On the continuum shape of gamma-ray burst energy spectra

Adv. Space Res. Vol. 6, No. 4, pp. 27—32, 1986

0273—1177/86 $0.00 + .50 Copyright © COSPAR

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ON THE CONTINUUM SHAPE OF GAMMA-RAY BURST ENERGY SPECTRA E. Eveno,* C. Barat,* G. Chambon,* J.-L. Attéia,* M. Niel,* R. Talon,* I. G. Mitrofanov,** V. Sh. Dolidze,** A. A. Kozlenkov** and A. S. Pozanenko** * Centre d’Etude Spatiale des Rayonnements (CNRS/UPS), B. P. 4346, 31029 Toulouse Cedex, France * *Institute for Space Research, U.S.S.R. Academy ofSciences, 117810 Moscow, U.S.S.R.

ABSTRACT The analysis of SIGNE experiment data shows that the theoretical laws we have tested assuming a single component continuum probably do not describe the real phenomenon of gamma emission and that there is a very fast spectral evolution, possibly on tiaescales down to 16 ms. Moreover we have not found a universal value of s for the relation F = (kT) /3/, but rather a gradual decrease of the spectral hardness index during pulses. INTRODUCTION This paper presents a preliminary spectral analysis of some gamma—ray bursts observed by the SIGNE experiment in order to reveal some basic patterns. It has been carried out in two ways. First we fitted our observed spectra to 4 theoretical models, using a least squares method. Then, we investigated a possible correlation between the time history and spectral evolution, defined either using complete spectra integrated over 0.5 s, or using a hardness ratio which was obtained with a time resolution down to 16 ms. INSTRUMENTATION The SIGNE instrument consists of two 90 mm diameter cylindrical Nal crystals, 37 mm thick, surrounded by plastic anticotncidence jackets. It detects ~ rays from .~50 keV to — 850 keY. It is calibrated in flight using the 511 keV line present in the detector background spectrum. Each detector has 5 channels of gamma burst spectral analysis for 64 s with 500 mm resolution (V13,V14), while a time analyser records the counts in the total range covered by the three lower energy channels every 1/64 s for lós. A more complete description of the instrumentation is given in Barat et al./1/ and Chambon Lt...~i.../2/. ANALYSIS AND RESULTS About 150 GRBs were detected aboard V13—V14, 10 of them on both spacecraft with a peak counting rate higher than 10~counts/s. Of these 10, we investigated 3 bursts which have been yet localised: 88820827, 68821104 1/3,4,5/) and G8821027. In order to minimize statistical errors and to increase the spectral range, the data from the 2 spacecraft were analysed together (10 energy channels) often integrated over 0.5 s, 1, or 2 s whenever the flux was not statistically significant (less than 4 channels for which the signal was above 3 ~ for 1 or 2 spacecraft). These 3 bursts thus yielded 50 high time resolution spectra. The 4 theoretical laws used for fitting were: —power law (PL): dN = A E~’dE —thermal breasstrahlung law (TB): dN = A E—~ e—~/kt dE /6/ —thermal synchrotron law (TS): dN = A e’~’~c’’’dE /7,8/ —exponential law (EL): dN .A e—~’~dE Each count spectrum was fitted by the convolution of each of these laws through the response function of the detector, previously determined by calibration. The goodness of the fit was measured by a least squares method in count space, and following Lampton et al./9/, we rejected all the fits yielding a minimum value of 12 greater than 13.4 (907. confidence for 8 degrees of freedom). 33 of the 50 spectra were satisfactorily fit by at least one of the above models. However, we noted that the bulk of the acceptable fits corresponded to spectra with lower fluxes) in particular no spectra with counting rate > 2000 counts/s had acceptable fits (Figure 1). On the other hand, the most frequently accepted model was the exponential law 152X of trial fits). However this model has no physical basis. For some of the rejected spectra the high value of the minimum chi—square may be explained by the presence of spectral features which are not consistent with our hypothesis of a single 27

28

E. Eveno et al.

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Fig. 1. Frequency of acceptable fits

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FLUX INTEGRATED OVER 0.5 S

component continuum. This is the case, for example, for spectrum n° 1 integrated at the onset of the 3rd peak of 98821104 (Figure 2.a) which displays an excess around 500 keV or a deep cut—off at E > 800 keY (Figure 2.b), and for spectrum no 2 integrated on the leading edge of the 4~’ pulse which displays a slight break in the slope below 100 keY and a suggestion of a high energy tail above 500 keV (Figure 2.c). On the other hand low energy absorption is visible in 9B820827 over a long portion of the burst (Figure 3.b, 3.c, 3.d). Since this is detected on both spacecraft the possibility of absorption by satellite material may be eliminated. However, models are sometimes rejected even after elimination 2 for theof channels spectrum which are contaminated by spectral features. For example,computing the X plotted on Figure 2.c, after elimination of the 3 lower and 2 higher energy bands, we can reject thermal models with confidence > 98%. Moreover, some spectra such as 98821027 no 17 (Figure 4.b), though smooth in shape, are not fit by any of the former models with 90% confidence. ‘1982 IJOU.

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We believed, at this point, that this study indicates that most spectra were fit by these theoretical laws only because of the poorness of statistics.

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Continuum Shape of Gamma-Ray Burst Spectra

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The fact that numerous spectra, particularly those with small error bars, cannot be fit by any simple model may be interpreted in one or more of 3 ways: 1— spectral evolution takes place on time scales much smaller than our integrating time of 0.5 5, 2— the spectra are composite spectra (more than one component), thus defined by more than two parameters, 3— the spectra correspond to a physical process not investigated in this analysis. A test for this last hypothesis is a study of the correlation between the flux and the spectral hardness. Such a correlation should exist on the basis of a few simple hypotheses such as: — evolution of radiating matter with constant mass: n • V = constant — adiabatic expansion of matter (1 or 3 dimensions): T . V’ = constant — constant magnetic field, — or similar hypotheses relating the fundamental parameters of the radiating matter: n (density), V (volume), I (temperature), B (magnetic field).

•

30

E. Eveno et al.

A search for this correlation has been undertaken in two ways. First, from the spectral data of V13—V14 with, as hardness index, either the best fitting value of kT for a thermal—breinsstrahlung law, or a count ratio (Table 1), chosen so as to limit the effect of any possible low energy absorption. Second, using the ratio of counts recorded by the fast time history mimories of V11—Vl2 in two energy windows with a small overlap (Table 1) -on timescales as short as 16 as. This is comparable to the method used by Golenetskii ~j_~j. /3/ but with some important differences: —we have searched for a correlation between the flux and the count ratio, and not a temperature derived from this ratio assuming a TB model, as in )3olenetskii ~ /3/, since there exists a limiting value of the ratio beyond which there is no equivalent temperature /3,5/, —The count ratio was not always defined over 16 ms~ intervals of 16 as were integrated until the signal was greater than 3 r.

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Using this method, we observe a suggestion of a very fast spectral down to 16 ms in some parts of the events (insert of Figure 6.c.).

evolution on timescales

On Figures 5 and 6, we can see a correlation between the hardness ratio and the time history. However it should be noted that for the 3 bursts detected on V13—V14, the main departure from a good correlation is observed in the rising edge of steep peaks, particularly at the beginning of the bursts for example, for 88821104 (Figure 5), flux and hardness are varying together for the 22~d peak, while there is a shift between these 2 values both for the 3’~ pulse, probably-due to the excess around 500 keV (Figure 2.b), and for the 4t1. pulse with sharp~ brief increases in hardness (however, the first increase is not very significant because it is almost in the background). Moreover, we observe on Figure 6 that the hardness

Continuum Shape of Gamma-Ray Burst Spectra

31

ratio decreases on the average both all along- the burst and during each pulse. This result seems to agree with the hard—to—soft evolution observed by Norris et al.15/, with the maximum of mean hardness (computed over 250 ms -— Figure 6.d) not really at the onset of the pulse but in the rising edge. This effect could be explained in terms of gamma emission resulting from relativistic particle injection in matter, with the cooling time for the initial radiation being longer than the injection time, so that the high energy bands fluctuates more rapidly than the lower ones, decreasing the correlation coefficient, and creating a phase shift between hardness and flux. We plan to investigate this effect, by mesuring the cross correlation between them. TABLE 1 Correlation Analysis of Venera Spectral Data Analysis of spectral data integrated over 0.5 s (V13 — V14) GB Spectral hardness index used * 82 08 27 • best fit thermal bremsstrahlung law parameter kT ( computed for energies > 125 keV 82 10 27 hardness ratio (338—786 keV cts) / (73—325 keV cts] 82 11 04 best fit thermal bremsstrahlung law parameter kT ~ we verified that these various methods gave substantially the same results in each case. Analysis of hardness ratio on time scales down to 16 ms (Vii — V12) GB • Vii Vl2 78 11 04 255 — 1230 keV 90 — 495 keV 78 11 19 280 — 1280 keV 38 — 169 keV 78 04 19 345 — 1635 keV 146 — 812 keV 79 11 05 340 — 1655 keV 120 — 665 keV The flux—hardness correlation can be analysed quantitatively. In Table 2, we present the correlation coefficient r obtained as well as the best fit value of a dLogF / dLogl, when the correlation is significant, where I is the temperature deduced either from the TB fit (V13—V14), or from the curve T = f(hardness ratio), assuming as in /3/ (but only at this point) that the emission mechanism is well described by an optically—thin TB emission. TABLE 2 Results of Correlation Analysis

78 78 79 79 82 82 82

GRB /3/ 11 04 11 19 04 lB 11 15 08 27 10 27 11 04

—

-

r 0.9 0.31 0.55 0 09 0:42 0.19 0.43 0.94

a 1.6 1.8 ±0.6 0.7 ~ 0.2 NS 1.4 ±0.6 NS NS 1 ±0.5 —

NS: non significant (significance < 99%)

The constancy of a in several events would be characteristic of a particular thermal mechanism: if there is no thermal equilibrium, there is no particle temperature and a can take any value as a function of the initial conditions or even the energy range. Yet as shown by the results of Table 2, no universal value of m exists, even with a time resolution down to 16 as. This seems to eliminate a thermal process as the basic energy production mechanism, the observed correlation being only a result of thermalisation and being by no mean universal. CONCLUSION -

From this study, we can conclude that: — The spectra are not well described by the trial models. It seems, at least in theses cases, that the emission mechanism is a sum of several distinct mechanisms, and the spectra probably have several components. Moreover, a very fast spectral evolution is observed. — From the study of the hardness index, it seems that there is a shift between hardness and luminosity with the hardness index reaching its maximum before the flux. Moreover, it is probable that there is no relationship between luminosity and energy in the basic energy production mechanism.

.

32

E. Eveno et al.

REFERENCES 1.

C. Barat, G. Chambon, K. Hurley, M. Niel, G. Vedrenne, LV. Estulin, and V.M. Zenchenko, The SIGNE—2 Franco—Soviet interplanetary gamma—ray network, Space Sci. Instr. 5, 229 (1981).

A.V. Kuznetsov, burst experiment

2.

6. Chambon, K. Hurley, (I. Niel, G. Vedrenne, V.M. Zenchenko, A.V. Kuznetsov, and I.V. Estulin, SIGNE II PIP: a Franco—Soviet gamma—ray burst satellite experiment, Smace Sci Instr. 5, 73 (1979).

3.

S.V. I3olenetskii, E.P. Mazets, R.L. Aptekar, and V.N. Ilyinskii, Correlation luminosity and temperature in gamma—ray burst sources, Nature 306, 451 (1983).

4.

E.P. Mazets, S.V. Golenetsk-ii, Vu.A. Guryan, R.L. Aptekar, V.N. Ilylnskii~ and V.N. Panov, Energy spectra of the cosmic gamma—ray bursts, in: positron—electron pairs in Astrophysics. AlP Conf. Proc. N°101, eds. M.L. Burns, A.K. Harding, and R. Ramaty, AlP Press, New York 1983, p. 36.

5.

J.P. Norris, G.H. Share, D.C. Messina, B.R. Dennis, U.D. Desai, T.L. Cline, S.M. Matz, and E.L. Chupp, Spectral evolution of pulse structures in gamma—ray bursts, Ao. 1. 301, 213 (1986).

6.

E.P. Mazets, S.V. Golenetskii, V.N. Ilyinskii, Yu.A. Guryan, R.L. Aptekar, V.N. Paflov, l.A. Sokolov, Z.Ya. Sokolova, and T.V. Kharitonova, Cosmic gamma—ray burst spectroscopy, Astrophys. Space Sci. 82, 261 (1982).

7.

E.P. Liang, Emission (1982).

between

mechanism and source distances of gamma—ray bursts, Nature 299, 321 -

B.

E.P. Liang, I.E. 3ernigan, R. Rodrigues, Analysis of the KONUS bursts with thermal synchrotron model, Ap. 1. 271, 766 (1983).

9.

M. Lampton, 8. Margon, 208, 177 (1976).

and

S. Bowyer,

catalog of

gamma—ray

Parameter estimation in X—Ray astronomy, An. .1.