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Register a new userShort and Long GRBs: same emission mechanism?
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Added by
Giancarlo Ghirlanda
Publication date
2011
fields
Physics
and research's language is
English
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We study the spectral evolution on second and sub--second timescales in 11 long and 12 short Gamma Ray Bursts (GRBs) with peak flux >8.5e-6 erg/cm2 s (8 keV-35 MeV) detected by the Fermi satellite. The peak flux correlates with the time-averaged peak energy in both classes of bursts. The peak energy evolution, as a function of time, tracks the evolution of the flux on short timescales in both short and long GRBs. We do not find evidence of an hard-to-soft spectral evolution. While short GRBs have observed peak energies larger than few MeV during most of their evolution, long GRBs can start with a softer peak energy (of few hundreds keV) and become as hard as short ones (i.e. with Ep,obs larger than few MeV) at the peak of their light curve. Six GRBs in our sample have a measured redshift. In these few cases we find that their correlations between the rest frame Ep and the luminosity Liso are less scattered than their correlations in the observer frame between the peak energy Ep,obs and the flux P. We find that the rest frame Ep of long bursts can be as high or even larger than that of short GRBs and that short and long GRBs follow the same Ep-Liso correlation, despite the fact that they likely have different progenitors.
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On the ground of the large number of gamma-ray bursts (GRBs) detected with cosmological redshift, we classified GRBs in seven subclasses, all with binary progenitors originating gravitational waves (GWs). Each binary is composed by combinations of carbon-oxygen cores (CO$_{rm core}$), neutron stars (NSs), black holes (BHs) and white dwarfs (WDs). The long bursts, traditionally assumed to originate from a BH with an ultra-relativistic jetted emission, not emitting GWs, have been subclassified as (I) X-ray flashes (XRFs), (II) binary-driven hypernovae (BdHNe), and (III) BH-supernovae (BH-SNe). They are framed within the induced gravitational collapse (IGC) paradigm with progenitor a CO$_{rm core}$-NS/BH binary. The supernova (SN) explosion of the CO$_{rm core}$ triggers an accretion process onto the NS/BH. If the accretion does not lead the NS to its critical mass, an XRF occurs, while when the BH is present or formed by accretion, a BdHN occurs. When the binaries are not disrupted, XRFs lead to NS-NS and BdHNe lead to NS-BH. The short bursts, originating in NS-NS, are subclassified as (IV) short gamma-ray flashes (S-GRFs) and (V) short GRBs (S-GRBs), the latter when a BH is formed. There are (VI) ultra-short GRBs (U-GRBs) and (VII) gamma-ray flashes (GRFs), respectively formed in NS-BH and NS-WD. We use the occurrence rate and GW emission of these subclasses to assess their detectability by Advanced LIGO-Virgo, eLISA, and resonant bars. We discuss the consequences of our results in view of the announcement of the LIGO-Virgo Collaboration of the source GW 170817 as being originated by a NS-NS.
We present a leptonic model on the external shock framework to describe the long- and short- lasting GeV component of some GRBs. This model was already applied successfully to GRB 090926A, and we extend it to describe the high-energy emission of GRB 090902B and GRB 090510. We argue that the high-energy emission consists of two components, one at MeV energies with a duration of a few seconds during the prompt phase, and a second GeV component lasting hundred of seconds after the prompt phase. The short high-energy component can be described as SSC emission from a reverse shock and the longer component arises from SSC emission of the forward shock. The main assumption of our model is that the jet is magnetized and evolves in the thick-shell case. The calculated fluxes and break energies are all consistent with the observed values.
We have investigated the time variations in the light curves from a sample of long and short Fermi/GBM Gamma ray bursts (GRBs) using an impartial wavelet analysis. The results indicate that in the source frame, the variability time scales for long bursts differ from that for short bursts, that variabilities on the order of a few milliseconds are not uncommon, and that an intriguing relationship exists between the minimum variability time and the burst duration.
In order to better understand the physical origin of short duration gamma-ray bursts (GRBs), we perform time-resolved spectral analysis on a sample of 70 pulses in 68 short GRBs with burst duration $T_{90}lesssim2$ s detected by the textit{Fermi}/GBM. We apply a Bayesian analysis to all spectra that have statistical significance $Sge15$ within each pulse and apply a cut-off power law (CPL) model. We then select in each pulse the timebin that has the maximal value of the low energy spectral index, %$alpha_{rm max}$, for further analysis. Under the assumption that the main emission mechanism is the same throughout each pulse, such an analysis is indicative of pulse emission. We find that $sim$1/3 of short GRBs are consistent with a pure, non-dissipative photospheric model, at least, around the peak of the pulse. This fraction is larger compare to the corresponding one (1/4) obtained for long GRBs. For these bursts, we find (i) a bi-modal distribution in the values of the Lorentz factors and the hardness ratios; (ii) an anti-correlation between $T_{90}$ and the peak energy, $E_{rm pk}$: $T_{90} propto E_{rm pk}^{-0.50pm0.19}$. This correlation disappears when we consider the entire sample. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one being a continuation of the long GRB population to shorter durations, and the other one being distinctly separate with different physical properties. Furthermore, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).
Due to their highly luminous nature, gamma-ray bursts (GRBs) are useful tools in studying the early Universe (up to z = 10). We consider whether the available subset of Swift high redshift GRBs are unusual when compared to analogous simulations of a bright low redshift sample. By simulating data from the Burst Alert Telescope (BAT; Barthelmy et al. 2005) the light curves of these bright bursts are obtained over an extensive range of redshifts, revealing complicated evolution in properties of the prompt emission such as T90.
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