884 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
New blazars from the cross-match of recent multi-frequency catalogs
Blazars are radio-loud active galactic nuclei well known for their non
thermal emission spanning a wide range of frequencies. The Roma-BZCAT is, to
date, the most comprehensive list of these sources. We performed the
cross-match of several catalogs obtained from recent surveys at different
frequencies to search for new blazars. We cross-matched the 1 Swift-XRT
Point Source catalog with the spectroscopic sample of the 9 Data Release
of the Sloan Digital Sky Survey. Then, we performed further cross-matches with
the catalogs corresponding to the Faint Images of the Radio Sky at Twenty cm
survey and to the AllWISE Data release, focusing on sources with infrared
colors similar to those of confirmed -ray blazars included in the
Second Fermi-LAT catalog. As a result, we obtained a preliminary list of
objects with all the elements needed for a proper blazar classification
according to the prescriptions of the Roma-BZCAT. We carefully investigated
additional properties such as their morphology and the slope of their spectral
energy distribution in the radio domain, the features shown in their optical
spectrum, and the luminosity in the soft X rays to exclude generic active
galactic nuclei and focus on authentic blazar-like sources. At the end of our
screening we obtained a list of 15 objects with firmly established blazar
properties.Comment: 8 pages, 3 figures, 1 table. Accepted for publication in Astrophysics
and Space Science on 2015 April 25. Corrected typo in Section
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