Gamma Ray Bursts (GRBs) are short bursts of very high energy photons which were discovered in the late 1960s. Ever since their discovery, scientists have wondered about their origin. Nowadays it is known that they originate outside the Milky Way because of their high red shift rst measured in the afterglows thanks to the Beppo-SAX satellite and ground-based observations. However, theoreticians still do not agree about the mechanism that generates the bursts, and different competing models are animatedly debated. Current GRB experiments include the Swift satellite and the Pierre Auger Observatory that could detect GRBs with an increase of the background. A forthcoming dedicated experiment is GLAST, a satellite observatory for detecting gamma rays with energies up to 300 GeV, whose launch is scheduled for early 2008

Alagoz, E

Boiti, D R

Carrillo, C

Gallo, Palomino J L

Golup, G T

Grimes, M

Herrera, Mora C

López, Vega A

Wicht, J

CERN Document Server

Gamma ray bursts: observations and theoretical conjectures∗E. Alagoz, D.R. Boito, C. Carrillo, G.T. Golup, M. Grimes, C. Mora Herrera, J.L. Palomino Gallo,A. Vega López, J. WichtStudent Discussion GroupAbstractGamma Ray Bursts (GRBs) are short bursts of very high energy photons whichwere discovered in the late 1960s. Ever since their discovery, scientists havewondered about their origin. Nowadays it is known that they originate outsidethe Milky Way because of their high red shift rst measured in the afterglowsthanks to the Beppo-SAX satellite and ground-based observations. However,theoreticians still do not agree about the mechanism that generates the bursts,and different competing models are animatedly debated. Current GRB exper-iments include the Swift satellite and the Pierre Auger Observatory that coulddetect GRBs with an increase of the background. A forthcoming dedicatedexperiment is GLAST, a satellite observatory for detecting gamma rays withenergies up to 300 GeV, whose launch is scheduled for early 2008.1 Observational progressGamma Ray Bursts (GRBs) were discovered by the US military satellite Vela in 1967 while monitoringfor clandestine tests of nuclear weapons. This information was not released to the scientic communityuntil the following decade. GRBs are among the most energetic known objects ever measured and theirorigin and mechanism became a puzzle to the community [1, 2].Fig. 1: GRBs measured by BATSE in Galactic coordinates [3]In the early 1990s the Compton Gamma Ray Observatory (CGRO) was launched and started tounravel these questions. It included four different detectors to measure gamma rays, one of which wasBATSE with a 4pi sr eld of view, 0.31.2 MeV energy range, and a precision of 2 degrees.At that moment, not only was the origin of GRBs unknown, but also their location in space, andmany scientists speculated that they were explosions occurring within our Galaxy. When the map show-ing an isotropical distribution of the GRBs as measured by BATSE was published (Fig. 1), it was clear*Work performed as a student project under the supervision of E. Nardi.287that there were two possibilities: either they were originating very near to us, or they had a cosmologi-cal origin. With a lack of candidates within such short distances, the latter represented the most likelyhypothesis, and the need to measure the red shift (z) of GRBs became evident to prove denitely theirextra-Galactic nature.An intriguing property of GRBs is the absence of any clear pattern associated with the gamma-rayemissions, that is all GRBs have different behaviour in time. This is apparent from Fig. 2 (left) wheredifferent examples of GRB brightness vs. time measured by BATSE are shown. The histogram in Fig. 2(right) illustrates the different durations of GRBs. There are two distinctive types: short GRBs (lastingless than 2 seconds) and long GRBs (more than 2 seconds). The time duration represents the mainclassication property of GRBs, and is thought to be related to the mechanism for their generation.Fig. 2: (Left) different examples of brightness vs. time [3] and (right) histogram of duration of GRBs [4]The rst experiment that allowed the measurement of the red shift of a GRB was the Beppo-SAXsatellite in 1997. Beppo-SAX carried out a GRB follow up in X-ray wavelengths, and its observationsallowed ground based telescopes to carry out follow ups in the optical and in other wavelengths, a fewhours after the burst (afterglow measurements). These combined observations were rendered possiblesince Beppo-SAX had arcminute precision in locating the position of the burst.In Fig. 3 the rst measurement of a GRB afterglow is shown. Two X-ray images are presented,corresponding to 8 hours and 3 days after the GRB. The red shift measured (z = 0.865) nally conrmedthe cosmological origin of GRBs.Since that rst measurement, several GRBs with higher red shift have been detected. This is shownin Fig. 4 where we can see that GRBs with z > 6 have been spotted recently.2 Overview of the theoretical statusAt present, since some of the GRBs have had their red shift measured, it is possible to estimate the amountof energy released in these processes. Assuming an isotropic emission, the radiated energy lies between2E. ALAGOZ, D.R. BOITO, C. CARRILLO, G.T. GOLUP, M. GRIMES, C. MORA HERRERA, . . .288Fig. 3: First afterglow measured by Beppo-SAX in 1997 [3]Fig. 4: Highest red shift z measured for GRBs, galaxies and quasars as a function of the year [5]1051 and 1054 ergs. This is of the same order of magnitude as the solar rest mass (m¯ = 2× 1054 ergs)and therefore GRBs are commonly associated with catastrophic stellar events [2, 6, 7]. Models aimed atexplaining the GRBs should address several questions, for instance they should predict the gamma-rayemission and the observed afterglow, they should state whether or not the emission is isotropic, theyshould explain the GRBs’ short time variability, the spectrum of the prompt GRBs’ emission and theobserved afterglows.Two main scenarios have been proposed to explain the progenitors of the bursts, namely the merg-ing of the partners of a binary system and the collapse of massive stars. The former is now assumed toproduce the short bursts and the latter the long ones [2]. In the following we briey describe two modelsthat have been put forward to explain the GRBs’ emissions for the long bursts: the cannonball model andthe reball model.The cannonball model, which was introduced by de Rœjula and collaborators [7], assumes that,after the explosion of a massive star, a remanent rotating compact core accelerates matter from a sur-rounding accretion disk and ejects it as high velocity ‘cannonballs’ (Lorentz γ-factor ∼ 1001000). Asthe expelled matter travels, its electrons scatter the ambient light up to gamma-ray energies through in-3GAMMA RAY BURSTS: OBSERVATIONS AND THEORETICAL CONJECTURES289verse Compton scattering [7]. The pulses observed in the GRB spectrum are due to the emission ofseveral cannonballs.In the reball model, a fraction of the gravitational energy from the collapse, together with theenergy from the matter accreted by the dense core (normally a black hole), is converted into the kineticenergy of a reball. The reball is a shell of matter consisting of e±, γ and baryons. The promptemission is due to collisions between different parts of the reball, and the afterglow emission is dueto the collisions between the fast moving reball and the interstellar medium. Within this framework,synchrotron radiation is emitted, and therefore the GRB spectrum should exhibit a power-law behaviour[2, 6].3 Present and future GRB observationsGRB prompt emissions have been studied extensively from keV and MeV energy ranges up to 10 GeV,but energies above this range still remain mysterious. There is ongoing research to explore the unknownhigher energy regions. In this section, a ground-based experiment at the Pierre Auger Observatory, andthe Swift and GLAST satellite experiments are briey described as examples of the current and futureexperiments to study GRBs.The Pierre Auger Observatory, which studies ultra-high-energy cosmic rays (energies above about1018 eV) could make a contribution in the detection of GRBs at ground level using a method known asthe ‘single particle technique’ [8, 9]. GRB emissions produce cascades in the atmosphere but they arenot directly detectable at ground level because of their low energies. However, these emissions increasethe background rate of all the running detectors at the same time, and this could allow their indirectdetection. The Pierre Auger Observatory is a 3000 km2 array located in Argentina with 1600 waterCherenkov tanks which are sensitive to photons. There is an ongoing project known as LAGO whoseobjective is to place Cherenkov tanks at higher altitudes (over 3000 m above sea level) which wouldincrease their sensitivity to these emissions.The running experiment Swift [10, 11] is a robotic spacecraft which was launched in November2004. The experiment was built and is operated by an international collaboration. Swift has alreadyobserved more than 270 GRBs. It has the particularity of being able to reorientate itself to point itsinstruments in the direction of a detected GRB.Swift is composed of three detectors: the Burst Alert Telescope (BAT), the X-Ray Telescope(XRT) and the UV/Optical Telescope (UVOT). BAT is a gamma-ray telescope operating in the 15150 keV energy range. Its goal is to detect GRBs and calculate their positions in the sky with a precisionof 1 to 4 arc-minutes in order to point the two other detectors in the burst direction. XRT is an X-Ray(0.310 keV) telescope used to take images and study spectra of the GRB afterglow. XRT enhancesthe precision on the GRB position to about 3.5 arc-seconds and then performs a long term study of theafterglow. UVOT is an UV/Optical Telescope (170650 nm) taking images and measuring spectra ofGRB afterglows. These images can again enhance the GRB position (0.32.5 arc-seconds) and are usedto measure red shifts.Swift is therefore a full-featured GRB observatory: it can detect GRBs, study their afterglows, andmeasure precisely their positions and red shifts in order to help in understanding their origins.The Gamma ray Large Area Space Telescope (GLAST) is scheduled for launch in early 2008. Itwill reside in a low Earth orbit of 550 km, with a minimum operational lifespan of 5 years (expected10 years). General Dynamics will build the spacecraft, and install the scientic instruments provided byseveral international institutions.The satellite will carry two instruments, the GLAST Burst Monitor (GBM), and the Large AreaTelescope (LAT). The observable energy range for GBM will be between 8 keV and 25 MeV, while LATwill extend the sensitivity to much higher energies, covering a range between 20 MeV and 300 GeV [12].In this respect LAT will provide an important extension in energy sensitivity with respect to4E. ALAGOZ, D.R. BOITO, C. CARRILLO, G.T. GOLUP, M. GRIMES, C. MORA HERRERA, . . .290AGILE, the Italian Space Agency satellite that was launched just a few months ago (on 23 April 2007)and that is currently taking data in the energy band 30 MeV to 50 GeV [13].Of the two GLAST instruments, LAT is intended as a multipurpose detector, while GBM is gearedparticularly towards GRB detection. The GBM will observe the whole sky not obscured by the Earth [14].GLAST is expected to observe upwards of 200 GRBs a year [15].For the rst year of operation, GLAST will perform a full sky survey by rocking the focus directionas it orbits. During this operation there will also be the opportunity for pointed observations, should anobject of high interest be seen. After the completion of the full sky survey, GLAST will operate in amixture of the two modes [12]. When either of the two instruments detects a GRB, the observatory willdirect itself towards the source automatically.AcknowledgementsWe would like to thank Professor Enrico Nardi for supervising us during the school discussion sessions,Dafne Guetta for revising the manuscript, and Egil Lillestłl and Danielle MØtral for organizing the 4thCERNCLAF School of High-Energy Physics 2007 in Chile. Some of the authors received nancialsupport from CERN.References[1] L. Bergstrom and A. Goobar, Cosmology and Particle Astrophysics, (1999) Wiley/Praxis.[2] T. Piran, Rev. Mod. Phys. 76 (2004) 1143; P. Meszaros, Rep. Prog. Phys. 69 (2006) 2259.[3] http://cossc.gsfc.nasa.gov/docs/cgro/images/epo/gallery/grbs/index.html[4] http://f64.nsstc.nasa.gov/batse/grb/duration/[5] N.R. Tanvir and P. Jakobsson, Observations of GRBs at High Redshift, arXiv:astro-ph/0701777(2007).[6] M. Topinka, Models of Gamma-ray Bursts, http://sirrah.troja.mff.cuni.cz/~toast/[7] A. Dar and A. De Rœjula, Phys. Rep. 405 (2004) 203.[8] D. Allard (Pierre Auger Collaboration), 29th International Cosmic Ray Conference, 2005, Pune,India.[9] S. Vernetto, Astropart. Phys. 13 (2000) 75.[10] N. Gehrels et al. (Swift Collaboration), The Swift Gamma-Ray Burst Mission, Astrophys. J. 611(2004) 1005.[11] Swift Collaboration website: http://swift.gsfc.nasa.gov.[12] GLAST Project, Science Requirements Document, http://glast.gsfc.nasa.gov/science/433-SRD-0001\_CH-03.pdf[13] AGILE website, http://agile.rm.iasf.cnr.it/index.html[14] NASA GLAST website, GBM Specications and Performance Compared with BATSE, http://glast.gsfc.nasa.gov/science/instruments/table1-2.html[15] J.P. Norris, J.T. Bonnell, G.F. Marani and J.D. Scargle, GLAST, GRBs, and Quantum Gravity,arXiv:astro-ph/9912136 (1999).5GAMMA RAY BURSTS: OBSERVATIONS AND THEORETICAL CONJECTURES291

Gamma Ray Bursts: Observations and Theoretical Conjectures

Gamma Ray Bursts: Observations and Theoretical Conjectures

Abstract

Similar works

Full text

Available Versions

CERN Document Server