We model the broad-band X-ray spectrum of Cyg X-3 in all states displayed by this source as observed by the Rossi X-ray Timing Explorer. From our models, we derive for the first time unabsorbed spectral shapes and luminosities for the full range of spectral states. We interpret the unabsorbed spectra in terms of Comptonization by a hybrid electron distribution and strong Compton reflection. We study the spectral evolution and compare with other black hole as well as neutron star sources. We show that a neutron star accretor is not consistent with the spectral evolution as a function of Ledd and especially not with the transition to a hard state. Our results point to the compact object in Cyg X-3 being a massive, ~30 Msun black hole.Comment: 14 pages, 9 figures, accepted for publication in MNRA

We report multifrequency radio observations of GRO J1655[40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 AugustÈ September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and signiÐcant deviations from a simple exponential decay. The polarization data show that the magnetic Ðeld is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid "" core-lobe ÏÏ model with a core which contributes both synchrotron and free-free emission and "" lobes,ÏÏ which are classical synchrotron emitters.

Abstract.We report on the analysis of 100 ks INTEGRAL observations of the Galactic microquasar GRS 1915+105. We focus on INTEGRAL Revolution number 48 when the source was found to exhibit a new type of variability as preliminarily reported in Hannikainen (2003, A&A, 411, L415). The variability pattern, which we name ξ, is characterized by a pulsing behaviour, consisting of a main pulse and a shorter, softer, and smaller amplitude precursor pulse, on a timescale of 5 min in the JEM-X 3-35 keV lightcurve. We also present simultaneous RXTE data. From a study of the individual RXTE/PCA pulse profiles we find that the rising phase is shorter and harder than the declining phase, which is opposite to what has been observed in other otherwise similar variability classes in this source. The position in the colour-colour diagram throughout the revolution corresponds to State A (Belloni et al. 2000, A&A, 355, 271) but not to any previously known variability class. We separated the INTEGRAL data into two subsets covering the maxima and minima of the pulses and fitted the resulting two broadband spectra with a hybrid thermal-non-thermal Comptonization model. The fits show the source to be in a soft state characterized by a strong disc component below ∼6 keV and Comptonization by both thermal and non-thermal electrons at higher energies.

Aims. The aim of this work is to investigate in a physical and quantitative way the spectral evolution of bright neutron star low-mass X-ray binaries (NS LMXBs) with special regard to the transient hard X-ray tails. Methods. We analyzed INTEGRAL data for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and built broad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source, X-ray spectra from different states were fitted with the recently proposed model compTB. Results. The spectra have been fit with a two-compTB model. In all cases the first compTB describes the dominant part of the spectrum that we interpret as thermal Comptonization of soft seed photons (<1 keV), likely from the accretion disk, by a 3−5 keV corona. In all cases, this component does not evolve much in terms of Comptonization efficiency, with the system converging to thermal equilibrium for an increasing accretion rate. The second compTB varies more dramatically, spanning from bulk plus thermal Comptonization of blackbody seed photons to the blackbody emission alone. These seed photons (R < 12 km, kT s > 1 keV), likely from the neutron star and the innermost part of the system, the transition layer, are Comptonized by matter in a converging flow. The presence and nature of this second compTB component (whether a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behavior of the hard tail: high values of accretion rates correspond to an efficient bulk Comptonization process (bulk parameter δ 0), while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (δ = 0). Conclusions. The spectral evolution of the sources has been successfully studied in terms of thermal and bulk Comptonization efficiency in relation to the physical conditions in the transition layer.

We have used the RXTE and INTEGRAL satellites simultaneously to observe the high‐mass X‐ray binary (HMXB) 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 38°–75°. This analysis leads to a wind mass‐loss rate from the companion star of ∼5 × 10−8 M⊙ yr−1, consistent with an OB I spectral type. We have detected a soft excess in at least four 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 around the superior conjunction, that it may be explained as the reprocessing of the X‐ray emission originating from the neutron star by the surrounding ionized gas.

Abstract. We report the detection of circular polarization during the 1994 outburst of the Galactic microquasar GRO J1655−40. The circular polarization is clearly detected at 1.4 and 2.4 GHz, but not at 4.8 and 8.4 GHz, where its magnitude never exceeds 5 mJy. Both the sign and magnitude of the circular polarization evolve during the outburst. The time dependence and magnitude of the polarized emission can be qualitatively explained by a model based on synchrotron emission from the outbursts, but is most consistent with circular polarization arising from propagation effects through the relativistic plasma surrounding the object.

We have fitted ∼200 RXTE and INTEGRAL spectra of the neutron star (NS) low‐mass X‐ray binary (LMXB) GX 9+9 from 2002 to 2007 with a model consisting of a disc blackbody and another blackbody representing the spreading layer (SL), i.e. an extended accretion zone on the NS surface as opposed to the more traditional disc‐like boundary layer. Contrary to theory, the SL temperature was seen to increase towards low SL luminosities, while the approximate angular extent had a nearly linear luminosity dependency. Comptonization was not required to adequately fit these spectra. Together with the ∼ 70° upper bound of inclination implied by the lack of eclipses, the best‐fitting normalization of the accretion disc blackbody component implies a distance of ∼10 kpc, instead of the usually quoted 5 kpc.

Aims. In this paper we study whether the shock-in-jet model, widely used to explain the outbursting behaviour of quasars, can be used to explain the radio flaring behaviour of the microquasar Cygnus X-3. Methods. We have used a method developed to model the synchrotron outbursts of quasar jets, which decomposes multifrequency lightcurves into a series of outbursts. The method is based on the Marscher & Gear (1985) shock model, but we have implemented the modifications to the model suggested by Björnsson & Aslaksen (2000), which make the flux density increase in the initial phase less abrupt. We study the average outburst evolution as well as specific characteristics of individual outbursts and physical jet properties of Cyg X-3. Results. We find that the lightcurves of the February-March 1994 and September 2001 outbursts can be described with the modified shock model. The average evolution shows that instead of the expected synchrotron plateau, the flux density is still increasing during the synchrotron stage. We also find that high frequency peaking outbursts are shorter in duration than the ones peaking at lower frequencies. Finally, we show that the method can be used, complementary to radio interferometric jet imaging, for deriving the physical parameters such as the magnetic field strength and the energy density of relativistic electrons in the jet of Cyg X-3.