In recent years, much effort has been devoted to unravelling the connection between the accretion flow and the jets in accreting compact objects. In the present work, we report new constraints on these issues, through the long‐term study of the radio and X‐ray behaviour of the black hole candidate H1743−322. This source is known to be one of the ‘outliers’ of the universal radio/X‐ray correlation, i.e. a group of accreting stellar‐mass black holes displaying fainter radio emission for a given X‐ray luminosity than expected from the correlation. Our study shows that the radio and X‐ray emission of H1743−322 are strongly correlated at high luminosity in the hard spectral state. However, this correlation is unusually steep for a black hole X‐ray binary: b∼ 1.4 (with Lradio∝LbX). Below a critical luminosity, the correlation becomes shallower until it rejoins the standard correlation with b∼ 0.6. Based on these results, we first show that the steep correlation can be explained if the inner accretion flow is radiatively efficient during the hard state, in contrast to what is usually assumed for black hole X‐ray binaries in this spectral state. The transition between the steep and the standard correlation would therefore reflect a change from a radiatively efficient to a radiatively inefficient accretion flow. Finally, we investigate the possibility that the discrepancy between ‘outliers’ and ‘standard’ black holes arises from the outflow properties rather than from the accretion flow.

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Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the International Gamma-Ray Astrophysics Laboratory Imager on Board the Integral Satellite (INTEGRAL/IBIS) telescope. Spectral modeling of the data reveals two emission mechanisms: The 250- to 400-keV (kilo-electron volt) data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400-keV to 2-MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.

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.Results. Through spectroscopic analysis of the most likely candidates we found the spectral types of IGR J16320-4751, IGR J16358-4726, IGR J16479-4514, IGR J17252-3616, IGR J18027-2016: They all host OB type supergiant companion stars, with IGR J16358-4726 likely hosting an sgB[e]. Our spectra also confirm the supergiant O and B nature of IGR J17391-3021 and IGR J19140+0951. From SED fitting we found that IGR J16418-4532 is a (likely OB supergiant) HMXB, IGR J16393-4643 a (likely BIV-V star) HMXB, and IGR J18483-0311 a likely HMXB system. Through accurate astrometry, we rejected the proposed counterparts of IGR J17091-3624 and IGR J17597-2201, and we discovered two new candidate counterparts for each source, both suggesting an LMXB from SED fitting. We confirm the AGN nature of IGR J16558-5203. Finally, we report that NIR fields of four sources of our sample exhibit large-scale regions of absorption. Conclusions. The majority of these systems are high-mass X-ray binaries hosting supergiant companion stars. We therefore confirm that INTEGRAL reveals a dominant class of obscured and short-lived high-energy binary systems, and we suggest an association of these systems with regions of the Galaxy exhibiting large-scale absorption. Stellar population models should take these systems into account for realistic estimates of high-energy binary systems in our Galaxy.

PSR B1259−63 is in a highly eccentric 3.4‐yr orbit with a Be star and crosses the Be star disc twice per orbit, just prior to and just after periastron. Unpulsed radio, X‐ray and gamma‐ray emission observed from the binary system is thought to be due to the collision of pulsar wind with the wind of Be star. We present here the results of new XMM–Newton observations of the PSR B1259−63 system during the beginning of 2004 as the pulsar approached the disc of the Be star. We combine these results with the earlier unpublished X‐ray data from BeppoSAX and XMM–Newton as well as with the ASCA data. The detailed X‐ray light curve of the system shows that the pulsar passes (twice per orbit) through a well‐defined Gaussian‐profile disc with the half‐opening angle (projected on the pulsar orbit plane) . The intersection of the disc middle plane with the pulsar orbital plane is inclined at θdisc≃ 70° to the major axis of the pulsar orbit. Comparing the X‐ray light curve to the TeV light curve of the the system, we find that the increase of the TeV flux some 10–100 d after the periastron passage is unambiguously related to the disc passage. At the moment of entrance to the disc, the X‐ray photon index hardens from Γ≃ 1.8 up to ≃1.2 before returning to the steeper value Γ≥ 1.5. Such behaviour is not easily accounted for by the model in which the X‐ray emission is synchrotron emission from the shocked pulsar wind. We argue that the observed hardening of the X‐ray spectrum is due to the inverse‐Compton or bremsstrahlung emission from 10–100 MeV electrons responsible for the radio synchrotron emission.

We present a scheme for determining the spectral state of the canonical black hole Cyg X-1 using data from previous and current X-ray all sky monitors (RXTE-ASM, Swift-BAT, MAXI, and Fermi-GBM). Determinations of the hard/intermediate and soft state agree to better than 10% between different monitors, facilitating the determination of the state and its context for any observation of the source, potentially over the lifetimes of different individual monitors. A separation of the hard and the intermediate states, which strongly differ in their spectral shape and short-term timing behavior, is only possible when data in the soft X-rays (<5 keV) are available. A statistical analysis of the states confirms the different activity patterns of the source (e.g., month-to year-long hard-state periods or phases during which numerous transitions occur). It also shows that the hard and soft states are stable, with the probability of Cyg X-1 remaining in a given state for at least one week to be larger than 85% in the hard state and larger than 75% in the soft state. Intermediate states are short lived, with a 50% probability that the source leaves the intermediate state within three days. Reliable detection of these potentially short-lived events is only possible with monitor data that have a time resolution better than 1 d.

We report on RXTE observations of the microquasar XTE J1550À564 during a $70 day outburst in 2000 April-June. We present the PCA+HEXTE 3-200 keV energy spectra of the source and study their evolution over the outburst. The spectra indicate that the source transited from an initial low hard state (LS) to an intermediate state (IS) characterized by a $1 crab maximum in the 1.5-12 keV band and then went back to the LS. The source shows a hysteresis effect such that the second transition occurs at a 2-200 keV flux that is half of the flux at the first transition. This behavior is similar to what is observed in other sources and favors a common origin for the state transitions in soft X-ray transients. In addition, the first transition occurs at an approximately constant 2-200 keV flux, which probably indicates a change in the relative importance of the emitting media, whereas the second transition occurs during a time when the flux gradually decreases, which probably indicates that it is driven by a drop in the mass accretion rate. In both LSs, the spectra are characterized by the presence of a strong power-law tail (Compton corona) with a variable high-energy cutoff. During the IS, the spectra show the presence of a $0.8 keV thermal component, which we attribute to an optically thick accretion disk. The inner disk radius as inferred from disk blackbody fits to the energy spectrum remains relatively constant throughout the IS. This suggests that the disk may be close to its last stable orbit during this period. We discuss the apparently independent evolution of the two media and show that right after the X-ray maximum on MJD 51,662 the decrease of the source luminosity is due to a decrease of the power-law luminosity, at a constant disk luminosity. The detection of radio emission with a spectrum typical of optically thin synchrotron emission soon after the X-ray peak and the sudden decrease of the power-law luminosity at the same time may suggest that the corona is ejected and further detected as a discrete radio ejection.

Abstract. The imager on board INTEGRAL (IBIS) presently provides the most detailed sky images ever obtained at energies above 30 keV. The telescope is based on a coded aperture imaging system which allows to obtain sky images in a large field of view (29with an angular resolution of 12 . The System Point Spread Function of the telescope and its detailed characteristics are here described along with the specific analysis algorithms used to derive the accurate point-like source locations. The derived location accuracy is studied using the first in-flight calibration data on strong sources for the IBIS/ISGRI system. The dependence of the calibrated location accuracy with the signal to noise ratio of the sources is presented. These preliminary studies demonstrate that the IBIS/ISGRI telescope and the standard scientific analysis software allow source localizations with accuracy at 90% confidence level better than 1 for sources with signal to noise ratios >30 over the whole field of view, in agreement with the expected performances of the instrument.