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Repo Rts from Observ Ers R E P O RTS FROM OBSERV E R S Gamma-Ray Bursts1 – Pushing Limits with the VLT H. Pedersen1a, J.-L. Atteia2, M. Boer 2, K. Beuermann3, A.J. Castro-Tirado 4, A. Fruchter 5, J. Greiner 6, R. Hessman2, J. Hjorth1b, L. Kaper 7, C. Kouveliotou 7,8, N. Masetti 9, E. Palazzi 9, E. Pian9, K. Reinsch 2, E. Rol 7, E. van den Heuvel7, P. Vreeswijk7, R. Wijers 10 1aCopenhagen University Observatory, [email protected]; 1bCopenhagen University Observatory; 2CESR, Toulouse; 3Universitäts-Sternwarte, Göttingen; 4LAEFF-INTA, Madrid, and IAA-CSIC, Granada, Spain; 5Space Telescope Science Institute, Baltimore, USA; 6Astrophysikalisches Institut Potsdam; 7Astronomical Institute “Anton Pannekoek”, Amsterdam; 8NASA/USRA, Huntsville, Alabama, USA; 9Ist. TeSRE, CNR, Bologna; 10Dept. of Physics and Astronomy, SUNY Stony Brook, USA 1. Introduction they could not be associated with any SAX, the simultaneous detection of known object population, be it solar- gamma-ray bursts by the GRBM and “New kids on the block” – perhaps system objects, stars from the Milky one of the WFCs proved to be crucial in this is the best way to describe cosmic Way, distant galaxies, or even quasars. identification of their counterparts at gamma-ray bursts among the multitude Their g-ray spectra do not show any ob- lower energies thanks to the rapid (4–5 of object types that can be studied by vious emission or absorption lines that hours) and accurate (~ 3¢) localisation optical telescopes. When they pop up, could help identify the physical proc- afforded by the WFCs. they give a lot of trouble, and for a while esses responsible for the phenomenon. The good scheduling flexibility of take the attention from all the work in Many gamma-ray bursts are surpris- BeppoSAX allows a prompt repointing progress. In this report we will describe ingly bright (in g-rays, 0.1–100 MeV) – of the NFIs at the gamma-ray burst er- how ESO observers are making most for a short moment they easily out- ror circle and efficient search of the out of these exciting opportunities, and shine the sum of all other celestial high- X-ray counterpart. In case of NFI de- how the VLT promises to integrate energy sources. Conversely, near the tection of a gamma-ray burst afterglow space and ground-based observations instrumental detection limit there are candidate in X-rays, the error circle is in a way never achieved before. But fewer events than expected from a ho- reduced to ~ 50² in radius. Meanwhile, first a few words on the historical back- mogeneous distribution through an the rapid dissemination of the error cir- ground. Euclidean space. This lack of faint cle centroid co-ordinates makes fast bursts could be explained by either a follow-up at ground-based telescopes 2. The Basics large Galactic halo population, or a cos- possible. mological origin for gamma-ray bursts. The first error circle thus established Cosmic gamma-ray bursts (GRB) During the early years, it was repeat- was for GRB 960720, but came 45 days are studied from spacecraft, since g-ra- ed over and over that optical observa- after trigger and did not result in the diation does not penetrate the Earth’s tions were needed to finally clinch the finding of a counterpart (in ’t Zand et al., atmosphere. The first detection was distance question. Only thereby could 1997). The next, GRB 970111, was an- made in 1967, but the phenomenon be- one know the total (absolute) amount of nounced promptly, and search in the came publicly known only in 1973, energy emitted, and then start dis- initially large WFC error circle turned up when 16 events were published, in the cussing in earnest, what is the funda- a number of sources, none of which lay Astrophysical Journal. This first paper mental physical process in gamma-ray in the refined error circle established a led to a flurry of theoretical works; for bursts. few days later2. Then, ‘third time lucky’, several years, the rate with which mod- came the burst of 28 February 1997, els were proposed kept pace with the 3. The BeppoSAX Legacy which revolutionised the field. It was de- detection of new events. Likewise, on tected in the WFC and located to a 3¢ the experimental side, numerous initia- In the beginning of 1997, this wish error circle. Prompt reorientation of the tives were taken. To date, more than 60 was fulfilled. The previous year, an satellite and activation of the NFIs 10 spacecraft have carried successful Italian-Dutch consortium had launched hours after the burst led to the localisa- gamma-ray burst detectors. The most the Satellite per Astronomia X, SAX, tion of a bright, previously unknown e fficient was BATSE on board the nicknamed BeppoSAX after the Italian X-ray source (Costa et al., 1997). Compton Gamma-Ray Observatory. Cosmic-ray and X-ray astronomy pio- A second NFI observation 3 days lat- This instrument has detected about one neer Giuseppe Occhialini. er showed that the source had faded by event every day, on average, since its The satellite was primarily intended a factor of 20. Subsequent observa- launch in April 1991. to study the 0.1–300 keV spectrum of tions with the ASCA and ROSAT satel- The hallmark of gamma-ray bursts is galactic and extragalactic sources with lites confirmed that the source faded a diversity: durations can be anything be- four Narrow-Field Instruments (NFIs). with a power-law, I µ (t – t GRB) where tween a few milliseconds and several The scientific payload onboard Beppo- time is counted in days, and a ~ –1.3. minutes. Gamma-ray burst ‘light- SAX also includes a Gamma-Ray Burst This type of decay had indeed been curves’ take numerous shapes, from Monitor (GRBM, 40–700 keV, all-sky predicted by Mészáros and Rees smooth, single peaks to very jagged, coverage), which is the non-imaging (1997). In their “fireball” model, a mix- multi-peaked graphs. They come from anticoincidence system of the high-en- ture of matter and photons expands at ever new directions, and until recently ergy NFI (the PDS), and two coded- mask Wi d e-Field Cameras (WFCs, 2–26 keV, 40° ´ 40° field), pointing at 2In hindsight, the NFI follow-up observation of this 1 This year’s Annual Review of Astronomy and right angles with respect to the NFIs. event shows a faint source. This source is consis- Astrophysics will include a review paper on tent with being an afterglow, given what we have Gamma-Ray Bursts, authored by Jan van Paradijs, Although gamma-ray burst investiga- learnt of afterglows from later events, but was not Chryssa Kouveliotou, and Ralph Wijers. tion was a secondary task of Beppo- recognised as such at the time. 32 tance, a supernova would have peaked Key to this process is the distribution at magnitude R ~ 25.2; i.e. about 50 via internet of positions derived from times fainter than observed for the OT. SAX, RXTE and from the Interplanetary Network, IPN, (consisting of Ulysses 5. A Treasure Hunt of Sorts and NEAR, in addition to near-Earth de- tectors like BATSE, SAX, and Wind). It had thus become obvious, that This service is operated by Goddard ground-based observers can harvest Space Flight Center (NASA) and has beautiful and important results, by more than 400 clients.3 maintaining close links to the space By the time this article goes to press, community. some 18 low-energy (optical, infrared, In this way, observations can start radio) counterparts have been identi- within hours from a gamma-ray burst, fied, worldwide.4 In a similar but larger perhaps even faster, hence raising the number of cases, no optical counterpart expectation that a counterpart will be encountered brighter than the 21st magnitude found for GRB 970228. 3http://gcn.gsfc.nasa.gov Such targets of opportunity are now 4For details, see Jochen Greiner’s site http://www.aip.de/~jcg/grb.html The present colla- Figure 1: The first optically identified gam - pursued with high priority at many ob- boration plans to establish a web site with informa- ma-ray burst, GRB 970228. This image, ob - servatories, including La Silla, Paranal tion on its activities and results; the URLwill be pro- tained at the NTT on March 13, 1997, gave and the Hubble Space Telescope. vided later. the earliest indication that the source was lo - cated in a remote galaxy (van Paradijs et al., 1997). “A” marks the combined light of the Figure 2a: The after - optical transient and candidate host galaxy, glow of GRB 970228. later verified by Hubble Space Telescope ob - The decay of the af - servations. terglow emission, over a wide range of energies is shown from -r a y s to the high speed and makes the interstellar near infrared (K- medium radiate synchrotron emission band). Time is mea - when it impacts on it; thus it is exactly sured in seconds, like a supernova remnant, except rela- following the mo - tivistic speeds are involved. ment of the gam - ma-ray burst, 1997 February 28.1236 4. The First Optical Transient UT. The lines indi - cate the power-law The initial 3¢ localisation from prediction for a spe - BeppoSAX’s WFC allowed independ- cific blast wave mod - ent ground-based optical observations el (from Wi j e r s , to start 20.8 hours after the gamma-ray Rees, and Mészá - burst. The late Jan van Paradijs, and ros, 1997, with re - his two students, Titus Galama and cent data added). Paul Groot, took this opportunity to make the observation, which forever changed gamma-ray burst astronomy.
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