802 research outputs found
Discovery of a Peculiar Dip from GX 301-2
We present temporal and spectral properties of a unique X-ray dip in GX 301-2
as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from
the source gradually declined prior to the dip, disappears for one spin cycle
during the dip and is abruptly restored in the spin cycle immediately after the
dip. Moreover, the phase-integrated spectrum of the source becomes softer
before and during the dip and it quickly hardens again following the dip. Our
findings indicate the fact that the mechanism for pulsations gradually turned
off briefly and underlying dim and softer emission likely from the accretion
column became observable in the brief absence of high level emission due to
wind accretion.Comment: Accepted for publication in A&A Letter
INTEGRAL observation of the high-mass X-ray transient V 0332+53 during the 2005 outburst decline
The decline of the high mass X-ray transient V 0332+53 during the Dec. 2004 to Feb. 2005 outburst is analysed from the data recorded by INTEGRAL. The flux is shown to decrease exponentially until 2005 Feb. 10, with a decay time scale of ∼30 days above 20 keV and ∼20 days at lower energies, and to decrease linearly thereafter.
The energy spectrum is well modelled throughout the decay by a power law with a folding energy of ∼7.5 keV, and with two cyclotron absorption features. The folding energy does not vary significantly over the decay, but the spectrum becomes harder with time. Most importantly, we show that the parameters describing the fundamental cyclotron line around 27 keV do vary with time: its energy and depth increase (by about 17% for the energy in ∼6 weeks), while its width decreases. These changes of the cyclotron line parameters are interpreted as resulting from a change in the extent of the cyclotron scattering region.
Two quasi-periodic oscillations are also observed at various times during the observations, one at 0.05 Hz and another one near the pulsation frequency around 0.23 Hz
Discovery of a Peculiar Dip from GX 301-2
We present temporal and spectral properties of a unique X-ray dip in GX 301-2
as seen with Rossi X-ray Timing Explorer in May 2010. The X-ray pulsation from
the source gradually declined prior to the dip, disappears for one spin cycle
during the dip and is abruptly restored in the spin cycle immediately after the
dip. Moreover, the phase-integrated spectrum of the source becomes softer
before and during the dip and it quickly hardens again following the dip. Our
findings indicate the fact that the mechanism for pulsations gradually turned
off briefly and underlying dim and softer emission likely from the accretion
column became observable in the brief absence of high level emission due to
wind accretion.Comment: Accepted for publication in A&A Letter
Discovery of a Cyclotron Resonance Scattering Feature in the X-ray Spectrum of XTE J1946+274
Observations of the transient accreting pulsar XTE J1946+274 made with the
Rossi X-ray Timing Explorer during the course of the 1998 September-November
outburst, reveal a cyclotron resonance scattering feature (or "cyclotron line")
in the hard X-ray spectrum near 35 keV. We determine a centroid energy of 36.2
+0.5/-0.7 keV, which implies a magnetic field strength of 3.1(1+z)x10^12 G,
where z is the gravitational redshift of the scattering region. The optical
depth, Tau = 0.33 +0.07/-0.06, and width, sigma = 3.37 +0.92/-0.75 keV, are
typical of known cyclotron lines in other pulsars. This discovery makes XTE
J1946+274 one of thirteen pulsars with securely detected cyclotron lines
resulting in direct magnetic field measurements.Comment: Five pages including four postscript figures and two tables. Uses
emulateapj5. Published in ApJ Letters:
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ApJ...563L..35
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
Witnessing the magnetospheric boundary at work in Vela X-1
We present an analysis of the Vela X-1's "off-states" based on Suzaku
observations taken in June 2008. Defined as states in which the flux sudden
decreases below the instrumental sensitivity, these "off-states" have been
interpreted by several authors as the onset of the "propeller regime". For the
first time ever, however, we find that the source does not turn off and,
although the flux drops by a factor of 20 during the three recorded
"off-states", pulsations are still observed. The spectrum and the
pulse-profiles of the "off-states" are also presented. Eventually, we discuss
our findings in framework of the "gated accretion" scenario and conclude that
most likely the residual flux is due to the accretion of matter leaking through
the magnetosphere by means of Kelvin-Helmholz instabilities (KHI).Comment: 4 pages 4 figures; accepted for publication in A&A letters
(20/02/2011); v1.1 - some changes in language + added 3 reference
Evidence for a Neutron Star in the non-pulsating massive X-ray binary 4U2206+54
We present an analysis of archival RXTE and BeppoSAX data of the X-ray source
4U2206+54 . For the first time, high energy data (> 30 kev) are analyzed for
this source. The data are well described by comptonization models (CompTT and
BMC) in which seed photons with temperatures between 1.1 kev and 1.5 kev are
comptonized by a hot plasma at 50 kev thereby producing a hard tail which
extends up to, at least, 100 kev. We offer a new method of identification of
neutron star systems using a temperature - luminosity relation. If a given
X-ray source is characterized by a low bolometric luminosity and a relatively
high color blackbody temperature (>1 kev) it has necessarily to be a neutron
star rather than a black hole. From these arguments it is shown that the area
of the soft photon source must be small (r ~ 1 km) and that the accretion disk,
if present, must be truncated very far from the compact object. Here we report
on the possible existence of a cyclotron line around 30 kev. The presence of a
neutron star in the system is strongly favored by the available data.Comment: Accepted for publication in A&A. 9 pages, 7 figures. Submitted to
journal in November 200
4U 1909+07: a well-hidden pearl
We present the first detailed spectral and timing analysis of the High Mass
X-ray Binary (HMXB) 4U 1909+07 with INTEGRAL and RXTE. 4U 1909+07 is detected
in the ISGRI 20-40 keV energy band with an average countrate of 2.6 cps. The
pulse period of ~604 sec is not stable, but changing erratically on timescales
of years. The pulse profile is strongly energy dependent: it shows a double
peaked structure at low energies, the secondary pulse decreases rapidly with
increasing energy and above 20 keV only the primary pulse is visible. This
evolution is consistent between PCA, HEXTE, and ISGRI. The phase averaged
spectrum can be well described by the sum of a photoabsorbed power law with a
cutoff at high energies and a blackbody component. To investigate the pulse
profile, we performed phase resolved spectral analysis. We find that the
changing spectrum can be best described with a variation of the folding energy.
We rule out a correlation between the black body component and the continuum
variation and discuss possible accretion geometries.Comment: 9 pages, 11 figures, accepted for publication in A&A Sect.
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
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