3,468 research outputs found
Searching for coherent pulsations in ultraluminous X-ray sources
Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large if
compared to the Eddington limit for isotropic accretion onto stellar-mass
object. Most often either supercritical accretion onto stellar mass black hole
or accretion onto intermediate mass black holes is invoked the high
luminosities of ULXs. However, the recent discovery of coherent pulsations from
M82 ULX with NuSTAR showed that another scenario implying accretion onto a
magnetized neutron star is possible for ULXs. Motivated by this discovery, we
re-visited the available XMM-Newton archival observations of several bright
ULXs with a targeted search for pulsations to check whether accreting neutron
stars might power other ULXs as well. We have found no evidence for significant
coherent pulsations in any of the sources including the M82 ULX. We provide
upper limits for the amplitude of possibly undetected pulsed signal for the
sources in the sample.Comment: 2 pages, 1 figure, submitted to A&
Ten years of INTEGRAL observations of the hard X-ray emission from SGR 1900+14
We exploited the high sensitivity of the INTEGRAL IBIS/ISGRI instrument to
study the persistent hard X-ray emission of the soft gamma-ray repeater SGR
1900+14, based on ~11.6 Ms of archival data. The 22-150 keV INTEGRAL spectrum
can be well fit by a power law with photon index 1.9 +/- 0.3 and flux F_x =
(1.11 +/- 0.17)E-11 erg/cm^2/s (20-100 keV). A comparison with the 20-100 keV
flux measured in 1997 with BeppoSAX, and possibly associated with SGR 1900+14,
shows a luminosity decrease by a factor of ~5. The slope of the power law above
20 keV is consistent within the uncertainties with that of SGR 1806-20, the
other persistent soft gamma-ray repeater for which a hard X-ray emission
extending up to 150 keV has been reported.Comment: Accepted for publication in Astronomy & Astrophysics. 4 page
Supergiant, fast, but not so transient 4U 1907+09
We have investigated the dipping activity observed in the high-mass X-ray
binary 4U 1907+09 and shown that the source continues to pulsate in the "off"
state, noting that the transition between the "on" and "off" states may be
either dip-like or flare-like. This behavior may be explained in the framework
of the "gated accretion" scenario proposed to explain the flares in supergiant
fast X-ray transients (SFXTs). We conclude that 4U 1907+09 might prove to be a
missing link between the SFXTs and ordinary accreting pulsars.Comment: 4 pages 5 figures, accepted in A&
Population of the Galactic X-ray binaries and eRosita
The population of the Galactic X-ray binaries has been mostly probed with
moderately sensitive hard X-ray surveys so far. The eRosita mission will
provide, for the first time a sensitive all-sky X-ray survey in the 2-10 keV
energy range, where the X-ray binaries emit most of the flux and discover the
still unobserved low-luminosity population of these objects. In this paper, we
briefly review the current constraints for the X-ray luminosity functions of
high- and low-mass X-ray binaries and present our own analysis based the
INTEGRAL 9-year Galactic survey, which yields improved constraints. Based on
these results, we estimate the number of new XRBs to be detected in the eRosita
all-sky surveyComment: accepted for publication in A&
Frustrated order on extrinsic geometries
We study, analytically and theoretically, defects in a nematically-ordered
surface that couple to the extrinsic geometry of a surface. Though the
intrinsic geometry tends to confine topological defects to regions of large
Gaussian curvature, extrinsic couplings tend to orient the nematic in the local
direction of maximum or minimum bending. This additional frustration is
unavoidable and most important on surfaces of negative Gaussian curvature,
where it leads to a complex ground state thermodynamics. We show, in
contradistinction to the well-known effects of intrinsic geometry, that
extrinsic curvature expels disclinations from the region of maximum curvature
above a critical coupling threshold. On catenoids lacking an "inside-outside"
symmetry, defects are expelled altogether.Comment: 4 pages, 3 figure
Exploring the role of X-ray reprocessing and irradiation in the anomalous bright optical outbursts of A0538-66
In 1981, the Be/X-ray binary (Be/XRB) A0538-66 showed outbursts characterized
by high peak luminosities in the X-ray and optical bands. The optical outbursts
were qualitatively explained as X-ray reprocessing in a gas cloud surrounding
the binary system. Since then, further important information about A0538-66
have been obtained, and sophisticated photoionization codes have been developed
to calculate the radiation emerging from a gas nebula illuminated by a central
X-ray source. In the light of the new information and tools available, we
studied again the enhanced optical emission displayed by A0538-66 to understand
the mechanisms responsible for these unique events among the class of Be/XRBs.
We performed about 10^5 simulations of a gas envelope photoionized by an X-ray
source. We assumed for the shape of the gas cloud either a sphere or a
circumstellar disc observed edge-on. We studied the effects of varying the main
properties of the envelope and the influence of different input X-ray spectra
on the optical/UV emission emerging from the photoionized cloud. We compared
the computed spectra with the IUE spectrum and photometric UBV measurements
obtained during the outburst of 29 April 1981. We also explored the role played
by the X-ray heating of the surface of the donor star irradiated by the X-ray
emission of the neutron star (NS). We found that reprocessing in a spherical
cloud with a shallow radial density distribution can reproduce the optical/UV
emission. To our knowledge, this configuration has never been observed either
in A0538-66 during other epochs or in other Be/XRBs. We found, contrary to the
case of most other Be/XRBs, that the optical/UV radiation produced by the X-ray
heating of the surface of the donor star irradiated by the NS is
non-negligible, due to the particular orbital parameters of this system that
bring the NS very close to its companion.Comment: Accepted for publication in Astronomy & Astrophysics. Abstract
abridged to meet arXiv requirement
Optical and near-infrared photometric monitoring of the transient X-ray binary A0538-66 with REM
The transient Be/X-ray binary A0538-66 shows peculiar X-ray and optical
variability. Despite numerous studies, the intrinsic properties underlying its
anomalous behaviour remain poorly understood. Since 2014 September we are
conducting the first quasi-simultaneous optical and near-infrared photometric
monitoring of A0538-66 in seven filters with the Rapid Eye Mount (REM)
telescope, aiming to understand the properties of this binary system. We found
that the REM lightcurves show fast flares lasting one or two days that repeat
almost regularly every ~16.6 days, the orbital period of the neutron star. If
the optical flares are powered by X-ray outbursts through photon reprocessing,
the REM lightcurves indicate that A0538-66 is still active in X-rays: bright
X-ray flares (L_x > 1E37 erg/s) could be observable during the periastron
passages. The REM lightcurves show a long-term variability that is especially
pronounced in the g band and decreases with increasing wavelength, until it no
longer appears in the near-infrared lightcurves. In addition, A0538-66 is
fainter with respect to previous optical observations most likely due to the
higher absorption of the stellar radiation of a denser circumstellar disc. On
the basis of the current models, we interpret these observational results with
a circumstellar disc around the Be star observed nearly edge-on during a
partial depletion phase. The REM lightcurves also show short-term variability
on timescales of ~1 day possibly indicative of perturbations in the density
distribution of the circumstellar disc caused by the tidal interaction with the
neutron star.Comment: Accepted for publication in Astronomy & Astrophysic
Properties and observability of glitches and anti-glitches in accreting pulsars
Several glitches have been observed in young, isolated radio pulsars, while a
clear detection in accretion-powered X-ray pulsars is still lacking. We use the
Pizzochero snowplow model for pulsar glitches as well as starquake models to
determine for the first time the expected properties of glitches in accreting
pulsars and their observability. Since some accreting pulsars show
accretion-induced long-term spin-up, we also investigate the possibility that
anti-glitches occur in these stars. We find that glitches caused by quakes in a
slow accreting neutron star are very rare and their detection extremely
unlikely. On the contrary, glitches and anti-glitches caused by a transfer of
angular momentum between the superfluid neutron vortices and the non-superfluid
component may take place in accreting pulsars more often. We calculate the
maximum jump in angular velocity of an anti-glitch and we find that it is
expected to be about 1E-5 - 1E-4 rad/s. We also note that since accreting
pulsars usually have rotational angular velocities lower than those of isolated
glitching pulsars, both glitches and anti-glitches are expected to have long
rise and recovery timescales compared to isolated glitching pulsars, with
glitches and anti-glitches appearing as a simple step in angular velocity.
Among accreting pulsars, we find that GX 1+4 is the best candidate for the
detection of glitches with currently operating X-ray instruments and future
missions such as the proposed Large Observatory for X-ray Timing (LOFT).Comment: Accepted for publication in Astronomy & Astrophysics. 6 pages. Minor
changes to match the final A&A versio
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