51 research outputs found
Probing the neutron star spin evolution in the young SMC Be/X-ray binary SXP 1062
The newly discovered Be/X-ray binary in the Small Magellanic Cloud, SXP 1062,
provides the first example of a robust association with a supernova remnant
(SNR). The short age estimated for the SNR qualifies SXP 1062 as the youngest
known source in its class, tau ~ 1e4 yr. As such, it allows to test current
models of magneto-rotational evolution of neutron stars ina still unexplored
regime. Here we discuss possible evolutionary scenarios for SXP 1062 in the
attempt to reconcile its long spin period, P=1062 s, and short age. Although
several options can be considered, like an anomalously long initial period or
the presence of a fossil disc, our results indicate that SXP 1062 may host a
neutron star born with a large initial magnetic field, typically in excess of ~
1e14 G, which then decayed to ~ 1e13 G.Comment: 5 pages, 1 figure, modified to incorporate referee's points, accepted
for publication in MNRA
Where Are Be/black-hole Binaries?
We apply the tidal truncation model proposed by Negueruela & Okazaki(2001) to
arbitrary Be/compact star binaries to study the truncation efficiency
dependance on the binary parameters. We find that the viscous decretion disks
around the Be stars could be truncated very effectively in narrow systems.
Combining this with the population synthesis results of Podsiadlowski,
Rappaport and Han (2003) that binary black holes are most likely to be born in
systems with orbital periods less than about 30 days, we suggest that most of
the Be/black-hole binaries may be transient systems with very long quiescent
states. This could explain the lack of observed Be/black-hole X-ray binaries.
We also discuss the evolution of the Be/black-hole binaries and their possible
observational features.Comment: 14 pages,3 figures, ApJ accepte
SSS in young stellar populations and the "prompt" component of Type Ia supernovae
We present the results of a search for UV and optical counterparts of the SSS
population in M31. We find that out of the 56 sources we included in our
search, 16 are associated with regions of ongoing or recent star formation. We
discuss two particularly interesting sources that are identified optically as
early type stars, one of which displayed long term X-ray evolution similar to
that observed in classical novae. We discuss the physical origin of supersoft
X-rays in these and the other SSS in young regions, and their possible link to
the so-called "prompt" component of the Type Ia supernova population.Comment: 5 pages, 3 figures; Proceedings of workshop "Supersoft X-ray Sources
- New Developments", ESAC, May 2009; accepted for publication in
Astronomische Nachrichte
Could SXP 1062 be an Accreting Magnetar?
In this work we explore the possible evolutionary track of the neutron star
in the newly discovered Be/X-ray binary SXP 1062, which is believed to be the
first X-ray pulsar associated with a supernova remnant. Although no cyclotron
feature has been detected to indicate the strength of the neutron star's
magnetic field, we show that it may be \ga 10^{14} G. If so SXP 1062 may
belong to the accreting magnetars in binary systems. We attempt to reconcile
the short age and long spin period of the pulsar taking account of different
initial parameters and spin-down mechanisms of the neutron star. Our calculated
results show that, to spin down to a period s within kyr
requires efficient propeller mechanisms. In particular, the model for angular
momentum loss under energy conservation seems to be ruled out.Comment: 19 pages, 4 figures, accepted for publication in Ap
Detection of compact objects by means of gravitational lensing in binary systems
We consider the gravitational magnification of light for binary systems
containing two compact objects: white dwarfs, a white dwarf and a neutron star
or a white dwarf and a black hole. Light curves of the flares of the white
dwarf caused by this effect were built in analytical approximations and by
means of numerical calculations. We estimate the probability of the detection
of these events in our Galaxy for different types of binaries and show that
gravitational lensing provides a tool for detecting such systems. We propose to
use the facilities of the Sloan Digital Sky Survey (SDSS) to search for these
flares. It is possible to detect several dozens compact object pairs in such a
programme over 5 years. This programme is apparently the best way to detect
stellar mass black holes with open event horizons.Comment: 15 pages, 11 figures; Accepted for publication in Astronomy &
Astrophysic
On the nature of the Be star HR 7409 (7 Vul)
HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt
and is the massive component of a 69-day spectroscopic binary. The binary
parameters and properties of the Be star measured using high-dispersion spectra
obtained at Ondrejov Observatory and at Rozhen Observatory imply the presence
of a low mass companion (~ 0.5-0.8 M_sun). If the pair is relatively young
(<50-80 Myr), then the companion is a K V star, but, following another, older
evolutionary scenario, the companion is a horizontal-branch star or possibly a
white dwarf star. In the latter scenario, a past episode of mass transfer from
an evolved star onto a less massive dwarf star would be responsible for the
peculiar nature of the present-day, fast-rotating Be star.Comment: Accepted for publication in MNRA
BeppoSAX survey of Be/X-ray binary candidates
We present a BeppoSAX survey of five Be/X-ray binary candidates. We report on
the identification of two of them, HD 110432 and HD 141926, as low luminosity
Be/X-ray binaries. For HD 110432 we report on the detection of a pulsation
period of ~14 ks. Because the luminosity of these sources is low and their
spectra do not require non-thermal emission models, these systems are good
Be+White Dwarf candidates. If the pulsation period for HD 110432 is confirmed,
this system would be the most firm Be+WD candidate found up to date. The other
three objects HD 65663, HD 249179 and BD+53 2262 did not show detectable X-ray
emission. We argue that, while the properties of BD+53 2262 are still
consistent with a quiescent Be+Neutron Star scenario, the lack of detection for
the other two objects implies that they are most probably not X-ray binaries.Comment: 8 pages, 5 figures, accepted for publication in A&
Luminosity function of binary X-ray sources calculated using the Scenario Machine
Using the ``Scenario Machine'' we have carried out a population synthesis of
X-ray binaries for the purpose of modelling of X-ray luminosity functions
(XLFs) in different types of galaxies: star burst, spiral, and elliptical. This
computer code allows to calculate, by using Monte Carlo simulations, the
evolution of a large ensemble of binary systems, with proper accounting for the
spin evolution of magnetized neutron stars.
We show that the XLF has no universal type. It depends on the star formation
rate in the galaxy. Also it is of importance to take into account the evolution
of binary systems and life times of X-ray stages in theoretical models of such
functions. We have calculated cumulative and differential XLFs for the galaxy
with the constant star formation rate. Also we have calculated cumulative
luminosity functions for different intervals of time after the star formation
burst in the galaxy and curves depicting the evolution of the X-ray luminosity
after the star formation burst in the galaxy.Comment: 22 pages, 13 figure
Supernova Kicks and Misaligned Be Star Binaries
Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas
in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a
neutron star. They are found to have non-zero eccentricities and there is
evidence that some systems have a misalignment between the spin axis of the
star and the spin axis of the binary orbit. The eccentricities in these systems
are thought to be caused by a kick to the neutron star during the supernova
that formed it. Such kicks would also give rise to misalignments. In this paper
we investigate the extent to which the same kick distribution can give rise to
both the observed eccentricity distribution and the observed misalignments. We
find that a Maxwellian distribution of velocity kicks with a low velocity
dispersion, , is consistent with the observed
eccentricity distribution but is hard to reconcile with the observed
misalignments, typically . Alternatively a higher velocity kick
distribution, , is consistent with the observed
misalignments but not with the observed eccentricities, unless post-supernova
circularisation of the binary orbits has taken place. We discuss briefly how
this might be achieved.Comment: Accepted for publication in MNRA
The transient X-ray pulsar 4U 0115+63 from quiescence to outburst through the centrifugal transition
We report on a BeppoSAX observation of the transient X-ray pulsar 4U 0115+63
close to periastron. This led to the discovery of a dramatic luminosity
variation from ~2x10^34 erg/s to ~5x10^36 erg/s (factor ~250) in less than 15
hr. The variation was accompanied by only minor (if any) changes in the emitted
spectrum and pulse fraction. On the contrary an observation near apastron
detected the source in a nearly constant state at a level of ~2x10^33 erg/s.
Direct accretion onto the neutron star surface encounters major difficulties in
explaining the source variability properties. When the different regimes
expected for a rotating magnetic neutron star subject to a variable inflow of
matter from its companion are taken into consideration, the results of BeppoSAX
observations of 4U 0115+63 can be explained naturally. In particular close to
apastron, the regime of centrifugal inhibition of accretion applies, whereas
the dramatic source flux variability observed close to periastron is readily
interpreted as the transition regime between direct neutron star accretion and
the propeller regime. In this centrifugal transition regime small variations of
the mass inflow rate give rise to very large luminosity variations. We present
a simple model for this transition, which we successfully apply to the X-ray
flux and pulse fraction variations measured by BeppoSAX.Comment: 7 pages, 3 figures, 1 table (requires emulateapj macro). Accepted for
publication on Ap
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