275 research outputs found
Unveiling black holes ejected from globular clusters
Was the black hole in XTE J1118+480 ejected from a globular cluster or kicked
away from the galactic disk?Comment: 2 pages, newpasp.sty. To appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, G.Djorgovski, M. Riell
Is NGC6752 hiding a double black hole binary in its core ?
NGC6752 hosts in its halo PSR J1911-5958A, a newly discovered binary
millisecond pulsar which is the most distant pulsar ever known from the core of
a globular cluster. Interestingly, its recycling history seems in conflict with
a scenario of ejection resulting from ordinary stellar dynamical encounters. A
scattering event off a binary system of two black holes with masses in the
range of 3-50 solar masses that propelled PSR J1911-5958A into its current
peripheral orbit seems more likely. It is still an observational challenge to
unveil the imprint(s) left from such a dark massive binary on cluster's stars:
PSR J1911-5958A may be the first case.Comment: 2 pages, newpasp.sty. To appear in "New Horizons in Globular Cluster
Astronomy", eds. G. Piotto, G. Meylan, G.Djorgovski, M. Riell
Is NGC 6752 Hosting a Single or a Binary Black Hole?
The five millisecond pulsars that inhabit NGC 6752 display locations or
accelerations remarkably different with respect to all other pulsars known in
globular clusters. This may reflect the occurrence of an uncommon dynamics in
the cluster core that could be attributed to the presence of a massive
perturber. We here investigate whether a single intermediate-mass black hole,
lying on the extrapolation of the mass versus sigma relation observed in galaxy
spheroids, or, a less massive black hole binary could play the requested role.Comment: To appear in "Carnegie Observatories Astrophysics Series, Vol. 1:
Coevolution of Black Holes and Galaxies," ed. L. C. Ho (Pasadena: Carnegie
Observatories,
http://www.ociw.edu/ociw/symposia/series/symposium1/proceedings.html
XMM-Newton observation of the double pulsar system J0737-3039
We report on a 50 ksec XMM-Newton observation of the double pulsar system
J0737-3039 performed on April 2004. We present results of the spectral analysis
of these data combined with the much shorter Chandra pointing performed on
January 2004. Black body emission with effective temperature of
0.20^{+0.02}_{-0.02} keV (90% confidence level) and emission radius
75^{+30}_{-9} m for a distance of 0.5 kpc (implying a 0.5-10 keV luminosity
\~6x10^{29} erg/s) is a viable interpretation, calling for a stream of
particles accelerated in the magnetosphere of PSR J0737-3039A and depositing
their kinetic energy in the magnetic polar cap of PSR J0737-3039A or of the
companion PSR J0737-3039B. A single power-law emission model implies a very
steep photon index Gamma=4.2^{+2.1}_{-1.2} and a suspiciously high hydrogen
column density, whereas a photon index Gamma=2 does not provide an adequate
description of the XMM-Newton and Chandra data. A two component model (a black
body plus a power-law with Gamma=2) is statistically acceptable, but the
additional power-law component is not required by the data.Comment: Accepted for publication on ApJ
Neutron star cooling in transiently accreting low mass binaries: a new tool for probing nuclear matter
We explore, using an exact cooling code, the thermal evolution of a neutron
star undergoing episodes of intense accretion, alternated by long periods of
quiescence (e.g. Soft X-Ray Transients). We find that the soft component of the
quiescent luminosity of Aql X-1, 4U 1608-522 and of SAX J1808.4-3658 can be
understood as thermal emission from a cooling neutron star with negligible
neutrino emission. In the case of Cen X-4 strong neutrino emission from the
inner core is necessary to explain the observation: this may indicate that the
neutron star of Cen X-4 is heavier than 1.4 Msun. This study opens the
possibility of using the quiescent emission of Soft X-Ray Transients as a tool
for probing the core superfluidity in relation to the mass of the neutron star.Comment: 5 pages, 3 embedded figures, latex style included. To appear in
"Evolution of Binary and Multiple Stars", 2001, eds. Ph. Podsiadlowski, S.
Rappaport, A. R. King, F. D'Antona & L. Burderi (San Francisco: ASP
The bright optical companion to the eclipsing millisecond pulsar in NGC 6397
We report the possible optical identification of the companion to the
eclipsing millisecond pulsar PSR J1740-5340 in the globular cluster NGC 6397. A
bright variable star with an anomalous red colour and optical variability which
nicely correlates to the orbital period of the pulsar has been found close to
the pulsar position. If confirmed, the optical light curve, reminiscent of
tidal distorsions similar to those observed in detached and contact binaries,
support the idea that this is the first case of a Roche lobe filling companion
to a millisecond pulsar.Comment: 9 pages, 4 embedded figures, submitted to ApJ Letter
Spin and magnetism in old neutron stars
The thermal, spin and magnetic evolution of neutron stars in the old low mass
binaries is first explored. Recycled to very short periods via accretion
torques, the neutron stars lose their magnetism progressively. If accretion
proceeds undisturbed for 100 Myrs these stars can rotate close to break up with
periods far below the minimum observed of 1.558 ms. We investigate their
histories using population synthesis models to show that a tail should exist in
the period distribution below 1.558 ms. The search of these ultrafastly
spinning neutron stars as pulsars can help discriminating among the various
equations of state for nuclear matter, and can shed light into the physics of
binary evolution.
The evolution of isolated neutron stars in the Galaxy is explored beyond the
pulsar phase. Moving through the tenuous interstellar medium, these old
solitary neutron stars lose their rotational energy. Whether also their
magnetism fades is still a mystery. A population synthesis model has revealed
that only a tiny fraction of them is able to accrete from the interstellar
medium, shining in the X-rays. There is the hope that these solitary stars will
eventually appear as faint sources in the Chandra sky survey. This might give
insight on the long term evolution of the magnetic field in isolated objects.Comment: 28 pages, 11 PostScript figures. To be published in "Physics of
Neutron Star Interiors" (Lecture Notes in Physics), ed. D. Blaschke, N.K.
Glendenning and A. Sedrakian (Springer, 2001
Recycling neutron stars to ultra short periods: a statistical analysis of their evolution in the mu-P plane
We investigate the statistical evolution of magnetic neutron stars, recycled
in binary systems, simulating synthetic populations. To bracket uncertainties,
we consider a soft (FP) and a stiff (PS) equation of state (EoS) for nuclear
matter and explore the hypothesis that the magnetic field is confined in the
stellar crust. We follow the magneto-rotational evolution within a simple
recycling scenario, including the possibility of magnetospheric propeller.
We find the presence of a tail in the period distribution of the synthetic
populations at periods shorter than 1.558 ms, the minimum detected so far. For
the soft EoS the recycling gives rise to a spin distribution which is
increasing monotonically toward short periods and a clear ``barrier'' forms at
the minimum period for the onset of mass shedding. For the stiff EoS the
distribution is flatter displaying a broad maximum about 2-4 ms.
The estimated fraction of neutron stars spinning close to their shedding
limit over the millisecond pulsar population is found to be significant.
Crustal magnetic field decay models predict the existence of massive (M>1.4
M_sun) rapidly spinning neutron stars with very low magnetic moment.Comment: 34 pages, 2 tables, 9 figures, Latex. Accepted (5 Jul 99) for
publication in the Astrophysical Journal Supplement
- …