128 research outputs found
Swift-XRT 6-year monitoring of the ultraluminous X-ray source M33-X8
The long term evolution of ULX with their spectral and luminosity variations
in time give important clues on the nature of ULX and on the accretion process
that powers them. We report here the results of a Swift-XRT 6-year monitoring
campaign of the closest example of a persistent ULX, M33 X-8, that extends to
16 years the monitoring of this source in the soft X-rays. The luminosity of
this source is a few 10^39 erg/s, marking the faint end of the ULX luminosity
function. We analysed the set of 15 observations collected during the Swift
monitoring. We searched for differences in the spectral parameters at different
observing epochs, adopting several models commonly used to fit the X-ray
spectra of ULX. The source exhibits flux variations of the order of 30%. No
significant spectral variations are observed along the monitoring. The average
0.5-10 keV spectrum can be well described by a thermal model, either in the
form of a slim disk, or as a combination of a Comptonized corona and a standard
accretion disk.Comment: 6 pages, 4 figures, 2 tables. Paper published in A&
Swift reveals the eclipsing nature of the high mass X-ray binary IGR~J16195-4945
IGR J16195-4945 is a hard X-ray source discovered by INTEGRAL during the Core
Program observations performed in 2003. We analyzed the X-ray emission of this
source exploiting the Swift-BAT survey data from December 2004 to March 2015,
and all the available Swift-XRT pointed observations. The source is detected at
a high significance level in the 123-month BAT survey data, with an average
15-150 keV flux of the source of ~1.6 mCrab. The timing analysis on the BAT
data reveals with a significance higher than 6 standard deviations the presence
of a modulated signal with a period of 3.945 d, that we interpret as the
orbital period of the binary system. The folded light curve shows a flat
profile with a narrow full eclipse lasting ~3.5% of the orbital period. We
requested phase-constrained XRT observations to obtain a more detailed
characterization of the eclipse in the soft X-ray range. Adopting resonable
guess values for the mass and radius of the companion star, we derive a
semi-major orbital axis of ~31 R_sun, equivalent to ~1.8 times the radius of
the companion star. From these estimates and from the duration of the eclipse
we derive an orbital inclination between 55 and 60 degrees. The broad band
time-averaged XRT+BAT spectrum is well modeled with a strongly absorbed flat
power law, with absorbing column N_H=7x 10^22 cm^(-2) and photon index
Gamma=0.5, modified by a high energy exponential cutoff at E_cut=14 keV.Comment: 5 pages, 5 figures, 2 tables. Published on MNRA
Temporal features of LS I +61303 in hard X-rays from the Swift/BAT survey data
We study the long-term spectral and timing behaviour of LS I +61303
in hard X-rays (15--150 keV) using 10 years of survey data from the
Burst Alert Telescope (BAT) monitor. We focus on the detection of long
periodicities known to be present in this source in multiple wavelengths. We
clearly detect three periods: the shorter one at 26.48 days is compatible with
the orbital period of the system; the second, longer, periodicity at 26.93
days, is detected for the first time in X-rays and its value is consistent with
an analogous temporal feature recently detected in the radio and in the
gamma-ray waveband, and we associate it with a modulation caused by a
precessing jet in this system. Finally, we find also evidence of the long-term
periodicity at 1667 d, that results compatible with a beat frequency of
the two close, and shorter, periodicities. We discuss our results in the
context of the multi-band behaviour of the physical processes of this source.Comment: 5 pages, 8 figures. Published in MNRA
The complex time behaviour of the microquasar GRS 1915+105 in the \rho-class observed with BeppoSAX. III: The hard X-ray delay and limit cycle mapping
The microquasar GRS1915+105 was observed by BeppoSAX in October 2000 for
about ten days while the source was in \rho-mode, which is characterized by a
quasi-regular type I bursting activity. This paper presents a systematic
analysis of the delay of the hard and soft X-ray emission at the burst peaks.
The lag, also apparent from the comparison of the [1.7-3.4] keV light curves
with those in the [6.8-10.2] keV range, is evaluated and studied as a function
of time, spectral parameters, and flux. We apply the limit cycle mapping
technique, using as independent variables the count rate and the mean photon
rate. The results using this technique were also cross-checked using a more
standard approach with the cross-correlation methods. Data are organized in
runs, each relative to a continuous observation interval. The detected
hard-soft delay changes in the course of the pointing from about 3 s to about
10 s and presents a clear correlation with the baseline count rate.Comment: accepted for publication in A&
Chandra X-ray spectroscopy of a clear dip in GX 13+1
The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an
accreting neutron star. It shows highly ionized absorption features, with a
blueshift of 400 km s and an outflow-mass rate similar to the
accretion rate. Many other X-ray sources exhibit warm absorption features, and
they all show periodic dipping behavior at the same time. Recently, a dipping
periodicity has also been determined for GX 13+1 using long-term X-ray folded
light-curves, leading to a clear identification of one of such periodic dips in
an archival Chandra observation. We give the first spectral characterization of
the periodic dip of GX 13+1 found in this archival Chandra observation
performed in 2010. We used Chandra/HETGS data (1.0-10 keV band) and
contemporaneous RXTE/PCA data (3.5-25 keV) to analyze the broadband X-ray
spectrum. We adopted different spectral models to describe the continuum
emission and used the XSTAR-derived warm absorber component to constrain the
highly ionized absorption features. The 1.0-25 keV continuum emission is
consistent with a model of soft accretion-disk emission and an optically thick,
harder Comptonized component. The dip event, lasting 450 s, is
spectrally resolved with an increase in the column density of the neutral
absorber, while we do not find significant variations in the column density and
ionization parameter of the warm absorber with respect to the out-of-dip
spectrum. We argue that the very low dipping duty-cycle with respect to other
sources of the same class can be ascribed to its long orbital period and the
mostly neutral bulge, that is relatively small compared with the dimensions of
the outer disk radius.Comment: 13 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
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