Synchrotron Radiation as the Source of GRB Spectra, Part I: Theory

We investigate synchrotron emission models as the source of gamma ray burst spectra. We show that allowing for synchrotron self absorption and a ``smooth cutoff'' to the electron energy distribution produces a wide range of low energy spectral behavior. We show that there exists a correlation between the value of the peak of the $\nu F_{\nu}$ spectrum, $E_{p}$, and the low energy spectral index $\alpha$ as determined by spectral fits over a finite bandwidth. Finally, we discuss the implications of synchrotron emission from internal shocks for GRB spectral evolution.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma Ray Burst

On the Kinetic Energy and Radiative Efficiency of Gamma-Ray Bursts

Using measured X-ray luminosities to 17 Gamma-Ray Bursts (GRBs) during the afterglow phase and accounting for radiative losses, we calculate the kinetic energy of these bursts and investigate its relation to other GRB properties. We then use the observed radiated energy during the prompt phase to determine the radiative efficiency of these bursts, and explore how the efficiency relates to other GRB observables. We find that the kinetic energy in the afterglow phase is directly correlated with the radiated energy, total energy as well as possibly the jet opening angle and spectral peak energy. More importantly, we find the intriguing fact that the efficiency is correlated with the radiated energy, and mildly with the total energy, jet opening angle and spectral peak energy. XRF020903 also seems to follow the trends we find for our GRB sample. We discuss the implications of these results for the GRB radiation and jet models.Comment: 9 pages, 7 figures; Revised version, accepted to Ap

Towards an Understanding of GRB Prompt Emission

We discuss the prompt emission of Gamma-Ray Bursts in different spectral energy bands. First, we suggest that a three-part synchrotron emission model is a good description of the ~20 keV - 1 MeV gamma-ray emission of GRBs. We show that this model provides excellent fits to the data and naturally explains the observed global correlations between spectral parameters. In particular, we show there exists a negative correlation between between the peak of the nu-Fnu spectrum, Ep, and the low energy photon index \alpha for bursts with -2/3 < \alpha < 0, and suggest that this correlation is due to the mechanism responsible for producing \alpha's above the value of -2/3 - namely, a decreasing mean pitch angle of the electrons. We then discuss the physical origin of the increasing number of GRBs that are observed to peak in the X-ray energy band (~5-40 keV). Although either a cosmological (i.e. high redshift) or intrinsic interpretation for the low values of Ep is viable at this point, the data appear to suggest that intrinsic effects are playing the dominant role. Finally, we briefly comment on the prompt GRB optical emission (~ eV) and very high energy emission (>10 MeV), and how these spectral bands may be used to place additional constraints on the physics of gamma-ray bursts.Comment: Invited talk at the 2001 Woodshole meeting,"Gamma-Ray Bursts and Afterglow Astronomy"; 8 pages including 8 postscript figure

Synchrotron Emission as the Source of GRB Spectra, Part II: Observations

We test the models of synchrotron emission presented in Part I of this series (Lloyd & Petrosian, these proceedings) against the distributions and evolution of GRB spectral parameters (particularly the low energy index, $\alpha$). With knowledge of the $E_{p}$ distribution and the correlation between $\alpha$ and $E_{p}$ presented in Part I, we show how to derive the expected distribution of $\alpha$ from fits to optically thin synchrotron spectra, and compare this with the observed distribution. We show that there is no difficulty explaining bursts below the ``line of death'', $\alpha < -2/3$, and that these bursts indicate that the spectrum of accelerated electrons must flatten or decline at low energies. Bursts with low energy spectral indices that fall above this limit are explained by the synchrotron self-absorption frequency entering the lower end of the BATSE window. Finally, we discuss a variety of spectral evolution behavior seen in GRBs and explain this behavior in the context of synchrotron emission from internal shocks.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma Ray Burst

Interpreting the Behavior of Time Resolved Gamma-Ray Burst Spectra

In this paper, we explore time resolved Gamma-Ray Burst (GRB) spectra in the context of the synchrotron emission model presented in Lloyd and Petrosian (2000; LP00). First, we show that our model - which involves three distinct emission regimes - can provide excellent fits to the time resolved spectra of GRBs, and we present these results for a few bursts. We then describe how the phenomenological Band spectrum (Band et al., 1993) can be interpreted in the context of our models based on the value of the low energy photon index $\alpha$. We discuss the types of correlations one would expect to observe among the Band parameters if these models are correct. We then compare these predictions to the existing data, combining a sample of 2,026 time resolved spectra (from approximately 80 bursts). We show that the correlations found in the data are consistent with the models, and discuss the constraints they place on the emission physics. In particular, we find a 4-sigma negative correlation between the peak of the nu-Fnu spectrum, Ep, and the low energy photon index $\alpha$ for bursts with $-2/3 < \alpha < 0$, in contrast to what is predicted by the instrumental effect discussed in LP00. We suggest that this correlation is simply due to the mechanism responsible for producing $\alpha$'s above the value of -2/3 - namely, a decreasing mean pitch angle of the electrons. We also show that Ep is correlated with the photon flux, and interpret this as a result of changing magnetic field or characteristic electron energy between emission episodes. Finally, we discuss the implications our results have on particle acceleration in GRBs, and prospects for further testing these models with the anticipated data from HETE-2, Swift and GLAST.Comment: 17 pages, including 13 figures. Accepted to Ap