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Register a new userAre GRBs the same at high and low redshift?
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Added by
Owen Littlejohns Dr
Publication date
2013
fields
Physics
and research's language is
English
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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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We present optical, near-infrared, and radio observations of the afterglow of GRB 120521C. By modeling the multi-wavelength dataset, we derive a photometric redshift of z~6.0, which we confirm with a low signal-to-noise ratio spectrum of the afterglow. We find that a model with a constant-density environment provides a good fit to the afterglow data, with an inferred density of 0.05/cm^3. The radio observations reveal the presence of a jet break at 7 d, corresponding to a jet opening angle of ~ 3 deg. The beaming-corrected gamma-ray and kinetic energies are both ~ 3e50 erg. We quantify the uncertainties in our results using a detailed Markov Chain Monte Carlo analysis, which allows us to uncover degeneracies between the physical parameters of the explosion. To compare GRB 120521C to other high-redshift bursts in a uniform manner we re-fit all available afterglow data for the two other bursts at z>6 with radio detections (GRBs 050904 and 090423). We find a jet break at ~ 15 d for GRB 090423, in contrast to previous work. Based on these three events, we find that GRBs at z>6 appear to explode in constant-density environments, and exhibit a wide range of energies and densities that span the range inferred for lower redshift bursts. On the other hand, we find a hint for narrower jets in the z>6 bursts, potentially indicating a larger true event rate at these redshifts. Overall, our results indicate that long GRBs share a common progenitor population at least to z~8.
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.
We present a template fitting algorithm for determining photometric redshifts, $z_{rm phot}$, of candidate high-redshift gamma-ray bursts (GRBs). Using afterglow photometry, obtained by the Reionization And Transients InfraRed (RATIR) camera, this algorithm accounts for the intrinsic GRB afterglow spectral energy distribution (SED), host dust extinction and the effect of neutral hydrogen (local and cosmological) along the line of sight. We present the results obtained by this algorithm and RATIR photometry of GRB 130606A, finding a range of best fit solutions $5.6 < z_{rm phot} < 6.0$ for models of several host dust extinction laws (none, MW, LMC and SMC), consistent with spectroscopic measurements of the redshift of this GRB. Using simulated RATIR photometry, we find our algorithm provides precise measures of $z_{rm phot}$ in the ranges $4 < z_{rm phot} lesssim 8$ and $9 < z_{rm phot} < 10$ and can robustly determine when $z_{rm phot}>4$. Further testing highlights the required caution in cases of highly dust extincted host galaxies. These tests also show that our algorithm does not erroneously find $z_{rm phot} < 4$ when $z_{rm sim}>4$, thereby minimizing false negatives and allowing us to rapidly identify all potential high-redshift events.
We study the high-energy properties of GRB 181123B, a short gamma-ray burst (sGRB) at redshift $zapprox$1.75. We show that, despite its nominal short duration with $T_{90}<$2 s, this burst displays evidence of a temporally extended emission (EE) at high energies and that the same trend is observed in the majority of sGRBs at $zgtrsim$1. We discuss the impact of instrumental selection effects on the GRB classification, stressing that the measured $T_{90}$ is not an unambiguous indicator of the burst physical origin. By examining their environment (e.g. stellar mass, star formation, offset distribution), we find that these high-$z$ sGRBs share many properties of long GRBs at a similar distance and are consistent with a short-lived progenitor system. If produced by compact binary mergers, these sGRBs with EE may be easier to localize at large distances and herald a larger population of sGRBs in the early universe.
At low redshift, a handful of gamma-ray bursts (GRBs) have been discovered with peak luminosities ($L_{rm iso} < 10^{48.5}~rm{erg,s}^{-1}$) substantially lower than the average of the more distant ones ($L_{rm iso} > 10^{49.5}~rm{erg,s}^{-1}$). The properties of several low-luminosity (low-$L$) GRBs indicate that they can be due to shock break-out, as opposed to the emission from ultrarelativistic jets. Owing to this, it is highly debated how both populations are connected, and whether there is a continuum between them. The burst at redshift $z=0.283$ from 2012 April 22 is one of the very few examples of intermediate-$L$ GRBs with a $gamma$-ray luminosity of $Lsim10^{48.9}~rm{erg,s}^{-1}$ that have been detected up to now. Together with the robust detection of its accompanying supernova SN 2012bz, it has the potential to answer important questions on the origin of low- and high-$L$ GRBs and the GRB-SN connection. We carried out a spectroscopy campaign using medium- and low-resolution spectrographs at 6--10-m class telescopes, covering the time span of 37.3 days, and a multi-wavelength imaging campaign from radio to X-ray energies over a duration of $sim270$ days. Furthermore, we used a tuneable filter centred at H$alpha$ to map star formation in the host galaxy and the surrounding galaxies. We used these data to extract and model the properties of different radiation components and incorporate spectral-energy-distribution fitting techniques to extract the properties of the host galaxy. Modelling the light curve and spectral energy distribution from the radio to the X-rays revealed the blast-wave to expand with an initial Lorentz factor of $Gamma_0sim60$, low for a high-$L$ GRB, and that the afterglow had an exceptional low peak luminosity-density of $lesssim2times10^{30}~rm{erg,s}^{-1},rm{Hz}^{-1}$ in the sub-mm. [Abridged]
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