Gamma-Ray Bursts: the Most Powerful Cosmic Explosions L. KAPER1, A. CASTRO-TIRADO 2, A. FRUCHTER 3, J. GREINER4, J. HJORTH 5, E. PIAN 6, M. ANDERSEN7, K. BEUERMANN 8, M. BOER9, I. BURUD 3, A. JAUNSEN 5, B. JENSEN 5, J.M. CASTRO CERÓN10, S. ELLISON11, F. FRONTERA12, J. FYNBO13, N. GEHRELS14, J. GOROSABEL2, J. HEISE15, F. HESSMAN16, K. HURLEY17, S. KLOSE16, C. KOUVELIOTOU18, N. MASETTI12, P. MØLLER13, E. PALAZZI12, H. PEDERSEN 5, L. PIRO19, K. REINSCH 8, J. RHOADS 3, E. ROL1, I. SALAMANCA1, N. TANVIR20, P.M. VREESWIJK1, R.A.M.J. WIJERS1, T. WIKLIND 21, A. ZEH16, E.P.J. VAN DEN HEUVEL1

1Astronomical Institute “Anton Pannekoek”, Amsterdam, The Netherlands; 2IAA-CSIC, Granada, Spain; 3STScI, Baltimore, USA; 4MPE Garching, Germany; 5Copenhagen University Observatory, Denmark; 6Trieste, Italy; 7Potsdam, Germany; 8Universitäts Sternwarte, Göttingen, Germany; 9CESR/CNRS, Toulouse, France; 10Real Instituto y Observatorio de la Armada, Cádiz, Spain; 11ESO Paranal, Chile; 12Bologna, Italy; 13ESO Garching, Germany; 14NASA/GSFC, Greenbelt, USA; 15SRON Utrecht, The Netherlands; 16Thüringer Landessternwarte (Tautenburg, Germany); 17UC Berkeley, Space Sciences Laboratory, USA; 18NASA/MSFC, Huntsville, USA; 19Rome, Italy; 20Hertfordshire, United Kingdom; 21Onsala, Sweden

1. Introduction Metzger et al. 1997), active collabora- States of America. The nodes take tions between many observatories turns for being “on duty” for periods of Gamma-ray bursts (GRBs) are brief around the world has resulted in a time- two weeks. Starting September 2002 flashes of cosmic γ-rays, first detected ly and detailed study of several dozen our collaboration will be supported by a in data from the US military Vela satel- GRBs. From the start ESO has played Research and Training Network funded lites in 1967 that were launched to a very important role in the identifica- by the European Commission for a pe- verify the Nuclear Test Ban Treaty tion and analysis of the optical and in- riod of four years. (Klebesadel et al. 1973). Lacking a dis- frared afterglows. Gamma-ray detec- tance scale, the physical nature of tors onboard spacecraft orbiting the 3. GRBs and their afterglows GRBs remained a mystery for thirty Earth or exploring the solar system pro- years. Their cosmological origin was vide the GRB alerts, which are prompt- GRBs are short flashes of γ-rays, suggested by their isotropic sky distri- ly announced on the Gamma-ray burst with a duration ranging from several bution, demonstrated in the early 1990s Circular Network (GCN); these trigger milliseconds to tens of minutes, and in by the BATSE experiment onboard the immediate follow-up observations at most cases an observed peak energy Compton Gamma-Ray Observatory ground-based observatories. By build- around 100 keV. The daily rate of (Fig. 1). ing up a network of astronomers at ob- GRBs, detectable from Earth, is about However, the definite proof of their servatories all around the world, it be- two (Paciesas et al. 1999). The γ-ray distant, extragalactic nature came from comes possible to quickly (within curves are extremely diverse, the discovery of their rapidly fading af- hours) respond to a GRB alert (from some very smooth, others with numer- terglows at X-ray, optical, and radio one or both hemispheres) and to locate ous spikes. BATSE data showed that wavelengths in 1997, thanks to the and monitor the afterglow. there are two distinct classes of GRBs: alerts of the Italian-Dutch BeppoSAX Here we report on behalf of the a class with a short duration (less satellite. Absorption and emission lines GRACE consortium, the Gamma-Ray than 2 seconds) and relatively hard in the afterglow spectra provided red- burst Afterglow Collaboration at ESO 1. spectra, and a class of long-duration shifts in the range z = 0.1–4.5, corre- The GRACE consortium was awarded bursts with softer spectra. It is impor- sponding to distances of several billion an ESO Large Programme that started tant to note that only afterglows of the light-years out to the edge of the visible in April 2000 and ended in March 2002. latter population have been observed universe. This made it clear that GRBs So far, the GRACE collaboration has so far: it is not known whether short represent the most powerful explosions identified most of the known GRB opti- bursts produce afterglows at all. The in the universe since the Big Bang. cal counterparts and has measured best limit obtained so far is for the There are strong indications that about two thirds of all known GRB red- short/hard HETE-II burst GRB 020531, GRBs are caused by highly relativistic, shifts. Currently, ESO observing time is for which Salamanca et al. (2002) did collimated outflows powered by the col- allocated to GRACE through normal not detect any afterglow candidate lapse of a massive star or by the merg- Target of Opportunity one-semester brighter than V ~ 25, just 20 hours after er of two compact objects. Their enor- programmes. Our collaboration is also the alert. mous brightness make GRBs powerful involved in GRB follow-up programmes In a previous Messenger paper probes of the distant and early uni- awarded observing time on the Hubble (Pedersen et al. 2000) the first scientif- verse, yielding information on the prop- Space Telescope (HST), the Chandra ic break-throughs in this field were re- erties of their host galaxies and the cos- X-ray observatory, and INTEGRAL. ported. The Italian-Dutch BeppoSAX mic star-formation history. GRACE consists of teams of as- satellite played a crucial role in these tronomers (“nodes”) based in Denmark, discoveries by rapidly determining the 2. The GRACE consortium Germany, Italy, Spain, the Netherlands, position of a GRB with arcminute preci- the United Kingdom and the United sion. Arcminute-sized error boxes Since the discovery of the first GRB match the typical field size of modern afterglow on 28 February 1997 (Costa (optical) detectors, so that it became et al. 1997, Van Paradijs et al. 1997, 1http://zon.wins.uva.nl/grb/grace/ feasible to detect the GRB afterglows,

37 Figure 1: This map shows the locations of a total of 2704 GRBs recorded with BATSE on board NASA’s Compton Gamma-Ray Observatory during its nine-year mission. The projection is in galactic coordinates; the plane of the Milky Way Galaxy is along the horizontal line at the middle of the figure. The burst locations are colour-coded based on the fluence, which is the energy flux of the burst integrated over the to- tal duration of the event. Long-duration, bright bursts appear in red, and short-duration, weak bursts appear in purple (credit BATSE team). which fade quickly, on a timescale of Besides HETE-II, satellites in the afterglow identification pipeline is nec- only a few days (the typical time profiles Interplanetary Network (IPN) provide essary. Our consortium has developed are t–α, with α ~ 1–2). burst positions, though at a low rate. such a pipeline using colour-colour dis- The High Energy Transient Explorer II The BeppoSAX mission was terminat- crimination techniques. The efficiency (HETE-II), launched in October 2000, ed on April 30, 2002, exactly 6 years af- of this procedure was demonstrated by was designed to provide very rapid (< 1 ter its launch. the discovery, at ESO, of the optical minute) and very precise positions (er- With Integral (launch October 2002) and near- counterpart of GRB ror boxes down to arcseconds) of both and Swift (2003) the rate of GRB alerts 001011 (Gorosabel et al. 2002a). long- and short-duration bursts. This will definitely increase again. In prepa- Since the advent of rapid (within a mission has been one of the main ration for these missions, robotic tele- few hours to days) GRB locations2, 39 drivers of our ESO Large Programme, scopes are installed at ESO La Silla to alerts resulted in the detection of an but so far only few accurate HETE-II perform prompt follow-up of GRB alerts. positions have become available. Given the expected high data rate, a fast 2http://www.aip.de/People/Greiner/grbgen.html

Figure 2: Left: VLT spectrum of GRB 990712 taken 12 hours after the burst. Absorption lines of Mg I and Mg II are detected, as well as sev- eral emission lines from the underlying bright (V ~ 22) host galaxy (from Vreeswijk et al. 2001). The redshift of the galaxy is z = 0.43. Right: A low-resolution FORS spectrum of the currently most-distant GRB 000131 at z = 4.5 (from Andersen et al. 2000). The redshift is determined from the Lyman break.

38 Figure 3: The fading afterglow of GRB 011121 at near-infrared wavelengths (1.2 µm, J band, Greiner et al. 2002). The field size is 40 × 40 arcseconds, North is up and East is to the left. On November 22, 2001, the afterglow is detected by NTT/SOFI at J = 17.5; 2 days later the afterglow has faded to J = 20.9. On February 9, 2002, the afterglow is not detected anymore (J > 24) in this superb VLT/ISAAC image (see- ing 0.4 arcsecond). The light sources near the position of GRB 011121 might represent some bright regions in the host galaxy.

X-ray afterglow; 32 optical afterglows will provide accurate burst positions 5. The origin of GRBs: possible have been found (of which more than within a few minutes, many such bright progenitors half were discovered by our collabora- early afterglows become detectable. tion), and 20 radio afterglows (status From a variety of arguments, such as July 1, 2002). For many GRBs no after- 4. Evidence for collimation their total energy and the evidence for glow is found. Adverse observing con- collimation, the general expectation is ditions can explain many of these Assuming isotropic emission, the that a system consisting of a black hole non-detections. For example, the opti- measured distances imply peak lumi- and a surrounding accretion torus is cal afterglow of GRB 000630 had faded nosities of 1052 erg/s (1045 Watt). Thus powering the GRB. Such a setting, just to an R-band magnitude of 23 just 21 the peak luminosity of each event cor- before the GRB goes off, can occur in hours after the burst (Fynbo et al. responds to about 1% of the luminosity several ways. One way is the merging 2001), and would certainly have re- of the visible universe! The resulting of a binary neutron-star system, like the mained undetected in searches initiat- energy budget is about 1053 erg, which Hulse-Taylor binary pulsar, or a neutron ed a bit later. In some cases, however, is actually comparable to the total star and a black hole (e.g. Lattimer & another explanation is needed. For ex- amount of energy released during a Schramm 1974, Eichler et al. 1989). ample, the extinction by gas and dust in stellar collapse (). The Another popular model involves the the circumburst environment might hin- measured rate of GRBs corresponds to core collapse of a rapidly rotating mas- der the detection of an afterglow at about one per million year per galaxy. sive star, the “collapsar” model (Woos- rest-frame UV and optical wavelengths, There is mounting evidence, howev- ley 1993, Paczynski 1998, MacFadyen or it may be too faint or even absent. er, that γ-ray bursts are collimated into & Woosley 1999). The nature of these dark bursts re- jets, with opening angles of a few de- There are several indications that mains to be resolved. grees only. This evidence comes from the observed population of GRB after- For 24 GRBs the distance has been the interpretation of the occurrence of a glows, i.e. the long-duration bursts, is determined. The GRB spectrum itself is kink in the slope of the afterglow light best explained by the latter model. featureless (consistent with optically curves, and from the detection (in a few The first indication comes from the thin synchrotron emission), but absorp- cases) of polarization (see ESO press models themselves. The collapsar tion and/or emission lines formed in the release 08/99). Also, the total isotropic model naturally produces bursts that GRB host galaxy, or the position of the energy inferred for GRB 990123 is un- have a duration longer than a few Lyman break (912 Å), provide the red- comfortably high (to be explained by a seconds, but cannot make short bursts. shift (Fig. 2). The majority of redshifts stellar-collapse model), but would be On the other hand, the merger model are in the range between 0.5 and 1.5. reduced by a factor of 500 if the energy can produce short bursts, but has prob- The current record holder, achieved were emitted into a cone with an open- lems keeping the engine on for longer with the VLT, is GRB 000131 with z = ing angle of 5 degrees. than a couple of seconds. The clear 4.5, corresponding to a “distance” Frail et al. (2001) determine the jet distinction between short- and long- (look-back time) of 13 billion light-years opening angle of several GRB after- duration bursts suggests that both (Andersen et al. 2000, ESO press re- glows and show that the spread in the progenitor models may be at work in lease 20/00). output energy distribution of their sam- nature. That GRBs can potentially probe the ple becomes much narrower when tak- very distant universe was demonstrat- ing the collimation into account, with a 6. The supernova connection ed by the impressive burst detected in mean energy output of 2 × 1051 erg. January 1999: GRB 990123 (Akerlof et They suggest that this may be the stan- Another indication that long-duration al. 1999). Within the first minutes fol- dard energy reservoir for all GRBs. GRBs are related to the core collapse lowing the burst, its optical afterglow Though speculative, the implications of of a massive star is that some GRBs reached visual magnitude V = 9, i.e. ob- this finding are great if these intrinsical- seem to be associated with a superno- servable with a pair of binoculars. It ly bright GRBs can be used as standard va (SN). The first evidence for a super- was briefly one million times brighter candles at high redshifts, e.g. to meas- nova connection came from GRB than a supernova. This particular burst, ure the expansion rate of the Universe. 980425/SN1998bw (Galama et al. at z = 1.6, would have been detectable Another consequence of the collimation 1998; ESO Press Release 15/98). This (at its maximum, in the K band) with is that the GRB rate also increases by supernova, approximately coincident in the Very Large Telescope up to a a factor of 500, and that the vast ma- time and position with GRB 980425, redshift of about 15. With Swift, which jority of bursts are not visible. was of the rare type Ic, and at radio

39 wavelengths the brightest supernova Figure 4: The light ever detected. Interpretation of the light curve of GRB 011121 curve indicated that during this super- at optical and nova a black hole was formed (Iwamoto near-infrared wave- lengths. The blue et al. 1998). However, the amount of dotted line prompt γ-ray emission was very mod- represents the contri- est, which makes GRB 980425, the bution to the light of closest GRB at a redshift of z = 0.0085, the GRB afterglow. a peculiar event. About 10 days after In the mean time, evidence has been the burst the light found that several GRB afterglow light curve shows a bump, curves show a so-called supernova indicative of an bump, i.e. a bump in the light curve at a accompanying super- nova,shown in time interval compatible with the rise magenta (Greiner et time of a SN, assuming it has gone off al. 2002). simultaneously with the GRB. The bump would thus represent the SN maximum light. Amongst them is the re- cent burst GRB 011121 (Fig. 3), which was followed up by our collaboration with ESO telescopes in several wave- length bands (Fig. 4, Greiner et al. 2002). Emission lines produced by the host galaxy indicate a redshift of 0.36. occur in regions where active star for- 150,000 light-years. Apparently, the The late-time light curve shows a mation is taking place (see, e.g., the GRB went off in the outskirts of one of bump, some 10 days after the burst VLT observations of the host of GRB the spiral arms. when the GRB afterglow has faded by 001007, Castro Cerón et al. 2002). For several host galaxies the star-for- a factor of 250. After correcting for the Neutron-star binaries do not neces- mation rate has been determined. The flux contribution and extinction due to sarily reside in star-forming regions. emission lines in the VLT spectrum of the host galaxy, the late-epoch light Due to the kick velocities received the host galaxy of GRB 990712 (Fig. 2) curve is consistent with a SN similar to during the two supernova explosions are produced by H II regions in that SN1998bw (taking into account the dif- forming the neutron stars, such bi- galaxy. The strengths of these lines in- ference in redshift). naries are high-velocity objects. As the dicate an (extinction-corrected) star- –1 merging process of the binary, driven formation rate of about 35 M yr by the emission of gravitational radia- (Vreeswijk et al. 2001). For some host 7. GRB host galaxies tion, can take up to a billion years, the galaxies even higher rates of star for- –1 binary may have travelled several thou- mation are claimed, up to 1000 M yr For practically all GRB afterglows sand light-years before producing a (e.g. Berger et al. 2001). These obser- with an accurate location, a host galaxy GRB. vations show that at least some of the has been detected. In nearly all cases With the Hubble Space Telescope GRB host galaxies belong to the class the burst is located within the optical (HST) the morphology of the GRB host of starburst galaxies. (rest frame UV) extent of the galaxy. galaxies is studied in detail. Figure 5 Thus, the observations of GRB host This, in combination with the blue shows an HST/STIS image of the galaxies support the collapsar model. colours of the galaxies, suggests that galaxy hosting GRB 990705 (Andersen Since these galaxies, due to their dis- GRBs originate in galaxies with a rela- et al. 2002). It is a giant grand-design tance, are often very faint, the bright tively high star-formation rate. The col- spiral at a distance of about 8 billion GRB afterglow provides a unique op- lapsar model predicts that GRBs will light-years with a diameter in excess of portunity to study the gas and dust con- tent of the host galaxy. The metallicity and star-formation rate of these rela- tively young galaxies can be measured. Figure 5: HST/STIS As part of our host-galaxy programme, image of the host galaxy of GRB the spectral energy distribution (SED) 990705. It is a of the host galaxy of GRB 000210 has grand-design spiral been determined. Fitting the observed at a distance of 7.6 SED with a grid of synthetic templates, billion light-years. the age (0.2 Gyr) of the dominant stel- North is up and East lar population and the galaxy’s photo- is to the right. The metric redshift (z = 0.84) is determined location of the GRB (Gorosabel et al. 2002b). If the collap- is marked with a sar model is right, the GRB rate is cross; the red circle a direct measure of the formation rate gives the 3σ error on its position. Note of massive stars in the early universe, that, due to the red- an important quantity for the study of shift, the image the star-formation rate as a function of shows the redshift. rest-frame UV light emitted by the galaxy (Andersen et 8. Remaining fundamental al. 2002). questions

Much progress has been made in un- derstanding the GRB phenomenon. However, many fundamental questions

40 remain to be addressed. What is the support which our collaboration has re- Gorosabel, J., et al. 2002b, in preparation. origin and nature of the short-duration ceived from the staff at several obser- Greiner, J., Klose, S., Zeh, A., et al. 2001, bursts? Do they produce afterglows, vatories. GCN 1166. like the long bursts? Are all GRBs Iwamoto, K., Mazzali, P.A., Nomoto, K., et al. associated with a supernova, and if 1998, Nature 395, 672. ApJ so, why do we rarely observe it? Is Lattimer, J.M., Schramm, D.N. 1974, References 192, L145. this related to the difference in collima- Klebesadel, R.W., Strong, I.B., Olson, R.A. tion of the γ-rays with respect to the op- Akerlof, C., Balsano, R., Berthelemy, S., et 1973, ApJ 182, L85. al. 1999, Nature 398, 400. tical light? Are GRBs preferentially Kouveliotou, C., Meegan, C.A., Fishman, Andersen, M.I., Hjorth, J., Pedersen, H., et found in galaxies undergoing a star- G.J., et al. 1993, ApJ 413, L101. al. 2000, A&A 364, L54. burst? MacFadyen, A.I., Woosley, S.E. 1999, ApJ Andersen, M.I., Hjorth, J., Gorosabel, J., et 524, 262. The number of GRBs with optical al. 2002, A&A, submitted. Meegan, C.A., Fishman, G.J., Wilson, R.B., counterparts roughly corresponds to Berger, E., Kulkarni, S.R., Frail, D.A. 2001, Nature 355 the number of supernovae observed ApJ 560, 652. et al. 1992, , 143. before 1934, when Baade and Zwicky Castro Cerón, J.M., Castro-Tirado, A.J., Metzger, M.R., Djorgovski, S.G., Kulkarni, suggested that supernovae might be Gorosabel, J., et al. 2002, A&A, in press. S.R., et al. 1997, Nature 387, 878. powered by the gravitational collapse to Costa, E., Frontera, F., Heise, J., et al. 1997, Paciesas, W.S., Meegan, C.A., Pendleton, G.N., et al. 1999, ApJS 122, 465. a neutron star. The collapsar model, a Nature 387, 783. Eichler, D., Livio, M., Piran, T., et al. 1989, Paczynski, B. 1998, ApJ 454, L45. massive star collapsing to a black hole, Pedersen, H., et al. 2000, The Messenger has now become widely accepted to Nature 340, 126. Fynbo, J.U., Jensen, B.L., Gorosabel, J., et 100, p. 32. explain GRBs. But, just as Baade and al. 2001, A&A 369, 373. Salamanca, I., et al. 2002, GCN 1443. Zwicky failed to anticipate Type Ia su- Frail, D.A., Kulkarni, S.R., Sari, R., et al. Van Paradijs, J.A., Groot, P.J., Galama, T., et pernovae, the collapsar model, even if 2001, ApJ 562, L55. al. 1997, Nature 386, 686. correct, may be incomplete. A challeng- Galama, T.J., Vreeswijk, P.M., Van Paradijs, Vreeswijk, P.M., Fruchter, A.S., Kaper, L., et ing future lies ahead. J., et al. 1998, Nature 395, 670. al. 2001, ApJ 546, 672. We would like to acknowledge the Gorosabel, J., et al. 2002a, A&A 384, 11. Woosley, S.E. 1993, ApJ 405, 273.

Cataclysmic Variables: Gladiators in the Arena R.E. MENNICKENT1, C. TAPPERT1, M. DIAZ 2

1Departamento de Física, Universidad de Concepción, Chile 2Instituto Astronomico e Geofisico, Universidade de São Paulo, Brazil

1. Warriors and weapons action. This is the case for cataclysmic cretion disc around the white dwarf. variables (CVs, Fig. 1), consisting of a The WD reacts by emitting X-rays and Life can be different for individuals red dwarf filling its Roche lobe and UV radiation from the region where the growing up alone or closely interacting transferring matter onto a white-dwarf inner disc reaches its surface. As a re- with members of their community. The (WD) companion. If the white dwarf is a sult, part of the red dwarf atmosphere is same is true for stars, which present in- non-magnetic one, the orbiting gas in- irradiated and mildly heated, and possi- teresting phenomena when they are teracts with itself dissipating energy by bly the upper accretion disc layers forced to evolve together in close inter- viscous forces, forming a luminous ac- evaporated. It has been suggested that after a long-time of having accreted Figure 1: Computer- matter, the outer layers of the white generated view of a dwarf eventually undergo a thermonu- cataclysmic variable clear runaway in a so-called nova ex- star by Andrew plosion. Beardmore. The Here we present some results of our donor, usually a recent research in the area of dwarf no- M-type star, trans- vae, a subclass of cataclysmic vari- fers matter onto a ables showing semi-regular outbursts white dwarf. Due to in time scales of days to years with typ- viscous forces, an ical amplitudes of 2–6 mags. The origin accretion disc is of these dwarf-nova outbursts is not a formed. Colours thermonuclear runaway as in the case represent different of a nova outburst, but a sudden jump temperatures, from in disc viscosity and mass transfer rate the higher (white) to as a result of the hydrogen ionization. the lower ones In this article we do not go into deep de- (red-black). tails. Instead, we will illustrate the ap- Irradiation of the plication of some standard techniques donor by the white dwarf and disc in the field of CVs aimed to explore disc shading are also dynamics and also to reveal the nature shown, as well as of the donor star. The latter point is es- the hotspot in the pecially important to constrain theories region where the of CV evolution. gas stream impacts A typical spectrum of a dwarf nova in the disc. quiescence is shown in Figure 2. It is

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