553 research outputs found
XMM-Newton Observations of the Be/X-ray transient A0538-66 in quiescence
We present XMM-Newton observations of the recurrent Be/X-ray transient
A0538-66, situated in the Large Magellanic Cloud, in the quiescent state.
Despite a very low luminosity state of (5-8)E33 ergs/s in the range 0.3-10 keV,
the source is clearly detected up to ~8 keV. and can be fitted using either a
power law with photon index alpha=1.9+-0.3 or a bremsstrahlung spectrum with
kT=3.9+3.9-1.7 keV. The spectral analysis confirms that the off-state spectrum
is hard without requiring any soft component, contrary to the majority of
neutron stars observed in quiescence up to now.Comment: Accepted for proceedings of 5th INTEGRAL Worksho
RX J0440.9+4431: a persistent Be/X-ray binary in outburst
The persistent Be/X-ray binary RX J0440.9+4431 flared in 2010 and 2011 and
has been followed by various X-ray facilities Swift, RXTE, XMM-Newton, and
INTEGRAL. We studied the source timing and spectral properties as a function of
its X-ray luminosity to investigate the transition from normal to flaring
activity and the dynamical properties of the system. We have determined the
orbital period from the long-term Swift/BAT light curve, but our determinations
of the spin period are not precise enough to constrain any orbital solution.
The source spectrum can always be described by a bulk-motion Comptonization
model of black body seed photons attenuated by a moderate photoelectric
absorption. At the highest luminosity, we measured a curvature of the spectrum,
which we attribute to a significant contribution of the radiation pressure in
the accretion process. This allows us to estimate that the transition from a
bulk-motion-dominated flow to a radiatively dominated one happens at a
luminosity of ~2e36 erg/s. The luminosity dependency of the size of the black
body emission region is found to be . This
suggests that either matter accreting onto the neutron star hosted in RX
J0440.9+4431 penetrates through closed magnetic field lines at the border of
the compact object magnetosphere or that the structure of the neutron star
magnetic field is more complicated than a simple dipole close to the surfaceComment: Accepted for publication by A&
Disappearing Pulses in Vela X-1
We present results from a 20 h RXTE observation of Vela X-1, ncluding a
peculiar low state of a few hours duration, during which the pulsation of the
X-ray emission ceased, while significant non-pulsed emission remained. This
``quiescent state'' was preceded by a ``normal state'' without any unusual
signs and followed by a ``high state'' of several hours of increased activity
with strong, flaring pulsations. while there is clear spectral evolution from
the normal state to the low state, the spectra of the following high state are
surprisingly similar to those of the low state.Comment: 5 pages, 5 figures, Proceedings of the 5th Compton Symposium, AIP, in
pres
Study of the cyclotron feature in MXB 0656-072
We have monitored a type II outburst of the Be/X-ray binary MXB 0656−072 in a series of pointed RXTE observations during October through December 2003. The source spectrum shows a cyclotron resonance scattering feature at 32.8 +0.5
−0.4 keV, corresponding to a magnetic field strength of 3.67
+0.06 −0.04 × 10 12 G and is stable through the outburst and over the pulsar spin phase. The pulsar, with an average pulse period of 160.4 ± 0.4s,shows a spin-up of 0.45 s over the duration of the outburst. From optical data, the source distance is estimated to be 3.9 ± 0.1 kpc and this is used to estimate the X-ray luminosity and a theoretical prediction of the pulsar spin-up during the outburst
Cyclotron lines in highly magnetized neutron stars
Cyclotron lines, also called cyclotron resonant scattering features (CRSF)
are spectral features, generally appearing in absorption, in the X-ray spectra
of objects containing highly magnetized neutron stars, allowing the direct
measurement of the magnetic field strength in these objects. Cyclotron features
are thought to be due to resonant scattering of photons by electrons in the
strong magnetic fields. The main content of this contribution focusses on
electron cyclotron lines as found in accreting X-ray binary pulsars (XRBP) with
magnetic fields on the order of several 1012 Gauss. Also, possible proton
cyclotron lines from single neutron stars with even stronger magnetic fields
are briefly discussed.With regard to electron cyclotron lines, we present an
updated list of XRBPs that show evidence of such absorption lines. The first
such line was discovered in a 1976 balloon observation of the accreting binary
pulsar Hercules X-1, it is considered to be the first direct measurement of the
magnetic field of a neutron star. As of today (mid 2018), we list 36 XRBPs
showing evidence of one ore more electron cyclotron absorption line(s). A few
have been measured only once and must be confirmed (several more objects are
listed as candidates). In addition to the Tables of objects, we summarize the
evidence of variability of the cyclotron line as a function of various
parameters (especially pulse phase, luminosity and time), and add a discussion
of the different observed phenomena and associated attempts of theoretical
modeling. We also discuss our understanding of the underlying physics of
accretion onto highly magnetized neutron stars. For proton cyclotron lines, we
present tables with seven neutron stars and discuss their nature and the
physics in these objects.Comment: 32 pages, 15 figures, 8 Tables, accepted by A&A 201
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