Detection of signature consistent with cosmological time dilation in gamma-ray bursts

Abstract

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 proles will be stretched in time, by an amount proportional to the redshift, 1 + z. We h a v e tested data from the Compton Gamma Ray Observatory's Burst and Transient Source Experiment (BATSE) for such time dilation. Out of 590 bursts observed by B A TSE, 131 bursts were analyzed; bursts with durations shorter than 1.5 s were excluded. We used three tests to compare the timescales of bright and dim bursts, the latter, on average, being more distant than the former. Our measures of timescale are constructed to avoid selection eects arising from intensity dierences by rescaling all bursts to ducial levels of peak intensity and noise bias. (1) We found that the total rescaled count a b o v e background for the dim burst ensemble is approximately twice that for the brightest bursts-translating into longer durations for the dim bursts. (2) Wavelet-transform decompositions of the burst proles conrmed that this dilation operates over a broad range of timescales. (3) Structure on the shortest timescales was examined using a procedure which aligns the highest peaks of proles from which the noise has been optimally removed using a wavelet thresholding technique. In all three tests, the dim bursts are stretched by a factor of 2 relative to the bright ones, over seven octaves of timescale. We calibrated the measurements by dilating synthetic bursts that approximate the temporal characteristics of bright B A TSE bursts. Results are consistent with bursts at BATSE's peak-ux completeness limit being at cosmological distances corresponding to z 1, and thus with independent cosmological interpretations of the BATSE number-intensity relation. Alternative explanations of our results, arising from the nature of physical processes in bursts, are still possible.

On the difference between the short and long gamma-ray bursts

“…This also implies that the cosmological time dilatation should not affect significantly the observed distribution of T 90 , which therefore is not expected to differ statistically from that of t 90 . We note that several other authors (Wijers & Paczyński 1994;Norris et al 1994;Norris et al 1995) have already suggested that the distribution of T 90 reflects predominantly the distribution of t 90 . Nevertheless, our argumentation based on the mathematical Cramér theorem is new.…”

“…As shown in Bagoly et al (1998), the stochastic variable corresponding to the duration is independent from that of the peak flux. This means that a fixed level of detection, given by the peak fluxes, does not have significant influence on the shape of the detected distribution of the durations (Efron & Petrosian 1992;Wijers & Paczyński 1994;Norris et al 1994Norris et al , 1995. In the case of the fluences, however, a detection threshold in the peak fluxes induces a bias on the true distribution, since fluences and durations are stochastically not independent.…”

Gamma-Ray Burst Afterglows

“…However, the interpretation of the duration differences between bright and dim bursts previously attributed to cosmological time dilation (Norris et al 1994) has now become very unclear.…”

Complex dynamics in a simple model of pulsations for super-asymptotic giant branch stars

“…The development of new nonlinear time-series analysis tools during the last decade has changed this situation. In particular, it has been found that these tools have rich applications in a broad range of astrophysical situations, which include time analysis of gamma-ray bursts [18], of gravitationally lensed quasars [19] or Xrays within galaxy clusters [20], and of variable white dwarfs [21]. These tools have also been used to analyze theoretical models of stellar pulsations.…”