3,511 research outputs found
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&
A calculation with a bi-orthogonal wavelet transformation
We explore the use of bi-orthogonal basis for continuous wavelet
transformations, thus relaxing the so-called admissibility condition on the
analyzing wavelet. As an application, we determine the eigenvalues and
corresponding radial eigenfunctions of the Hamiltonian of relativistic
Hydrogen-like atoms.Comment: 18 pages, see instead physics/970300
Probing the stellar wind environment of Vela X-1 with MAXI
Vela X-1 is among the best studied and most luminous accreting X-ray pulsars.
The supergiant optical companion produces a strong radiatively-driven stellar
wind, which is accreted onto the neutron star producing highly variable X-ray
emission. A complex phenomenology, due to both gravitational and radiative
effects, needs to be taken into account in order to reproduce orbital spectral
variations. We have investigated the spectral and light curve properties of the
X-ray emission from Vela X-1 along the binary orbit. These studies allow to
constrain the stellar wind properties and its perturbations induced by the
compact object. We took advantage of the All Sky Monitor MAXI/GSC data to
analyze Vela X-1 spectra and light curves. By studying the orbital profiles in
the and keV energy bands, we extracted a sample of orbital light
curves (% of the total) showing a dip around the inferior
conjunction, i.e., a double-peaked shape. We analyzed orbital phase-averaged
and phase-resolved spectra of both the double-peaked and the standard sample.
The dip in the double-peaked sample needs cm to
be explained by absorption solely, which is not observed in our analysis. We
show how Thomson scattering from an extended and ionized accretion wake can
contribute to the observed dip. Fitted by a cutoff power-law model, the two
analyzed samples show orbital modulation of the photon index, hardening by
around the inferior conjunction, compared to earlier and later
phases, hinting a likely inadequacy of this model. On the contrary, including a
partial covering component at certain orbital phase bins allows a constant
photon index along the orbital phases, indicating a highly inhomogeneous
environment. We discuss our results in the framework of possible scenarios.Comment: 10 pages, 9 figures, accepted for publication in A&
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
Observing Ultra High Energy Cosmic Particles from Space: SEUSO, the Super Extreme Universe Space Observatory Mission
The experimental search for ultra high energy cosmic messengers, from eV to beyond eV, at the very end of the known energy
spectrum, constitutes an extraordinary opportunity to explore a largely unknown
aspect of our universe. Key scientific goals are the identification of the
sources of ultra high energy particles, the measurement of their spectra and
the study of galactic and local intergalactic magnetic fields. Ultra high
energy particles might, also, carry evidence of unknown physics or of exotic
particles relics of the early universe. To meet this challenge a significant
increase in the integrated exposure is required. This implies a new class of
experiments with larger acceptances and good understanding of the systematic
uncertainties. Space based observatories can reach the instantaneous aperture
and the integrated exposure necessary to systematically explore the ultra high
energy universe. In this paper, after briefly summarising the science case of
the mission, we describe the scientific goals and requirements of the SEUSO
concept. We then introduce the SEUSO observational approach and describe the
main instrument and mission features. We conclude discussing the expected
performance of the mission
Footprints in the wind of Vela X-1 traced with MAXI
The stellar wind around the compact object in luminous wind-accreting high
mass X-ray binaries is expected to be strongly ionized with the X-rays coming
from the compact object. The stellar wind of hot stars is mostly driven by
light absorption in lines of heavier elements, and X-ray photo-ionization
significantly reduces the radiative force within the so-called Stroemgren
region leading to wind stagnation around the compact object. In close binaries
like Vela X-1 this effect might alter the wind structure throughout the system.
Using the spectral data from Monitor of All-sky X-ray Image (MAXI), we study
the observed dependence of the photoelectric absorption as function of orbital
phase in Vela X-1, and find that it is inconsistent with expectations for a
spherically-symmetric smooth wind. Taking into account previous investigations
we develop a simple model for wind structure with a stream-like photoionization
wake region of slower and denser wind trailing the neutron star responsible for
the observed absorption curve.Comment: 5 pages, 3 figures, accepted in A&
- …