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  • Other Sources  (3)
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  • 1
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    The Clumpy Absorber in the High-Mass X-Ray Binary Vela X-1 (2017)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase approximately 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium, and neon whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN59935 , Astronomy & Astrophysics (ISSN 0004-6361) (e-ISSN 2197-3504); 608; A143
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    NASA TECHNICAL REPORTS
  • 2
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    A Precessing Be Disc as a Possible Model for Occultation Events in GX 304-1 (2017)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: We report on the RXTE detection of a sudden increase in the absorption column density, NH, during the 2011 May outburst of GX 3041. The NH increased up to 16 10(exp 22) atoms cm(exp 2), which is a factor of 34 larger than what is usually measured during the outbursts of GX 3041 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-Proportional Counter Array light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disc around the Be-type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be disc density to be of the order of 10(exp 11)g cm(exp 3). Our model strengthens the idea of inclined Be discs as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN60528 , Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711) (e-ISSN 1365-2966); 471; 2; 1553–1564
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    NASA TECHNICAL REPORTS
  • 3
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    INTEGRAL Long-Term Monitoring of the Supergiant Fast X-Ray Transient XTE J1739-302 (2008)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Context. In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds. Aims. Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. Methods. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1 - 200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. Results. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity.
    Keywords: Astronomy
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