UvA-DARE (Digital Academic Repository) Gamma-Ray Burst afterglows Galama, T.J. Publication date 1999 Link to publication Citation for published version (APA): Galama, T. J. (1999). Gamma-Ray Burst afterglows. General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulations If you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: https://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. UvA-DARE is a service provided by the library of the University of Amsterdam (https://dare.uva.nl) Download date:26 Sep 2021 Introduction Introduction 1 1 Introduction n 1.11 What are 7-ray bursts? Gamma-rayy bursts (GRBs) are the strongest phenomenon seen at 7-ray wavelengths; bright GRBss are as bright in 7 rays as the brightest star in the sky, Sirius, is in visible light. GRBs weree discovered with the Vela satellites, whose main purpose was to verify compliance with the 19633 Limited Nuclear Test Ban Treaty. Since their discovery (Klebesadel, Strong and Olson 1973)) these events, which emit the bulk of their energy in the 0.1 — 1.0 MeV range, and whose durationss span milliseconds to tens of minutes, posed one of the great unsolved problems in astrophysics.. Until recently, no counterparts (quiescent as well as transient) could be found andd observations did not provide a direct measurement of their distance. The breakthrough camee in early 1997, when the Wide Field Cameras aboard the Italian-Dutch BeppoSAX satellite allowedd rapid and accurate localization of GRBs. Follow-up on these positions resulted in the discoveryy of X-ray, optical and radio afterglows. These observations revealed that GRBs come fromm 'cosmological' distances, and that they are by far the most luminous photon sources in thee Universe, with peak luminosities in 7 rays up to 1052 erg/s, and total energy budgets up too several times 1053-54 erg (for assumed isotropic emission). The optical signal from GRB is regularlyy seen to be 10 magnitudes brighter (absolute) than the brightest supernovae, and once evenn 18 magnitudes brighter. 1.1.11 Vela satellites Thee Limited Nuclear Test Ban Treaty prohibits nuclear weapons tests "or any other nuclear explosion"" in the atmosphere, in outer space, and underr water. The Vela satellites were designed suchh that they could verify compliance with the treaty by detecting the 7 rays from nuclear testss outside the Earth's atmosphere. Vela 5A, 5B, 6A and 6B each carried six 10 cm3 Csl scintillationn counters; they could detect photons in the 0.2-1.0 MeV (Vela 5) and 0.3-1.5 MeV (Velaa 6) energy range (see Fig. 1.1). AA search for 7-ray bursts was started by R. Klebesabel because of the prediction that 7-ray emissionn would be observable during the initial stages of supernova explosions (Colgate 1968). Noo indications for such a phenomenon were found. It was in 1969 that, embedded in Vela spacecraftt data from 1967, a 7-ray burst was found. At that time the Sun could not be exluded ass a source. With the launch of a new generation of Vela satellites (the Vela 6) sufficient timing accuracyy made it possible to exclude the Sun as the source of these events. Once about 16 eventss were found (and believed) a paper was published announcing the discovery of cosmic 7 7 ChapterChapter I 7-rayy bursts (Klebesadel, Strong and Olson 1973). The original July 1967 event is not in that paperr because it could have come from the Sun (although its characteristics are like a GRB and itt is now considered the oldest known GRB)'. A time history of the 1967 event can be found in Strongg and Klebesabel (1976). Figuree 1.1: Left figure: Vela-SA and 5B Satellites in the Clean Room. Right figure: animation ofof Vela-5B in low Earth orbit (courtesy of Los Alamos National Laboratory). 1.1.22 Light curves and spectra of GRBs Thee Burst And Transient Source Experiment (BATSE; Fishman et al. 1989) on board the Comp- tonn Gamma-Ray Observatory (CGRO; see Fig. 1.3) observes about one GRB a day. The CGRO providess a wealth of information on GRB light curves and spectra; here I summarize some of thee most important results. A more extensive discussion on temporal properties and spectra of GRBss can be found in the review by Fishman and Meegan (1995). GRBs have durations ranging from milliseconds to ~ 103 s. Their time histories display aa great diversity of structure, with single or multiple peaks, with smooth profiles, profiles withh sub-peaks, profiles with well separated peaks and profiles with overlapping peaks andd spikes (see Fig. 1.2). GRB light curves show rapid variability, on time scales sometimes less than a millisecond (Bhatt 1992; Schaefer and Walker 1999). As light travels a distance L = ct = 3-107cmin onee millisecond, about 10 neutron star radii, it is generally believed that GRBs originate fromm compact objects, such as neutron stars (NS) or black holes (BH). Several attempts have been made to categorize GRB time histories. No other observa- tionall parameters appear to be correlated with temporal morphologies (e.g., Fishman and Meegann 1995), i.e., there is little morphological evidence for distinct burst classes within 11 It is often believed that the publication was delayed until 1973 because the data were classified. In fact it was thee better timins of GRBs which was needed to rule out the Sun. 8 8 Introduction Introduction thee GRBs. There is one exception: the distribution of burst durations is bimodal and separatess GRBs into two classes, the short events (<2s) and the longer ones (>2s) (Kou- veliotouu et al. 1993; see Fig. 1.4). The duration appears to be anticorrelated with spectral hardness:: short bursts are predominantly harder than long ones (Kouveliotou et al. 1993). Norriss et al. (1995) find evidence for time dilation by comparison of samples of bright andd dim BATSE GRBs; the centroids and widths of the duration distribution for the dim samplee are scaled by a factor of two relative to the bright sample. They interpret this as a resultt of cosmological redshift: the dim bursts are, on average, located at larger distance (i.e.,, redshifts) than the bright bursts. The sources of dimmer bursts would lie at redshifts off order 2. High-energyy emission is a unique feature of GRBs. Spectral measurements extend from a feww keV to ~ GeV (see e.g., Fig. 1.5). The continuum spectra of GRBs are very broad and hard;; most of the power is emitted above 50 keV. GRB spectra are well described by an empiricall function, the so called Band function (Band et al. 1993). This function consists off a low- and high-energy power law, smoothly joined by an exponential turnover. It has threee parameters, the peak energy, and the low- and high-energy photon index (for details seee Bandetal. 1993). Delayedd very high-energy 7 rays have been observed from GRBs with EGRET onboard thee CGRO (Hurley et al. 1994; Sommer et al. 1994). In the case of GRB 940217, 200 MeV-200 GeV photons were observed with EGRET up to 90 minutes after the event onset (Hurleyy et al. 1994), i.e., much longer than the duration of the lower photon energies of thee GRB itself (~ 200 seconds; 30-2000 keV). Fordd et al. (1995) investigated the evolution of the peak energy for long and bright GRBs andd found that the peak energy decreases with time. Liang and Kargatis (1996) found thatt the peak energy decreases exponentially with the photon fluence. This 'hard-to-soft' spectrall evolution (see also Norris et al. 1986) is also found within individual pulses of GRBss (Ford et al. 1995). Timee histories of GRBs are different for different energy bands. At higher energies the overalll burst duration as well as the rise and fall time scales of pulses are shorter than thosee at lower energies (Link, Epstein and Priedhorsky 1994). Typically, the low-energy emissionn in GRBs persists longer than the high-energy emission. 1.1.33 Distribution on the sky and in space Priorr to the launch of the CGRO, in 1991, it was found, from various experiments, that the burstt population is approximately uniform in space. Detector sensitivities were still too small to detectt an 'edge' to the distribution of burst sources (Higdon, Meegan and Cline 1984; Hurley, Clinee and Epstein 1984). Thee space distribution of GRB sources is related to their apparent flux distribution, as can bee seen from the following simplified argument. We assume that bursts are standard candles, i.e.,, they emit the same amount of luminosity at the same wavelengths. A burst at a distance 2 dodo is observed with a peak flux P0, P0 ^c d^ . The cumulative rate of bursts, with peak fluxes 9 9 ChapterChapter 1 Burstt number : : 105 5 Burstt number: 249 bU U -- 50 0 fl l 40 0r r -. -. 30 0r r -_ -_ 20 0 -- 10 0 0 0 ... i... i .. i . : : -55 0 5 10 15 -100 0 10 20 30 40 50 60 Timee (seconds since trigger) Timee (seconds since trigger) Burstt number: 257 B ursB t t number: : 408 8 14 4 ...,...,...,...,.