Investigating the Possibility of Screening High-z GRBs based on BAT Prompt Emission Properties

📝 Abstract

Being able to quickly select among gamma-ray bursts (GRBs) seen by the Swift satellite those which are high-z candidates would give ground-based observers a better chance to determine a redshift for such distant GRBs. Information about these high-z GRBs is important in helping to resolve questions about the early universe such as the formation rate of high-z GRBs, the re-ionization period of the universe, the metallicity of the early universe, and the Hubble expansion. Initially using a sample of 51 GRBs with previously measured redshifts, we have developed high-z screening criteria employing the GRB spectral as well as temporal characteristics of the prompt emission from the Burst Alert Telescope (BAT) on Swift. Now that the sample has increased to 81 GRBs, we have revisited the screening criteria and our methodology. Our updated high-z screening criteria are presented in this paper.

💡 Analysis

Being able to quickly select among gamma-ray bursts (GRBs) seen by the Swift satellite those which are high-z candidates would give ground-based observers a better chance to determine a redshift for such distant GRBs. Information about these high-z GRBs is important in helping to resolve questions about the early universe such as the formation rate of high-z GRBs, the re-ionization period of the universe, the metallicity of the early universe, and the Hubble expansion. Initially using a sample of 51 GRBs with previously measured redshifts, we have developed high-z screening criteria employing the GRB spectral as well as temporal characteristics of the prompt emission from the Burst Alert Telescope (BAT) on Swift. Now that the sample has increased to 81 GRBs, we have revisited the screening criteria and our methodology. Our updated high-z screening criteria are presented in this paper.

📄 Content

arXiv:0901.2928v1 [astro-ph.HE] 19 Jan 2009 Investigating the Possibility of Screening High-z GRBs based on BAT Prompt Emission Properties T. N. Ukwatta∗,†, T. Sakamoto†,∗∗, K. S. Dhuga∗, W. C. Parke∗, S. D. Barthelmy†, N. Gehrels†, M. Stamatikos†,‡ and J. Tueller† ∗The George Washington University, Washington, D.C. 20052 †NASA Goddard Space Flight Center, Greenbelt, MD 20771 ∗∗The University of Maryland, Baltimore County, Baltimore, MD 21250 ‡Oak Ridge Associated Universities, P.O. Box 117, Oak Ridge, Tennessee 37831-0117 Abstract. Being able to quickly select among gamma-ray bursts (GRBs) seen by the Swift satellite those which are high-z candidates would give ground-based observers a better chance to determine a redshift for such distant GRBs. Information about these high-z GRBs is important in helping to resolve questions about the early universe such as the formation rate of high-z GRBs, the re-ionization period of the universe, the metallicity of the early universe, and the Hubble expansion. Initially using a sample of 51 GRBs with previously measured redshifts, we have developed high-z screening criteria employing the GRB spectral as well as temporal characteristics of the prompt emission from the Burst Alert Telescope (BAT) on Swift. Now that the sample has increased to 81 GRBs, we have revisited the screening criteria and our methodology. Our updated high-z screening criteria are presented in this paper. Keywords: Gamma-ray Bursts, High-z GRBs PACS: 98.70.Rz, 98.62.Ai INTRODUCTION The detection of high redshift (z) Gamma-Ray Burst (GRBs) promises to give us valuable information about the early universe. Both GRB prompt emission and afterglows are so powerful that they should be detectable out to redshift of z

10 [1]. Being the brightest explosions yet seen in the universe since the Big Bang, GRBs are well-suited objects to probe the evolution of cosmic star formation, re-ionization of the of the intergalactic medium, and metallicity histories of the universe. The Swift Gamma-Ray Burst Mission [2] opens a window to explore the high redshift universe using GRBs. Currently, there are only five GRBs with spectroscopically-confirmed bursts with z > 5. The highest redshift GRB detected thus far is GRB 080913 with redshift ∼6.7 [3]. The next highest redshift burst is GRB 050904 which has redshift of ∼6.3 [4]. The remaining three are GRB 060927 with z = 5.47 [5], GRB 050814 with z = 5.3 [6] and GRB 060522 with z = 5.11 [7]. All these high-z GRBs have been discovered by the Swift mission. Even though the number of high-z GRBs are a handful, the Burst Alert Telescope (BAT; [8]), the primary instrument on board Swift, has enough sensitivity to detect more high-z GRBs. At present, the most difficult aspect of obtaining a spectroscopically-measured redshift for a high-z GRB is to observe the same object over the infrared band within a day after the burst is detected by a space telescope. Useful infrared spectra of GRBs require a large telescope, but the large telescopes in the world have very limited available observing time and as such are not in a position to do follow-up measurements on all GRB triggers. In this context, it would be useful to provide alerts to ground-based observers when possible high-z GRBs are detected. Previously, we have presented criteria [9] which utilize trends with redshift to predict high-z GRBs with some confidence. Now, with a larger sample some of those trends have disappeared, and predicting high-z bursts requires utilization of more observable parameters than before. In this paper, we present revised criteria that enable the selection of possible high-z GRBs using the prompt emission data from BAT. SCREENING HIGH-Z BURSTS USING BAT DATA In order to be effective, any high-z criteria need to be fast, automated and reliable. We should also be able to make a decision based on one or two orbits worth of data, which constrains the number of available observable properties. We have looked at six GRB BAT prompt emission properties to check for potential high-z screening criteria. These properties are:

1. Peak Photon Flux: Peak photon flux of the GRB measured in photons cm−2 s−1.
2. T90/Peak Photon Flux: T90 is the time needed to accumulate from 5% to 95% of the counts in the 15–150 keV band. We normalize T90 by the peak photon flux to compensate for any detector threshold effects of BAT.
3. FFT Cutoff Frequency: Cutoff frequency of the power spectrum when fitted by a broken power law curve.
4. Tau 50: Sum of all burst time durations for which emission is greater that 50 % of the total fluence.
5. Photon Index: Photon index of the time integrated spectrum when fitted by a simple power law.
6. 1-s Peak Photon Index: Photon index of the peak 1 sec spectrum when fitted with a simple power law. We have used BAT event-by-event data to derive all the observable parameters. The redshift measurements were taken from online archives of the Gamma-Ray Burst Online Index (GRBOX1) and verified them using t

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