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Register a new userOn The Origin Of High Energy Correlations in Gamma-ray Bursts
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Daniel Kocevski
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I investigate the origin of the observed correlation between a GRBs nuFnu spectral peak Epk and its isotropic equivalent energy Eiso through the use of a population synthesis code to model the prompt gamma-ray emission from GRBs. By using prescriptions for the distribution of prompt spectral parameters as well as the populations luminosity function and co-moving rate density, I generate a simulated population of GRBs and examine how bursts of varying spectral properties and redshift would appear to a gamma-ray detector here on Earth. I find that a strong observed correlation can be produced between the source frame Epk and Eiso for the detected population despite the existence of only a weak and broad correlation in the original simulated population. The energy dependance of a gamma-ray detectors flux-limited detection threshold acts to produce a correlation between the source frame Epk and Eiso for low luminosity GRBs, producing the left boundary of the observed correlation. Conversely, very luminous GRBs are found at higher redshifts than their low luminosity counterparts due to the standard Malquest bias, causing bursts in the low Epk, high Eiso regime to go undetected because their Epk values would be redshifted to energies at which most gamma-ray detectors become less sensitive. I argue that it is this previously unexamined effect which produces the right boundary of the observed correlation. Therefore, the origin of the observed correlation is a complex combination of the instruments detection threshold, the intrinsic cutoff in the GRB luminosity function, and the broad range of redshifts over which GRBs are detected. These simulations serve to demonstrate how selection effects caused by a combination of instrumental sensitivity and the cosmological nature of an astrophysical population can act to produce an artificially strong correlation between observed properties.
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Gamma-ray bursts (GRBs) have been an enigma since their discovery forty years ago. However, considerable progress unraveling their mysteries has been made in recent years. Developments in observations, theory, and instrumentation have prepared the way so that the next decade can be the one in which we finally answer the question, What are gamma-ray bursts? This question encompasses not only what the progenitors are that produce the GRBs, but also how the enormous luminosity of the GRBs, concentrated in gamma rays, is achieved. Observations across the electromagnetic spectrum, from both the ground and space, will be required to fully tackle this important question. This white paper, mostly distilled from a recent study commissioned by the Division of Astrophysics of the American Physical Society, focuses on what very high energy (~100 GeV and above) gamma-ray observations can contribute. Very high energy gamma rays probe the most extreme high energy particle populations in the burst environment, testing models of lepton and proton acceleration in GRBs and constraining the bulk Lorentz factor and opacity of the outflow. Sensitivity improvements of more than an order of magnitude in the very high energy gamma-ray band can be achieved early in the next decade, in order to contribute to this science.
The number of Gamma-Ray Bursts (GRBs) detected at high energies ($sim,0.1-100$ GeV) has seen a rapid increase over the last decade, thanks to observations from the Fermi-Large Area Telescope. The improved statistics and quality of data resulted in a better characterisation of the high-energy emission properties and in stronger constraints on theoretical models. In spite of the many achievements and progresses, several observational properties still represent a challenge for theoretical models, revealing how our understanding is far from being complete. This paper reviews the main spectral and temporal properties of $sim,0.1-100$ GeV emission from GRBs and summarises the most promising theoretical models proposed to interpret the observations. Since a boost for the understanding of GeV radiation might come from observations at even higher energies, the present status and future prospects for observations at very-high energies (above $sim$ 100 GeV) are also discussed. The improved sensitivity of upcoming facilities, coupled to theoretical predictions, supports the concrete possibility for future ground GRB detections in the high/very-high energy domain.
We collect and reanalyze about 200 GRB data of prompt-emission with known redshift observed until the end of 2009, and select 101 GRBs which were well observed to have good spectral parameters to determine the spectral peak energy ($E_p$), 1-second peak luminosity ($L_p$) and isotropic energy ($E_{rm iso}$). Using our newly-constructed database with 101 GRBs, we first revise the $E_p$--$L_p$ and $E_p$--$E_{rm iso}$ correlations. The correlation coefficients of the revised correlations are 0.889 for 99 degree of freedom for the $E_p$--$L_p$ correlation and 0.867 for 96 degree of freedom for the $E_p$--$E_{rm iso}$ correlation. These values correspond to the chance probability of $2.18 times 10^{-35}$ and $4.27 times 10^{-31}$, respectively. It is a very important issue whether these tight correlations are intrinsic property of GRBs or caused by some selection effect of observations. In this paper, we examine how the truncation of the detector sensitivity affects the correlations, and we conclude they are surely intrinsic properties of GRBs. Next we investigate origins of the dispersion of the correlations by studying their brightness and redshift dependence. Here the brightness (flux or fluence) dependence would be regarded as an estimator of the bias due to the detector threshold. We find a weak fluence-dependence in the $E_p$--$E_{rm iso}$ correlations and a redshift dependence in the $E_p$--$L_p$ correlation both with 2 $sigma$ statistical level. These two effects may contribute to the dispersion of the correlations which is larger than the statistical uncertainty. We discuss a possible reason of these dependence and give a future prospect to improve the correlations.
The synchrotron self-Compton (SSC) emission from Gamma-ray Burst (GRB) forward shock can extend to the very-high-energy (VHE; $E_gamma > $100 GeV) range. Such high energy photons are rare and are attenuated by the cosmic infrared background before reaching us. In this work, we discuss the prospect to detect these VHE photons using the current ground-based Cherenkov detectors. Our calculated results are consistent with the upper limits obtained with several Cherenkov detectors for GRB 030329, GRB 050509B, and GRB 060505 during the afterglow phase. For 5 bursts in our nearby GRB sample (except for GRB 030329), current ground-based Cherenkov detectors would not be expected to detect the modeled VHE signal. Only for those very bright and nearby bursts like GRB 030329, detection of VHE photons is possible under favorable observing conditions and a delayed observation time of $la$10 hours.
We examine 288 GRBs detected by the Fermi Gamma-ray Space Telescopes Gamma-ray Burst Monitor (GBM) that fell within the field-of-view of Fermis Large Area Telescope (LAT) during the first 2.5 years of observations, which showed no evidence for emission above 100 MeV. We report the photon flux upper limits in the 0.1-10 GeV range during the prompt emission phase as well as for fixed 30 s and 100 s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the { u}F{ u} spectra (Epk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above Epk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cut-off in their high-energy spectra, which if assumed to be due to {gamma}{gamma} attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT- detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.
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