IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array

📝 Abstract

Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB detection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger effective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay between the burst onset and the start of ground observations. We also consider the possibility of using GBM on Fermi as a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM localization is problematic, the small field-of-view for IACTs can potentially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20 to 30 months with our baseline instrument model, assuming consistently rapid pursuit of GRB alerts, and provided that spectral breaks below 100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year.

💡 Analysis

Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB detection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger effective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statistics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma-ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay between the burst onset and the start of ground observations. We also consider the possibility of using GBM on Fermi as a finder of GRBs for rapid ground follow-up. While the uncertainty of GBM localization is problematic, the small field-of-view for IACTs can potentially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20 to 30 months with our baseline instrument model, assuming consistently rapid pursuit of GRB alerts, and provided that spectral breaks below 100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year.

📄 Content

arXiv:1201.0010v2 [astro-ph.HE] 26 Mar 2012 Noname manuscript No. (will be inserted by the editor) IACT observations of gamma-ray bursts: prospects for the Cherenkov Telescope Array Rudy C. Gilmore · Aurelien Bouvier · Valerie Connaughton · Adam Goldstein · Nepomuk Otte · Joel R. Primack · David A. Williams November 2, 2018 Abstract Gamma rays at rest frame energies as high as 90 GeV have been reported from gamma-ray bursts (GRBs) by the Fermi Large Area Telescope (LAT). There is considerable hope that a confirmed GRB de- tection will be possible with the upcoming Cherenkov Telescope Array (CTA), which will have a larger ef- fective area and better low-energy sensitivity than current-generation imaging atmospheric Cherenkov telescopes (IACTs). To estimate the likelihood of such a detection, we have developed a phenomenological model for GRB emission between 1 GeV and 1 TeV that is motivated by the high-energy GRB detections of Fermi-LAT, and allows us to extrapolate the statis- tics of GRBs seen by lower energy instruments such as the Swift-BAT and BATSE on the Compton Gamma- ray Observatory. We show a number of statistics for detected GRBs, and describe how the detectability of GRBs with CTA could vary based on a number of parameters, such as the typical observation delay be- tween the burst onset and the start of ground observa- tions. We also consider the possibility of using GBM Rudy C. Gilmore Santa Cruz Institute for Particle Physics, University of Cal- ifornia, Santa Cruz, CA 95064, USA Scuola Internazionale Superiore di Studi Avanzati (SISSA), Via Bonomea 265, 34136, Trieste, Italy E-mail: rgilmore@physics.ucsc.edu Aurelien Bouvier Santa Cruz Institute for Particle Physics, University of Cal- ifornia, Santa Cruz, CA 95064, USA E-mail: apbouvie@ucsc.edu Valerie Connaughton · Adam Goldstein University of Alabama, Huntsville, AL 35899, USA Nepomuk Otte School of Physics & Center for Relativistic Astrophysics, Georgia Institute of Technology, Atlanta, GA 30332-0430 Joel R. Primack · David A. Williams Santa Cruz Institute for Particle Physics, University of Cal- ifornia, Santa Cruz, CA 95064, USA on Fermi as a finder of GRBs for rapid ground follow- up. While the uncertainty of GBM localization is prob- lematic, the small field-of-view for IACTs can poten- tially be overcome by scanning over the GBM error region. Overall, our results indicate that CTA should be able to detect one GRB every 20 to 30 months with our baseline instrument model, assuming consis- tently rapid pursuit of GRB alerts, and provided that spectral breaks below ∼100 GeV are not a common feature of the bright GRB population. With a more optimistic instrument model, the detection rate can be as high as 1 to 2 GRBs per year. Keywords gamma rays: bursts · telescopes 1 Introduction The observation of gamma-ray bursts (GRBs) with ground-based atmospheric Cherenkov telescopes has been a tantalizing possibility in recent years. Pow- erful >10-meter telescope arrays such as H.E.S.S., MAGIC, and VERITAS have come online in the last decade, and satellite detectors such as the Swift Burst Alert Telescope (BAT) are capable of pro- viding the necessary localization of GRB events within seconds over the Gamma-ray burst Coordi- nates Network (GCN1). Despite major campaigns to respond to satellite burst alerts at all three of these instruments (Aharonian et al 2009; Albert et al 2007; Garczarczyk et al 2008; Acciari et al 2011), and dozens of follow-up attempts, no conclusive detection of a GRB with an IACT has yet been made. Air shower arrays have also played a complementary role in the search for GRBs. A hint of emission was de- tected by the Milagrito air-shower array (Atkins et al 1 http://gcn.gsfc.nasa.gov/ 2 R. C. Gilmore, et al. 2003); however no detections were found by the later Milagro experiment (Abdo et al 2007). Prior to the launch of Fermi on June 11, 2008, knowledge about the emission of GRBs above 100 MeV was limited to a small number of events ob- served simultaneously in the EGRET and BATSE in- struments on the Compton Gamma-ray Observatory (CGRO) (Dingus 1995; Le and Dermer 2009). One fascinating finding by EGRET was the discovery of an 18 GeV photon associated with GRB 940217, 1.5 hours after the event. This was a much longer time than the duration of the burst as measured at lower energies by BATSE, which determined a T90 of 150 seconds2 (defined as the time between the arrival of 5 percent and 95 percent of the observed fluence). Though the statistics of these EGRET observations were quite limited, they suggested that high energy emission in GRBs did occur in some fraction of events, and that it could last longer than the lower energy emission. Fermi-LAT, covering the energy range of 20 MeV – 300 GeV (Atwood et al 2009), has now detected emis- sion from over 20 GRBs, of the some 800 detected by GBM at 8 keV – 40 MeV energy range (Meegan et al 2009). Photons from four of these LAT GRBs were detected above 10 GeV, and 2 above 30 GeV.

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