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Register a new userPeak energy of the prompt emission of long Gamma Ray Bursts vs their fluence and peak flux
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Lara Nava
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The spectral-energy and (luminosity) correlations in long GRBs are being hotly debated to establish, first of all, their reality against possible selection effects. These are best studied in the observer planes, namely the peak energy E_peak_obs vs the fluence F or the peak flux P. In a recent paper we started to attack this problem considering all GRBs with known z and spectral properties. Here we consider instead all bursts with known E_peak_obs, irrespective of z, adding to those a sample of 100 faint BATSE bursts representative of a larger population. This allows us to construct a complete, fluence limited, sample, to study the selection/instrumental effects. We found that fainter bursts have smaller E_peak_obs than those of bright events. As a consequence, the E_peak_obs of these bursts is correlated with the fluence, though with a slope flatter than that defined by bursts with z. Selection effects, which are present, are shown not to be responsible for the existence of such a correlation. About 6% of these bursts are surely outliers of the E_peak-E_iso correlation (updated to include 83 bursts), since they are inconsistent with it for any z. E_peak_obs correlates also with P, with a slope similar to the E_peak-L_iso correlation.In this case there is only one sure outlier.The scatter of the E_peak_obs-P correlation defined by the BATSE bursts of our sample is smaller than the E_peak_obs-F correlation of the same bursts, while for the bursts with known z the E_peak-E_iso correlation is tighter than the E_peak-L_iso one. Once a very large number of bursts with E_peak_obs and z will be available, we thus expect that the E_peak-L_iso correlation will be similar to that currently found, whereas it is likely that the E_peak-E_iso correlation will become flatter and with a larger scatter.
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Owing to narrow energy band of textit{Swift}/BAT, several urgent issues are required to pay more attentions but unsolved so far. We systematically study the properties of a refined sample of 283 textit{Swift}/BAT gamma-ray bursts with well-measured spectral peak energy ($E_{text p}$) at a high confidence level larger than 3$sigma$. It is interestingly found that duration ($T_{90}$) distribution of textit{Swift} bursts still exhibits an evident bimodality with a more reliable boundary of $T_{90}simeq$1.06 s instead of 2 s for previously contaminated samples including bursts without well-peaked spectra, which is very close to $sim$1.27 s and $sim$0.8 s suggested by some authors for Fermi/GBM and textit{Swift}/BAT catalogs, respectively. The textit{Swift}/BAT short and long bursts have comparable mean $E_{text p}$ values of $87^{+112}_{-49}$ and $85^{+101}_{-46}$ keV in each, similar to what found for both types of BATSE bursts, which manifests the traditional short-hard/long-soft scheme may not be tenable for the certain energy window of a detector. In statistics, we also investigate the consistency of distinct methods for the $E_{text p}$ estimates and find that Bayesian approach and BAND function can always give consistent evaluations. In contrast, the frequently-used cut-off power-law model matches two other methods for lower $E_{text p}$ and will overestimate the $E_{text p}$ more than 70% as $E_{text p}>$100 keV. Peak energies of X-ray flashes, X-ray rich bursts and classical gamma-ray bursts could have an evolutionary consequence from thermal-dominated to non-thermal-dominated radiation mechanisms. Finally, we find that the $E_{text p}$ and the observed fluence ($S_{gamma}$) in the observer frame are correlated as $E_psimeq [S_{gamma}/(10^{-5} erg cm^{-2})]^{0.28}times 117.5^{+44.7}_{-32.4}$ keV proposed to be an useful indicator of GRB peak energies.
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.
Gamma-ray bursts (GRBs) were first detected thanks to their prompt emission, which was the only information available for decades. In 2010, while the high-energy prompt emission remains the main tool for the detection and the first localization of GRB sources, our understanding of this crucial phase of GRBs has made great progress. We discuss some recent advances in this field, like the occasional detection of the prompt emission at all wavelengths, from optical to GeV; the existence of sub-luminous GRBs; the attempts to standardize GRBs; and the possible detection of polarization in two very bright GRBs. Despite these advances, tantalizing observational and theoretical challenges still exist, concerning the detection of the faintest GRBs, the panchromatic observation of GRBs from their very beginning, the origin of the prompt emission, or the understanding of the physics at work during this phase. Significant progress on this last topic is expected with SVOM thanks to the observation of dozens of GRBs from optical to MeV during the burst itself, and the measure of the redshift for the majority of them. SVOM will also change our view of the prompt GRB phase in another way. Within a few years, the sensitivity of sky surveys at optical and radio frequencies, and outside the electromagnetic domain in gravitational waves or neutrinos, will allow them to detect several new types of transient signals, and SVOM will be uniquely suited to identify which of these transients are associated with GRBs. This radically novel look at GRBs may elucidate the complex physics producing these bright flashes.
Correlation studies of prompt and afterglow emissions from gamma-ray bursts (GRBs) between different spectral bands has been difficult to do in the past because few bursts had comprehensive and intercomparable afterglow measurements. In this paper we present a large and uniform data set for correlation analysis based on bursts detected by the Swift mission. For the first time, short and long bursts can be analyzed and compared. It is found for both classes that the optical, X-ray and gamma-ray emissions are linearly correlated, but with a large spread about the correlation line; stronger bursts tend to have brighter afterglows, and bursts with brighter X-ray afterglow tend to have brighter optical afterglow. Short bursts are, on average, weaker in both prompt and afterglow emissions. No short bursts are seen with extremely low optical to X-ray ratio as occurs for dark long bursts. Although statistics are still poor for short bursts, there is no evidence yet for a subgroup of short bursts with high extinction as there is for long bursts. Long bursts are detected in the dark category at the same fraction as for pre-Swift bursts. Interesting cases are discovered of long bursts that are detected in the optical, and yet have low enough optical to X-ray ratio to be classified as dark. For the prompt emission, short and long bursts have different average tracks on flux vs fluence plots. In Swift, GRB detections tend to be fluence limited for short bursts and flux limited for long events.
The prompt emission of gamma-ray bursts (GRBs) is widely thought to be radiation from accelerated electrons, but an appreciably larger amount of energy could be carried by accelerated protons, particularly if GRBs are the sources of ultra-high-energy cosmic rays (UHECRs). We model the expected photon spectra for such proton-dominated GRBs in the internal shock scenario through Monte Carlo simulations, accounting for various processes related to high-energy electrons and protons. Besides proton and muon synchrotron components, emission from photomeson-induced secondary pair cascades becomes crucial, generally enhancing the GeV-TeV and/or eV-keV photons and offering a signature of UHE protons. In some cases, it can overwhelm the primary electron component and result in GRBs peaking in the 10 MeV - 1 GeV range, which may be relevant to some bursts discussed in a recent re-analysis of EGRET TASC data. The dependence of the spectra on key quantities such as the bulk Lorentz factor, magnetic field and proton-to-electron ratio is nontrivial due to the nonlinear nature of cascading and the interplay of electron- and proton-induced components. Observations by {it Fermi}, ground-based telescopes and other facilities should test these expectations and provide critical constraints on the proton acceleration efficiency.
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