503 research outputs found
Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
We report on the long-term average spin period, rate of change of spin period
and X-ray luminosity during outbursts for 42 Be X-ray binary systems in the
Small Magellanic Cloud. We also collect and calculate parameters of each system
and use these data to determine that all systems contain a neutron star which
is accreting via a disc, rather than a wind, and that if these neutron stars
are near spin equilibrium, then over half of them, including all with spin
periods over about 100 s, have magnetic fields over the quantum critical level
of 4.4x10^13 G. If these neutron stars are not close to spin equilibrium, then
their magnetic fields are inferred to be much lower, of the order of 10^6-10^10
G, comparable to the fields of neutron stars in low-mass X-ray binaries. Both
results are unexpected and have implications for the rate of magnetic field
decay and the isolated neutron star population.Comment: 22 pages, 50 figures; to appear in MNRA
Spin equilibrium in strongly-magnetized accreting stars
Strongly magnetized accreting stars are often hypothesized to be in `spin
equilibrium' with their surrounding accretion flows, which requires that the
accretion rate changes more slowly than it takes the star to reach spin
equilibrium. This is not true for most magnetically accreting stars, which have
strongly variable accretion outbursts on time-scales much shorter than the time
it would take to reach spin equilibrium. This paper examines how accretion
outbursts affect the time a star takes to reach spin equilibrium and its final
equilibrium spin period. I consider several different models for angular
momentum loss -- either carried away in an outflow, lost to a stellar wind, or
transferred back to the accretion disc (the `trapped disc'). For transient
sources, the outflow scenario leads to significantly longer times to reach spin
equilibrium (10x), and shorter equilibrium spin periods than would be
expected from spin equilibrium arguments, while the `trapped disc' does not.
The results suggest that disc trapping plays a significant role in the spin
evolution of strongly magnetic stars, with some caveats for young stellar
objects.Comment: version accepted by MNRAS; some significant changes to conclusions
about FU Ori star
Transient High Mass X-ray Binaries
High Mass X-ray Binaries (HMXBs) are interesting objects that provide a wide
range of observational probes to the nature of the two stellar components,
accretion process, stellar wind and orbital parameters of the systems. A large
fraction of the transient HMXBs are found to be Be/X-ray binaries in which the
companion Be star with its circumstellar disk governs the outburst. These
outbursts are understood to be due to the sudden enhanced mass accretion to the
neutron star and is likely to be associated with changes in the circumstellar
disk of the companion. In the recent years, another class of transient HMXBs
have been found which have supergiant companions and show shorter bursts.
X-ray, infrared and optical observations of these objects provide vital
information regarding these systems. Here we review some key observational
properties of the transient HMXBs and also discuss some important recent
developments from studies of this class of sources. The X-ray properties of
these objects are discussed in some detail whereas the optical and infrared
properties are briefly discussed.Comment: 21 Pages, 8 Figures, To appear in the special issue of the Bulletin
of the Astronomical Society of India on Transients at different wavelengths,
eds D.J. Saikia and D.A. Gree
On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud
We explore the possibility to explain the properties of the Be/X-ray pulsars
observed in the Small Magellanic Cloud within the magnetic levitation accretion
scenario. This implies that their X-ray emission is powered by a wind-fed
accretion onto a neutron star (NS) which captures matter from a magnetized
stellar wind. The NS in this case is accreting matter from a non-keplerian
magnetically levitating disc (ML-disc) which is surrounding its magnetosphere.
This allows us to explain the observed periods of the pulsars in terms of spin
equilibrium without the need of invoking dipole magnetic fields outside the
usual range ~ 10^11- 10^13 G inferred from cyclotron features of Galactic high
mass X-ray binaries. We find that the equilibrium period of a NS, under certain
conditions, depends strongly on the magnetization of the stellar wind of its
massive companion and, correspondingly, on the magnetic field of the massive
companion itself. This may help to explain why similar NSs in binaries with
similar properties rotate with different periods yielding a large scatter of
periods of the accretion-powered pulsar observed in SMC and our galaxy.Comment: 6 pages, 1 figure, Published in MNRAS 454, 3760-3765 (2015
X-Ray Timing, Spectroscopy and Photometry of the Anomalous X-Ray Pulsar Candidate CXOU J010043.1-721134
We present new X-ray timing and spectral results on the 8.0-second X-ray
pulsar CXOU J010043.1-721134 from a series of observations using the Chandra
X-ray Observatory. We find a spin period in 2004 January of 8.020392pm0.000009
seconds. Comparison of this to 2001 Chandra observations implies a period
derivative dot{P} = (1.88 pm 0.08) times 10^{-11} s s^{-1}, leading to an
inferred dipole surface magnetic field of 3.9 times 10^{14} G. The spectrum is
well fit to an absorbed blackbody of temperature kT = 0.38pm0.02 keV with a
power law tail of photon index Gamma = 2.0pm0.6. We find that the source has an
unabsorbed X-ray flux (0.5-10 keV) of 4(+2-1) times 10^{-13} erg cm^{-2} s^{-1}
and a corresponding X-ray luminosity of ~2 times 10^{35} erg s^{-1} for a
distance of 60 kpc. These properties support classification of CXOU
J010043.1-721134 as the seventh confirmed anomalous X-ray pulsar,the eleventh
confirmed magnetar, and the first magnetar to be identified in the Small
Magellanic Cloud.Comment: 5 pages, plus 1 embedded eps figure. Refined coordinates of source,
including typo in declination. ApJ Letters, in pres
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