2,005 research outputs found
The quark strange star in the enlarged Nambu-Jona-Lasinio model
The strange quark star is investigated within the enlarged SU(3)
Nambu-Jona-Lasinio (NJL). The stable quark star exists till maximal
configutation with rho_m=3.1, 10^15, g/cm^3 with M_m=1.61, M_Sun and R_m=8.74,
km is reached. Strange quarks appear for density above rho_c=9.84 g/cm^3 for
the quark star with radius R_c=8.003 km and M_c=0.77, M_Sun. The comparison of
a quark star properties obtained in the Quark Mean Field (QMF) approach to a
neutron star model constructed within the Relativistic Mean Field (RMF) theory
is presented.Comment: 23 pages, iop latex2e + hyperref, 11 figures, to appear in NJ
One-dimensional pair cascade emission in gamma-ray binaries
In gamma-ray binaries such as LS 5039 a large number of electron-positron
pairs are created by the annihilation of primary very high energy (VHE)
gamma-rays with photons from the massive star. The radiation from these
particles contributes to the total high energy gamma-ray flux and can initiate
a cascade, decreasing the effective gamma-ray opacity in the system. The aim of
this paper is to model the cascade emission and investigate if it can account
for the VHE gamma-ray flux detected by HESS from LS 5039 at superior
conjunction, where the primary gamma-rays are expected to be fully absorbed. A
one-dimensional cascade develops along the line-of-sight if the deflections of
pairs induced by the surrounding magnetic field can be neglected. A
semi-analytical approach can then be adopted, including the effects of the
anisotropic seed radiation field from the companion star. Cascade equations are
numerically solved, yielding the density of pairs and photons. In LS 5039, the
cascade contribution to the total flux is large and anti-correlated with the
orbital modulation of the primary VHE gamma-rays. The cascade emission
dominates close to superior conjunction but is too strong to be compatible with
HESS measurements. Positron annihilation does not produce detectable 511 keV
emission. This study provides an upper limit to cascade emission in gamma-ray
binaries at orbital phases where absorption is strong. The pairs are likely to
be deflected or isotropized by the ambient magnetic field, which will reduce
the resulting emission seen by the observer. Cascade emission remains a viable
explanation for the detected gamma-rays at superior conjunction in LS 5039.Comment: 8 pages, 7 figures, 1 table, accepted for publication in Astronomy
and Astrophysic
The synchrotron peak shift during high-energy flares of blazars
A prediction for the energy shift of the synchrotron spectrum of
flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If
the -ray emission of FSRQs is produced by Comptonization of external
radiation, then the peak of the synchrotron spectrum is predicted to move to
lower energies in the flare state. This is opposite to the well-known broadband
spectral behavior of high-frequency peaked BL-Lac objects where the external
radiation field is believed to be weak and synchrotron-self Compton scattering
might be the dominant -ray radiation mechanism. The synchrotron peak
shift, if observed in FSRQs, can thus be used as a diagnostic to determine the
dominant radiation mechanism in these objects. I suggest a few FSRQs as
promising candidates to test the prediction of the external-Comptonization
model.Comment: 9 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for
ApJ Letters; minor revision
Recommended from our members
Beyond Borders: Charting the Changing Global Reinsurance Landscape
This report is the result of a three-year study of the global reinsurance industry, covering the main stakeholders of cedents, reinsurers, and brokers. It examines the implications of profound change arising from shifts in regulation, consolidation in the key players, and increasing competition both within the reinsurance industry and from alternative capital providers. These changes are driving convergence in a market that was, until recently, characterised by significant cultural variation in buying and selling reinsurance. The findings in this report will help industry participants to take stock of their current position in the industry, the specific types of differentiation available, and how these differentiators can create advantage. Specifically, the report provides evidence-based frameworks and models that firms can use to diagnose their existing strategies and structures and consider alternatives
Gamma-rays from binary system with energetic pulsar and Be star with aspherical wind: PSR B1259-63/SS2883
At least one massive binary system containing an energetic pulsar, PSR
B1259-63/SS2883, has been recently detected in the TeV gamma-rays by the HESS
telescopes. These gamma-rays are likely produced by particles accelerated in
the vicinity of the pulsar and/or at the pulsar wind shock, in comptonization
of soft radiation from the massive star. However, the process of gamma-ray
production in such systems can be quite complicated due to the anisotropy of
the radiation field, complex structure of the pulsar wind termination shock and
possible absorption of produced gamma-rays which might initiate leptonic
cascades. In this paper we consider in detail all these effects. We calculate
the gamma-ray light curves and spectra for different geometries of the binary
system PSR B1259-63/SS2883 and compare them with the TeV gamma-ray
observations. We conclude that the leptonic IC model, which takes into account
the complex structure of the pulsar wind shock due to the aspherical wind of
the massive star, can explain the details of the observed gamma-ray light
curve.Comment: 12 pages, 11 figures, accepted for publication in MNRA
Very high energy gamma-ray emission from X-ray transients during major outbursts
Context: Some high mass X-ray binaries (HMXB) have been recently confirmed as
gamma-ray sources by ground based Cherenkov telescopes. In this work, we
discuss the gamma-ray emission from X-ray transient sources formed by a Be star
and a highly magnetized neutron star. This kind of systems can produce variable
hadronic gamma-ray emission through the mechanism proposed by Cheng and
Ruderman, where a proton beam accelerated in the pulsar magnetosphere impacts
the transient accretion disk. We choose as case of study the best known system
of this class: A0535+26. Aims: We aim at making quantitative predictions about
the very high-energy radiation generated in Be-X ray binary systems with
strongly magnetized neutron stars. Methods: We study the gamma-ray emission
generated during a major X-ray outburst of a HMXB adopting for the model the
parameters of A0535+26. The emerging photon signal from the disk is determined
by the grammage of the disk that modulates the optical depth. The
electromagnetic cascades initiated by photons absorbed in the disk are
explored, making use of the so-called "Approximation A" to solve the cascade
equations. Very high energy photons induce Inverse Compton cascades in the
photon field of the massive star. We implemented Monte Carlo simulations of
these cascades, in order to estimate the characteristics of the resulting
spectrum. Results: TeV emission should be detectable by Cherenkov telescopes
during a major X-ray outburst of a binary formed by a Be star and a highly
magnetized neutron star. The gamma-ray light curve is found to evolve in
anti-correlation with the X-ray signal.Comment: 8 pages, 7 figures, accepted for publication in Astronomy and
Astrophysical journa
Constraints on the Physical Parameters of TeV Blazars
We consider the constraints on the physical parameters of a homogeneous SSC
model that can be derived from the spectral shape and variability of TeV
blazars. Assuming that the relativistic electron spectrum is a broken power
law, where the break energy is a free parameter, we write the
analytical formulae that allow to connect the physical parameters of the model
to observable quantities. The constraints can be summarized in a plane where
the coordinates are the Doppler factor and the magnetic field. The consistency
between the break energy and the balance between cooling and escape and the
interpretation of the soft photon lags measured in some sources as radiative
cooling times are treated as additional independent constraints.
We apply themethod to the case of three well known blazars, PKS 2155-304, Mrk
421 and Mrk 501.Comment: 36 pages, incl. 6 figures in PS format, AAS LaTeX, to be published in
ApJ, Dec 199
- …