1,394 research outputs found
The Extraordinary Abundances of QSO Broad Absorption Line Regions: A Matter of Novae?
The broad absorption lines (BALs) of QSOs indicate abundances of heavy
elements, relative to hydrogen, that are 1 to 2 orders of magnitude higher than
the solar values. In at least one QSO, an especially large enhancement of
phosphorus is observed. These abundances resemble those in Galactic novae, and
this suggests that novae may produce the BAL gas. The needed rate of nova
outbursts may come from single white dwarfs that accrete gas as they pass
through a supermassive accretion disk around a central black hole.Comment: 9 pages including 1 Postscript figure. Uses aaspp4.sty and
flushrt.sty. Uuencoded, gzipped tarfile. To appear in Astrophys. J.
(Letters), 1996 April 1
Strange stars in low-mass binary pulsar systems
Based on observational facts and a variety of theoretical arguments we
discuss in this work the possibility that pulsars in Low-Mass Binary Pulsar
systems could be strange stars rather than neutron stars. It is shown that,
although subject to reasonable uncertainties, the consideration of the physics
of the SQM core and thin normal crusts leads to the prediction of several
observed features of the magnetic field history of these systems whitin this
working hypothesis.Comment: 6 pages, no figures, PlainTex file submitted to IJMP
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
Neutron Star Masses and Radii as Inferred from kilo-Hertz QPOs
Kilo-Hertz (kHz) Quasi-periodic oscillations (QPOs) have been discovered in
the X-ray fluxes of 8 low-mass X-ray binaries (LMXBs) with the Rossi X-ray
Timing Explorer (RXTE). The characteristics of these QPOs are remarkably
similar from one source to another. In particular, the highest observed QPO
frequencies for 6 of the 8 sources fall in a very narrow range: 1,066 to 1,171
Hz. This is the more remarkable when one considers that these sources are
thought to have very different luminosities and magnetic fields, and produce
very different count rates in the RXTE detectors. Therefore it is highly
unlikely that this near constancy of the highest observed frequencies is due to
some unknown selection effect or instrumental bias. In this letter we propose
that the highest observed QPO frequency can be taken as the orbital frequency
of the marginally stable orbit. This leads to the conclusions that the neutron
stars in these LMXBs are inside their marginally stable orbits and have masses
in the vicinity of 2.0 solar masses. This mass is consistent with the
hypothesis that these neutron stars were born with about 1.4 solar masses and
have been accreting matter at a fraction of the Eddington limit for 100 million
years.Comment: 7 pages, uses aas2pp4.sty, Accepted by ApJ
The Physics of Supernova Remnant Blast Waves. I. Kinematics of DEM L71 in the Large Magellanic Cloud
We present the results from Fabry-Perot imaging spectroscopy of the
Balmer-dominated supernova remnant DEM L71 (0505-67.9) in the LMC. Spectra
extracted from the entire circumference of the blast wave reveal the broad and
narrow component H-alpha line emission characteristic of non-radiative shocks
in partially neutral gas. The new spectra of DEM L71 include portions of the
rim that have not been previously observed. We find that the broad component
width varies azimuthally along the edge of DEM L71, ranging from 450+/-60 km/s
along the eastern edge to values as high as 985 (+210)(-165) km/s along the
faint western edge. In part of the faint northern rim the broad component is
not detected, possibly indicating a lower density in these regions and/or a
broad component width in excess of 1000 km/s. Between the limits of zero and
full electron-ion temperature equilibration at the shock front, the allowed
range of shock velocities is 430-560 km/s along the east rim and 700-1250 km/s
along other parts of the blast wave. The H-alpha broad-to-narrow flux ratios
vary considerably around the remnant, ranging from 0.4 to 0.8. These ratios lie
below the values predicted by our shock models. We find that narrow component
H-alpha emission from a cosmic ray precursor may be the cause of the
discrepancy. The least decelerated portions of the blast wave (i.e., regions
excluding the brightest filaments) are well characterized by Sedov models with
a kinetic energy E_51= (0.37+/-0.06)*D_50**(5/2), where D_50 is the LMC
distance in units of 50 kpc. The corresponding age for DEM L71 is
(4360+/-290)*D_50 yr. This is the first time that velocity information from the
entire blast wave has been utilized to study the global kinematics of a
non-radiative SNR at a known distance.Comment: 21 pages, including 8 postscript figures and 4 tables, LaTeX,
accepted to ApJ; see companion pape
BASEL III: Long-term impact on economic performance and fluctuations
We assess the long-term economic impact of the new regulatory standards (the Basel III reform), answering the following questions. (1) What is the impact of the reform on long-term economic performance? (2) What is the impact of the reform on economic fluctuations? (3) What is the impact of the adoption of countercyclical capital buffers on economic fluctuations? The main results are the following. (1) Each percentage point increase in the capital ratio causes a median 0.09 percent decline in the level of steady state output, relative to the baseline. The impact of the new liquidity regulation is of a similar order of magnitude, at 0.08 percent. This paper does not estimate the benefits of the new regulation in terms of reduced frequency and severity of financial crisis, analysed in Basel Committee on Banking Supervision (BCBS, 2010b). (2) The reform should dampen output volatility; the magnitude of the effect is heterogeneous across models; the median effect is modest. (3) The adoption of countercyclical capital buffers could have a more sizeable dampening effect on output volatility. These conclusions are fully consistent with those of the reports by the Long-term Economic Impact group (BCBS, 2010b) and Macro Assessment Group (MAG, 2010b).Basel III, countercyclical capital buffers, financial (in)stability, procyclicality, macroprudential policy.
The Temperature and Cooling Age of the White-Dwarf Companion to the Millisecond Pulsar PSR B1855+09
We report on Keck and {\em Hubble Space Telescope} observations of the binary
millisecond pulsar PSR B1855+09. We detect its white-dwarf companion and
measure \mv=25.90\pm0.12 and \mi=24.19\pm0.11 (Vega system). From the
reddening-corrected color, (\mv-\mi)_0=1.06\pm0.21, we infer a temperature
\Teff=4800\pm800 K. The white-dwarf mass is known accurately from
measurements of the Shapiro delay of the pulsar signal,
\Mcomp=0.258^{+0.028}_{-0.016} \Msun. Hence, given a cooling model, one can
use the measured temperature to determine the cooling age. The main uncertainty
in the cooling models for such low-mass white dwarfs is the amount of residual
nuclear burning, which is set by the thickness of the hydrogen layer
surrounding the helium core. From the properties of similar systems, it has
been inferred that helium white dwarfs form with thick hydrogen layers, with
mass \simgt3\times10^{-3} \Msun, which leads to significant additional
heating. This is consistent with expectations from simple evolutionary models
of the preceding binary evolution. For PSR B1855+09, though, such models lead
to a cooling age of Gyr, which is twice the spin-down age of the
pulsar. It could be that the spin-down age were incorrect, which would call the
standard vacuum dipole braking model into question. For two other pulsar
companions, however, ages well over 10 Gyr are inferred, indicating that the
problem may lie with the cooling models. There is no age discrepancy for models
in which the white dwarfs are formed with thinner hydrogen layers
(\simlt3\times10^{-4} \Msun).Comment: 7 pages, 1 figure, aas4pp2.sty. Accepted for publication in ApJ
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
Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic
We have computed, based on 17 infrared radial velocities, the first set of
orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The
giant's companion is a neutron star, the bright X-ray source GX 1+4. We find an
orbital period of 1161 days by far the longest of any known X-ray binary. The
orbit has a modest eccentricity of 0.10 with an orbital circularization time of
less than 10^6 years. The large mass function of the orbit significantly
restricts the mass of the M giant. Adopting a neutron-star mass of 1.35M(Sun),
the maximum mass of the M giant is 1.22M(Sun), making it the less massive star.
Derived abundances indicate a slightly subsolar metallicity. Carbon and
nitrogen are in the expected ratio resulting from the red-giant first dredge-up
phase. The lack of O-17 suggests that the M-giant has a mass less than
1.3M(Sun), consistent with our maximum mass. The red giant radius is 103R(Sun),
much smaller than the estimated Roche lobe radius. Thus, the mass loss of the
red giant is via a stellar wind. Although the M giant companion to the neutron
star has a mass similar to the late-type star in low-mass X-ray binaries, its
near-solar abundances and apparent runaway velocity are not fully consistent
with the properties of this class of stars.Comment: In press to The Astrophysical Journal (10 April 2006 issue). 23 page
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
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