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We carry out a search for signatures of cosmological time dilation in the light curves of Gamma Ray Bursts (GRBs), detected by the Neil Gehrels Swift Observatory. For this purpose, we calculate two different durations ($T_{50}$ and $T_{90}$) for a sample of 247 GRBs in the fixed rest frame energy interval of 140-350 keV, similar to Zhang et al. We then carry out a power law-based regression analysis between the durations and redshifts. This search is done using both the unbinned as well as the binned data, where both the weighted mean and the geometric mean was used. For each analysis, we also calculate the intrinsic scatter to determine the tightness of the relation. We find that weighted mean-based binned data for long GRBs and the geometric mean-based binned data is consistent with the cosmological time dilation signature, whereas the analyses using unbinned durations show a very large scatter. We also make our analysis codes and the procedure for obtaining the light curves and estimation of $T_{50}$/$T_{90}$ publicly available.
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The detection of six Fast Radio Bursts (FRBs) has recently been reported. FRBs are short duration ($sim$ 1 ms), highly dispersed radio pulses from astronomical sources. The physical interpretation for the FRBs remains unclear but is thought to involve highly compact objects at cosmological distance. It has been suggested that a fraction of FRBs could be physically associated with gamma-ray bursts (GRBs). Recent radio observations of GRBs have reported the detection of two highly dispersed short duration radio pulses using a 12 m radio telescope at 1.4 GHz. Motivated by this result, we have performed a systematic and sensitive search for FRBs associated with GRBs. We have observed five GRBs at 2.3 GHz using a 26 m radio telescope located at the Mount Pleasant Radio Observatory, Hobart. The radio telescope was automated to rapidly respond to Gamma-ray Coordination Network notifications from the Swift satellite and slew to the GRB position within $sim$ 140 s. The data were searched for pulses up to 5000 pc $rm cm^{-3}$ in dispersion measure and pulse widths ranging from 640 $rm mu$s to 25.60 ms. We did not detect any events $rm geq 6 sigma$. An in-depth statistical analysis of our data shows that events detected above $rm 5 sigma$ are consistent with thermal noise fluctuations only. A joint analysis of our data with previous experiments shows that previously claimed detections of FRBs from GRBs are unlikely to be astrophysical. Our results are in line with the lack of consistency noted between the recently presented FRB event rates and GRB event rates.
If gamma-ray bursters are at cosmological distances - as suggested by their isotropic distribution on the sky and by their number-intensity relation - then the burst profiles will be stretched in time, by an amount proportional to the redshift, 1 + $z$. We have tested data from the {it Compton} Gamma Ray Observatorys Burst and Transient Source Experiment (BATSE) for such time dilation. Our measures of time scale are constructed to avoid selection effects arising from intensity differences by rescale all bursts to fiducial levels of peak intensity and noise bias. The three tests involved total count rate above background, wavelet decomposition, and alignment of the highest peaks. In all three tests, the dim bursts are stretched by a factor of about two relative to the bright ones, over seven octaves of time scale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright BATSE bursts. Results are consistent with bursts of BATSEs peak-flux completeness limit being at cosmological distances corresponding to $z sim 1$, and thus with independent cosmological interpretations of the BATSE number-intensity relation.
We study the emission observed at energies greater than 100 MeV of 11 Gamma Ray Bursts (GRBs) detected by the Fermi/Large Area Telescope (LAT) until October 2009. The GeV emission has three main properties: (i) its duration is often longer than the duration of the softer emission detected by the Gamma Burst Monitor (GBM) onboard Fermi [this confirms earlier results from the Energetic Gamma-Ray Experiment Telescope (EGRET)]; (ii) its spectrum is consistent with F(v) propto v^(-1) and does not show strong spectral evolution; (iii) for the brightest bursts, the flux detected by the LAT decays as a power law with a typical slope: t^(-1.5). We argue that the observed >0.1 GeV flux can be interpreted as afterglow emission shortly following the start of the prompt phase emission as seen at smaller frequencies. The decay slope is what expected if the fireball emission is produced in the radiative regime, i.e. all dissipated energy is radiated away. We also argue that the detectability in the GeV energy range depends on the bulk Lorentz factor Gamma of the bursts, being strongly favoured in the case of large Gamma. This implies that the fraction of bursts detected at high energies corresponds to the fraction of bursts having the largest Gamma. The radiative interpretation can help to explain why the observed X-ray and optical afterglow energetics are much smaller than the energetics emitted during the prompt phase, despite the fact that the collision with the external medium should be more efficient than internal shocks in producing the radiation we see.
Recent Swift observations suggest that the traditional long vs. short GRB classification scheme does not always associate GRBs to the two physically motivated model types, i.e. Type II (massive star origin) vs. Type I (compact star origin). We propose a new phenomenological classification method of GRBs by introducing a new parameter epsilon=E_{gamma, iso,52}/E^{5/3}_{p,z,2}, where E_{gamma,iso} is the isotropic gamma-ray energy (in units of 10^{52} erg), and E_{p,z} is the cosmic rest frame spectral peak energy (in units of 100 keV). For those short GRBs with extended emission, both quantities are defined for the short/hard spike only. With the current complete sample of GRBs with redshift and E_p measurements, the epsilon parameter shows a clear bimodal distribution with a separation at epsilon ~ 0.03. The high-epsilon region encloses the typical long GRBs with high-luminosity, some high-z rest-frame-short GRBs (such as GRB 090423 and GRB 080913), as well as some high-z short GRBs (such as GRB 090426). All these GRBs have been claimed to be of the Type II origin based on other observational properties in the literature. All the GRBs that are argued to be of the Type I origin are found to be clustered in the low-epsilon region. They can be separated from some nearby low-luminosity long GRBs (in 3sigma) by an additional T_{90} criterion, i.e. T_{90,z}<~ 5 s in the Swift/BAT band. We suggest that this new classification scheme can better match the physically-motivated Type II/I classification scheme.
There exists an inevitable scatter in intrinsic luminosity of Gamma Ray Bursts(GRBs). If there is relativistic beaming in the source, viewing angle variation necessarily introduces variation in the intrinsic luminosity function(ILF). Scatter in the ILF can cause a selection bias where distant sources that are detected have a larger median luminosity than those detected close by. Median luminosity, as we know, divides any given population into equal halves. When the functional form of a distribution is unknown, it can be a more robust diagnostic than any that use trial functional forms. In this work we employ a statistical test based on median luminosity and apply it to test a class of models for GRBs. We assume that the GRB jet has a finite opening angle and that the orientation of the GRB jet is random relative to the observer. We parameterize the jet with constant Lorentz factor $Gamma$ and opening angle $theta_0$. We calculate $L_{median}$ as a function of redshift with an average of 17 grbs in each redshift bin($dz=0.01$) empirically, theoretically and use Fermi GBM data, noting that SWIFT data is problematic as it is biased, specially at high redshifts. We find that $L_{median}$ is close to $L_{max}$ for sufficiently extended GRB jet and does not fit the data. We find an acceptable fit with the data when $Gamma$ is between $100$ and $200$, $theta_0leq 0.1$, provided that the jet material along the line of sight to the on axis observer is optically thick, such that the shielded maximum luminosity is well below the bare $L_{max}$. If we associate an on-axis observer with a classically projected monotonically decreasing afterglow, we find that their ILF is similar to those of off-jet observer which we associate with flat phase afterglows.
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