What is the Astrophysical Meaning of the Intermediate Subgroup of GRBs?

Published articles concerning the intermediate (third) subgroup of GRBs are surveyed. From a statistical perspective this subgroup may exist, however its significance depends on which data set is used. Its astrophysical meaning is unclear because the occurrence of this subgroup can also be an artificial selection effect. Hence, GRBs from this subgroup need not be given by a physically different phenomenon. The aim of this contribution is to search for the answer to the question in the title.Comment: journal: Proceedings of Science, Swift: 10 Years of Discovery; conference date: 2-5 December 2014; location: La Sapienza University, Rome, Italy; 6 pages, 4 figures, 1 table; accepted for publication in July 9 201

A curious relation between the flat cosmological model and the elliptic integral of the first kind

The dependence of the luminosity distance on the redshift has a key importance in the cosmology. This dependence can well be given by standard functions for the zero cosmological constant. The purpose of this article is to present such a relation also for the non-zero cosmological constant, if the universe is spatially flat. A definite integral is used. The integration ends in the elliptic integral of the first kind. The result shows that no numerical integration is needed for the non-zero cosmological constant, if the universe is spatially flat.Comment: Accepted for publication in Astronomy & Astrophysics. 2 page

GLAST Prospects for Swift-Era Afterglows

We calculate the GeV spectra of GRB afterglows produced by inverse Compton scattering of the sub-MeV emission of these objects. We improve on earlier treatments by using refined afterglow parameters and new model developments motivated by recent Swift observations. We present time-dependent GeV spectra for standard, constant parameter models, as well as for models with energy injection and with time-varying parameters, for a range of burst parameters. We evaluate the limiting redshift to which such afterglows can be detected by the GLAST LAT, as well as AGILE.Comment: 19 pages, 5 figures, ApJ, in pres

Delayed Gev Emission from Cosmological Gamma-Ray Bursts : Impact of a Relativistic Wind on External Matter

Sudden collapse of a compact object, or coalescence of a compact binary, can generate an unsteady relativistic wind that lasts for a few seconds. The wind is likely to carry a high magnetic field; and its Lorentz factor depends on the extent to which it is 'loaded' with baryons. If the Lorentz factor is $\sim 100$, internal dissipation and shocks in this wind produce a non-thermal gamma-ray burst, detectable in the range 0.1\MeV \siml E_\gamma \siml 0.1-1\GeV out to cosmological distances. The cooled wind ejecta would subsequently be decelerated by the external medium. The resultant blast wave and reverse shock can then give rise to a second burst component, mainly detectable in the GeV range, with a time delay relative to the MeV burst ranging from minutes to hours.Comment: 5 pages, plain Te

Gamma-Ray Bursts: Multiwaveband Spectral Predictions for Blast Wave Models

In almost any scenario for 'cosmological' gamma-ray bursts (and in many models where they originate in our own Galaxy), the initial energy density is so large that the resulting relativistic plasma expands with $v\sim c$ producing a blast wave ahead of it and a reverse shock moving into the ejecta, as it ploughs into the external medium. We evaluate the radiation expected from these shocks,for both cosmological and galactic bursts, for various assumptions about the strength of the magnetic field and the particle acceleration mechanisms in the shocks. The spectra are evaluated over the whole range from the IR to $>$ GeV, and are compared with the variety of spectral behavior reported by BATSE, and with the X-ray and optical constraints. For bursts of duration \simg 1\s acceptable $\gamma$-ray spectra and $L_x/L_\gamma$ ratios are readily obtained for 'cosmological' models. Blast waves in galactic models can produce bursts of similar gamma-ray fluence and duration, but they violate the X-ray paucity constraint, except for the shorter bursts (\siml 1\s). We discuss the prospects for using O/UV and X-ray observations to discriminate among alternative models.Comment: 7 pages with one figure (figure in uuencoded compressed postscript file),te

The Edge of a Gamma Ray Burst Afterglow

We discuss the formation of spectral features in the decelerating ejecta of gamma-ray bursts, including the possible effect of inhomogeneities. These should lead to blueshifted and broadened absorption edges and resonant features, especially from H and He. An external neutral ISM could produce detectable H and He, as well as Fe X-ray absorption edges and lines. Hypernova scenarios may be diagnosed by Fe K-$\alpha$ and H Ly-$\alpha$ emission lines.Comment: M.N.R.A.S., accepted July 16 1998; submitted June 4 1998; latex, 11 page

Gamma-ray bursts as X-ray depth-gauges of the Universe

We discuss the X-ray flux of gamma-ray burst afterglows at redshifts in the range 3-30, including the effects of the intergalactic He II absorption. We point out that strong X-ray lines may form locally in burst afterglows starting minutes after the trigger. This can provide distinctive X-ray distance indicators out to the redshifts where the first generation of massive stars form.Comment: ApJ(Lett) in press 5/31/03; subm. 5/7/0