246,920 research outputs found
High Energy Continuum Spectra from X-Ray Binaries
A variety of high energy (>1 keV) spectra have been observed in recent years
from Black Hole (BH) and Neutron Star (NS) X-ray Binaries (XB). Some common
physical components exist between BHXBs and NSXBs, resulting in some high
energy spectral features. A common component between a BHXB and a weakly
magnetized NSXB is the inner accretion disk region extending very close to the
surface (for a NS) or the horizon (for a BH). The inner disk radiation can be
described by a multi-color blackbody (MCB) spectral model. The surface
radiation of the NS can be approximated by a Single Color Blackbody (SCB)
spectrum. For a strongly magnetized NSXB, the high energy emission is from its
magnetosphere, characterised by a thermal bremsstrahlung (TB) spectrum. In both
BHXBs and weakly magnetized NSXBs, a hot electron cloud may exist, producing
the hard X-ray power law (photon index -1.5 to -2.0) with thermal cutoff
(50-200 keV). It has been recently proposed that a converging flow may be
formed near the horizon of a BH, producing a softer power law (photon index
about -2.5) without cutoff up to several hundred keV. Based on these concepts
we also discuss possible ways to distinguish between BH and NS XBs. Finally we
discuss briefly spectral state transitions in both BH and NS XBs.Comment: 10 pages, PostScript file ps.gz file. Invited review talk at the IAU
Colloquium 163 `Accretion Phenomena and Related Outflows', PASP Conference
Series, in pres
QSO-Galaxy Association and Gravitational Lensing
The amplification caused by gravitational light bending by compact objects in
a foreground galaxy can affect the apparent number density of background QSOs,
as well as their distribution in the fields of galactic halos. In this work we
investigate the number enhancement of QSOs in the fields of galactic halos
caused by point mass lensing effect and singular isothermal lensing effect, and
apply the microlensing effect due to dark compact objects in the halo to NGC
3628. NGC 3628 is a well-studied nearby edge-on Sbc peculiar galaxy, where QSOs
are shown to be concentrated around the galaxy with a density much higher than
background. We show that if present understanding of the luminosity function of
QSOs is right, such concentration could not be caused by gravitational lensing.Comment: 6 pages, 4 figures, accepted for publication in Chinese Journal of
Astronomy and Astrophysics, 2004 Supplement
Radiation Properties of GeV Narrow Line Seyfert 1 Galaxies
The broadband SEDs of four gamma-ray NLS1s are compiled and explained with
the leptonic model. It is found that their characteristics and fitting
parameters of the observed SEDs are more like FSRQs than BL Lacs.Comment: 2 pages; 1 figure; to appear in the proceedings of the IAU Symposium
290, "Feeding compact objects: accretion on all scales", Beijing, 20-24 Aug
201
Maser mechanism of optical pulsations from anomalous X-ray pulsar 4U0142+61
A maser curvature emission mechanism in the presence of curvature drift is
used to explain the optical pulsations from anomalous X-ray pulsars. For the
source of AXP0142+61,the optical pulsation occurs at the radial distance
cm to the neutron star. The corresponding
curvature maser frequency is about Hz. The
result is consistent with the observation of the optical pulsations from the
anomalous X-ray pulsar 4U0142+61.Comment: 18pages,LaTeX2e, accetpetd for publication in MNRA
The Black Hole Mass and Magnetic Field Correlation in Active Galactic Nuclei
The observed optical luminosity in 5100 angstorm and black hole mass
correlation is used to probe the magnetic field of black holes harbored in
active galactic nuclei(AGNs). The model is based on the assumption that the
disk is heated by energy injection due to the magnetic coupling(MC) process and
the gravitational dissipation due to accretion. The MC process can transfer
energy and angular momentum from a rotating Kerr black hole to its surrounding
disk. The relation of optical luminosity in 5100 angstorm and black hole mass
as functions of the spin and magnetic field of the black hole is modelled. The
model predicts that optical luminosity in 5100 angstorm emitted from the disk
is sensitive to the strength of the poloidal component of the magnetic field on
the BH horizon. Based on the observations of optical luminosity in 5100
angstorm for 143 AGN sources, we obtain the correlation between mass and
magnetic field of black hole. And we compared out result with the approximate
result between mass and magnetic field of black hole derived from the condition
in the standard accretion disc theory.Comment: 7 pages, 4 figures, accepted by Chinese Journal of Astronomy and
Astrophysics, 2004 Supplement
Fermi surface of a trapped dipolar Fermi gas
Under the framework of the semiclassical theory, we investigate the
equilibrium-state properties of a spin polarized dipolar Fermi gas through full
numerical calculation. We show that the Fermi surfaces in both real and
momentum spaces are stretched along the attractive direction of dipolar
interaction. We further verify that the deformed Fermi surfaces can be well
approximated by ellipsoids. In addition, the deformation parameters slightly
depend on the local real- and momentum-space densities. We also study the
interaction strength dependence of the energy and real- and momentum-space
densities. By comparing them with variational results, we find that the
ellipsoidal ansatz usually generates accurate results for weak dipolar
interaction, while under strong dipolar interaction limit, notable discrepancy
can be observed. Finally, we map out the stability boundary of the system
Black Holes in Brans-Dicke Theory with a Cosmological Constant
Since the Brans-Dicke theory is conformal related to the dilaton gravity
theory, by applying a conformal transformation to the dilaton gravity theory,
we derived the cosmological constant term in the Brans-Dicke theory and the
physical solution of black holes with the cosmological constant. It is found
that, in four dimensions, the solution is just the Kerr-Newman-de Sitter
solution with a constant scalar field. However, in dimensions, the
solution is not yet the dimensional Kerr-Newman-de Sitter solution and the
scalar field is not a constant in general. In Brans-Dicke-Ni theory, the
resulting solution is also not yet the Kerr-Newman-de Sitter one even in four
dimensions. The higher dimensional origin of the Brans-Dicke scalar field is
briefly discussed.Comment: 5 page
Phantom Black Holes
The exact solutions of electrically charged phantom black holes with the
cosmological constant are constructed. They are labelled by the mass, the
electrical charge, the cosmological constant and the coupling constant between
the phantom and the Maxwell field. It is found that the phantom has important
consequences on the properties of black holes. In particular, the extremal
charged phantom black holes can never be achieved and so the third law of
thermodynamics for black holes still holds. The cosmological aspects of the
phantom black hole and phantom field are also briefly discussed.Comment: 8 pages,some references adde
RX J1856.5-3754: A strange star with solid quark surface?
Within the realms of the possibility of solid quark matter, we fitted the
500ks Chandra LETG/HRC data for RX J1856.5-3754 with a phenomenological
spectral model, and found that electric conductivity of quark matter on the
stellar surface is about > 1.2 x 10^{18} s^{-1}.Comment: 2 page and no figure, IAU Symposium No.218, or at
http://vega.bac.pku.edu.cn/~rxxu/publications/index_C.ht
The High Energy cosmic-Radiation Detection (HERD) Facility onboard China's Future Space Station
The High Energy cosmic-Radiation Detection (HERD) facility is one of several
space astronomy payloads of the cosmic lighthouse program onboard China's Space
Station, which is planned for operation starting around 2020 for about 10
years. The main scientific objectives of HERD are indirect dark matter search,
precise cosmic ray spectrum and composition measurements up to the knee energy,
and high energy gamma-ray monitoring and survey. HERD is composed of a 3-D
cubic calorimeter (CALO) surrounded by microstrip silicon trackers (STKs) from
five sides except the bottom. CALO is made of about 10 cubes of LYSO
crystals, corresponding to about 55 radiation lengths and 3 nuclear interaction
lengths, respectively. The top STK microstrips of seven X-Y layers are
sandwiched with tungsten converters to make precise directional measurements of
incoming electrons and gamma-rays. In the baseline design, each of the four
side SKTs is made of only three layers microstrips. All STKs will also be used
for measuring the charge and incoming directions of cosmic rays, as well as
identifying back scattered tracks. With this design, HERD can achieve the
following performance: energy resolution of 1\% for electrons and gamma-rays
beyond 100 GeV, 20\% for protons from 100 GeV to 1 PeV; electron/proton
separation power better than ; effective geometrical factors of 3
for electron and diffuse gamma-rays, 2 for cosmic ray nuclei. R\&D is under way for reading out the
LYSO signals with optical fiber coupled to image intensified CCD and the
prototype of one layer of CALO.Comment: 9 pages, 6 figures, 2 tables. Oral talk presented at SPIE
Astronomical Telescopes and Instrumentation, June, 2014, Montrea
- âŠ