During the first year of operation, the COMPTEL instrument on board the Compton Gamma Ray Observatory detected 22 γ‐ray bursts within its field of view. Spectra and time histories for the strongest 7 of these bursts have been obtained from both the main instrument (0.75–30 MeV) and the burst modules (0.1–10 MeV). The deconvolved photon spectra for the majority of bursts are fit by a single power law model with spectral index between −1.6 and −2.8. One strong burst, GRB 910814, exhibited significant curvature and could not be fit by a single power law model. A broken power law model with a break in slope at ∼2 MeV is a good fit to the time averaged spectrum of this burst. There is evidence, at the 2.8σ level, for a change in the break energy of GRB 910814, from above 2 MeV to below 1 MeV during the first 9 s of the burst

Bennett, K

Collmar, W

Connors, A

den Herder, J W

Diehl, R

Greiner, J

Hanlon, L O

Hermsen, W

Kippen, R M

Kuiper, L

McConnell, Mark L

Ryan, James M.

Schonfelder, V

Steinle, H

Strong, A W

VanDijk, R

Varendorff, M

Williams, O R

Winkler, C

English

UNH Scholars' Repository

University of New HampshireUniversity of New Hampshire Scholars' RepositorySpace Science Center Institute for the Study of Earth, Oceans, and Space(EOS)1994Spectral properties of gamma‐ray bursts observedby COMPTELL O. HanlonESTECK BennettESTECO R. WilliamsESTECC WinklerESTECW CollmarMax-Planck-Institut für extraterrestriche PhysikSee next page for additional authorsFollow this and additional works at: https://scholars.unh.edu/sscPart of the Astrophysics and Astronomy CommonsThis Conference Proceeding is brought to you for free and open access by the Institute for the Study of Earth, Oceans, and Space (EOS) at University ofNew Hampshire Scholars' Repository. It has been accepted for inclusion in Space Science Center by an authorized administrator of University of NewHampshire Scholars' Repository. For more information, please contact nicole.hentz@unh.edu.Recommended CitationSpectral properties of gamma‐ray bursts observed by COMPTEL Hanlon, L. O. and Bennett, K. and Williams, O. R. and Winkler, C.and Collmar, W. and Diehl, R. and Greiner, J. and Schönfelder, V. and Steinle, H. and Strong, A. and Varendorff, M. and van Dijk, R.and den Herder, J. W. and Hermsen, W. and Kuiper, L. and Connors, A. and Kippen, R. M. and McConnell, M. and Ryan, J., AIPConference Proceedings, 307, 275-279 (1994), DOI:http://dx.doi.org/10.1063/1.45809AuthorsL O. Hanlon, K Bennett, O R. Williams, C Winkler, W Collmar, R Diehl, J Greiner, V Schonfelder, H Steinle,A W. Strong, M Varendorff, R VanDijk, J W. den Herder, W Hermsen, L Kuiper, A Connors, R M. Kippen,Mark L. McConnell, and James M. RyanThis conference proceeding is available at University of New Hampshire Scholars' Repository: https://scholars.unh.edu/ssc/179Spectral properties of gammaray bursts observed by COMPTELL. O. Hanlon, K. Bennett, O. R. Williams, C. Winkler, W. Collmar, R. Diehl, J. Greiner,V. Schönfelder, H. Steinle, A. Strong, M. Varendorff, R. van Dijk, J. W. den Herder,W. Hermsen, L. Kuiper, A. Connors, R. M. Kippen, M. McConnell, and J. Ryan  Citation: AIP Conference Proceedings 307, 275 (1994); doi: 10.1063/1.45809 View online: http://dx.doi.org/10.1063/1.45809 View Table of Contents:http://scitation.aip.org/content/aip/proceeding/aipcp/307?ver=pdfcov Published by the AIP Publishing  Articles you may be interested in Spectral curvature in highenergy gamma ray bursts observed by the BATSE largearea detectors AIP Conf. Proc. 307, 318 (1994); 10.1063/1.45886  The energy emission of gammaray bursts and solar flares AIP Conf. Proc. 307, 308 (1994); 10.1063/1.45884  Continuum evolution of bright gamma ray bursts observed by BATSE AIP Conf. Proc. 307, 298 (1994); 10.1063/1.45882  Comparison of BATSE, COMPTEL, EGRET, and OSSE spectra of GRB 910601 AIP Conf. Proc. 307, 283 (1994); 10.1063/1.45880  Observation of gammaray burst spectral at high energy by the Phebus experiment AIP Conf. Proc. 265, 43 (1991); 10.1063/1.42776   This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26SPECTRAL PROPERTIES OF GAMMA-RAY BURSTS OBSERVED BY COMPTEL L.O. Hanlon, K. Bennett, O.R. Williams, C. Winkler Astrophysics Division, ESTEC, NL-2200 AG Noordwijk, The Netherlands W. Collmar, R. Diehl, J. Greiner, V. SchSnfelder, H. Steinle, A. Strong, M. Varendorff Max-Planck-Institut fiir Extraterrestrische Physik, 85740 Garching, Germany R. van Dijk,* J.W. den Herder, W. Hermsen, L. Kuiper SRON-Leiden, P.O. Box 9504, 2300 RA Leiden, The Netherlands A. Connors, R.M. Kippen, M. McConnell, J. Ryan University of New Hampshire, Durham NH 03824, USA ABSTRACT During the first year of operation, the COMPTEL instrument on board the Compton Gamma Ray Observatory detected 22 v-ray bursts within its field of view. Spectra and time histories for the strongest 7 of these bursts have been obtained from both the main instrument (0.75-30 MeV) and the burst modules (0.1-10 MeV). The deconvolved photon spectra for the majority of bursts are fit by a single power law model with spectral index between -1.6 and -2.8. One strong burst, GRB 910814, exhibited significant curvature and could not be fit by a single power law model. A broken power law model with a break in slope at ~ 2 MeV is a good fit to the time averaged spectrum of this burst. There is evidence, at the 2.8a level, for a change in the break energy of GRB 910814, from above 2 MeV to below 1 MeV during the first 9 s of the burst. INTRODUCTION COMPTEL is an imaging v-ray telescope on board the Compton Gamma Ray Observatory (CGRO) operating in the energy range 0.75-30 MeV. The instrument consists of an upper layer of seven detectors (total area 4188 cm2), made from the low-Z liquid scintillator material NE213A along with a lower layer of fourteen NaI cells (total area 8620 cm2). Two independent modes of operation are employed by COMPTEL for the detection of ")'-ray bursts. The "double scatter" (or "telescope") mode, which is the normal imaging mode of the telescope 1, is used to produce images, spectra nd time histories of bursts which occur within the field of view (FOV). , ,  , ,  2 The "single detector" (or burst ) mode is triggered upon receipt of a signal from CGRO-BATSE that a burst has commenced. In this instance, two of the lower NaI detectors (called D2-7 and D2-14) accumulate 6 high time resolution ('burst') spectra for an integration time of 0.5 s, followed by up to 255 'tail' spectra, each of 6 s integration time. Thereafter the burst modules return to ~background mode', integrating spectra for 100 s and awaiting the next BATSE trigger. Also: Astronomical Institute, University of Amsterdam, Kruislaan 403, NL-1098 S J, Amsterdam, The Netherlands 9 1994 American Institute of Physics 275  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26276 Spectral Properties of Gamma-Ray Bursts The burst modules (each with a detecting area of 615 cm 2) operate in overlapping energy regions. The low range detector (D2-14) is sensitive between 0.1 - 1.3 MeV while the high range module (D2-7) covers the energy interval 0 .4 -  10 MeV (122 and 128 energy channels per module respectively). The spectral resolution is 9.6% at 0.5 MeV and 7.0% at 1.5 MeV. This paper presents a summary of spectroscopic results from the burst module data of v-ray bursts which were detected in the FOV of the COMPTEL instrument in the first year of operation. OBSERVATIONS Table I Bright COMPTEL FOV Bursts BATSE Date Seconds (UT) Zenith Fluence (> 0.6 MeV) Trigger (Degrees) (ergs cm -2) 109 910425 2265.75 44.8 5.5x10 -5 143 910503 25452.7 23.0 1.5x10 -4 249 910601 69734.55 8.0 3.5x10 -~ 451 910627 16157.78 10.0 2.6x10 -6 503 910709 41602.13 36.7 2.8x10 -6 678 910814 69273.04 29.2 1.3x10 -4 1085 911118 68258.06 35.7 6.6x10 -6 The BATSE all-sky monitors detected ,-~ 300 cosmic v-ray bursts between April 17 1991 and April 16 1992. The COMPTEL burst detectors (modules D2- 7 and D2-14) are, in principle, 47r sensitive. In practice, however, their field of view is obstructed by a considerable amount of mass from both COMPTEL itself and the rest of the CGRO spacecraft. Only bursts with a zenith angle less than 45 ~ are considered in this paper because the sensitivity of the instrument in the forward direction is high and relatively uniform so the same detector esponse matrix may be used in the spectral deconvolution of these events. COMPTEL detected 22 events within its FOV during the first year of the CGRO mission, of which 7 were bright enough to be subjected to detailed spectral analysis s (see Table I). RESULTS Single power-law, broken power-law (BPL) and optically thin thermal bremsstrahlung (OTTB) functions were used as trial photon models. The latter two were only applied when a single power law resulted in all unacceptable fit. The single power law model is of the form a x E(MeV) -a,  where a is the nor- malisation, or flux at 1 MeV (in photons/(cm 2 s MeV)). The low range module dD2-14) was switched off between May 25 1991 and May 13 1992, so low range ata are only available for GRB 910425 and 910503. The results of the spectral  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26L. O. Han lon  et al. 277 fitting using a power law model are shown in Table II. Table  I I :  Results of spectral fitting Date Duration 0.3-1.3 MeV 0.6-10 MeV (sec) I I 910425 39 2.45 + 0 .2328/382 .1  • 09 13/15 910503 57 2.03 • 0 .0999/812 .12  + 0.09 39/36 910601 33 - - 2.82 • 0.27 9.5/10 910627 2 - - 16 +1"1 - 9 - -0 .8  910709 0.5 - - 2.0 • 09 7/5 910814 33 - - 29 • 0  9  911118 9 - - ~."-*-0.a89 ~A+0.56 7/6 DISCUSSION The 7-ray bursts shown in Table II are adequately described by a single power law model over their entire duration, with the exception of GRB 9108149 The best fit to the single detector data in this case was obtained using a BPL or an OTTB model. Neither model is rejectable on the basis of X 2 statistics9 The spectral break is confirmed by BATSE 4 and SIGMA s. The best fit BPL photon spectrum, averaged over the entire 33 s duration of the event, is shown in Figure 1. Figure 1. Photon spectrum of GRB 910814 derived from the best fit BPL model, l a  errors for one interesting parameter are given for the fit results.  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26278 Spectral Properties of Gamma-Ray Bursts GRB 910814 was intense nough to be analysed in individual time intervals as shown in Figure 2. The intervals marked 1, 2 and 3 represent 1 s time bins, while 4, 5, 6 and 7 are each 6 s in duration. These intervals have been used in the time-resolved spectral fitting of the burst mode spectra to investigate the spectral evolution in this burst. The spectral curvature, requiring a BPL or OTTB model, is only signifi- cant during the first 9 s of the burst (intervals 1 to 4) after which time a single power law model is an acceptable fit. The deconvolved spectra for intervals 1 through 5 are shown in Figure 3. The variation in break energy, from above 2 MeV to below 1 MeV during the first 9 s of the burst is significant at the 2.8a level. Figure 2. Background subtracted time history of GRB 910814 (0.6- 10 MeV) showing time intervals used in spectral deconvolution Figure 3. Photon spectra for time intervals 1-5 of GRB 910814 The time resolved spectral analysis of GRB 910814 leads to the possibility  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26L. O. Hanlon et al. 279 that the spectral break energy decreases in the course of the burst, such that after the first 9 s, it has shifted to below the energy range of the instrument. The EGRET data 6 from this burst support this conclusion, since for the first 7 s of the burst, a single power law fits their data only if the first few points (~2 MeV) are not included in the fit. In subsequent time intervals, a single power law fits the data well over the whole energy range. Thus, the combined COMPTEL and EGRET results provide clear evidence for a time dependent spectral break at MeV energies in this 7-ray burst. CONCLUSIONS The majority of bright GRBs observed by COMPTEL had single power law spectra, with 1.6 < a < 2.82. Only one of the seven showed definite curva- ture, unlike the BATSE sample 4 in which roughly a quarter of bursts exhibited curvature. These are not incompatible statistics, since some of the breaks ob- served by BATSE are below the COMPTEL energy range (e.g. GRB 910601 and GRB 910709). One bright burst, GRB 910814, exhibited a spectral turnover at , ,~  2 MeV which is confirmed by other experiments. There is evidence in the COMPTEL data, supported by EGRET observations, that the spectral break energy of this burst is time dependent. ACKNOWLEDGEMENTS L.H. gratefully acknowledges an ESA fellowship. This research was sup- ported in part by the Deutsche Agentur fiir Raumfahrtangelegenheiten (DARA) under the grant 50 QV 90968. REFERENCES 1. Schgnfelder, V., et al, Ap. J. Suppl. 86, 629 (1993). 2. Winkler, C., et al, Adv. Sp. Res. 6, 113 (1986). 3. Hanlon, L.O., et al, Accepted for publication in A&A , (1993). 4. Schaefer, B.E., et al, Ap. J. 393, L51 (1992). 5. Pelaez, F., et al, Proceedings of INTEGRAL Workshop , (1993). 6. Kwok, P.W., et al, Compton Symposium (AIP 280, 1993), p. 855.  This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at:http://scitation.aip.org/termsconditions. Downloaded to  IP:  132.177.229.79 On: Mon, 24 Nov 2014 18:05:26

Spectral properties of gamma‐ray bursts observed by COMPTEL

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Spectral properties of gamma‐ray bursts observed by COMPTEL

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