Manchanda, R K

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29th International Cosmic Ray Conference Pune (2005) 4, 439442Galactic and Extragalactic components in Gamma ray burstsR. K. ManchandaDept. of Astronomy Astrophysics, Tata Institute of Fundamental ResearchHomi Bhabha Road, Mumbai 400 005, IndiaPresenter: R. K. Manchanda (ravi@tifr.res.in), ind-manchanda-R-abs1-og24-oralThe origin of the Gamma ray bursts is still an unresolved issue even after 32 years since the discovery of thefirst event. From the recent data from the HETE satellite, it has become obvious that some of these eventsare definitely associated with Supernova explosions and some of these have cosmological distances. However,no simple classification scheme fit the data. We present an analysis of the 	 distribution of theGRBs and argue that data suggest the presence of two components which may be associated with galactic andextragalactic origin.IntroductionGamma ray bursts are the most energetic explosions known in the Universe since the Big Bang. GRBs areunpredictable flashes of electromagnetic radiation in the X- and  -ray energy region around 100 keV, comingfrom any direction, their time durations range from few millisec to seconds and during the time they last, theyoutshine all other celestial sources combined. The mean rate of detected GRBs is around one per day. If the skyexposure time and the actual solid angle of the detectors are taken into account, this mean rate typically raisesto a few (2-3) per day. Since its discovery in 1973, the gamma ray burst astronomy has gone through manyrevolutions. The first radical change was brought about by the BATSE detectors on board CGRO satellitewhich was launched on 1991. In its 9 years operation, the BATSE team cataloged 2704 events of differentshapes, sizes and peak fluences (see fig 1). Statistical studies show that about 80% of all GRB last less thanone minute, and that there is a class of short-duration GRB which last less than a few seconds and that thedistribution is well described by a bimodal function, with the two peaks around 0.3s and 30s, respectively. Abreakthrough in the understanding of GRB came with the discovery of GRB afterglow in the X-ray [1], optical[2], and radio [3] bands. The detection of the host galaxies and the measurement of their redshifts showed thatsome of the long duration GRB events are related to the final stage in the evolution of massive stars, takingplace at cosmological distances. High Energy Explorer Satellite 2 (HETE-2) has dramatically changed ourunderstanding of GRB phenomena. Apart from the discovery of short-hard bursts, X-ray rich bursts, opticaldark bursts, it confirmed the GRB-SN connection and that some of the long bursts are associated with Type 1ccore collapse supernovae. It has provided evidence that the isotropic-equivalent energies and luminosities ofGRBs are correlated with the redshift, implying that GRBs and their progenitors evolve strongly with redshift.Both of these results have profound implications for the nature of GRB progenitors and for the use of GRBsas a probe of cosmology and the early universe [4]. However, 8 years after the discovery of the first afterglow,spectroscopic redshifts have been determined only for 40 bursts among 263 localized GRBs of which about100 have an optical, X-ray or radio afterglow. The spectral data too does not provide clue to the origin ofgamma ray bursts. Although GRBs release their energy mostly in the 50-1000 keV band, photons down to afew keV and up to 18 GeV have been detected in some events. The non-thermal spectra are well described bythe Band law (5], that is a smoothed broken power law. This functional form fits the burst spectra satisfactorilyin the energy range from 10 keV to 100 MeV and the break energy typically ranges from 100 keV up to someMeV. The properties derived from the afterglow observations also reveal only limited information about thedistribution of GRB sources. Though the temporal nature of the afterglow can be explained globally, within theframework of the relativistic fireball models, first proposed by [6] [7] [8], the extreme characteristics of GRBsseem to lead to a paradox called “the compactness problem” and also the re-brightening of the afterglow is not440 R. K. Manchandayet understood. The gamma rays in a GRB are believed to be produced by internal shocks taking place in acollimated jet directed close to the line of sight, while the afterglow would result from the interaction of the jetwith the surrounding medium. However, the energy release of about  ergs cm  , the time durationof ﬀﬂﬁﬃ sec and temporal structures of millisec duration within the time history, remain some of thenagging problems still to be resolved. The successful phenomenological models too, can only produce globalpicture and a composite understanding of all observed parameters and the uniqueness and individual nature ofeach of these events depends on the nature of the progenitors and their distribution in the universe.Figure 1. A collage of the Spatial, Temporal and Spectral properties of GRBs (top) time duration, uence distribution,(bottom) typical energy spectrum, variety in pulse shapes, angular distribution and log N-log S for T ! events,Source distribution of GRBsThe angular distribution of the 2704 bursts from BATSE catalog as seen in Fig 1 suggests an apparent isotropicdistribution [9], although earlier data did indicate concentration near the galactic plane [10] [11]. The observeddipole and quadrupole relative to the galaxy are perfectly consistent with null values, i.e. with an isotropicdistribution. This property clearly supports an extragalactic origin or, at least, from an extended dark halosurrounding our Galaxy. However, the number of faint bursts in the BATSE GRB sample are fewer thanexpected for a homogeneous distribution if one supposes that GRBs are standard candles. Taking that allbursts have the same luminosity L in a Euclidean geometry and if S is the peak flux, V the volume of thesphere whose radius R is the distance to the GRB source; then for a minimum detectable flux S "ﬂ#%$ for a givendetector, there is the maximum volume V "'&)( , whose radius can be obtained from+*-, .0/2143657ﬀ8 and underthese assumptions, 939*-:<;/>=?*-, .3ﬃ+@ACBDGalactic and Extragalactic components in Gamma ray bursts 441The ratio expressed by above equation can be used for testing the spatial distribution of a GRB events for ahomogeneous distribution within a Euclidean space. The E3EF"'&)( distribution is uniform in the allowed rangeHG939*-:;GI with a mean value939*-:; /+JLK . However, in the case of BATSE bursts the correspondingmean value is939*-:; /+J MMNOPJ P [ 12]. Similarly, the hypothesis for isotropic or a disk distribution of theGRBs can be tested with the log N-log S curve by considering the cumulative distribution N( Q S) representingthe number of bursts, whose flux intensity is greater than S and which in the case of a homogeneous distributionis characterized by the N( Q S) = S ACBD . The best fit parameters for a large sample of BATSE bursts, clearlyshow that the spatial distribution of GRBs is not consistent with the homogeneous case since there are fewerfaint bursts than expected on this hypothesis. The distribution of short duration bursts in the BATSE data isconsistent with the homogeneous distribution and the short bursts also show more symmetrical profiles than thelong ones. Similarly, the observed time durations of the events in the BATSE sample, there is some evidencefor the existence of two classes. The T R(calculated by taking the interval going from the time at which thetotal fluence is at 5% to the time at which it is at 95% of the overall burst fluence) distribution shows a bimodalbehaviour, i.e. short bursts T RTSMU and long ones with T RQVMU [13].The significant deviation from the homogeneous distribution of sources therefore requires a fresh look to ex-plain the observed data. The fundamental questions to be answered are whether, the present sensitivity of theinstruments has been sufficient to detect true GRB frequency and if there is a class of events which have defieddetection. For example, some of the weak burst events reported in literature have no correspondence in theBATSE data. Second, whether observed transient events are a mixture of completely independent phenom-ena only related in their transient nature or third, if the log N-log S curve consists of a mixture of differentpopulations having different spatial distribution.Two component modelWe propose that GRB sample indeed consists of two populations namely, extragalactic and galactic. Thecontribution from the galactic component is small, such that observed angular distribution has an apparentisotropy. In figure 2, I have plotted the GRB frequency distribution of peak fluxes for the events published in4B BATSE catalog. It is seen from the figure that a simple 3/2 scaling law does not fit the data. The solid linein the figure is the best fit for a mixture of two populations, one of which follows a 3/2 scaling law and thesecond, a Gaussian distribution with a peak flux at 28 counts and a relative number density of 7.5% of the firstcomponent. The composite functional form is given by;0=Q+@W/YXKﬀCZ [	\^]'_a` b%cdeZ fDghB)R<ikjIt is seen from Fig. 2 that a mixed population functional form fits the data quite well. A cumulative Gaussiandistribution of galactic component simply reflects the radial distribution of matter in the galaxy and is similarto the distribution of galactic supernovae. In addition, a small spread in the absolute luminosity of GRBevents will further broaden the source distribution [14]. Assuming that the peak in the gaussian distributioncorresponding to 28 BATSE counts represents an average GRB luminosity of l>) ergs, similar to the energyrelease in type II supernovae and that the bursts are located at an average galactic distance of 6 kpc, then theobserved %mn %distribution of the GRB peak fluxes corresponds to an absolute luminosity scale betweenloMqprRﬁlsMqpVD ergs. The average distance for the weakest burst thus corresponds to loMK kpc,thereby, favouring a halo model for the source distribution.442 R. K. ManchandaFigure 2. Log N-Log S curve for the gamma ray bursts, The dotted line represent the -3/2 distribution while the solid linegives the composite t.In conclusion, irrespective of the fact that some of the long bursts are associated with the supernovae in othergalaxies and may be at cosmological distance, the statistical analysis of the observed burst parameters clearlysuggest that the observed sample consists of both galactic and extragalactic components.AcknowledgementsIt is a pleasure to thank BATSE team for making the archival data available on the WEB. I also than themembers of the SIX team who provide a continuous support for research activities.References[1] E. Costa, F. Frontera, J. Heise et al., Nature, 387, 783 (1997).[2] J. van Paradijs, P. J. Groot, T. Galama et al., Nature 386, 686 (2003).[3] D. A. Frail, S. L. Kulkarni, S. R. Nicastro et al., Nature, 389, 261 (1997).[4] D. Q. Lamb et al., Nuclear Physics B (Proc. Suppl.) 132, 279, (2004).[5] D. Band, J. Matteson, L. Ford et al., Ap. J., 413, 281 (1983).[6] G. Cavallo & M. Rees, MNRAS, 183, 359 (1978).[7] J. Goodman, Ap. J., 308, L43 (1986).[8] B. Paczynski, Ap. J., 308, L43 (1986).[9] W. S. Paciesas, Baltic Astronomy, vol. 13 pp. 187-192 (2004)[10] E. P. Mazets et al., Ap & SS, 80, 3 (1981).[11] J-L Attia et al., Ap. J., 320, L105 (1987).[12] C. Kouveliotou et al., Ap. J., 413, L101 (1993).[13] T. Cline et al., Ap. J., 527, 827 (1999).[14] R. K. Manchanda & D. Ramsden, Nature, 266, 425 (1977).

Galactic and Extragalactic components in Gamma ray bursts

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