3,355 research outputs found
Estimation and prediction of credit risk based on rating transition systems
Risk management is an important practice in the banking industry. In this
paper we develop a new methodology to estimate and predict the probability of
default (PD) based on the rating transition matrices, which relates the rating
transition matrices to the macroeconomic variables. Our method can overcome the
shortcomings of the framework of Belkin et al. (1998), and is especially useful
in predicting the PD and doing stress testing. Simulation is conducted at the
end, which shows that our method can provide more accurate estimate than that
obtained by the method of Belkin et al. (1998).Comment: 15 page
Formation of the Double Neutron Star System PSR J19301852
The spin period (185 ms) and period derivative () of the double neutron star (DNS) system PSR J19301852 recently
discovered indicate that the pulsar was mildly recycled through the process of
Roche-lobe overflow. This system has the longest orbital period (45 days) of
the known DNS systems, and can be formed from a helium star-NS binary if the
initial mass of the helium star was ; otherwise the
helium star would never fill its Roche-lobe \citep{t15}. At the moment of the
supernova explosion, the mass of the helium star was .
We find that the probability distribution of the velocity kick imparted to the
new-born neutron star has a maximum at about (and a tail
up to ), indicating that this NS most probably received
a low kick velocity at birth.Comment: 9 pages, 4 figures, accepted for publication in Ap
On the formation of Be stars through binary interaction
Be stars are rapidly rotating B type stars. The origin of their rapid
rotation is not certain, but binary interaction remains to be a possibility. In
this work we investigate the formation of Be stars resulting from mass transfer
in binaries in the Galaxy. We calculate the binary evolution with both stars
evolving simultaneously and consider different possible mass accretion
histories for the accretor. From the calculated results we obtain the critical
mass ratios that determine the stability of mass transfer. We also
numerically calculate the parameter in common envelope evolution, and
then incorporate both and into the population synthesis
calculations. We present the predicted numbers and characteristics of Be stars
in binary systems with different types of companions, including helium stars,
white dwarfs, neutron stars, and black holes. We find that in Be/neutron star
binaries the Be stars can have a lower limit of mass if
they are formed by stable (i.e., without the occurrence of common envelope
evolution) and nonconservative mass transfer. We demonstrate that isolated Be
stars may originate from both mergers of two main-sequence stars and disrupted
Be binaries during the supernova explosions of the primary stars, but mergers
seem to play a much more important role. Finally the fraction of Be stars which
have involved binary interactions in all B type stars can be as high as , implying that most of Be stars may result from binary interaction.Comment: 38 pages, 14 figures, 1 table, accepted for publication in Ap
A Population of Ultraluminous X-ray Sources with An Accreting Neutron Star
Most ultraluminous X-ray sources (ULXs) are believed to be X-ray binary
systems, but previous observational and theoretical studies tend to prefer a
black hole rather than a neutron star accretor. The recent discovery of 1.37 s
pulsations from the ULX M82 X-2 has established its nature as a magnetized
neutron star. In this work we model the formation history of neutron star ULXs
in an M82- or Milky Way-like galaxy, by use of both binary population synthesis
and detailed binary evolution calculations. We find that the birthrate is
around for the incipient X-ray binaries in both cases.
We demonstrate the distribution of the ULX population in the donor mass -
orbital period plane. Our results suggest that, compared with black hole X-ray
binaries, neutron star X-ray binaries may significantly contribute to the ULX
population, and high-mass and intermediate-mass X-ray binaries dominate the
neutron star ULX population in M82- and Milky Way-like galaxies, respectively.Comment: 13 pages, 5 figures, accepted for publication in Ap
Formation and Evolution of Galactic Intermediate/Low-Mass X-ray Binaries
We investigate the formation and evolutionary sequences of Galactic
intermediate- and low-mass X-ray binaries (I/LMXBs) by combining binary
population synthesis (BPS) and detailed stellar evolutionary calculations.
Using an updated BPS code we compute the evolution of massive binaries that
leads to the formation of incipient I/LMXBs, and present their distribution in
the initial donor mass vs. initial orbital period diagram. We then follow the
evolution of I/LMXBs until the formation of binary millisecond pulsars (BMSPs).
We find that the birthrate of the I/LMXB population is in the range of , compatible with that of
BMSPs which are thought to descend from I/LMXBs. We show that during the
evolution of I/LMXBs they are likely to be observed as relatively compact
binaries with orbital periods 1 day and donor masses . The resultant BMSPs have orbital periods ranging from less than 1
day to a few hundred days. These features are consistent with observations of
LMXBs and BMSPs. We also confirm the discrepancies between theoretical
predications and observations mentioned in the literature, that is, the
theoretical average mass transfer rates (\,yr) of LMXBs are considerably lower than observed, and the number of
BMSPs with orbital periods day is severely underestimated. These
discrepancies imply that something is missing in the modeling of LMXBs, which
is likely to be related to the mechanisms of the orbital angular momentum loss.Comment: 30 pages, 13 figure
On the role of supernova kicks in the formation of Galactic double neutron star systems
In this work we focus on a group of Galactic double neutron star (DNS)
systems with long orbital periods of day and low eccentricities of
. The feature of these orbital parameters is used to constrain
the evolutionary processes of progenitor binaries and the supernova (SN) kicks
of the second born NSs. Adopting that the mass transfer during primordial
binary evolution is highly non-conservative (rotation-dependent), the formation
of DNS systems involves a double helium star binary phase, the common envelope
(CE) evolution initiates before the first NS formation. During the CE evolution
the binary orbital energy is obviously larger when using a helium star rather
than a NS to expel the donor envelope, this can help explain the formation of
DNS systems with long periods. SN kicks at NS birth can lead to eccentric
orbits and even the disruption of binary systems, the low eccentricities
require that the DNSs receive a small natal kick at the second collapse.
Compared with the overall distribution of orbital parameters for observed DNS
binaries, we propose that the second born NSs in most DNS systems are subject
to small natal kicks with the Maxwellian dispersion velocity of less than , which can provide some constraints on the SN explosion
processes. The mass distribution of DNS binaries is also briefly discussed. We
suggest that the rotation-dependent mass transfer mode and our results about SN
kicks should be applied to massive binary evolution and population synthesis
studies.Comment: 7 figures, 2 tables, accepted by Ap
Can the Subsonic Accretion Model Explain the Spin Period Distribution of Wind-fed X-ray Pulsars?
Neutron stars in high-mass X-ray binaries (HMXBs) generally accrete from the
wind matter of their massive companion stars. Recently Shakura et al. (2012)
suggested a subsonic accretion model for low-luminosity (
ergs), wind-fed X-ray pulsars. To test the feasibility of this model, we
investigate the spin period distribution of wind-fed X-ray pulsars with a
supergiant companion star, using a population synthesis method. We find that
the modeled distribution of supergiant HMXBs in the spin period - orbital
period diagram is consistent with observations provided that the winds from the
donor stars have relatively low terminal velocities (
kms). The measured wind velocities in several supergiant HMXBs seem to
favor this viewpoint. The predicted number ratio of wind-fed X-ray pulsars with
persistent X-ray luminosities higher and lower than
ergs is about .Comment: 17 pages, 3 figures, accepted for publication in Ap
Microscopic explanation for black hole phase transitions via Ruppeiner geometry: two competing factors-the temperature and repulsive interaction among BH molecules
Charged dilatonic black hole (BH) has rather rich phase diagrams which may
contain zeroth-order, first-order as well as reentrant phase transitions (RPTs)
depending on the value of the coupling constant between the
electromagnetic field and the dilaton. We try to give a microscopic explanation
for these phase transitions by adopting Ruppeiner's approach. By studying the
behaviors of the Ruppeiner invariant along the co-existing lines, we find
that the various phase transitions may be qualitatively well explained as a
result of two competing factors: the first one is the low-temperature effect
which tends to shrink the BH and the second one is the repulsive interaction
between the BH molecules which, on the contrary, tends to expand the BH. In the
standard phase transition without RPT, as temperature is lowered, the first
kind of factor dominates over the second one, so that large black hole (LBH)
tends to shrink and thus transits to small black hole (SBH); While in the RPT,
after the LBH-SBH transition, as temperature is further decreased, the strength
of the second factor increases quickly and finally becomes strong enough to
dominate over the first factor, so that SBH tends to expand to release the high
repulsion and thus transits back to LBH. Moreover, by comparing the behavior of
versus the temperature with fixed pressure to that of ordinary
two-dimensional thermodynamical systems but with fixed specific volume, it is
interesting to see that SBH behaves like a Fermionic gas system in cases with
RPT, while it behaves oppositely to an anyon system in cases without RPT. And
in all cases, LBH behaves like a nearly ideal gas system.Comment: 16 pages, 7 figures;v2:minor modifications, refs added;v3:minor
modifications, more refs added; v4:minor modifications to match published
versio
Population Synthesis of Black Hole X-Ray Binaries
We present a systematic study of the X-ray binaries (XRBs) containing a black
hole (BH) and a non-degenerate companion, in which mass transfer takes place
via either capturing the companion's wind or Roche lobe overflow (RLO). As
shown in our previous work that focusing on the formation and evolution of
detached BH binaries, our assumed models relevant to BH's progenitors predicted
significantly different binary properties \citep{sl19}. In this paper, we
further follow the evolutionary paths of the BH systems that appearing as XRBs.
By use of both binary population synthesis and detailed binary evolution
calculations, we can obtain the potential population of BH XRBs. Distributions
at the current epoch of various binary parameters have been computed. The
observed sample of wind-fed XRBs can be well reproduced under assumption of all
our models. The wind-fed XRBs are expected to be so rare ()
that only a couple of such systems have been detected. Comparison of known RLO
XRBs with the calculated distributions of various binary parameters indicates
that only the models assuming relatively small masses for BH progenitors can
roughly match the observations. Accordingly we estimate that there are hundreds
of RLO XRBs in the Milky Way, of which the majority are low-mass XRBs. The RLO
systems may become ultraluminous X-ray sources (ULXs) if the BH accretes at a
very high rate, we expect that about a dozen ULXs with a BH accretor may exist
in a Milky Waylike galaxy.Comment: 17 pages, 15 figures, 2 tables, accepted by Ap
Population Synthesis of Galactic Be-star Binaries with A Helium-star Companion
LB-1 was originally suggested to harbour a very massive ()
black hole, but was recently suggested to be a post-mass transfer binary
containing a Be star and a helium (He) star. In this paper, we use the binary
population synthesis method to simulate the potential population of the BeHe binaries in the Milky Way. Mass transfer process during the progenitor
binary evolution plays a vital role in determining the possible properties of
the BeHe binary population. By constructing a range of physical models
with significantly different mass-transfer efficiencies, we obtain the
predicted distributions at the current epoch of the component masses and the
orbital periods for the BeHe binaries. In particular, we show that, LB-1
very likely has evolved through non-conservative mass transfer if it is indeed
a BeHe system. We estimate that there are more than BeHe
binaries with V-band apparent magnitudes brighter than LB-1.Comment: 8 pages, 5 figures, accepted by Ap
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