Gamma Ray Bursts in the Era of Rapid Followup

arXiv:1003.3573v1 [astro-ph.CO] 18 Mar 2010 Gamma Ray Bursts in the Era of Rapid Followup C.G. Mundell1, C. Guidorzi2,1 and I.A. Steele1 on behalf of the Liverpool GRB team 1 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Birkenhead, CH41 1LD, U.K. cgm@astro.livjm.ac.uk 2 Physics Department, University of Ferrara, via Saragat 1, 44122, Ferrara, Italy. guidorzi@fe.infn.it We present a status report on the study of gamma-ray bursts (GRB) in the era of rapid follow-up using the world’s largest robotic optical telescopes - the 2-m Liverpool and Faulkes telescopes. Within the context of key unsolved issues in GRB physics, we describe (1) our innovative software that allows real- time automatic analysis and interpretation of GRB light curves, (2) the novel instrumentation that allows unique types of observations (in particular, early time polarisation measurements), (3) the key science questions and discoveries to which robotic observations are ideally suited, concluding with a summary of current understanding of GRB physics provided by combining rapid optical observations with simultaneous observations at other wavelengths. 1 Introduction Gamma-Ray Bursts (GRBs) are the most powerful explosions in the Universe and, arguably, represent the most significant new astrophysical phenomenon since the discovery of quasars and pulsars. As their name suggests, GRBs are detected as brief, intense and totally unpredictable flash of high-energy gamma rays, thought to be produced during the core collapse of massive stars (long-soft bursts, Tγ>2 s) or the merger of two compact objects such as two neutron stars or a neutron star and a stellar-mass black hole (short- hard bursts, Tγ<2 s). Although discovered through their γ-ray emission [1], they are now known to emit non-thermal radiation detectable across the elec- tromagnetic spectrum [2, 3, 4]. However, despite their enormous luminosity, their unpredictability and short duration limit rapid, accurate localisation and observability with traditional telescopes. Consequently, new ground and space-based facilities have been developed over the past decade; dedicated satellites optimised for GRB detection and followup, such as Swift [5], are 2 Mundell, Guidorzi & Steele revolutionizing GRB studies by locating ∼100 bursts per year with γ-ray po- sitions accurate to ∼3′ and X-ray positions accurate to 5” within seconds or minutes of the burst. Here we describe the automatic ground-based follow-up of GRBs with the world’s largest robotic optical telescopes that use intelligent software and innovative instruments. The Era of Rapid Follow-up: Predictions and Outcomes Before the launch of current satellites such as Swift, Integral and Fermi, signif- icant progress in understanding GRBs had been made since their discovery, in particular the general γ and X-ray properties. The first crucial step in dissem- inating real-time GRB positions to ground observers was triggered by BATSE on the CGRO [6] through the GRB Coordinates Network (GCN) [7] via in- ternet socket connection (no humans-in-the-loop). This drove development of the first generation of wide-field robotic followup ground-based facilities, such as GROCSE, ROTSE, and LOTIS, culminating with the discovery of the op- tical flash associated with GRB 990123 [8]. BATSE provided an invaluable catalogue of prompt γ-ray profiles, whose isotropic sky distribution and in- homogeneous intensity distribution suggested a cosmological origin [6], and BeppoSAX [9] revolutionised the cosmological study of GRBs by providing sub-arcmin (∼50”) localisation of X-ray afterglows that enabled late-time (∼ hours) optical followup with traditional ground-based telescopes and redshift determinations. Collimation of the ejecta (i.e. jets) was inferred from tempo- ral breaks - steepening - of optical light curves at ∼1 day post-burst and the concept of a universal central engine and the use of GRBs as standardisable cosmological candles was introduced [10, 11]. 0 5 10 15 20 25 30 0 1 2 3 4 5 6 7 8 9 # GRBs Redshift All Swift GRBs (as of Jan 2010) T90>3 s Fig. 1. Redshift distribution of Swift GRBs detected to-date. Gamma Ray Bursts in the Era of Rapid Followup 3 The possibility for great advances with the launch of Swift was fully recog- nised. Optical counterparts were expected to be found for all GRBs with many GRBs expected to exhibit bright optical flashes from reverse shock emission at early times, similar to GRB 990123 [8]. An increase in the num- ber of GRBs detected would lead to many jet breaks being identified, short GRBs would be easily observed and understood and identification of GRBs at very high redshift would be routine. Instead, 50% of GRBs remain opti- cally dark, despite deep, rapid followup [12, 13, 14, 15]; there is a dearth of bright reverse-shock optical emission [16]; light curves are complex in all bands with a variety of chromatic and achromatic breaks and flares observed (e.g., [17, 1

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