Jose M. Torrejon

High variability in Vela X-1: giant flares and off states

Aims. We investigate the spectral and temporal behavior of the high mass X-ray binary Vela X-1 during a phase of high activity, with special focus on the observed giant flares and off states. Methods. INTEGRAL observed Vela X-1 in a long almost uninterrupted observation for two weeks in 2003 Nov/Dec. The data were analyzed with OSA 7.0 and FTOOLS 6.2. We derive the pulse period, light curves, spectra, hardness ratios, and hardness intensity diagrams, and study the eclipse. Results. In addition to an already high activity level, Vela X-1 exhibited several intense flares, the brightest ones reaching a maximum intensity of more than 5 Crab in the 20–40 keV band and several off states where the source was no longer detected by INTEGRAL. We determine the pulse period to be 283.5320 ± 0.0002 s, which is stable throughout the entire observation. Analyzing the eclipses provided an improvement in the ephemeris. Spectral analysis of the flares indicates that there appear to be two types of flares: relatively brief flares, which can be extremely intense and show spectral softening, in contrast to high intensity states, which are longer and show no softening. Conclusions. Both flares and off states are interpreted as being due to a strongly structured wind of the optical companion. When Vela X-1 encounters a cavity with strongly reduced density, the flux will drop triggering the onset of the propeller effect, which inhibits further accretion, giving rise to off states. The sudden decrease in the density of the material required to trigger the propeller effect in Vela X-1 is of the same order as predicted by theoretical papers about the densities in OB star winds. A similarly structured wind can produce giant flares when Vela X-1 encounters a dense blob in the wind.

The Optical Counterpart to the Peculiar X-Ray Transient XTE J1739–302*

The weak X-ray transient XTE J1739-302, characterized by extremely short outbursts, has recently been identified with a reddened star. Here we present spectroscopy and photometry of the counterpart, identifying it as a O8 Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 thus becomes the prototype of the new class of supergiant fast X-ray transients (SFXTs). The optical and infrared spectra of the counterpart to XTE J1739-302 do not reveal any obvious characteristics setting it apart from other X-ray binaries with supergiant companions, which display a very different type of X-ray light curve.

A CHANDRA SURVEY OF FLUORESCENCE Fe LINES IN X-RAY BINARIES AT HIGH RESOLUTION

Fe K line fluorescence is commonly observed in the X-ray spectra of many X-ray binaries (XRBs) and represents a fundamental tool to investigate the material surrounding the X-ray source. In this paper, we present a comprehensive survey of 41 XRBs (10 HMXBs and 31 LMXBs) with Chandra with specific emphasis on the Fe K region and the narrow Fe Kα line, at the highest resolution possible. We find that (1) the Fe Kα line is always centered at λ = 1.9387 ± 0.0016 A, compatible with Fei up to Fex; we detect no shifts to higher ionization states nor any difference between high mass X-ray binaries (HMXBs) and low mass X-ray binaries (LMXBs). (2) The line is very narrow, with FWHM < 5 mA, normally not resolved by Chandra which means that the reprocessing material is not rotating at high speeds. (3) Fe Kα fluorescence is present in all the HMXBs in the survey. In contrast, such emissions are astonishingly rare (∼10%) among LMXBs where only a few out of a large number showed Fe K fluorescence. However, the line and edge properties of these few are very similar to their high mass cousins. (4) The lack of Fe line emission is always accompanied by the lack of any detectable K edge. (5) We obtain the empirical curve of growth of the equivalent width of the Fe Kα line versus the density column of the reprocessing material, i.e., EWKα versus NH, and show that it is consistent with a reprocessing region spherically distributed around the compact object. (6) We show that fluorescence in XRBs follows the X-ray Baldwin effect as previously only found in the X-ray spectra of active galactic nuclei. We interpret this finding as evidence of decreasing neutral Fe abundance with increasing X-ray illumination and use it to explain some spectral states of Cyg X-1 as a possible cause of the lack of narrow Fe line emission in LMXBs. (7) Finally, we study anomalous morphologies such as Compton shoulders and asymmetric line profiles associated with the line fluorescence. Specifically, we present the first evidence of a Compton shoulder in the HMXB X1908+075. Also, the Fe Kα lines of 4U1700−37 and LMC X-4 present asymmetric wings, suggesting the presence of highly structured stellar winds in these systems.

Near-infrared survey of high mass X-ray binary candidates

Context. The INTEGRAL satellite is detecting a large population of new X-ray sources that were missed by previous missions because of high obscuration and, in some cases, very short duty cycles. The nature of these sources must be addressed by characterizing their optical and/or infrared counterparts. Aims. We investigate the nature of the optical counterparts to five of these newly discovered X-ray sources. Methods. We combine infrared spectra in the I, J,H ,a ndK bands with JHK photometry to characterize the spectral type, luminosity class, and distance to the infrared counterparts to these systems. For IGR J19140+0951, we present spectroscopy from the red to the K band and new red and infrared photometry. For SAX J18186−1703 and IGR J18483−0311, we present the first intermediateresolution spectroscopy to be published. Finally, for IGR J18027−2016, we present new I and K band spectra. Results. We find that four systems harbour early-type B supergiants. All of them are heavily obscured, with E(B − V) ranging between 3 and 5, implying visual extinctions of ∼ 9 to 15 mag. We refine the published classifications of IGR J18027−2016 and IGR J19140+0951 by constraining their luminosity class. In the first case, we confirm the supergiant nature but exclude a class III. In the second case, we propose a slightly higher luminosity class (Ia instead of Iab) and provide an improved value of the distance based on new optical photometry. Two other systems, SAX J18186−1703 and IGR J18483−0311 are classified as supergiant fast X-ray transients (SFXTs). XTE J1901+014, on the other hand, contains no bright infrared source in its error circle. Conclusions. Owing to their infrared and X-ray characteristics, IGR J18027−2016 and IGR J19140+0951, emerge as supergiant X-ray binaries with X-ray luminosities of the order of LX ∼ [1 − 2] × 10 36 erg s −1 , while SAX J1818.6−1703 and IGR J18483−0311, are found to be SFXTs at 2 and 3 kpc, respectively. Finally, XTE J1901+014 emerges as a puzzling source: its X-ray behaviour is strongly reminiscent of SFXTs, but a supergiant nature is firmly excluded for the counterpart. We discuss several alternative scenarios to explain its behaviour.

Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

Context. The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 10 −3 –10 3 s. A cyclotron line at ∼30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Aims. The main objective of our RXTE monitoring of 4U 2206+54 is to study the X-ray orbital variability of the spectral and timing parameters. The new long and uninterrupted RXTE observations allow us to search for long (∼1 h) pulsations for the first time. Methods. We divided the ∼7-day observation into five intervals and obtained time-averaged energy spectra and power spectral density for each observation interval. We also searched for pulsations using various algorithms. Results. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2–10 keV is 1.5 × 10 35 erg s −1 with a ratio Fmax/Fmin ≈ 5. This ratio implies an eccentricity of ∼0.4, somewhat higher than previously suggested. The power spectrum is dominated by red noise that can be fitted with a single power law whose index and strength decrease with X-ray flux. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. Conclusions. The discovery of X-ray pulsations in 4U 2206+54 confirms the neutron star nature of the compact companion and definitively rules out the presence of a black hole. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit. If current models for the spin evolution in X-ray pulsars are correct, then the magnetic field of 4U 2206+54 at birth must have been B 10 14 G.

Supergiant Fast X‐ray Transients and Other Wind Accretors

Supergiant Fast X‐ray Transients are obviously related to persistent Supergiant X‐ray Binaries. Any convincing explanation for their behaviour must consistently take into account all types of X‐ray sources powered by wind accretion. Here we present a common framework for wind accreting sources, within the context of clumpy wind models, that allows a coherent interpretation of their different behaviours as an immediate consequence of diverse orbital geometries.

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

Massive stars, at least ∼10

Further evidence for the presence of a neutron star in 4U 2206+54. INTEGRAL and VLA observations

\sim10

INTEGRAL long-term monitoring of the supergiant fast X-ray transient XTE J1739-302

times more massive than the Sun, have two key properties that make them the main drivers of evolution of star clusters, galaxies, and the Universe as a whole. On the one hand, the outer layers of massive stars are so hot that they produce most of the ionizing ultraviolet radiation of galaxies; in fact, the first massive stars helped to re-ionize the Universe after its Dark Ages. Another important property of massive stars are the strong stellar winds and outflows they produce. This mass loss, and finally the explosion of a massive star as a supernova or a gamma-ray burst, provide a significant input of mechanical and radiative energy into the interstellar space. These two properties together make massive stars one of the most important cosmic engines: they trigger the star formation and enrich the interstellar medium with heavy elements, that ultimately leads to formation of Earth-like rocky planets and the development of complex life. The study of massive star winds is thus a truly multidisciplinary field and has a wide impact on different areas of astronomy.In recent years observational and theoretical evidences have been growing that these winds are not smooth and homogeneous as previously assumed, but rather populated by dense “clumps”. The presence of these structures dramatically affects the mass loss rates derived from the study of stellar winds. Clump properties in isolated stars are nowadays inferred mostly through indirect methods (i.e., spectroscopic observations of line profiles in various wavelength regimes, and their analysis based on tailored, inhomogeneous wind models). The limited characterization of the clump physical properties (mass, size) obtained so far have led to large uncertainties in the mass loss rates from massive stars. Such uncertainties limit our understanding of the role of massive star winds in galactic and cosmic evolution.Supergiant high mass X-ray binaries (SgXBs) are among the brightest X-ray sources in the sky. A large number of them consist of a neutron star accreting from the wind of a massive companion and producing a powerful X-ray source. The characteristics of the stellar wind together with the complex interactions between the compact object and the donor star determine the observed X-ray output from all these systems. Consequently, the use of SgXBs for studies of massive stars is only possible when the physics of the stellar winds, the compact objects, and accretion mechanisms are combined together and confronted with observations.This detailed review summarises the current knowledge on the theory and observations of winds from massive stars, as well as on observations and accretion processes in wind-fed high mass X-ray binaries. The aim is to combine in the near future all available theoretical diagnostics and observational measurements to achieve a unified picture of massive star winds in isolated objects and in binary systems.

ON THE RADIAL ONSET OF CLUMPING IN THE WIND OF THE B0I MASSIVE STAR QV NOR

The majority of High Mass X-ray Binaries (HMXBs) behave as X-ray pulsars, revealing that they contain a magne- tised neutron star. Among the four HMXBs not showing pulsations, and that do not show the characteristics of accreting bl ack holes, there is the unusual HMXB 4U 2206+54. Here we present contemporaneous high-energy and radio observations of this system conducted with INTEGRALand the VLA in order to unveil its nature. The high-energy spectra show clear indications of the presence of an absorption feature at∼32 keV. This is the third high-energy observatory which reveals marginal evidence of this feature, giving strong support to the existence of a cyc lotron resonance scattering feature, which implies a magnetic field of 3.6×10 12 G. On the other hand, the source is not detected at centimetre radio wavelengths with a 3σ upper limit of 0.039 mJy. The expected radio emission for an accreting black hole in the low/hard state, inferred from X-ray flux measurements, would be at least 60 times greater than the measured upper limit. Both results firmly indicate that, in spite of the absence of pul sations, 4U 2206+54 hosts a magnetic accreting neutron star, the first one not t o be observed as an X-ray pulsar.