Scientists TCB With Simulated GRBs

May 5, 2015

By realistically simulating a population of gamma-ray bursts, KIPAC scientists have demonstrated the extent to which these explosions can be mischaracterized when they are far away.

The observed duration of a GRB pulse as a function of distance (redshift) for both an ideal (Without Noise) and a realistic (With Noise) observing instrument. The actual observed duration deviates from the expected duration.

As their name itself almost implies, gamma-ray bursts (GRBs), where a massive star or binary system almost instantly collapses to a black hole sending out enormous jets of particles and radiation, are the biggest and brightest explosions in nature. As such, we can see them when they are really far away, and because of the time it takes light to travel the distances, really far back in time. Because of this, observed GRBs span almost the entire length of the observable Universe and almost the entire history of time, making them ideal for investigating the effects that should arise from the integrated expansion of the Universe, as well as many other changes in the cosmos over time.
Interestingly, on the former point, observations of GRBs to date have not shown evidence of the time dilation effects that are expected to arise from the large expansion of the Universe since the early GRBs. Because of the expansion of the Universe stretching light on its way to us, the farthest away GRBs should appear to last longer than nearby ones, but in hundreds of GRBs observed by the SWIFT satellite and the Fermi Gamma-ray Space Telescope this is not seen to be the case. GRBs are fleeting, with the main burst output lasting minutes at most, and no correlation of duration with distance has been observed.
KIPAC postdoc Dan Kocevski and Professor Vahe Petrosian set out to explain this puzzling situation, and in the process have delivered some cautionary refinements for GRB analysis. In a recent work they carry out a simulation of GRBs at different distances detected by an observing instrument. They use a simulated population of GRBs that is faithful to those known in the real world, as described in a second paper by Kocevski. The GRB properties simulated include such things as the spectral shape, energy profile as a function of time, and total energy, and correlations among those and other characteristics. These  simulated GRBs include characteristics of the way GRBs evolve over the course of the Universe, some of which were determined in a previous work by Kocevski using analysis techniques pioneered by Petrosian.
The duo find that when a distribution of GRBs corresponding to those in the real Universe is observed by an instrument which has a finite energy band sensitivity and a finite flux limit, these effects combine to remove the correlation of observed burst duration and distance from the data. In simplified terms, because farther away things appear dimmer than close things, a distant GRB will tend to be seen above the detection threshold of an instrument for less time than a more nearby one. The quantification of this effect is subtle and relies on precise inputs for the burst properties and detector characteristics.
Critically for GRB science, Kocevski and Petrosian's results show that a commonly applied correction for the known time dilation due to the expansion of the Universe can lead to very incorrect estimates of inherent GRB properties such as the total energy output.  Additionally, although GRBs are known to be divided into two classes based on their duration - known creatively as 'long' and 'short' - the authors state that a classification into long or short based solely on the measured duration is not reliable for the farther GRBs, in which the observed duration and actual duration are related in a complicated way. This work will help guide scientists in their quest to further understand these distant explosions and what they encode about the expansion of the Universe.
This work is described in two papers submitted to the Astrophysical Journal and available from astro-ph at arXiv:1110.6173 and 1110.6175.
Science Contact:
Dan Kocevski