1,324 research outputs found
GRAIL, an omni-directional gravitational wave detector
A cryogenic spherical and omni-directional resonant-mass detector proposed by
the GRAIL collaboration is described.Comment: 5 pages, 4 figs., contribution to proceedings GW Data Analysis
Workshop, Paris, nov. 199
Magnetically-driven explosions of rapidly-rotating white dwarfs following Accretion-Induced Collapse
We present 2D multi-group flux-limited diffusion magnetohydrodynamics (MHD)
simulations of the Accretion-Induced Collapse (AIC) of a rapidly-rotating white
dwarf. We focus on the dynamical role of MHD processes after the formation of a
millisecond-period protoneutron star. We find that including magnetic fields
and stresses can lead to a powerful explosion with an energy of a few Bethe,
rather than a weak one of at most 0.1 Bethe, with an associated ejecta mass of
~0.1Msun, instead of a few 0.001Msun. The core is spun down by ~30% within
500ms after bounce, and the rotational energy extracted from the core is
channeled into magnetic energy that generates a strong magnetically-driven
wind, rather than a weak neutrino-driven wind. Baryon loading of the ejecta,
while this wind prevails, precludes it from becoming relativistic. This
suggests that a GRB is not expected to emerge from such AICs during the early
protoneutron star phase, except in the unlikely event that the massive white
dwarf has sufficient mass to lead to black hole formation. In addition, we
predict both negligible 56Ni-production (that should result in an
optically-dark, adiabatically-cooled explosion) and the ejection of 0.1Msun of
material with an electron fraction of 0.1-0.2. Such pollution by neutron-rich
nuclei puts strong constraints on the possible rate of such AICs. Moreover,
being free from ``fallback,'' such highly-magnetized millisecond-period
protoneutron stars may later become magnetars, and the magnetically-driven
winds may later transition to Poynting-flux-dominated, relativistic winds,
eventually detectable as GRBs at cosmological distances. However, the low
expected event rate of AICs will constrain them to be, at best, a small subset
of GRB and/or magnetar progenitors.Comment: 16 pages, 8 figures, paper accepted to ApJ; High resolution version
available at http://hermes.as.arizona.edu/~luc/aic_mhd/aic_mhd.htm
The Role of Helium Stars in the Formation of Double Neutron Stars
We have calculated the evolution of 60 model binary systems consisting of
helium stars in the mass range of M_He= 2.5-6Msun with a 1.4Msun neutron star
companion to investigate the formation of double neutron star systems.Orbital
periods ranging from 0.09 to 2 days are considered, corresponding to Roche lobe
overflow starting from the helium main sequence to after the ignition of carbon
burning in the core. We have also examined the evolution into a common envelope
phase via secular instability, delayed dynamical instability, and the
consequence of matter filling the neutron star's Roche lobe. The survival of
some close He-star neutron-star binaries through the last mass transfer episode
(either dynamically stable or unstable mass transfer phase) leads to the
formation of extremely short-period double neutron star systems (with
P<~0.1days). In addition, we find that systems throughout the entire calculated
mass range can evolve into a common envelope phase, depending on the orbital
period at the onset of mass transfer. The critical orbital period below which
common envelope evolution occurs generally increases with M_He. In addition, a
common envelope phase may occur during a short time for systems characterized
by orbital periods of 0.1-0.5 days at low He-star masses (~ 2.6-3.3Msun).
The existence of a short-period population of double neutron stars increases
the predicted detection rate of inspiral events by ground-based
gravitational-wave detectors and impacts their merger location in host galaxies
and their possible role as gamma-ray burst progenitors. We use a set of
population synthesis calculations and investigate the implications of the
mass-transfer results for the orbital properties of DNS populations.Comment: 30 pages, Latex (AASTeX), 1 table, 8 figures. To appear in ApJ, v592
n1 July 20, 200
Does Sub-millisecond Pulsar XTE J1739-285 Contain a Low Magnetic Neutron Star or Quark Star ?
With the possible detection of the fastest spinning nuclear-powered pulsar
XTE J1739-285 of frequency 1122 Hz (0.8913 ms), it arouses us to constrain the
mass and radius of its central compact object and to imply the stellar matter
compositions: neutrons or quarks. Spun-up by the accreting materials to such a
high rotating speed, the compact star should have either a small radius or
short innermost stable circular orbit. By the empirical relation between the
upper kHz quasi-periodic oscillation frequency and star spin frequency, a
strong constraint on mass and radius is obtained as 1.51 solar masses and 10.9
km, which excludes most equations of states (EOSs) of normal neutrons and
strongly hints the star promisingly to be a strange quark star. Furthermore,
the star magnetic field is estimated to be about , which reconciles with those of millisecond radio pulsars,
revealing the clues of the evolution linkage of two types of astrophysical
objects.Comment: 10 pages, 2 figures, accepted by PASP 200
Magnetic fileds of coalescing neutron stars and the luminosity function of short gamma-ray burst
Coalescing neutron star binaries are believed to be the most reliable sources
for ground-based detectors of gravitational waves and likely progenitors of
short gamma-ray bursts. In the process of coalescence, magnetic fields of
neutron stars can induce interesting observational manifestations and affect
the form of gravitational wave signal. In this papaer we use the population
synthesis method to model the expected distribution of neutron star magnetic
fields during the coalescence under different assumptions on the initial
parameters of neutron stars and their magnetic field evolution. We discuss
possible elecotrmagnetic phenomena preceding the coalescence of magnetized
neutron star binaries and the effect of magnetic field on the gravitational
wave signal. We find that a log-normal (Gaussian in logarithms) distribution of
the initial magnetic fields of neutron stars, which agrees with observed
properties of radio pulsars, produces the distribution of the magnetic field
energy during the coalescence that adequately describes the observed luminosity
function of short gamma-ray bursts under different assumptions on the field
evolution and initial parameters of neutron stars. This agreement lends further
support to the model of coalescing neutron star binaries as progenitors of
gamma-ray bursts.Comment: v.2, LATEX, 25 pages, inc. 7 ps figures, Astron. Lett., in press.
Typos corrected, reference adde
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
Discovery of Five Binary Radio Pulsars
We report on five binary pulsars discovered in the Parkes multibeam Galactic
plane survey. All of the pulsars are old, with characteristic ages 1-11 Gyr,
and have relatively small inferred magnetic fields, 5-90e8 G. The orbital
periods range from 1.3 to 15 days. As a group these objects differ from the
usual low-mass binary pulsars (LMBPs): their spin periods of 9-88 ms are
relatively long; their companion masses, 0.2-1.1 Msun, are, in at least some
cases, suggestive of CO or more massive white dwarfs; and some of the orbital
eccentricities, 1e-5 < e < 0.002, are unexpectedly large. We argue that these
observed characteristics reflect binary evolution that is significantly
different from that of LMBPs. We also note that intermediate-mass binary
pulsars apparently have a smaller scale-height than LMBPs.Comment: 5 pages, 4 embedded EPS figs, accepted for publication by ApJ Letter
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
Identification of the Mass Donor Star's Spectrum in SS 433
We present spectroscopy of the microquasar SS 433 obtained near primary
eclipse and disk precessional phase Psi = 0.0, when the accretion disk is
expected to be most ``face-on''. The likelihood of observing the spectrum of
the mass donor is maximized at this combination of orbital and precessional
phases since the donor is in the foreground and above the extended disk
believed to be present in the system. The spectra were obtained over four
different runs centered on these special phases. The blue spectra show clear
evidence of absorption features consistent with a classification of A3-7 I. The
behavior of the observed lines indicates an origin in the mass donor. The
observed radial velocity variations are in anti-phase to the disk, the
absorption lines strengthen at mid-eclipse when the donor star is expected to
contribute its maximum percentage of the total flux, and the line widths are
consistent with lines created in an A supergiant photosphere. We discuss and
cast doubt on the possibility that these lines represent a shell spectrum
rather than the mass donor itself. We re-evaluate the mass ratio of the system
and derive masses of 10.9 +/- 3.1 Msun and 2.9 +/- 0.7 Msun for the mass donor
and compact object plus disk, respectively. We suggest that the compact object
is a low mass black hole.
In addition, we review the behavior of the observed emission lines from both
the disk/wind and high velocity jets.Comment: submitted to ApJ, 24 pages, 7 figure
Survey of Chickpea (Cicer arietinum L.) for Chickpea Stunt Disease and Associated Viruses in India and Pakistan
Chickpea chlorotic dwarf geminivirus (CCDV) and some luteoviruses were associated with chickpea stunt disease in India and Pakistan. One thousand eight hundred and four plants with stunt disease symptoms were collected and tested with poly- and monoclonal antibodies. Bean leafroll luteovirus (BLRV)-like luteoviruses and viruses reacting with an antiserum to a luteovirus isolate from chickpea, tentatively referred to as chickpea luteovirus (CpLV), were involved. Relative prevalence of the viruses varied among the different chickpea-growing areas. The BLRV-like viruses were of minor importance, while CCDV and CpLV-like viruses were widely distributed. The reaction patterns of the luteoviruses from chickpea with monoclonal antibodies differed from those of some known luteoviruses. In addition to CpLV, BLRV, and other luteoviruses, an unidentified, graft-transmissible agent may be involved in the etiology, which is more complex than reported initiall
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