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

The proton microquasar

We present a model for high-energy emission in microquasars where the energy content of the jets is dominated by relativistic protons. We also include a primary leptonic component. Particles are accelerated up to relativistic energies in a compact region located near the base of the jet, where most of the emission is produced. We calculate the production spectrum due to proton and electron synchrotron radiation and photohadronic interactions. The target field for proton-photon collisions is provided by the synchrotron radiation in the acceleration region. In models with a significant leptonic component, strong internal photon-photon absorption can attenuate the emission spectrum at high energies. Depending on the values of the parameters, our model predicts luminosities in the range 10^34-10^37 erg s^-1 up to GeV energies, with a high-energy tail that can extend up to 10^16 eV. In some cases, however, absorption effects can completely suppress the emission above 10 GeV, giving rise to different spectral shapes. These results can be tested in the near future by observations with instruments like GLAST-Fermi, HESS II and MAGIC II.Comment: 4 pages, 3 figures, contribution to the "Fourth Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy 2008

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