3,341 research outputs found
Modeling the light curves of ultraluminous X-ray sources as precession
We present a freely available xspec model for the modulations seen in the
long-term light curves of multiple ultraluminous X-ray sources (ULXs). By
incorporating the physics of multiple electron scatterings (ray traced with a
Monte-Carlo routine), we go beyond analytical predictions and show that the
geometrical beaming of radiation in the conical outflow can be more than a
factor of 100 for opening angles smaller than . We apply our new
model to the long-term, well sampled Swift light curve of the recently
confirmed ULX pulsar NGC 5907 X-1 with an established period of 78 days. Our
results suggest that geometrical beaming together with a slight precession of
the conical wind can describe the light curve with a consistent set of
parameters for the wind. The small opening angle of roughly
implies a highly super-critical flow and boosting
factors at the order of that would yield a fairly
low surface magnetic field strength of Gauss.Comment: accepted by MNRAS for publication (7 pages, 6 figures
Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state
Hard X-ray spectra of black hole binaries in the low/hard state are well
modeled by thermal Comptonization of soft seed photons by a corona-type region
with \thinspace{\thinspace}keV and optical depth around 1.
Previous spectral studies of 1E{\thinspace}1740.72942, including both the
soft and the hard X-ray bands, were always limited by gaps in the spectra or by
a combination of observations with imaging and non-imaging instruments. In this
study, we have used three rare nearly-simultaneous observations of
1E{\thinspace}1740.71942 by both XMM-Newton and INTEGRAL satellites to
combine spectra from four different imaging instruments with no data gaps, and
we successfully applied the Comptonization scenario to explain the broadband
X-ray spectra of this source in the low/hard state. For two of the three
observations, our analysis also shows that, models including Compton reflection
can adequately fit the data, in agreement with previous reports. We show that
the observations can also be modeled by a more detailed Comptonization scheme.
Furthermore, we find the presence of an iron K-edge absorption feature in one
occasion, which confirms what had been previously observed by Suzaku. Our
broadband analysis of this limited sample shows a rich spectral variability in
1E{\thinspace}1740.72942 at the low/hard state, and we address the possible
causes of these variations. More simultaneous soft/hard X-ray observations of
this system and other black-hole binaries would be very helpful in constraining
the Comptonization scenario and shedding more light on the physics of these
systems.Comment: 6 pages, two figures, accepted for publication in A&
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
Chandra and RXTE spectroscopy of the accreting msec pulsar IGR J00291+5934
We report on an observation of the recently discovered accreting millisecond X-ray pulsar IGR J00291+5934 performed with the RXTE-Proportional Counter Array (PCA) and Chandra-High Energy Transmission Grating Spectrometer (HETGS). The RXTE data are from a twoweek follow-up of the source, while the Chandra observation took place around the end of the follow-up, about 12 days after the discovery of
the source, when the source flux had decreased already by a factor of ten. The analysis of the Chandra data allowed us to extract the most precise X-ray position of IGR J00291+5934, RA = 00h 29m 03.08s, and Dec =+59â—¦ 34 19.2 (0.6 error), compatible with the optical and radio ones.
We find that the spectra of IGR J00291+5934 can be described by a combination of a thermal component and a power-law. Along the outburst detected by PCA, the power-law photon index showed no particular trend, while the thermal component (∼1 keV, interpreted as a hot spot on
the neutron star surface) became weaker until non-detection. In the simultaneous observation of the weak Chandra /RXTE spectrum, there was no longer any indication of the ∼1 keV thermal component, while we detected a colder thermal component (∼0.4 keV) that we interpret as the
emission from the cold disc. A hint of a 6.4 keV iron line was detected, together with an excess around 6.8 keV and absorption feature around 7.1 keV. The last two features have never been detected in the spectra of accretion-driven millisecond pulsars before and, if confirmed, would
suggest the presence of an expanding hot corona with high outflow velocities
Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1
Because of their inherently high flux allowing the detection of clear
signals, black hole X-ray binaries are interesting candidates for polarization
studies, even if no polarization signals have been observed from them before.
Such measurements would provide further detailed insight into these sources'
emission mechanisms. We measured the polarization of the gamma-ray emission
from the black hole binary system Cygnus X-1 with the INTEGRAL/IBIS telescope.
Spectral modeling of the data reveals two emission mechanisms: The 250-400 keV
data are consistent with emission dominated by Compton scattering on thermal
electrons and are weakly polarized. The second spectral component seen in the
400keV-2MeV band is by contrast strongly polarized, revealing that the MeV
emission is probably related to the jet first detected in the radio band.Comment: 11 pages, 3 figures, to be published in Science in April 22nd 2011,
available on Science Express Web site (March 24th edition
Suzaku observation of IGR J16318-4848
We report on the first Suzaku observation of IGR J16318-4848, the most
extreme example of a new group of highly absorbed X-ray binaries that have
recently been discovered by the International Gamma-Ray Astrophysics Laboratory
INTEGRAL. The Suzaku observation was carried out between 2006 August 14 and 17,
with a net exposure time of 97 ks.
The average X-ray spectrum of the source can be well described with a
continuum model typical for neutron stars i.e., a strongly absorbed power law
continuum with a photon index of 0.676(42) and an exponential cutoff at 20.5(6)
keV. The absorbing column is 1.95(3)x10e24 cm-2. Consistent with earlier work,
strong fluorescent emission lines of Fe Kalpha, Fe Kbeta, and Ni Kalpha are
observed. Despite the large absorbing column, no Compton shoulder is seen in
the lines, arguing for a non-spherical and inhomogeneous absorber.
Seen at an average 5-60 keV absorbed flux of 3.4x10e-10 erg cm-2 s-1, the
source exhibits significant variability on timescales of hours.Comment: 5 pages, 5 figures, 1 table. Accepted for publication in A&
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