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Register a new userGamma-ray Bursts: Light on the distant Universe
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Added by
Jonathan E. Grindlay
Publication date
2008
fields
Physics
and research's language is
English
Authors
Jonathan Grindlay
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Observations of a long-lasting Gamma-ray burst, one that has the brightest optical counterpart yet discovered, challenge theoretical understanding of these bursts but may enhance their usefulness as cosmic probes.
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We review recent results on the high-redshift universe and the cosmic evolution obtained using Gamma Ray Bursts (GRBs) as tracers of high-redshift galaxies. Most of the results come from photometric and spectroscopic observations of GRB host galaxies once the afterglow has faded away but also from the analysis of the GRB afterglow line of sight as revealed by absorptions in their optical spectrum.
We present the analysis of a large sample of gamma-ray burst (GRB) X-ray light curves in the rest frame to characterise their intrinsic properties in the context of different theoretical scenarios. We determine the morphology, time scales, and energetics of 64 long GRBs observed by emph{Swift}/XRT emph{without} flaring activity. We furthermore provide a one-to-one comparison to the properties of GRBs emph{with} X-ray flares. We find that the steep decay morphology and its connection with X-ray flares favour a scenario in which a central engine origin. We show that this scenario can also account for the shallow decay phase, provided that the GRB progenitor star has a self-similar structure with a constant envelope-to-core mass ratio $sim 0.02-0.03$. However, difficulties arise for very long duration ($t_pgtrsim10^4$ s) shallow phases. Alternatively, a spinning-down magnetar whose emitted power refreshes the forward shock can quantitatively account for the shallow decay properties. In particular we demonstrate that this model can account for the plateau luminosity vs. end time anticorrelation.
If gamma-ray burst prompt emission originates at a typical radius, and if material producing the emission moves at relativistic speed, then the variability of the resulting light curve depends on the viewing angle. This is due to the fact that the pulse evolution time scale is Doppler contracted, while the pulse separation is not. For off-axis viewing angles $theta_{rm view} gtrsim theta_{rm jet} + Gamma^{-1}$, the pulse broadening significantly smears out the light curve variability. This is largely independent of geometry and emission processes. To explore a specific case, we set up a simple model of a single pulse under the assumption that the pulse rise and decay are dominated by the shell curvature effect. We show that such a pulse observed off-axis is (i) broader, (ii) softer and (iii) displays a different hardness-intensity correlation with respect to the same pulse seen on-axis. For each of these effects, we provide an intuitive physical explanation. We then show how a synthetic light curve made by a superposition of pulses changes with increasing viewing angle. We find that a highly variable light curve, (as seen on-axis) becomes smooth and apparently single-pulsed (when seen off-axis) because of pulse overlap. To test the relevance of this fact, we estimate the fraction of off-axis gamma-ray bursts detectable by textit{Swift} as a function of redshift, finding that a sizable fraction (between 10% and 80%) of nearby ($z<0.1$) bursts are observed with $theta_{rm view} gtrsim theta_{rm jet} + Gamma^{-1}$. Based on these results, we argue that low luminosity gamma-ray bursts are consistent with being ordinary bursts seen off-axis.
It is now more than 40 years since the discovery of gamma-ray bursts (GRBs) and in the last two decades there has been major progress in the observations of bursts, the afterglows and their host galaxies. This recent progress has been fueled by the ability of gamma-ray telescopes to quickly localise GRBs and the rapid follow-up observations with multi-wavelength instruments in space and on the ground. A total of 674 GRBs have been localised to date using the coded aperture masks of the four gamma-ray missions, BeppoSAX, HETE II, INTEGRAL and Swift. As a result there are now high quality observations of more than 100 GRBs, including afterglows and host galaxies, revealing the richness and progress in this field. The observations of GRBs cover more than 20 orders of magnitude in energy, from 10^-5 eV to 10^15 eV and also in two non-electromagnetic channels, neutrinos and gravitational waves. However the continuation of progress relies on space based instruments to detect and rapidly localise GRBs and distribute the coordinates.
Gamma-Ray Bursts (GRBs) are the most powerful cosmic explosions since the Big Bang, and thus act as signposts throughout the distant Universe. Over the last 2 decades, these ultra-luminous cosmological explosions have been transformed from a mere curiosity to essential tools for the study of high-redshift stars and galaxies, early structure formation and the evolution of chemical elements. In the future, GRBs will likely provide a powerful probe of the epoch of reionisation of the Universe, constrain the properties of the first generation of stars, and play an important role in the revolution of multi-messenger astronomy by associating neutrinos or gravitational wave (GW) signals with GRBs. Here, we describe the next steps needed to advance the GRB field, as well as the potential of GRBs for studying the Early Universe and their role in the up-coming multi-messenger revolution.
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