129 research outputs found
A search for lines in the bright X-ray afterglow of GRB120711A
GRB120711A, discovered and rapidly localized by the INTEGRAL satellite,
attracted particular interest due to its high gamma-ray fluence, very bright
X-ray afterglow, and the detection of a prompt optical transient and of
long-lasting emission at GeV energies. A follow-up observation carried out with
the XMM-Newton satellite has provided an X-ray spectrum in the 0.3-10 keV with
unprecedented statistics for a GRB afterglow 20 hours after the burst. The
spectrum is well fit by a power-law with photon index 1.87+-0.01, modified by
absorption in our Galaxy and in the GRB host at z=1.4. A Galactic absorption
consistent with that estimated from neutral hydrogen observations is obtained
only with host metallicity lower than 0.05 of the Solar value. We report the
results of a sensitive search for emission and absorption lines using the
matched filter smoothing method (Rutledge and Sako 2003). No statistically
significant lines were found. The upper limits on the equivalent width of
emission lines, derived through Monte Carlo simulations, are few tens of eV, a
factor about 10 lower than that of the possible lines reported in the
literature for other bursts.Comment: Accepted for publication in A&
A new investigation of the possible X-ray counterparts of the magnetar candidate AX J1845-0258
AX J1845-0258 is a transient X-ray pulsar, with spin period of 6.97s,
discovered with the ASCA satellite in 1993. Its soft spectrum and the possible
association with a supernova remnant suggest that AX J1845-0258 might be a
magnetar, but this has not been confirmed yet. A possible counterpart one order
of magnitude fainter, AX J184453-025640, has been found in later X-ray
observations, but no pulsations have been detected. In addition, some other
X-ray sources are compatible with the pulsar location, which is in a crowded
region of the Galactic plane. We have carried out a new investigation of all
the X-ray sources in the ASCA error region of AX J1845-0258, using archival
data obtained with Chandra in 2007 and 2010, and with XMM-Newton in 2010. We
set an upper limit of 6% on the pulsed fraction of AX J184453-025640 and
confirmed its rather hard spectrum (power law photon index of 1.2 +\- 0.3). In
addition to the other two fainter sources already reported in the literature,
we found other X-ray sources positionally consistent with AX J1845-0258.
Although many of them are possibly foreground stars likely unrelated to the
pulsar, at least another new source, CXOU J184457.5-025823, could be a
plausible counterpart of AX J1845-0258. It has a flux of 6x10^{-14} erg cm^{-2}
s^{-1} and a spectrum well fit by a power law with photon index ~1.3 and Nh ~
10^{22} cm^{-2}.Comment: 6 pages, 6 figures, 3 tables; accepted for publication in MNRA
A new X-ray look into four old pulsars
We report on the X-ray properties of four rotation-powered pulsars with
characteristic ages in the range 0.3-5 Myr, derived from the analysis of
XMM-Newton archival observations. We found convincing evidence of thermal
emission only in the phase-averaged spectrum of PSR B0114+58, that is well
fitted by a blackbody with temperature kT = keV and emitting
radius R = m, consistent with the size of its polar cap. The
other three considered pulsars, PSR B0628-28, PSR B0919+06 and PSR B1133+16,
have phase-averaged spectra well described by single power-laws with photon
index {\Gamma} ~ 3. The 3{\sigma} upper limits on the bolometric luminosity of
a possible thermal component with temperature in the range ~ 0.05-2 keV are
L_bol erg/s and L_bol
erg/s, for PSR B0628-28 and PSR B0919+06, respectively. On the other hand, we
found possible evidence that the pulsed emission of PSR B0628-28 is thermal.
Two absorption lines at ~0.22 keV and ~0.44 keV are detected in the spectrum of
PSR B1133+16. They are best interpreted as proton cyclotron features, implying
the presence of multipolar components with a field of a few G at the
neutron star polar caps. We discuss our results in the context of high-energy
emission models of old rotation-powered pulsars.Comment: Accepted by A&A, 15 pages, 11 figures, 7 table
The radio and X-ray mode-switching pulsar PSR B0943+10
Observations obtained in the last years challenged the widespread notion that
rotation-powered neutron stars are steady X-ray emitters. Besides a few
allegedly rotation-powered neutron stars that showed "magnetar-like"
variability, a particularly interesting case is that of PSR B0943+10. Recent
observations have shown that this pulsar, well studied in the radio band where
it alternates between a bright and a quiescent mode, displays significant X-ray
variations, anticorrelated in flux with the radio emission. The study of such
synchronous radio/X-ray mode switching opens a new window to investigate the
processes responsible for the pulsar radio and high-energy emission. Here we
review the main X-ray properties of PSR B0943+10 derived from recent
coordinated X-ray and radio observations.Comment: Published in Journal of Astrophysics and Astronomy special issue on
'Physics of Neutron Stars and Related Objects', celebrating the 75th
birth-year of G. Srinivasa
Magnetic fields of neutron stars in X-ray binaries
A substantial fraction of the known neutron stars resides in X-ray binaries
-- systems in which one compact object accretes matter from a companion star.
Neutron stars in X-ray binaries have magnetic fields among the highest found in
the Universe, spanning at least the range from to several 10
G. The magnetospheres around these neutron stars have a strong influence on the
accretion process, which powers most of their emission. The magnetic field
intensity and geometry, are among the main factors responsible for the large
variety of spectral and timing properties observed in the X-ray energy range,
making these objects unique laboratories to study the matter behavior and the
radiation processes in magnetic fields unaccessible on Earth. In this paper we
review the main observational aspects related to the presence of magnetic
fields in neutron star X-ray binaries and some methods that are used to
estimate their strength.Comment: 16 pages, 9 figures, invited topical review, to be published in The
Strongest Magnetic Fields in the Universe (Space Sciences Series of ISSI,
Springer), Space Science Reviews, accepte
X-rays from Hot Subdwarfs
Thanks to the high sensitivity of the instruments on board the XMM-Newton and
Chandra satellites, it has become possible to explore the properties of the
X-ray emission from hot subdwarfs. The small but growing sample of hot
subdwarfs detected in X-rays includes binary systems, in which the X-rays
result from wind accretion onto a compact companion (white dwarf or neutron
star), as well as isolated sdO stars in which X-rays are probably due to shock
instabilities in the wind. X-ray observations of these low mass stars provide
information which can be useful also for our understanding of the winds of more
luminous and massive early-type stars and can lead to the discovery of
particularly interesting binary systems.Comment: Review paper to be published in Advances in Space Research (special
issue on "X-ray emission from hot stars and their winds"
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