Evolution Of GRBs Research Articles

TheSwift Burst Analyser

<i>Context. <i/>Gamma ray burst models predict the broadband spectral evolution and the temporal evolution of the energy flux. In contrast, standard data analysis tools and data repositories provide count-rate data, or use single flux conversion factors for all of the data, neglecting spectral evolution.<i>Aims. <i/>We produce <i>Swift<i/> BAT and XRT light curves in flux units, where the spectral evolution is accounted for.<i>Methods. <i/>We have developed software to use the hardness ratio information to track spectral evolution of GRBs, and thus to convert the count-rate light curves from the BAT and XRT instruments on <i>Swift<i/> into accurate, evolution-aware flux light curves.<i>Results. <i/>The <i>Swift<i/> Burst Analyser website (http://www.swift.ac.uk/burst_analyser) contains BAT, XRT and combined BAT-XRT flux light curves in three energy regimes for all GRBs observed by the <i>Swift<i/> satellite. These light curves are automatically built and updated when data become available, are presented in graphical and plain-text format, and are available for download and use in research.

On the multiwavelength emission from gamma ray burst afterglows

Aims: Drawing an analogy with Active Galactic Nuclei, we investigate the one-zone SSC model of Gamma Ray Bursts afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.

The Nonrelativistic Evolution of GRBs 980703 and 970508: Beaming‐independent Calorimetry

We use the Sedov-Taylor self-similar solution to model the radio emission from the γ-ray bursts GRB 980703 and GRB 970508 when the blast wave has decelerated to nonrelativistic velocities. This approach allows us to infer the energy of the GRBs independent of jet collimation. We find that for GRB 980703 the kinetic energy at the time of the transition to nonrelativistic evolution, tNR ≈ 40 days, is EST ≈ (1-6) × 1051 ergs. For GRB 970508 we find EST ≈ 3 × 1051 ergs at tNR ≈ 100 days, nearly an order of magnitude higher than the energy formerly derived by Frail, Waxman, and Kulkarni. This is due primarily to revised cosmological parameters and partly to the maximum likelihood fit we use here. Taking into account radiative losses prior to tNR, the inferred energies agree well with those derived from the early, relativistic evolution of the afterglow. Thus, the analysis presented here provides a robust, geometry-independent confirmation that the energy scale of cosmological GRBs is about 5 × 1051 ergs and also shows that the central engine in these two bursts did not produce a significant amount of energy in mildly relativistic ejecta at late times. Furthermore, a comparison to the prompt energy release reveals a wide dispersion in the γ-ray efficiency, strengthening our growing understanding that Eγ is a not a reliable proxy for the total energy.

Wind Interaction Models for Gamma‐Ray Burst Afterglows: The Case for Two Types of Progenitors

Beginning with the $\gamma$-ray bursts GRB 970228 and GRB 970508, a standard model for the interpretation of GRB afterglows emerged involving synchrotron emission from a constant energy blast wave expanding into a constant density, ``interstellar'' medium. However, a massive star origin for GRBs implies a stellar wind environment, probably a Wolf-Rayet star, and we have previously suggested wind interaction models for the afterglows of GRBs 980326, 980519, and 980425/SN 1998bw. Here, we extend the theory of afterglows in winds, considering strong cooling phases, the transition to nonrelativistic motion, jets, and prompt, reverse shock emission. We find that, compared to the interstellar case, the optical prompt emission in the wind case could have a comparable magnitude, but would die off faster. We examine the afterglows of other well-observed GRBs in the context of wind interaction models, and find that GRBs 970228 and 970508 are likely wind interactors. The revision in the nonthermal afterglow emission from GRB 970228 caused by the recognition of late supernova emission favors wind interaction. The radio evolution of GRB 970508 provides especially strong evidence for wind interaction. For GRB 970508, the observations suggest a density that is compatible with that expected in a Wolf-Rayet star wind. Finally, observations of the afterglow evolution of GRBs 990123 and 990510 and the prompt optical emission from GRB 990123 favor interstellar interaction models, which would suggest compact star merger progenitors for these objects.

Spectral evolution of GRBs and the "death line” of the synchrotron shock model

I calculate spectral evolution series for pulses of GRBs, in the BATSE spectral range, for continuous particle injection and cooling by synchrotron, inverse Compton, and adiabatic expansion. The hydrodynamic properties of the relativistic outflow are homogeneous across the emitting region which is a conical jet. The flow is viewed at an angle off its symmetry axis; time delays are taken into account. I discuss the low energy slope part of the spectrum, in view of the recent claim of photon slopes in bright BATSE bursts that are inconsistent with the optically thin synchrotron shock model (SSM).

Distribution of Spectral Characteristics and the Cosmological Evolution of Gamma‐Ray Bursts

We investigate the cosmological evolution of GRBs, using the total gamma ray fluence as a measure of the burst strength. This involves an understanding of the distributions of the spectral parameters of GRBs as well as the total fluence distribution - both of which are subject to detector selection effects. We present new non-parametric statistical techniques to account for these effects, and use these methods to estimate the true distribution of the peak of the nu F_nu spectrum, E_p, from the raw distribution. The distributions are obtained from four channel data and therefore are rough estimates. Here, we emphasize the methods and present qualitative results. Given its spectral parameters, we then calculate the total fluence for each burst, and compute its cumulative and differential distributions. We use these distributions to estimate the cosmological rate evolution of GRBs, for three cosmological models. Our two main conclusions are the following: 1) Given our estimates of the spectral parameters, we find that there may exist a significant population of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving rate density quite different from that of other extragalactic objects; in particular, it is different from the recently determined star formation rate.