An Interpretation of the Evidence for TeV Emission from Gamma-Ray Burst 970417a

The Milagrito collaboration recently reported evidence for emission of very high energy gamma-rays in the TeV range from one of the BATSE GRBs, GRB 970417a. Here I discuss possible interpretations of this result. Taking into account the intergalactic absorption of TeV gamma-rays by the cosmic infrared background, I found that the detection rate (one per 54 GRBs observed by the Milagrito) and energy fluence can be consistently explained with the redshift of this GRB at z \sim 0.7 and the isotropic total energy in the TeV range, E_{TeV, iso} >~ 10^{54} erg. This energy scale is not unreasonably large, but interestingly similar to the maximum total GRB energy observed to date, in the sub-MeV range for GRB 990123. On the other hand, the energy emitted in the ordinary sub-MeV range becomes E_{MeV, iso} \sim 10^{51} erg for the GRB 970417a, which is much smaller than the total energy in the TeV range by a factor of about 10^3. I show that the proton-synchrotron model of GRBs provides a possible explanation for these observational results. I also discuss some observational signatures expected in the future experiments from this model.Comment: 6 pages, 2 figures, Accepted by ApJ Letter

Cosmological Fast Radio Bursts from Binary Neutron Star Mergers

Fast radio bursts (FRBs) at cosmological distances have recently been discovered, whose duration is about milliseconds. We argue that the observed short duration is difficult to explain by giant flares of soft gamma-ray repeaters, though their event rate and energetics are consistent with FRBs. Here we discuss binary neutron star (NS-NS) mergers as a possible origin of FRBs. The FRB rate is within the plausible range of NS-NS merger rate and its cosmological evolution, while a large fraction of NS-NS mergers must produce observable FRBs. A likely radiation mechanism is coherent radio emission like radio pulsars, by magnetic braking when magnetic fields of neutron stars are synchronized to binary rotation at the time of coalescence. Magnetic fields of the standard strength (~ 10^{12-13} G) can explain the observed FRB fluxes, if the conversion efficiency from magnetic braking energy loss to radio emission is similar to that of isolated radio pulsars. Corresponding gamma-ray emission is difficult to detect by current or past gamma-ray burst satellites. Since FRBs tell us the exact time of mergers, a correlated search would significantly improve the effective sensitivity of gravitational wave detectors.Comment: 4 pages, no figure. Matches the published version in PASJ. References added. This is an open access paper at the PASJ website http://pasj.asj.or.jp/v65/n5/65L012/65L012.pd

Preheating in the Universe Suppressing High Energy Gamma-rays from Structure Formation

Structure formation in the universe can produce high energy gamma-rays from shock-accelerated electrons, and this process may be the origin of the extragalactic gamma-ray background (EGRB) as well as a part of the unidentified sources detected by EGRET in the GeV band, if about 5% of the kinetic energy of the shock is going into electron acceleration. However, we point out that the production of gamma-rays may be severely suppressed if the collapsing matter has been preheated by external entropy sources at the time of gravitational collapse, as can be inferred from the luminosity-temperature (LT) relation of galaxy clusters and groups. We also make a rough estimate of this effect by a simple model, showing that the EGRB flux may be suppressed by a factor of about 30. Hence structure formation is difficult to be the dominant origin of EGRB if preheating is actually responsible for the observed anomary in the LT relation. The detectable number of gamma-ray clusters is also reduced, but about 5-10 forming clusters should still be detectable by EGRET all sky, and this number is similar to that of the steady and high-latitude unidentified sources in the EGRET catalog. The future GLAST mission should detect 10^2-10^3 gamma-ray clusters of galaxies even if the intergalactic medium has been preheated.Comment: References added for relevant work. 5 pages, 4 figures, accepted in Astroparticle Physic

Constraints on Models for TeV Gamma Rays from Gamma-Ray Bursts

We explore several models which might be proposed to explain recent possible detections of high-energy (TeV) gamma rays in association with low-energy gamma-ray bursts (GRBs). Likely values (and/or upper limits) for the source energies in low- and high-energy gamma rays and hadrons are deduced for the burst sources associated with possible TeV gamma-ray detections by the Project GRAND array. Possible spectra for energetic gammas are deduced for three models: 1) inverse-Compton scattering of ambient photons from relativistic electrons; 2) proton-synchrotron emission; and 3) inelastic scattering of relativistic protons from ambient photons creating high-energy neutral pions, which decay into high-energy photons. These models rely on some basic assumptions about the GRB properties, e.g. that: the low- and high-energy gamma rays are produced at the same location; the time variability of the high-energy component can be estimated from the FWHM of the highest peak in the low-energy gamma ray light curve; and the variability-luminosity relation of Fenimore & Ramirez-Ruiz (2000) gives a reliable estimate of the redshifts of these bursts. We also explore the impact of each of these assumptions upon our models. We conclude that the energetic requirements are difficult to satisfy for any of these models unless, perhaps, either the photon beaming angle is much narrower for the high-energy component than for the low-energy GRB or the bursts occur at very low redshifts (z<0.01). Nevertheless, we find that the energetic requirements are most easily satisfied if TeV gamma rays are produced predominantly by inverse-Compton scattering with a magnetic field strength well below equipartition or by proton-synchrotron emission with a magnetic field strength near equipartition.Comment: 39 pages, 7 figures, accepted for publication in Astroparticle Physic