149,759 research outputs found
Observations of Cygnus X-1 in the MeV band by the INTEGRAL imager
The spectrum of the MeV tail detected in the black-hole candidate Cygnus X-1
remains controversial as it appeared much harder when observed with the
INTEGRAL Imager IBIS than with the INTEGRAL spectrometer SPI or CGRO. We
present an independent analysis of the spectra of Cygnus X-1 observed by IBIS
in the hard and soft states. We developed a new analysis software for the
PICsIT detector layer and for the Compton mode data of the IBIS instrument and
calibrated the idiosyncrasies of the PICsIT front-end electronics. The spectra
of Cygnus X-1 obtained for the hard and soft states with the INTEGRAL imager
IBIS are compatible with those obtained with the INTEGRAL spectrometer SPI,
with CGRO, and with the models that attribute the MeV hard tail either to
hybrid thermal/non-thermal Comptonisation or to synchrotron emission.Comment: 6 pages, 7 figure
Toward an understanding of thermal X-ray emission of pulsars
We present a theoretical model for the thermal X-ray emission and cooling of
isolated pulsars, assuming that pulsars are solid quark stars. We calculate the
heat capacity for such a quark star, and the results show that the residual
thermal energy cannot sustain the observed thermal X-ray luminosities seen in
typical isolated X-ray pulsars. We conclude that other heating mechanisms must
be in operation if the pulsars are in fact solid quark stars. Two possible
heating mechanisms are explored. Firstly, for pulsars with little
magnetospheric activities, accretion from the interstellar medium or from the
material in the associated supernova remnants may power the observed thermal
emission. In the propeller regime, a disk-accretion rate 1% of
the Eddington rate with an accretion onto the stellar surface at a rate of
could explain the observed emission luminosities of the
dim isolated neutron stars and the central compact objects. Secondly, for
pulsars with significant magnetospheric activities, the pulsar spindown
luminosities may have been as the sources of the thermal energy via reversing
plasma current flows. A phenomenological study between pulsar bolometric X-ray
luminosities and the spin energy loss rates presents the probable existence of
a 1/2-law or a linear law, i.e. or
. This result together with the thermal
properties of solid quark stars allow us to calculate the thermal evolution of
such stars. Thermal evolution curves, or cooling curves, are calculated and
compared with the `temperature-age' data obtained from 17 active X-ray pulsars.
It is shown that the bolometric X-ray observations of these sources are
consistent with the solid quark star pulsar model.Comment: Astroparticle Physics Accepte
Multiple passages of light through an absorption inhomogeneity in optical imaging of turbid media
The multiple passages of light through an absorption inhomogeneity of finite
size deep within a turbid medium is analyzed for optical imaging using the
``self-energy'' diagram. The nonlinear correction becomes more important for an
inhomogeneity of a larger size and with greater contrast in absorption with
respect to the host background. The nonlinear correction factor agrees well
with that from Monte Carlo simulations for CW light. The correction is about
in near infrared for an absorption inhomogeneity with the typical
optical properties found in tissues and of size of five times the transport
mean free path.Comment: 3 figure
The optical/UV excess of isolated neutron stars in the RCS model
The X-ray dim isolated neutron stars (XDINSs) are peculiar pulsar-like
objects, characterized by their very well Planck-like spectrum. In studying
their spectral energy distributions, the optical/UV excess is a long standing
problem. Recently, Kaplan et al. (2011) have measured the optical/UV excess for
all seven sources, which is understandable in the resonant cyclotron scattering
(RCS) model previously addressed. The RCS model calculations show that the RCS
process can account for the observed optical/UV excess for most sources . The
flat spectrum of RX J2143.0+0654 may due to contribution from bremsstrahlung
emission of the electron system in addition to the RCS process.Comment: 6 pages, 2 figures, 1 table, accepted for publication in Research in
Astronomy and Astrophysic
AXPs and SGRs in the outer gap model: confronting Fermi observations
Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are
magnetar candidates, i.e., neutron stars powered by strong magnetic field. If
they are indeed magnetars, they will emit high-energy gamma-rays which are
detectable by Fermi-LAT according to the outer gap model. However, no
significant detection is reported in recent Fermi-LAT observations of all known
AXPs and SGRs. Considering the discrepancy between theory and observations, we
calculate the theoretical spectra for all AXPs and SGRs with sufficient
observational parameters. Our results show that most AXPs and SGRs are
high-energy gamma-ray emitters if they are really magnetars. The four AXPs 1E
1547.0-5408, XTE J1810-197, 1E 1048.1-5937, and 4U 0142+61 should have been
detected by Fermi-LAT. Then there is conflict between out gap model in the case
of magnetars and Fermi observations. Possible explanations in the magnetar
model are discussed. On the other hand, if AXPs and SGRs are fallback disk
systems, i.e., accretion-powered for the persistent emissions, most of them are
not high-energy gamma-ray emitters. Future deep Fermi-LAT observations of AXPs
and SGRs will help us make clear whether they are magnetars or fallback disk
systems.Comment: 15 pages, 3 figures, 1 table, accepted for publication in The
Astrophysical Journa
Understanding the different rotational behaviors of No and No
Total Routhian surface calculations have been performed to investigate
rapidly rotating transfermium nuclei, the heaviest nuclei accessible by
detailed spectroscopy experiments. The observed fast alignment in No
and slow alignment in No are well reproduced by the calculations
incorporating high-order deformations. The different rotational behaviors of
No and No can be understood for the first time in terms of
deformation that decreases the energies of the
intruder orbitals below the N=152 gap. Our investigations reveal the importance
of high-order deformation in describing not only the multi-quasiparticle states
but also the rotational spectra, both providing probes of the single-particle
structure concerning the expected doubly-magic superheavy nuclei.Comment: 5 pages, 4 figures, the version accepted for publication in Phys.
Rev.
Phenomenological analysis of the double pion production in nucleon-nucleon collisions up to 2.2 GeV
With an effective Lagrangian approach, we analyze several NN \to NN\pi\pi
channels by including various resonances with mass up to 1.72 GeV. For the
channels with the pion pair of isospin zero, we confirm the dominance of
N*(1440)\to N\sigma in the near threshold region. At higher energies and for
channels with the final pion pair of isospin one, we find large contributions
from N*(1440)\to \Delta\pi, double-Delta, \Delta(1600) \to N*(1440)\pi,
\Delta(1600) \to \Delta\pi and \Delta(1620) \to \Delta\pi. There are also
sizeable contributions from \Delta \to \Delta\pi, \Delta \to N\pi, N \to
\Delta\pi and nucleon pole at energies close to the threshold. We well
reproduce the total cross sections up to beam energies of 2.2 GeV except for
the pp\to pp\pi^0\pi^0 channel at energies around 1.1 GeV and our results agree
with the existing data of differential cross sections of pp \to pp\pi^+\pi^-,
pp \to nn\pi^+\pi^+ and pp \to pp\pi^0\pi^0 which are measured at CELSIUS and
COSY.Comment: 36 pages, 18 figure
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