SHORT-TERM VARIABILITY OF X-RAYS FROM ACCRETING NEUTRON STAR VELA X-1. I. SUZAKU OBSERVATIONS

We have analyzed the time variability of the wide-band X-ray spectrum of Vela X-1, the brightest wind-fed accreting neutron star, on a short timescale of 2 ks by using Suzaku observations with an exposure of 100 ks. During the observation, the object showed strong variability, including several flar...

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Bibliographic Details
Published in:The Astrophysical journal 2013-04, Vol.767 (1), p.1-17
Main Authors: Odaka, Hirokazu, Khangulyan, Dmitry, Tanaka, Yasuyuki T, Watanabe, Shin, Takahashi, Tadayuki, Makishima, Kazuo
Format: Article
Language:English
Subjects:
ABSORPTION
Accretion
ACCRETION DISKS
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
Broadband
COMPACTS
CYCLOTRON RESONANCE
Flares
LUMINOSITY
NEUTRON STARS
Orbitals
PHOTONS
PULSARS
SCATTERING
SHOCK WAVES
Spectra
SPECTRAL HARDENING
SPIN
STELLAR WINDS
X RADIATION
X-RAY SPECTRA
X-rays
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Summary:We have analyzed the time variability of the wide-band X-ray spectrum of Vela X-1, the brightest wind-fed accreting neutron star, on a short timescale of 2 ks by using Suzaku observations with an exposure of 100 ks. During the observation, the object showed strong variability, including several flares and so-called "low states," in which the X-ray luminosity decreases by an order of magnitude. Although the spectral hardness increases with the X-ray luminosity, the majority of the recorded flares do not show any significant changes in circumstellar absorption. However, a sign of heavy absorption was registered immediately before one short flare that showed a significant spectral hardening. In the low states, the flux level is modulated with the pulsar spin period, indicating that even at this state the accretion flow reaches the close proximity of the neutron star. Phenomenologically, the broadband X-ray spectra, which are integrated over the entire spin phase, are well represented by the "NPEX" function (a combination of negative and positive power laws with an exponential cutoff by a common folding energy) with a cyclotron resonance scattering feature at 50 keV. Fitting of the data allowed us to infer a correlation between the photon index and X-ray luminosity. Finally, the circumstellar absorption shows a gradual increase in the orbital phase interval 0.25-0.3, which can be interpreted as an impact of a bow shock imposed by the motion of the compact object in the supersonic stellar wind.
ISSN:0004-637X
1538-4357
DOI:10.1088/0004-637X/767/1/70