The past decade has produced a wealth of observational data on the energy spectra of prompt emission from gamma-ray bursts. Most of the data cover the energy range from a few to several hundred keV. One set of higher energy

observations comes from the Imaging Compton Telescope COMPTEL on the Compton Observatory, which measured in the energy range from 0.75 to 30MeV. We analyzed the full 9.2 years COMPTEL data to reveal the significant detection of 44 gamma-ray bursts. We present preliminary results obtained in the process of preparing a final catalog of the spectral analysis of these events. In addition, we compare

the COMPTEL spectra to simultaneous BATSE measurements for purposes of cross-calibration

Hoover, A. S.

Kippen, R. M.

McConnell, M. L.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2005-10165-6IL NUOVO CIMENTO Vol. 28 C, N. 4-5 Luglio-Ottobre 2005The MeV spectra of gamma-ray bursts measured withCOMPTEL(∗)A. S. Hoover(1), R. M. Kippen(1) and M. L. McConnell(2)(1) Los Alamos National Laboratory - Los Alamos, NM 87545, USA(2) University of New Hampshire - Durham, NH 03824, USA(ricevuto il 23 Maggio 2005; pubblicato online il 28 Ottobre 2005)Summary. — The past decade has produced a wealth of observational data onthe energy spectra of prompt emission from gamma-ray bursts. Most of the datacover the energy range from a few to several hundred keV. One set of higher en-ergy observations comes from the Imaging Compton Telescope COMPTEL on theCompton Observatory, which measured in the energy range from 0.75 to 30MeV.We analyzed the full 9.2 years COMPTEL data to reveal the significant detectionof 44 gamma-ray bursts. We present preliminary results obtained in the process ofpreparing a final catalog of the spectral analysis of these events. In addition, we com-pare the COMPTEL spectra to simultaneous BATSE measurements for purposes ofcross-calibration.PACS 98.70.Rz – γ-ray sources; γ-ray bursts.PACS 01.30.Cc – Conference proceedings.1. – IntroductionThe Compton Gamma-Ray Observatory (CGRO) was launched in April 1991, andmade observations until June, 2000. CGRO was comprised of four detector subsytems,including the Burst and Transient Source Experiment (BATSE) and the Compton Tele-scope (COMPTEL). BATSE was the all-sky burst monitor, and covered the 20 keV–1MeV range. COMPTEL consisted of a layer of liquid-scintillator scattering detectorsand a second layer of NaI absorbing detectors, and covered the 0.8–30MeV range. WhenCOMPTEL was operated in “telescope mode”, gamma-rays Compton scattered in thefirst layer and were completely absorbed in the second layer. This paper discusses resultsobtained from telescope mode observations, where the use of imaging information leadsto nearly background-free data, and a strongly diagonal instrument response function.2. – COMPTEL Gamma-ray BurstsCOMPTEL observed 44 significant gamma-ray bursts during its nine year lifetime.Bursts were identified by searching for excess numbers of measured photons at the times(∗) Paper presented at the “4th Workshop on Gamma-Ray Burst in the Afterglow Era”, Rome,October 18-22, 2004.c© Societa` Italiana di Fisica 825826 A. S. HOOVER, R. M. KIPPEN and M. L. MCCONNELLFig. 1. – Statistical location contours in galactic coordinates for the 44 observed COMPTELbursts.of known BATSE bursts using a maximum-likelihood imaging technique [1]. From theBATSE catalog of 2703 bursts, 828 were within the COMPTEL field of view (directionangle < 65o), 633 had COMPTEL data available with no gaps, and 44 had a signifi-cant point source in the image with a likelihood ratio > 20 (formally corresponding to3.8σ detection significance). Figure 1 shows the COMPTEL statistical (1σ, 2σ, and 3σ)location contours in Galactic coordinates for the 44 observed bursts. Within statisti-cal uncertainties, there are no significant deviations from large-scale isotropy of burstlocations in either galactic or celestial coordinates [2].Figures 2 and 3 show how the 44 COMPTEL bursts fit into the larger sample ofBATSE bursts. Figure 2 plots burst hardness versus duration, and fig. 3 plots burst hard-ness versus fluence. The COMPTEL events tend to have large fluence and populate theharder region of long bursts. These selection biases are expected because the COMPTELburst detection sensitivity depends strongly on the total number of > 700 keV photons.3. – COMPTEL/BATSE joint fit spectral analysisIt is possible that our understanding of the high-energy behavior of burst spectraobtained using only BATSE data could be improved by considering a joint analysis ofthe low-energy BATSE data combined with the high-energy COMPTEL data. BATSEand COMPTEL data were combined for the 44 bursts and jointly fitted with Band’sGRB function [3]. In addition to the four Band function parameters, a fifth parameterwas used in the fit to account for an effective area correction between the BATSE andCOMPTEL detectors. Seven of the bursts could not be fit without imposing a lowerlimit on the β parameter, and these bursts were not used in the analysis, leaving uswith a sample of 37 bursts. All results are time-averaged over the full burst duration,using 16-channel continuous BATSE LAD data and telescope mode binned COMPTELdata. An example of how the COMPTEL data complement the BATSE data at higherenergies is shown for GRB 990728 in fig. 4. Figure 5 shows the time history of GRB990728 as measured jointly by BATSE and COMPTEL. For each of the 37 bursts, thedata were fit once using BATSE data alone, and again using the joint BATSE plusCOMPTEL data. We are interested to see if the addition of the high-energy COMPTELdata will significantly change the results obtained from the low-energy BATSE data,THE MeV SPECTRA OF GAMMA-RAY BURSTS MEASURED WITH COMPTEL 827Fig. 2 Fig. 3Fig. 2. – Hardness versus duration for BATSE and COMPTEL bursts.Fig. 3. – Hardness versus fluence for BATSE and COMPTEL bursts.particularly the β parameter which describes the high-energy tail. In the case of the αparameter, there was no change in the distribution when COMPTEL data was addedto BATSE data, as expected. The β (fig. 6) and Epeak distributions changed slightly,and the χ2/DOF values for the fits improved. A Kolmogorov-Smirnov (KS) statisticaltest indicates that the significance level for the null hypothesis that the BATSE-only andBATSE+COMPTEL data sets are drawn from the same distribution is 1.0 for α andFig. 4 Fig. 5Fig. 4. – GRB 990728 joint fit using BATSE and COMPTEL data.Fig. 5. – Time history of GRB 990728 as measured jointly by BATSE and COMPTEL (dashedlines indicate the interval used for spectral fitting).828 A. S. HOOVER, R. M. KIPPEN and M. L. MCCONNELLβ-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0Events02468101214 BATSEonlyBATSE +COMPTELβ-4 -3.5 -3 -2.5 -2Cumulative Probability (%)00.20.40.60.81BATSEonlyBATSE +COMPTEL = 0.135KSDSignif. = 0.888Fig. 6 Fig. 7Fig. 6. – β parameter for BATSE data only fit (line) and BATSE+COMPTEL data joint fit(dash).Fig. 7. – Cumulative probability distribution for the β parameter. DKS is the KS test statistic.Epeak, and 0.89 for β (fig. 7). Since β describes the high-energy tail, it seems reasonablethat addition of the high-energy COMPTEL data might change the results obtained fromthe BATSE-only data, although it appears the change is not statistically significant. Anidentical analysis was performed using a broken power law spectrum instead of the Bandfunction, and the results were consistent, giving a significance level of 1.0 for the low-energy index, 1.0 for the break energy, and 0.81 for the high-energy index.4. – ConclusionsThe COMPTEL experiment detected 44 gamma-ray bursts during its operationallifetime. It is found that COMPTEL bursts tend to occupy the large hardness ratio/highfluence regions of the BATSE burst dataspace. The BATSE spectral data were extendedto higher energies by combining them with the corresponding COMPTEL data for 37bursts. Kolmogorov-Smirnov tests show that the α, β, and Epeak parameters had nosignificant change due to the addition of COMPTEL data. These results indicate thatfor this sample of 37 detected COMPTEL bursts, BATSE data alone provide a fairrepresentation of the MeV spectral properties. Future work to strengthen this argumentincludes examining whether BATSE spectral data are consistent with the more numerousCOMPTEL non-detections.∗ ∗ ∗This work was supported in part through a grant from the NASA Astrophysics DataProgram.REFERENCES[1] deBoer H. et al., Data Analysis in Astronomy - IV, 59 (1992) 241.[2] Kippen R. M. et al., ApJ, 492 (1998) 246.[3] Band D. et al., ApJ, 413 (1993) 281.

The MeV spectra of gamma-ray bursts measured with COMPTEL

The past decade has produced a wealth of observational data on the energy spectra of prompt emission from gamma-ray bursts. Most of the data cover the energy range from a few to several hundred keV. One set of higher energy observations comes from the Imaging Compton Telescope COMPTEL on the Compton Observatory, which measured in the energy range from 0.75 to 30MeV. We analyzed the full 9.2 years COMPTEL data to reveal the significant detection of 44 gamma-ray bursts. We present preliminary results obtained in the process of preparing a final catalog of the spectral analysis of these events. In addition, we compare the COMPTEL spectra to simultaneous BATSE measurements for purposes of cross-calibration

The MeV spectra of gamma-ray bursts measured with COMPTEL

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