Radio Afterglows Of Gamma-ray Bursts Research Articles

Radio afterglow rebrightening: evidence for multiple active phases in gamma-ray burst central engines

The rebrightening phenomenon is an interesting feature in some X-ray, optical, and radio afterglows of gamma-ray bursts (GRBs). Here, we propose a possible energy-supply assumption to explain the rebrightenings of radio afterglows, in which the central engine with multiple active phases can supply at least two GRB pulses in a typical GRB duration time. Considering the case of double pulses supplied by the central engine, the double pulses have separate physical parameters, except for the number density of the surrounding interstellar medium (ISM). Their independent radio afterglows are integrated by the ground detectors to form the rebrightening phenomenon. In this work, we firstly simulate diverse rebrightening light curves under consideration of different and independent physical parameters. Using this assumption, we also give our best fit to the radio afterglow of GRB 970508 at three frequencies of 1.43, 4.86, and 8.46 GHz. We suggest that the central engine may be active continuously at a timescale longer than that of a typical GRB duration time as many authors have suggested, and that it may supply enough energy to cause the long-lasting rebrightenings observed in some GRB afterglows.

Detecting radio afterglows of gamma-ray bursts with FAST

Using the generic hydrodynamic model of gamma-ray burst (GRB) afterglows, we calculate the radio afterglow light curves of low luminosity, high luminosity, failed and standard GRBs in different observational bands of FAST's energy window. The GRBs are assumed to be located at different distances from us. Our results rank the detectability of GRBs in descending order as high luminosity, standard, failed and low luminosity GRBs. We predict that almost all types of radio afterglows except those of low luminosity GRBs could be observed by a large radio telescope as long as the domains of time and frequency are appropriate. It is important to note that FAST can detect relatively weak radio afterglows at a higher frequency of 2.5 GHz for very high redshift up to z = 15 or even more. Radio afterglows of low luminosity GRBs can only be detected after the completion of the second phase of FAST. FAST is expected to significantly expand the sample of GRB radio afterglows in the near future.

The earliest galaxies seen in 21 cm line absorption

We investigate the 21 cm absorption lines produced by non-linear structures during the early stage of reionization, i.e. the starless minihaloes and the dwarf galaxies. After a detailed modelling of their properties, with particular attention to the coupling physics, we determine their 21 cm absorption line profiles. The infalling gas velocity around minihaloes/dwarf galaxies strongly affects the line shape and, with the low spin temperatures outside the virial radii of the systems, gives rise to horn-like line profiles. The optical depth of a dwarf galaxy is reduced for lines of sight penetrating through its H ii region and, especially, a large H ii region created by a dwarf galaxy with higher stellar mass and/or a top-heavy initial mass function results in an optical depth trough rather than an absorption line. We compute synthetic spectra of 21 cm forest for both high-redshift quasars and radio afterglows of gamma-ray bursts (GRBs). Even with the planned Square Kilometre Array (SKA), radio afterglows of most if not all GRBs would still be too dim to be the background sources for high-resolution (1 kHz) observations, but absorption lines can be easily detected towards a high-z quasar. Broadband observation against GRB afterglows can also be used to reveal the evolving 21 cm signal from both minihaloes and dwarf galaxies if there was no X-ray background or it was extremely weak, but it becomes difficult if an early X-ray background existed. Hence, the 21 cm absorption could be a powerful probe of the presence/intensity of the X-ray background and the thermal history of the early Universe.

Radio and millimeter continuum surveys and their astrophysical implications

We review the statistical properties of the main populations of radio sources, as emerging from radio and millimeter sky surveys. Recent determinations of local luminosity functions are presented and compared with earlier estimates still in widespread use. A number of unresolved issues are discussed. These include: the (possibly luminosity-dependent) decline of source space densities at high redshifts; the possible dichotomies between evolutionary properties of low- versus high-luminosity and of flat- versus steep-spectrum AGN-powered radio sources; and the nature of sources accounting for the upturn of source counts at sub-mJy levels. It is shown that straightforward extrapolations of evolutionary models, accounting for both the far-IR counts and redshift distributions of star-forming galaxies, match the radio source counts at flux-density levels of tens of microJy remarkably well. We consider the statistical properties of rare but physically very interesting classes of sources, such as GHz Peak Spectrum and ADAF/ADIOS sources, and radio afterglows of gamma-ray bursts. We also discuss the exploitation of large-area radio surveys to investigate large scale structure through studies of clustering and the Integrated Sachs-Wolfe effect. Finally we briefly describe the potential of the new and forthcoming generations of radio telescopes. A compendium of source counts at different frequencies is given in an appendix.

The Radio Afterglow and Host Galaxy of the Dark GRB 020819

Of the fourteen gamma-ray bursts (GRBs) localized to better than 2' radius with the SXC on HETE-2, only two lack optical afterglow detections, and the high recovery rate among this sample has been used to argue that the fraction of truly dark bursts is ~10%. While a large fraction of earlier dark bursts can be explained by the failure of ground-based searches to reach appropriate limiting magnitudes, suppression of the optical light of these SXC dark bursts seems likely. Here we report the discovery and observation of the radio afterglow of GRB 020819, an SXC dark burst, which enables us to identify the likely host galaxy (probability of 99.2%) and hence the redshift (z=0.41) of the GRB. The radio light curve is qualitatively similar to that of several other radio afterglows, and may include an early-time contribution from the emission of the reverse shock. The proposed host is a bright R = 19.5 mag barred spiral galaxy, with a faint R ~ 24.0 mag "blob'' of emission, 3" from the galaxy core (16 kpc in projection), that is coincident with the radio afterglow. Optical photometry of the galaxy and blob, beginning 3 hours after the burst and extending over more than 100 days, establishes strong upper limits to the optical brightness of any afterglow or associated supernova. Combining the afterglow radio fluxes and our earliest R-band limit, we find that the most likely afterglow model invokes a spherical expansion into a constant-density (rather than stellar wind-like) external environment; within the context of this model, a modest local extinction of A_V ~ 1 mag is sufficient to suppress the optical flux below our limits.

Radio Afterglows of Gamma‐Ray Bursts and Hypernovae at High Redshift and Their Potential for 21 Centimeter Absorption Studies

We investigate the radio afterglows of gamma-ray bursts (GRBs) and hypernovae (HNe) at high redshifts and quantify their detectability, as well as their potential usefulness for 21 cm absorption line studies of the intergalactic medium (IGM) and intervening structures. We examine several sets of source and environment model parameters that are physically plausible at high redshifts. The radio afterglows of GRBs would be detectable out to z ~ 30, while the energetic HNe could be detectable out to z ~ 20 even by the current Very Large Array (VLA). We find that the 21 cm absorption line due to the diffuse neutral IGM is difficult to detect even by the proposed Square Kilometer Array (SKA), except for highly energetic sources. We also find that the 21 cm line due to collapsed gas clouds with high optical depth may be detected on rare occasions.

Gamma-Ray bursts: accumulating afterglow implications, progenitor clues, and prospects.

Gamma-ray bursts (GRBs) are sudden, intense flashes of gamma rays that, for a few blinding seconds, light up in an otherwise fairly dark gamma-ray sky. They are detected at the rate of about once a day, and while they are on, they outshine every other gamma-ray source in the sky, including the sun. Major advances have been made in the last 3 or 4 years, including the discovery of slowly fading x-ray, optical, and radio afterglows of GRBs, the identification of host galaxies at cosmological distances, and evidence showing that many GRBs are associated with star-forming regions and possibly supernovae. Progress has been made in understanding how the GRB and afterglow radiation arises in terms of a relativistic fireball shock model. These advances have opened new vistas and questions on the nature of the central engine, the identity of their progenitors, the effects of the environment, and their possible gravitational wave, cosmic ray, and neutrino luminosity. The debates on these issues indicate that GRBs remain among the most mysterious puzzles in astrophysics.