Lionel Prat

Peering through the stellar wind of IGR J19140+0951 with simultaneous INTEGRAL/RXTE observations

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 x 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.

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.

IS THE CORONA THE SOURCE OF THE EJECTED MATERIAL IN MICROQUASARS

Microquasar outbursts are characterized by spectral state transitions. The transitions between states characterized by a hard spectrum and those characterized by a soft spectrum are of particular interest. Besides drastic spectral and timing changes, these transitions often show discrete ejections detectable in the radio domain. The mechanisms giving birth to the ejections, the links with accretion and the exact nature of the ejected material are still largely unknown. We present systematic X–ray spectral analysis prior to the ejection in several microquasars, and show that, in each case, the properties of the corona drastically evolve, while that of the disc remain roughly constant. We intepret this behavior as possibly due to an ejection of the corona at the spectral transition.

Ejection of the Corona at State Transitions: a Common Behavior in Microquasars?

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 behavior of at least 4 microquasars points in the direction of an ejection of the corona at the state transition, feature that is ...

On the nature of discrete radio ejection events in microquasars

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 spe tral and timing changes, this transition often shows a discrete ejection event detectable in t he radio range. However, the exact nature of the ejected material and the mechanisms that give b irth to these phenomena are yet to be unraveled. Recent simultaneous radio and X-ray observat ions on several sources point to a coronal nature of the ejected material. First, in the case of XTE J1550-564, the hard X-ray tail, usually attributed to an inverse Compton effect, disappear s just before an ejection is detected at radio wavelengths. Then, in the case of GRS 1915+105, the flux of the Compton component decreases sharply just prior to an ejection. We interpret th is behaviour as evidence of the disappearance of the corona, which is ejected under the form of blo bs. In the case of the 2002 outburst of GX 339-4, the first radio detection occurs after a decrease of the disc flux, which may indicate that the radio events are initiated by the disc. The peak of th e radio flare occurs several day after, and in particular after a large decrease of the coronal flux. F inally, in the case of XTE J1859+226, drastic physical changes occurred in the corona just before the state transition, compatible with the disappearance of part of this medium. Thus, a total of 4 mi croquasars point in the direction of an ejection of the corona, feature that is yet to be confirme d (or infirmed) in the case of other available sources.

RXTE and INTEGRAL Observations of IGR J19140+0951

We have simultaneously used the RXTE and INTEGRAL satellites 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 ∼10−7 M⊙/year, consistent with its expected 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 prior to the superior conjunction, that it may be explained...