A. Paizis

An alternative hypothesis for the outburst mechanism in Supergiant Fast X-ray Transients: the case of IGR J11215-5952

Context. The physical mechanism responsible for the short outbursts in a recently recognised class of high mass X-ray binaries, the supergiant fast X-ray transients (SFXTs), is still unknown. Two main hypotheses have been proposed to date: the sudden accretion by the compact object of small ejections originating in a clumpy wind from the supergiant donor, or outbursts produced at (or near) the periastron passage in wide and eccentric orbits, to explain the low (∼10 32 erg s −1 ) quiescent emission. Neither proposed mechanisms explain the whole phenomenology of these sources. IGR J11215-5952, discovered in April 2005 by the INTEGRAL satellite, is a SFXT which undergoes an outburst every 329 days, a periodicity likely associated with the orbital period of the binary system. Aims. We propose a new explanation for the outburst mechanism, based on the X-ray observations of the unique SFXT known to display periodic outbursts, IGR J11215-5952. Methods. We performed three Target of Opportunity (ToO) observations with Swift, XMM-Newton and INTEGRAL at the time of the fifth outburst, expected on 2007 February 9. Swift observations of the February 2007 outburst have been reported elsewhere. Another ToO with Swift was performed in July 2007, to monitor the supposed “apastron” passage. Results. XMM-Newton observed the source on 2007 February 9, for 23 ks, at the peak of the outburst, while INTEGRAL started the observation two days later, failing to detect the source, which had already undergone the decaying phase of the fast outburst. XMM-Newton data show large variability, with a bright flare at the beginning of the observation (lasting about 1 h), followed by a lower intensity phase (about one order of magnitude fainter) with a large variability as well as low level flaring activity. The spin periodicity discovered by RXTE is confirmed, and a spin-phase spectral variability is observed and studied in detail. The Swift campaign performed in July 2007 reveals a second outburst on 2007 July 24, as bright as that observed about 165 days before. Conclusions. The new X-ray observations allow us to propose an alternative hypothesis for the outburst mechanism in SFXTs, linked to the possible presence of a second wind component, in the form of an equatorial disc from the supergiant donor. We discuss the applicability of the model to the short outburst durations of all other SFXTs, where a clear periodicity in the outbursts has not been found yet. The new outburst from IGR J11215–5952 observed in July suggests that the true orbital period is ∼165 days, instead of 329 days, as previously thought.

Chandra Deep X-ray Observation of a Typical Galactic Plane Region and Near-Infrared Identification

Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), we have carried out a deep hard X-ray observation of the Galactic plane region at (l,b) ≈ (285,00), where no discrete X-ray source had been reported previously. We have detected 274 new point X-ray sources (4 σ confidence), as well as strong Galactic diffuse emission within two partially overlapping ACIS-I fields (~250 arcmin2 in total). The point-source sensitivity was ~3 × 10-15 ergs s-1 cm-2 in the hard X-ray band (2-10 keV) and ~2 × 10-16 ergs s-1 cm-2 in the soft band (0.5-2 keV). The sum of all the detected point-source fluxes accounts for only ~10% of the total X-ray flux in the field of view. Even hypothesizing a new population of much dimmer and numerous Galactic point sources, the total observed X-ray flux cannot be explained. Therefore, we conclude that X-ray emission from the Galactic plane has a truly diffuse origin. Removing point sources brighter than ~3 × 10-15 ergs s-1 cm-2 (2-10 keV), we have determined the Galactic diffuse X-ray flux to be 6.5 × 10-11 ergs s-1 cm-2 deg-2 (2-10 keV). Only 26 point sources were detected in both the soft and hard bands, indicating that there are two distinct classes of X-ray sources distinguished by their spectral hardness ratios. The surface number density of the hard sources is only slightly higher than that measured at the high Galactic latitude regions, indicating that the majority of the hard sources are background AGNs. Following up the Chandra observation, we have performed a near-infrared (NIR) survey with SofI at ESO/NTT. Almost all the soft X-ray sources have been identified in the NIR, and their spectral types are consistent with main-sequence stars, suggesting that most of them are nearby X-ray-active stars. On the other hand, only 22% of the hard sources had NIR counterparts, which are presumably Galactic. From X-ray and NIR spectral study, they are most likely to be quiescent cataclysmic variables. Our observation suggests a population of 104 cataclysmic variables in the entire Galactic plane fainter than ~2 × 1033 ergs s-1. We have carried out a precise spectral study of the Galactic diffuse X-ray emission excluding the point sources. Confirming previous results, we have detected prominent emission lines from highly ionized heavy elements in the diffuse emission. In particular, the central energy of the iron emission line was determined to be 6.52 keV (90% confidence), which is significantly lower than what is expected from a plasma in thermal equilibrium. The downward shift of the iron line center energy suggests nonequilibrium ionization states of the plasma or the presence of a nonthermal process to produce 6.4 keV fluorescent lines.

The structure of blue supergiant winds and the accretion in supergiant High Mass X-ray Binaries

We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of High Mass X‐ray Binaries. Assuming that the clumps are confined by ram pressu re of the ambient gas and exploring different distributions for their mass and radii, w e computed the expected X‐ray light curves in the framework of the Bondi-Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properti es are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X-ray light curves of three representative High Mass X-ray Binaries: two persistent supergiant systems (Vela X-1 and 4U 1700‐377) and the Supergiant Fast X-ray Transient IGR J11215 5952. The model can reproduce well the observed light curves, but requiring in all cases an overall mass loss from the supergiant about a factor 3 10 smaller than the values inferred from UV lines studies that assume a homogeneous wind.

INTEGRAL results on supergiant fast X-ray transients and accretion mechanism interpretation: ionization effect and formation of transient accretion discs

We performed a systematic analysis of all INTEGRAL observations from 2003 to 2009 of 14 supergiant fast X-ray transients (SFXTs), implying a net exposure time of about 30 Ms. For each source we obtained light curves and spectra (3–100 keV), discovering several new outbursts. We discuss the X-ray behaviour of SFXTs emerging from our analysis in the framework of the clumpy wind accretion mechanism we proposed. We discuss the effect of X-ray photoionization on accretion in close binary systems such as IGR J16479−4514 and IGR J17544−2619. We show that, because of X-ray photoionization, there is a high probability of an accretion disc forming from the capture of angular momentum in IGR J16479−4514, and we suggest that the formation of transient accretion discs could be partly responsible for the flaring activity in SFXTs with narrow orbits. We also propose an alternative way to explain the origin of flares with peculiar shapes observed in our analysis applying the model of Lamb et al., which is based on accretion via the Rayleigh–Taylor instability and was originally proposed to explain Type II bursts.

First simultaneous multi-wavelength observations of the black hole candidate IGR J17091 3624 ATCA, INTEGRAL, Swift, and RXTE views of the 2011 outburst

We present the results of the first four (quasi-)simultaneous radio (ATCA), X-ray (Swift, RXTE), and γ-ray (INTEGRAL) observations of the black hole candidate IGR J17091−3624, performed in February and March 2011. The X-ray analysis shows that the source was in the hard state, and then it transited to a soft intermediate state. We study the correlated radio/X-ray behaviour of this source for the first time. The radio counterpart to IGR J17091−3624 was detected during all four observations with the ATCA. In the hard state, the radio spectrum is typical of optically thick synchrotron emission from a self-absorbed compact jet. In the soft intermediate state, the detection of optically thin synchrotron emission is probably due to a discrete ejection event associated with the state transition. The position of IGR J17091−3624 in the radio versus X-ray luminosity diagram (aka fundamental plane) is compatible with that of the other black hole sources for distances greater than 11 kpc. IGR J17091−3624 also appears as a new member of the few sources that show a strong quenching of radio emission after the state transition. Using the estimated luminosity at the spectral transition from the hard state, and for a typical mass of 10 M� , we estimate a distance to the source between ∼11 and ∼17 kpc, compatible with the radio behaviour of the source.

Bright flares in supergiant fast X-ray transients

At steady low-luminosity states, Supergiant Fast X-ray Transients (SFXTs) can be at the stage of quasi-spherical settling accretion onto slowly rotatin g magnetized neutron stars from the OB-companion winds. At this stage, a hot quasi-static shell is formed above the magnetosphere, the plasma entry rate into magnetosphere is controlled by (ineffi cient) radiative plasma cooling, and the accretion rate onto the neutron star is supp ressed by a factor of∼ 30 relative to the Bondi-Hoyle-Littleton value. Changes in the local wind velocity and density due to, e.g., clumps, can only slightly increase the mass accretion rate (a factor of∼ 10) bringing the system into the Compton cooling dominated regime and led to the production of moderately bright flares ( Lx . 10 36 erg/s). To interpret the brightest flares ( Lx > 10 36 erg/s) displayed by the SFXTs within the quasi-spherical settling accretion regimes, we propose that a larger increase in the mass accretion rate can be produced by sporadic capture of magnetized stellar wind plasma. At suffi cently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical e nergy released in an SFXT bright flare corresponds to the mass of the shell. This view is consis tent with the energy released in SFXT bright flares (∼ 10 38 − 10 40 ergs), their typical dynamic range (∼ 100), and with the observed dependence of these characteristics on the average unflaring X-ray luminosity of SFXTs. Thus the flaring behaviour of SFXTs, as opposed to stea dy HMXBs, may be primarily related to their low X-ray luminosity allowing sporadic magnetic reconnection to occur during magnetized plasma entry into the magnetosphere.

Supergiant Fast X-ray Transients in outburst: new Swift observations of XTE J1739−302, IGR J17544−2619 and IGR J08408−4503

We report on new X‐ray outbursts observed with Swift from three Supergiant Fast X‐ ray Transients (SFXTs): XTE J1739 302, IGR J17544 2619, and IGR J08408 4503. XTE J1739 302 underwent a new outburst on 2008, August 13, IGR J17544 2619 on 2008, September 4, while IGR J08408 4503 on 2008, September 21. While the XTE J1739 302 and IGR J08408 4503 bright emission triggered the Swift/Burst Alert Telescope, IGR J17544 2619 did not, thus we could perform a spectral investigation only of the spectrum below 10 keV. The broad band spectra from XTE J1739 302 and IGR J08408 4503 were compatible with the X‐ray spectral shape displayed during the previous flares. A variable absorbing column density during the flare was observed in XTE J1739 302 for the first time. The broad-band spectrum of IGR J08408 4503 requires the presence of two distinct photon populations, a cold one (�0.3 keV) most likely from a thermal halo around the neutron star and a hotter one (1.4‐1.8 keV) from the accreting column. The outburst from XTE J1739 302 could be monitored with a very good sampling, thus revealing a shape which can be explained with a second wind component in this SFXT, in analogy to what we have suggested in the periodic SFXT IGR J11215‐5952. The outburst recurrence timescale in IGR J17544 2619 during our monitoring campaign with Swift suggests a long orbital period of �150 days (in an highly eccentric orbit), compatible with what previously observed with INTEGRAL.

INTEGRAL and XMM-Newton observations of the X-ray pulsar IGR J16320-4751/AX J1631.9-4752

We report on observations of the X-ray pulsar IGR J16320−4751 (a.k.a. AX J1631.9−4752) performed simultaneously with INTEGRAL and XMM-Newton. We refine the source position and identify the most likely infrared counterpart. Our simultaneous coverage allows us to confirm the presence of X-ray pulsations at ∼ 1300 s, that we detect above 20 keV with INTEGRAL for the first time. The pulse fraction is consistent with bein g constant with energy, which is compatible with a model of polar accretion by a pulsar. We study the spectral properties of IGR J16320−4751 during two major periods occurring during the simultaneous coverage with both satellites, namely a flare and a non-flare p eriod. We detect the presence of a narrow 6.4 keV iron line in both periods. The presence of such a feature is typical of supergiant wind accretors such as Vela X-1 or GX 301−2. We inspect the spectral variations with respect to the pulse phase during the non-flare period, a nd show that the pulse is solely due to variations of the X-ray flux emitted by the source and no t to variations of the spectral parameters. Our results are therefore compatible with the source being a pulsar in a High Mass X-ray Binary. We detect a soft excess appearing in the spectra as a blackbody with a temperature of∼0.07 keV. We discuss the origin of the X-ray emission in IGR J16320−4751: while the hard X-rays are likely the result of Compton emission produced in the close vicinity of the pulsar, based on energy argument we suggest that the soft excess is likely the emission by a collisionally energised cloud in which the compact object is embedded.

Cumulative luminosity distributions of supergiant fast X-ray transients in hard X-rays

We have analyzed in a systematic way about nine years of INTEGRAL data (17-100 keV) focusing on Supergiant Fast X-ray Transients (SFXTs) and three classical High Mass X-ray Binaries (HMXBs). Our approach has been twofold: image based analysis, sampled over a ~ks time frame to investigate the long-term properties of the sources, and lightcurve based analysis, sampled over a 100s time frame to seize the fast variability of each source during its ~ks activity. We find that while the prototypical SFXTs (IGR J17544-2619, XTE J1739-302 and SAX J1818.6-1703) are among the sources with the lowest ~ks based duty cycle (

The INTEGRAL Galactic bulge monitoring program: the first 1.5 years

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