2,597 research outputs found
Galactic Models of Gamma-Ray Bursts
We describe observational evidence and theoretical calculations which support
the high velocity neutron star model of gamma-ray bursts. We estimate the
energetic requirements in this model, and discuss possible energy sources. we
also consider radiative processes involved in the bursts.Comment: 16 pages Latex file in revtex format. Fourteen postscript figures
come in a separate file. To appear in the Proceedings of the 1995 La Jolla
Workshop "High Velocity Neutron Stars and Gamma-Ray Bursts", eds. R.
Rorschild etal., AIP, New Yor
Distribution of compact object mergers around galaxies
Compact object mergers are one of the currently favored models for the origin
of GRBs. The discovery of optical afterglows and identification of the nearest,
presumably host, galaxies allows the analysis of the distribution of burst
sites with respect to these galaxies. Using a model of stellar binary evolution
we synthesize a population of compact binary systems which merge within the
Hubble time. We include the kicks in the supernovae explosions and calculate
orbits of these binaries in galactic gravitational potentials. We present the
resulting distribution of merger sites and discuss the results in the framework
of the observed GRB afterglows.Comment: 8 pages, 5 figures, submitted to MNRA
Population synthesis of neutron stars, strange (quark) stars and black holes
We compute and present the distribution in mass of single and binary neutron
stars, strange stars, and black holes. The calculations were performed using a
stellar population synthesis code. We follow all phases of single and binary
evolution, starting from a ZAMS binary and ending in the creation of one
compact object (neutron star, black hole, strange star) and a white dwarf, or
two compact objects (single or binary). We assume that neutron stars are formed
in the collapse of iron/nickel cores in the mass range M0 < M < M1, quark stars
in the range M1 M2 and find that
the population of quark stars can easily be as large as the population of black
holes, even if there is only a small mass window for their formation.Comment: 4 pages, 4 figures, to appear in the proceedings of "The 4th Integral
Workshop
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Resolving dichotomy in compact objects through continuous gravitational waves observation
More than two dozen soft gamma-ray repeaters (SGRs) and anomalous X-ray
pulsars (AXPs) have been detected so far. These are isolated compact objects.
Many of them are either found to be associated with supernova remnants or their
surface magnetic fields are directly measured, confirming that they are neutron
stars (NSs). However, it has been argued that some SGRs and AXPs are highly
magnetized white dwarfs (WDs). Meanwhile, the existence of super-Chandrasekhar
WDs has remained to be a puzzle. However, not even a single such massive WD has
been observed directly. Moreover, some WD pulsars are detected in
electromagnetic surveys and some of their masses are still not confirmed. Here
we calculate the signal-to-noise ratio for all these objects, considering
different magnetic field configurations and thereby estimate the required time
for their detection by various gravitational wave (GW) detectors. For SGRs and
AXPs, we show that, if these are NSs, they can hardly be detected by any of the
GW detectors, while if they are WDs, Big Bang Observer (BBO), DECi-hertz
Interferometer Gravitational wave Observatory (DECIGO) and Advanced Laser
Interferometer Antenna (ALIA) would be able to detect them within a few days to
a year of integration, depending on the magnetic field strength and its
configuration. Similarly, if a super-Chandrasekhar WD has a dominant toroidal
field, we show that even Laser Interferometer Space Antenna (LISA) and TianQin
would be able to detect it within one year of integration. We also discuss how
GWs can confirm the masses of the WD pulsars
Gravitational lensing as a probe of compact object population in the Galaxy
The population of solitary compact objects in the Galaxy is very diffcult to
investigate. In this paper we analyze the possibility of using microlensing
searches to detect and to analyze the properties of the solitary black holes
and neutron stars. Evolution of single and binary stars is considered using the
StarTrack population synthesis code. We investigate the properties of the
Galactic population of compact objects numerically. We find that the compact
object lensing events are concentrated in a region with the radius of degrees around the Galactic center. The distribution of masses of the lenses
for the models we consider differs but only slightly from the underlying
massdistribution. The expected detection rates are of the order of a few per
year.Comment: Submitted to Astronomy and Astrophysic
On The Maximum Mass of Stellar Black Holes
We present the spectrum of compact object masses: neutron stars and black
holes that originate from single stars in different environments. In
particular, we calculate the dependence of maximum black hole mass on
metallicity and on some specific wind mass loss rates (e.g., Hurley et al. and
Vink et al.). Our calculations show that the highest mass black holes observed
in the Galaxy M_bh = 15 Msun in the high metallicity environment (Z=Zsun=0.02)
can be explained with stellar models and the wind mass loss rates adopted here.
To reach this result we had to set Luminous Blue Variable mass loss rates at
the level of about 0.0001 Msun/yr and to employ metallicity dependent
Wolf-Rayet winds. With such winds, calibrated on Galactic black hole mass
measurements, the maximum black hole mass obtained for moderate metallicity
(Z=0.3 Zsun=0.006) is M_bh,max = 30 Msun. This is a rather striking finding as
the mass of the most massive known stellar black hole is M_bh = 23-34 Msun and,
in fact, it is located in a small star forming galaxy with moderate
metallicity. We find that in the very low (globular cluster-like) metallicity
environment the maximum black hole mass can be as high as M_bh,max = 80 Msun
(Z=0.01 Zsun=0.0002). It is interesting to note that X-ray luminosity from
Eddington limited accretion onto an 80 Msun black hole is of the order of about
10^40 erg/s and is comparable to luminosities of some known ULXs. We emphasize
that our results were obtained for single stars only and that binary
interactions may alter these maximum black hole masses (e.g., accretion from a
close companion). This is strictly a proof-of-principle study which
demonstrates that stellar models can naturally explain even the most massive
known stellar black holes.Comment: 15 pages, ApJ accepte
Couple-based interventions in the treatment of adult anorexia nervosa: A brief case example of UCAN.
Adult anorexia nervosa (AN) is a serious and often fatal illness that significantly erodes quality of life for both the patient and loved ones. Treatment of adults with AN has focused largely on individual therapy, with recent findings suggesting that improvement is limited and dropout rates are high. In an effort to improve treatment response, we developed a couple-based intervention, Uniting Couples in the treatment of Anorexia Nervosa (UCAN) as an adjunct treatment to standard multidisciplinary care. UCAN leverages the support of a partner and the relationship in treatment by decreasing avoidance around AN, teaching the couple how to effectively address the eating disorder, and helping to foster a more satisfying relationship. This paper presents a case study of a couple who completed UCAN, “Laura and Steve”, including their experiences in treatment and outcome measures at pretest, posttest, and three-month follow-up. Laura showed clinically significant change on the Restraint subscale of the EDE at follow-up, and both partners showed clinically significant improvements in relationship satisfaction, as well as on self-reported and observed communication. Both partners reported very high satisfaction with the treatment. A discussion of therapists' experiences in delivering UCAN is provided, including common challenges for therapists with primarily a couple therapy or an individual CBT for eating disorders background, as well as important factors for therapists to consider in order to optimally leverage the benefits of including partners in treatment for AN
Innermost stable circular orbits around magnetized rotating massive stars
In 1998, Shibata and Sasaki [Phys. Rev. D 58, 104011 (1998)] presented an
approximate analytical formula for the radius of the innermost stable circular
orbit (ISCO) of a neutral test particle around a massive, rotating and deformed
source. In the present paper, we generalize their expression by including the
magnetic dipole moment. We show that our approximate analytical formulas are
accurate enough by comparing them with the six-parametric exact solution
calculated by Pach\'on et. al. [Phys. Rev. D 73, 104038 (2006)] along with the
numerical data presented by Berti and Stergioulas [MNRAS 350, 1416 (2004)] for
realistic neutron stars. As a main result, we find that in general, the radius
at ISCO exhibits a decreasing behavior with increasing magnetic field. However,
for magnetic fields below 100GT the variation of the radius at ISCO is
negligible and hence the non-magnetized approximate expression can be used. In
addition, we derive approximate analytical formulas for angular velocity,
energy and angular momentum of the test particle at ISCO.Comment: 8 pages, 3 figure
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