Strong GeV Emission Accompanying TeV Blazar H1426+428

For High frequency BL Lac objects (HBLs) like H1426+428, a significant fraction of their TeV gamma-rays emitted are likely to be absorbed in interactions with the diffuse IR background, yielding $e^\pm$ pairs. The resulting $e^\pm$ pairs generate one hitherto undiscovered GeV emission by inverse Compton scattering with the cosmic microwave background photons (CMBPs). We study such emission by taking the 1998-2000 CAT data, the reanalyzed 1999 & 2000 HEGRA data and the corresponding intrinsic spectra proposed by Aharonian et al. (2003a). We numerically calculate the scattered photon spectra for different intergalactic magnetic field (IGMF) strengths. If the IGMF is about $10^{-18}{\rm G}$ or weaker, there comes very strong GeV emission, whose flux is far above the detection sensitivity of the upcoming satellite GLAST! Considered its relatively high redshift ($z=0.129$), the detected GeV emission in turn provides us a valuable chance to calibrate the poor known spectral energy distribution of the intergalactic infrared background, or provides us some reliable constraints on the poorly known IGMF strength.Comment: 5 pages, 1 figure. A&A in Pres

Hyperaccretion Disks around Neutron Stars

(Abridged) We here study the structure of a hyperaccretion disk around a neutron star. We consider a steady-state hyperaccretion disk around a neutron star, and as a reasonable approximation, divide the disk into two regions, which are called inner and outer disks. The outer disk is similar to that of a black hole and the inner disk has a self-similar structure. In order to study physical properties of the entire disk clearly, we first adopt a simple model, in which some microphysical processes in the disk are simplified, following Popham et al. and Narayan et al. Based on these simplifications, we analytically and numerically investigate the size of the inner disk, the efficiency of neutrino cooling, and the radial distributions of the disk density, temperature and pressure. We see that, compared with the black-hole disk, the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity. Finally, we consider an elaborate model with more physical considerations about the thermodynamics and microphysics in the neutron star disk (as recently developed in studying the neutrino-cooled disk of a black hole), and compare this elaborate model with our simple model. We find that most of the results from these two models are basically consistent with each other.Comment: 44 pages, 10 figures, improved version following the referees' comments, main conclusions unchanged, accepted for publication in Ap

Spectrum and Duration of Delayed MeV-GeV Emission of Gamma-Ray Bursts in Cosmic Background Radiation Fields

We generally analyze prompt high-energy emission above a few hundreds of GeV due to synchrotron self-Compton scattering in internal shocks. However, such photons cannot be detected because they may collide with cosmic infrared background photons, leading to electron/positron pair production. Inverse-Compton scattering of the resulting electron/positron pairs off cosmic microwave background photons will produce delayed MeV-GeV emission, which may be much stronger than a typical high-energy afterglow in the external shock model. We expand on the Cheng & Cheng model by deriving the emission spectrum and duration in the standard fireball shock model. A typical duration of the emission is ~ 10^3 seconds, and the time-integrated scattered photon spectrum is nu^{-(p+6)/4}, where p is the index of the electron energy distribution behind internal shocks. This is slightly harder than the synchrotron photon spectrum, nu^{-(p+2)/2}. The lower energy property of the scattered photon spectrum is dependent on the spectral energy distribution of the cosmic infrared background radiation. Therefore, future observations on such delayed MeV-GeV emission and the higher-energy spectral cutoff by the Gamma-Ray Large Area Space Telescope (GLAST) would provide a probe of the cosmic infrared background radiation.Comment: 5 pages, accepted for publication in Ap

Can the Bump be Observed in the Early Afterglow of GRBS with X-Ray Line Emission Features?

Extremely powerful emission lines are observed in the X-ray afterglow of several GRBs. The energy contained in the illuminating continuum which is responsible for the line production exceeds 10$^{51}$ erg, much higher than that of the collimated GRBs. It constrains the models which explain the production of X-ray emission lines. In this paper, We argue that this energy can come from a continuous postburst outflow. Focusing on a central engine of highly magnetized millisecond pulsar or magnetar we find that afterglow can be affected by the illuminating continuum, and therefore a distinct achromatic bump may be observed in the early afterglow lightcurves. With the luminosity of the continuous outflow which produces the line emission, we define the upper limit of the time when the bump feature appears. We argue that the reason why the achromatic bumps have not been detected so far is that the bumps should appear at the time too early to be observed.Comment: 13 pags, 2 tables, appear in v603 n1 pt1 ApJ March 1, 2004 issu

Early photon-shock interaction in stellar wind: sub-GeV photon flash and high energy neutrino emission from long GRBs

For gamma-ray bursts (GRBs) born in a stellar wind, as the reverse shock crosses the ejecta, usually the shocked regions are still precipitated by the prompt MeV \gamma-ray emission. Because of the tight overlapping of the MeV photon flow with the shocked regions, the optical depth for the GeV photons produced in the shocks is very large. These high energy photons are absorbed by the MeV photon flow and generate relativistic e^\pm pairs. These pairs re-scatter the soft X-ray photons from the forward shock as well as the prompt \gamma-ray photons and power detectable high energy emission, significant part of which is in the sub-GeV energy range. Since the total energy contained in the forward shock region and the reverse shock region are comparable, the predicted sub-GeV emission is independent on whether the GRB ejecta are magnetized (in which case the reverse shock IC and synchrotron self-Compton emission is suppressed). As a result, a sub-GeV flash is a generic signature for the GRB wind model, and it should be typically detectable by the future {\em Gamma-Ray Large Area Telescope} (GLAST). Overlapping also influence neutrino emission. Besides the 10^{15} \sim 10^{17} eV neutrino emission powered by the interaction of the shock accelerated protons with the synchrotron photons in both the forward and reverse shock regions, there comes another $10^{14}$eV neutrino emission component powered by protons interacting with the MeV photon flow. This last component has a similar spectrum to the one generated in the internal shock phase, but the typical energy is slightly lower.Comment: 7 pages, accepted for publication in Ap

Gamma-ray bursts: postburst evolution of fireballs

The postburst evolution of fireballs that produce $\gamma$-ray bursts is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Due to adoption of a new relation among $t$, $R$ and $\gamma$ (see the text), our results differ markedly from the previous studies. Synchrotron radiation from the shocked interstellar medium is attentively calculated, using a convenient set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory approach to observations. It is also found that the expansion will no longer be highly relativistic about 4 days after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.Comment: 17 pages, 4 figures, MNRAS in pres