uncertain. Methods. Parameters that are available in the literature, such as pos itions, photoelectric absorption ( NH), spin and orbital periods, and distances or redshifts, were collected for all sources d etected by ISGRI. These values and their references are provided online. Results. ISGRI has detected similar numbers of X-ray Binaries and Active Galactic Nuclei (AGN). The former group contains new members of the class of HMXBs with supergiant stellar companions. Usually, this type of object presents strong intrinsi c absorption which leads to a peak emission in an energy range that ISGRI is ideally suited to detect. Thanks to these additional system s, we are able to show that HMXBs are generally segregated in plots of intrinsic NH versus the orbital period of the system and versus the spin period of the pulsar, based on whether the companion is a Be or an OB supergiant star. We also find a tentative but expected an ticorrelation between NH and the orbital period, and a possible and unexpected correlation between the NH and the spin period. While only a handful of new Low-Mass X-ray Binaries (LMXBs) have been discovered, there are many sources that remain unclassifi ed and they appear to follow a spatial distribution typical of Gala ctic sources (especially LMXBs) rather than extragalactic sources.

Detailed Radio to Soft Gamma-ray Studies of the 2005 Outburst of the New X-ray Transient XTE J1818-245

Context. XTE J1818−245 is an X-ray nova that experienced an outburst in 2005, as first seen by the RXTE satellite. The source was observed simultaneously at various wavelengths up to soft γ-rays with the INTEGRAL satellite, from 2005 February to September, during our INTEGRAL Target of Opportunity program dedicated to new X-ray novae and during Galactic Bulge observations. Aims. X-ray novae are extreme systems that often harbor a black hole, and are known to emit throughout the electromagnetic spectrum when in outburst. The goals of our programme are to understand the physical processes close to the black hole and to study the possible connection with the jets that are observed in the radio. Methods. We analysed radio, (N)IR, optical, X-ray and soft γ-ray observations. We constructed simultaneous broad-band X-ray spectra covering a major part of the outburst, which we fitted with physical models. Analyzing both the light curves in various energy ranges and the hardness-intensity diagram enabled us to study the long-term behaviour of the source. Results. Spectral parameters were typical of the soft intermediate states and the high soft states of a black hole candidate. The source showed relatively small spectral variations in X-rays with considerable flux variation in radio. Spectral studies showed that the accretion disc cooled down from 0.64 to 0.27 keV in ∼100 days and that the total flux decreased while the relative flux of the hot medium increased. Radio emission was detected several times, and, interestingly, five days after entering the HSS. Modeling the spectral energy distribution from the radio to the soft γ-rays reveals that the radio flares arise from several ejection events. Conclusions. XTE J1818−245 probably belongs to the class of low-mass X-ray binaries and is likely a black hole candidate transient source that might be closer than the Galactic Bulge. The results from the data analysis trace the physical changes that took place in the system (disc, jet/corona) at a maximum bolometric luminosity of 0.4−0.9 × 10 38 erg s −1 (assuming a distance between 2.8–4.3 kpc) and they are discussed within the context of disc and jet models.

The early phase of a H1743 322 outburst observed by INTEGRAL, RXTE, Swift, and XMM/Newton

Aims. We investigate the early phase of the first state change durin g the 2008 September-November outburst of H1743-322, first detected by the INTEGRAL satellite. We focus on the preliminary hard X-ray state with the aim of investigating the possible influence of this phase on the subsequent evolution during the outburst. Methods. The outburst started on MJD 54732, and remains ongoing at the time of writing this paper (MJD 54770). We analyse INTEGRAL, RXTE, Swift, and XMM/Newton observations, which provide coverage of the quiescence to outburst evolution in the 3‐200 keV range every few days. We present both the spectral and timing analysis. We compare these parameters with the behaviour of the source during a previous outburst in 2003, which was observed by INTEGRAL and RXTE. Results. The energy spectra are well fitted by a phenomenological mode l consisting of an exponentially cut-off power law plus a disc component. A more physical model of thermal Comptonisation (and a disc) represents the spectra equally well. In a first ph ase (up to MJD 54760), the photon index and temperature of the disc do not vary significantly, and have values reminiscent of the Hard State (HS). The timing analysis is also consistent with that of a HS, and shows in particular a rather high degree of variability (∼30%), and a strong∼0.5‐1 Hz QPO with its first harmonic. After MJD 54760, a change to softer spectra and a∼5‐6 Hz QPO indicate that the source underwent a state transition into a Hard-Intermediate State (HIMS). Conclusions. The timing and spectral characteristics of H1743−322 are similar to those of the first HS during its 2003 outburs t. We observe a correlation between the QPO frequency and the photon index, which indicates a strong link between the accretion disc, generally understood to determine the QPO frequency, and the corona, which determines the QPO power. The gradual disappearance of the QPO harmonic, and the slowly decreasing hard X-ray flux , imply that the accretion disc slowly moved inwards during the HS.

Average hard X‐ray emission from NS LMXBs

We studied and compared the long term average hard X‐ray (>20 keV) spectra of a sample of twelve bright low‐mass X‐ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright (“GX”) bulge sources while two are weaker ones in the 2–10 keV range (H 1750‐440 and H 1608‐55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. We extend the detection of H 1750‐440 up to 150 keV and discover a hard tail in the spectrum of the Atoll source GX 13+1. We can describe all the spectral states as well as the long known variability of the hard X‐ray tails detected in some of these objects in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion, at the origin of the hard X‐ray tail, is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local lumin...

The Accretion-Ejection Instability: Observational Tests

QPOs are commonly considered to originate in the disks of X-ray binaries. One thus expects, and most often observes, that the QPO frequency decreases as the disk inner radius increases. However Sobczak et al. (1999) pointed out an opposite behavior in the case of GRO J1655-40. This might prove usefull in testing the theory of the Accretion-Ejection Instability (see contributions by Caunt, Tagger and Varniere, these proceedings). At the same time, Merloni, Fabian, and Ross (1999) showed that, when fitting the sources spectra with the standard model (multicolor blackbody+ power law), severe distorsions could be due to the vertical structure of the disk. In that view we re-did the spectral analysis of the observations in which Sobczak et al. found QPO’s.