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Register a new userA complete sample of bright Swift Long Gamma Ray Bursts: testing the spectral-energy correlations
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Added by
Giancarlo Ghirlanda
Publication date
2011
fields
Physics
and research's language is
English
Authors
L. Nava
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We use a nearly complete sample of Gamma Ray Bursts (GRBs) detected by the Swift satellite to study the correlations between the spectral peak energy Ep of the prompt emission, the isotropic energetics Eiso and the isotropic luminosity Liso. This GRB sample is characterized by a high level of completeness in redshift (90%). This allows us to probe in an unbiased way the issue related to the physical origin of these correlations against selection effects. We find that one burst, GRB 061021, is an outlier to the Ep-Eiso correlation. Despite this case, we find strong Ep-Eiso and Ep-Liso correlations for the bursts of the complete sample. Their slopes, normalisations and dispersions are consistent with those found with the whole sample of bursts with measured redshift and Ep. This means that the biases present in the total sample commonly used to study these correlations do not affect their properties. Finally, we also find no evolution with redshift of the Ep-Eiso and Ep-Liso correlations.
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We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of our sample (52 objects out of 58) allow us to establish the existence of a genuine dark population and we are able to estimate the maximum fraction of dark burst events (~30%) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to the one of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the sub-class of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.
We present a carefully selected sub-sample of Swift Long Gamma-ray Bursts (GRBs), that is complete in redshift. The sample is constructed by considering only bursts with favorable observing conditions for ground-based follow-up searches, that are bright in the 15-150 keV Swift/BAT band, i.e. with 1-s peak photon fluxes in excess to 2.6 ph s^-1 cm^-2. The sample is composed by 58 bursts, 52 of them with redshift for a completeness level of 90%, while another two have a redshift constraint, reaching a completeness level of 95%. For only three bursts we have no constraint on the redshift. The high level of redshift completeness allows us for the first time to constrain the GRB luminosity function and its evolution with cosmic times in a unbiased way. We find that strong evolution in luminosity (d_l=2.3pm 0.6) or in density (d_d=1.7pm 0.5) is required in order to account for the observations. The derived redshift distribution in the two scenarios are consistent with each other, in spite of their different intrinsic redshift distribution. This calls for other indicators to distinguish among different evolution models. Complete samples are at the base of any population studies. In future works we will use this unique sample of Swift bright GRBs to study the properties of the population of long GRBs.
Starting from the Swift sample we define a complete sub-sample of 58 bright long Gamma Ray Bursts (GRB), 55 of them (95%) with a redshift determination, in order to characterize their properties. Our sample (BAT6) allows us to study the properties of the long GRB population and their evolution with cosmic time. We focus in particular on the GRB luminosity function, on the spectral-energy correlations of their prompt emission, on the nature of dark bursts, on possible correlations between the prompt and the X-ray afterglow properties, and on the dust extinction.
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 update a flux-limited complete sample of Swift-based SGRBs (SBAT4, DAvanzo et al. 2014), bringing it to 25 events and doubling its previous redshift range. We then evaluate the column densities of the events in the updated sample, in order to compare them with the NH distribution of LGRBs, using the sample BAT6ext (Arcodia et al. 2016). We rely on Monte Carlo simulations of the two populations and compare the computed NH distributions with a two sample Kolmogorov Smirnov (K-S) test. We then study how the K-S probability varies with respect to the redshift range we consider. We find that the K-S probability keeps decreasing as redshift increases until at z$sim$1.8 the probability that short and long GRBs come from the same parent distribution drops below 1$%$. This testifies for an observational difference among the two populations. This difference may be due to the presence of highly absorbed LGRBs above z$sim$1.3, which have not been observed in the SGRB sample yet, although this may be due to our inability to detect them, or to the relatively small sample size.
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