41,216 research outputs found
Pulsating White Dwarfs
The Sloan Digital Sky Survey has allowed us to increase the number of known
white dwarfs by a factor of five and consequently the number of known pulsating
white dwarfs also by a factor of five. It has also led to the discovery of new
types of variable white dwarfs, as the variable hot DQs, and the pulsating
Extremely Low Mass white dwarfs. With the Kepler Mission, it has been possible
to discover new phenomena, the outbursts present in a few pulsating white
dwarfs.Comment: 10 pages, 2 figures, Wide-field variability surveys: a 21st-century
perspective, 22nd Los Alamos Stellar Pulsation Conference Series meeting, San
Pedro de Atacama, Chile, Nov. 28 - Dec. 2, 201
A model for jets of low-mass microquasars
In this work we present a new jet model for the non-thermal broadband
emission of low-mass microquasars. We calculate the contribution of
relativistic particles, primary electrons and protons as well as secondary
muons, charged pions and electron-positron pairs, to the electromagnetic
spectrum of the sources. The distribution in energy of all particle species is
obtained for an extended, inhomogeneous region. We include detailed analysis of
particle energy losses, injection, decay and escape from the acceleration zone.
We also calculate absorption effects due to photon-photon annihilation. As an
application, we consider the case of XTE J1118+480, a well-known low-mass X-ray
binary in the galactic halo, and we present predictions about its high-energy
radiationComment: 7 pages, 6 figures. Proceedings of the 25th Texas Symposium on
Relativistic Astrophysics - TEXAS 2010, December 06-10, 2010, Heidelberg,
German
Leptonic/hadronic models for electromagnetic emission in microquasars: the case of GX 339-4
We present a general self-consistent lepto/hadronic jet model for the
non-thermal electromagnetic emission of microquasars. The model is applied to
the low-mass microquasar (LMMQ) GX 339-4 and predicts its high-energy features.
We assume that both leptons and hadrons are accelerated up to relativistic
energies by diffusive shock acceleration, and calculate their contribution to
the electromagnetic spectrum through all main radiative processes. The
radiative contribution of secondary particles (pions, muons and
electron-positron pairs) is included. We use a set of simultaneous observations
in radio and X-rays to constrain the model parameters and find the best fit to
the data. We obtain different spectral energy distributions that can explain
the observations, and make predictions for the high-energy emission.
Observations with gamma-ray instruments like Fermi can be used to test the
model and determine the proton content of the jets. Finally, we estimate the
positron injection in the surrounding medium. Our findings support the
suggested association between LMMQs and the observed distribution of the 511
keV line flux observed by INTEGRAL.Comment: 13 pages, 7 figures, accepted for publication in MNRA
Quantum transport through single and multilayer icosahedral fullerenes
We use a tight-binding Hamiltonian and Green functions methods to calculate
the quantum transmission through single-wall fullerenes and bilayered and
trilayered onions of icosahedral symmetry attached to metallic leads. The
electronic structure of the onion-like fullerenes takes into account the
curvature and finite size of the fullerenes layers as well as the strength of
the intershell interactions depending on to the number of interacting atom
pairs belonging to adjacent shells. Misalignment of the symmetry axes of the
concentric icosahedral shells produces breaking of the level degeneracies of
the individual shells, giving rise some narrow quasi-continuum bands instead of
the localized discrete peaks of the individual fullerenes. As a result, the
transmission function for non symmetrical onions are rapidly varying functions
of the Fermi energy. Furthermore, we found that most of the features of the
transmission through the onions are due to the electronic structure of the
outer shell with additional Fano-like antiresonances arising from coupling with
or between the inner shells.Comment: 16 pages, 5 figur
Models for gamma-ray production in low-mass microquasars
Unlike high-mass gamma-ray binaries, low-mass microquasars lack external
sources of radiation and matter that could produce high-energy emission through
interactions with relativistic particles. In this work we consider the
synchrotron emission of protons and leptons that populate the jet of a low-mass
microquasar. In our model photohadronic and inverse Compton (IC) interactions
with synchrotron photons produced by both protons and leptons result in a
high-energy tail of the spectrum. We also estimate the contribution from
secondary pairs injected through photopair production. The high-energy emission
is dominated by radiation of hadronic origin, so we can call these objects
proton microquasars.Comment: 4 pages, 2 figures, accepted for publication in the International
Journal of Modern Physics D, proceedings of HEPRO meeting, held in Dublin, in
September 200
High-energy signatures of binary systems of supermassive black holes
Context. Binary systems of supermassive black holes are expected to be strong
sources of long gravitational waves prior to merging. These systems are good
candidates to be observed with forthcoming space-borne detectors. Only a few of
these systems, however, have been firmly identified to date.
Aims. We aim at providing a criterion for the identification of some
supermassive black hole binaries based on the characteristics of the
high-energy emission of a putative relativistic jet launched from the most
massive of the two black holes.
Methods. We study supermassive black hole binaries where the less massive
black hole has carved an annular gap in the circumbinary disk, but nevertheless
there is a steady mass flow across its orbit. Such a perturbed disk is hotter
and more luminous than a standard thin disk in some regions. Assuming that the
jet contains relativistic electrons, we calculate its broadband spectral energy
distribution focusing on the inverse Compton up-scattering of the disk photons.
We also compute the opacity to the gamma rays produced in the jet by photon
annihilation with the disk radiation and take into account the effects of the
anisotropy of the target photon field as seen from the jet.
Results. We find that the excess of low-energy photons radiated by the
perturbed disk causes an increment in the external Compton emission from the
jet in the X-ray band, and a deep absorption feature at energies of tens of
TeVs for some sets of parameters. According to our results, observations with
Cherenkov telescopes might help in the identification of supermassive black
hole binaries, especially those black hole binaries that host primaries from
tens to hundreds of million of solar masses.Comment: 12 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
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