Class Of High-mass X-ray Binaries Research Articles

Multiwavelength study of the fast rotating supergiant high-mass X-ray binary IGR J16465−4507

Since its launch, the X-ray and gamma-ray observatory INTEGRAL satellite has revealed a new class of high-mass X-ray binaries (HMXB) displaying fast flares and hosting supergiant companion stars. Optical and infrared (OIR) observations in a multi-wavelength context are essential to understand the nature and evolution of these newly discovered celestial objects. The goal of this multiwavelength study (from ultraviolet to infrared) is to characterise the properties of IGR J16465-4507, to confirm its HMXB nature and that it hosts a supergiant star. We analysed all OIR, photometric and spectroscopic observations taken on this source, carried out at ESO facilities. Using spectroscopic data, we constrained the spectral type of the companion star between B0.5 and B1 Ib, settling the debate on the true nature of this source. We measured a high rotation velocity of v = 320 +/- 8 km/s from fitting absorption and emission lines in a stellar spectral model. We then built a spectral energy distribution from photometric observations to evaluate the origin of the different components radiating at each energy range. We finally show that, having accurately determined the spectral type of the early-B supergiant in IGR J16465-4507, we firmly support its classification as an intermediate supergiant fast X-ray transient (SFXT).

Retrograde wind accretion – an alternative mechanism for long spin period of SFXTs

A new class of high-mass X-ray binaries (HMXBs) --- supergiant fast X-ray transients (SFXTs) --- are discovered by INTEGRAL, which are associated with OB supergiants and present long spin periods. Observational evidence indicates that some accreting neutron stars in HMXBs display accretion reversals. It has been suggested that the inverted torque can lead to a very slow rotator. According to three characteristic radii in wind-fed accretion, we developed a retrograde accretion scenario and divided the accretion phase into three regimes, to interpret the formation of the long spin period of SFXTs. The accretion regime in some SFXT systems can be determined by their spin and orbital periods.

Accurate localization and identification of six hard X-ray sources from Chandra and XMM-Newton data

We present the results of Chandra and XMM-Newton observations for six hard X-ray sources (IGR J12134-6015, IGR J18293-1213, IGR J18219-1347, IGR J17350-2045, IGR J18048-1455, XTE J1901+014) from the INTEGRAL all-sky survey. Based on these observations, we have improved significantly the localization accuracy of the objects and, therefore, have managed to identify their optical counterparts. Using data from the publicly available 2MASS and UKIDSS infrared sky surveys as well as data from the SOFI/NTT telescope (European Southern Observatory), we have determined the magnitudes of the optical counterparts, estimated their types and (in some cases) the distances to the program objects. A triplet of iron lines with energies of 6.4, 6.7, and 6.9 keV has been detected in the X-ray spectrum of IGR J18048-1455; together with the detection of pulsations with a period of ~1440 s from this source, this has allowed it to be classified as a cataclysmic variable, most likely an intermediate polar. In addition, broadband X-ray spectra of IGR J12134-6015 and IGR J17350-2045 in combination with infrared and radio observations suggest an extragalactic nature of these objects. The source IGR J18219-1347 presumably belongs to the class of high-mass X-ray binaries.

Monitoring Supergiant Fast X‐Ray Transients withSwift. I. Behavior Outside Outbursts

Supergiant fast X-ray transients (SFXTs) are a new class of high-mass X-ray binaries (HMXBs) discovered thanks to the monitoring of the Galactic plane performed with the INTEGRAL satellite in the last 5 years. These sources display short outbursts (significantly shorter than typical Be/X-ray binaries) with a peak luminosity of a few 1036 erg s−1. The quiescent level, measured only in a few sources, is around 1032 erg s−1. The X-ray spectral properties are reminiscent of those of accreting pulsars; thus, it is likely that all the members of the new class are indeed HMXBs hosting a neutron star, although only two SFXTs have a measured pulse period, IGR J11215–5952 (~187 s) and IGR J18410–0535 (~4.7 s). Several competing mechanisms have been proposed to explain the shortness of these outbursts, mostly involving the structure of the wind from the supergiant companion. To characterize the properties of these sources on timescales of months (e.g., the quiescent level and the outburst recurrence), we are performing a monitoring campaign with Swift of four SFXTs (IGR J16479–4514, XTE J1739–302, IGR J17544–2619, and AX J1841.0–0536/IGR J18410–0535). We report on the first 4 months of Swift observations, which started on 2007 October 26. We detect low-level X-ray activity in all four SFXTs, which demonstrates that these transient sources accrete matter even outside their outbursts. This fainter X-ray activity is composed of many flares with a large flux variability, on timescales of thousands of seconds. The light-curve variability is also evident on larger timescales of days, weeks, and months, with a dynamic range of more than 1 order of magnitude in all four SFXTs. The X-ray spectra are typically hard, with an average 2-10 keV luminosity during this monitoring of about 1033-1034 erg s−1. We detected pulsations from the pulsar AX J1841.0–0536/IGR J18410–0535, with a period of 4.7008 ± 0.0004 s. This monitoring demonstrates that these transients spend most of the time accreting matter, although at a much lower level (~100-1000 times lower) than during the bright outbursts, and that the true quiescence, characterized by a soft spectrum and a luminosity of a few 1032 erg s−1, observed in the past in only a couple of members of this class, is probably a very rare state.

IGR J16479–4514: the first eclipsing supergiant fast X-ray transient?

Abstract Supergiant fast X-ray transients (SFXTs) are a new class of high-mass X-ray binaries recently discovered with INTEGRAL. Hours-long outbursts from these sources have been observed on numerous occasions at luminosities of ∼1036–1037 erg s−1, whereas their low-level activity at ∼1032–1034 erg s−1 has not been deeply investigated yet due to the paucity of long pointed observations with high-sensitivity X-ray telescopes. Here, we report on the first long (∼32 ks) pointed XMM–Newton observation of IGR J16479−4514, a member of this new class. This observation was carried out in 2008 March, shortly after an outburst from this source, with the main goal of investigating its low-level emission and physical mechanisms that drive the source activity. Results from the timing, spectral and spatial analysis of the EPIC-PN XMM–Newton observation show that the X-ray source IGR J16479−4514 underwent an episode of sudden obscuration, possibly an X-ray eclipse by the supergiant companion. We also found evidence for a soft X-ray extended halo around the source that is most readily interpreted as due to scattering by dust along the line of sight to IGR J16479−4514. We discuss this result in the context of the gated accretion scenarios that have been proposed to interpret the behaviour of SFXT.

Monitoring Supergiant Fast X-Ray Transients with Swift . II. Rise to the Outburst in IGR J16479-4514

IGR J16479–4514 is a supergiant fast X-ray transient (SFXT), a new class of high-mass X-ray binaries, whose number is rapidly growing thanks to the INTEGRAL observations of the Galactic plane. It has been regularly monitored with Swift XRT since 2007 November to study the quiescent emission, the outburst properties, and their recurrence. A new bright outburst, reaching fluxes above 10−9 erg cm−2 s−1, was caught by the Swift BAT. Swift immediately repointed at the target with the narrow-field instruments so that, for the first time, an outburst from an SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2-150 keV energy band. The X-ray emission is highly variable and spans almost 4 orders of magnitude in count rate during the Swift XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of 0.98 ± 0.07, and in an absorbing column density of ~5 × 1022 cm−2. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215–5952), the accretion phase lasts much longer than a few hours.

A New Class of High‐Mass X‐Ray Binaries: Implications for Core Collapse and Neutron Star Recoil

We investigate an interesting new class of high-mass X-ray binaries (HMXBs) with long orbital periods (P_orb > 30 days) and low eccentricities (e <~ 0.2). The orbital parameters suggest that the neutron stars in these systems did not receive a large impulse, or ``kick,'' at the time of formation. We develop a self-consistent phenomenological picture wherein the neutron stars born in the observed wide HMXBs receive only a small kick (<~ 50 km/s), while neutron stars born in isolation, in the majority of low-mass X-ray binaries, or in many of the well-known HMXBs with P_orb <~ 30 days receive the conventional large kicks, with a mean speed of ~ 300 km/s. We propose that the magnitude of the natal kick to a neutron star born in a binary system depends on the rotation rate of the pre-collapse core. We further suggest that the rotation rate of the core is a strong, well-defined function of the evolutionary path of the progenitor star.

The Superluminal Source GRS 1915+105: A High-Mass X-Ray Binary?

We obtained K-band (2.0-2.5 μm) infrared spectra of the visually obscured superluminal source GRS 1915+105 at three different epochs of 1995, at times when it was detected as a hard X-ray source. The three spectra exhibit H I Brγ (2.167 μm) and He I (2.058 μm) emission lines. The spectral morphology, absolute magnitude, and time variability of the infrared counterpart are consistent with the class of high-mass X-ray binaries that contain late Oe and early Be stars as mass donors. In GRS 1915+105 we have not detected the Doppler-shifted emission lines observed in the classic stellar source of relativistic jets, SS 433. We discuss why it may be more difficult in GRS 1915+105 than in SS 433 to detect emission lines from the jets.