Gamma-ray burst localization with the international solar polar mission

Adv. Space flee. Vol.3,

pp.203—206, 1983 All rights reserved.

0273—1177/83 $0.00 + .50 Copyright ©COSPAR

No.4,

Printed in Great Britain.

GAMMA-RAY BURST LOCALIZATION WITH THE INTERNATIONAL SOLAR POLAR MISSION K. Hur1ey~ On behalf of the JSPM collaboration (CESR, Toulouse, France; MPI, Garching, FR. G.; Observatoire de Meudon, Meudon, France; SRL, Utrecht, Holland; GSFC, Greenbelt, MD, U.S.A.; UCSSL, Berkeley, CA, U.S.A.) *Centre d’Etude Spatiale des Rayonnements, CNRS/UPS, B. P. 4346, 31029 Toulouse, Cedex, France ABSTRACT The European Space Agency’s Solar Polar spacecraft is scheduled for launch in 1986. A solar X—ray and cosmic gamma ray burst detector will be aboard. Although the solar polar mission will not provide the long baselines originally planned, due to the cancellation of the NASA spacecraft, it is shown that arrival time analysis between the remaining ESA spacecraft and other missions will nevertheless achieve extremely precise localizations. I. INTRODUCTION The Solar X-Ray and Cosmic Gamma Ray Burst experiment on the ESA International Solar Polar Mission spacecraft is being built by the laboratories listed in Table I. The design of the instrument was carried out in close collaboration with R. Lin of the University of California Space Sciences Laboratory, Berkeley, T. Cline and U. Desai of Goddard Space Flight Center, Greenbelt, and J.—C. Heinoux of the Observatoire de Meudori, Meudon. The scientific obTABLE I The Solar X—Ray/Cosmic Gamma Ray Burst Experiment Collaboration

Institute

Personnel

Centre d’Etude Spatiale des Rayonnements, Toulouse, France

K. M. G. F.

Burley Niel Vedrenne Cotin

Max—Planck Institut ft~rExtraterrestrische Physik, Garching, Germany

N. Sommer G. Paschmann

Space Research Laboratory, Utrecht, Holland

C. deJager J. Heise J. van Rooijen

jectives are the localization of gamma ray bursts to accuracies of 10’s of arc seconds, and the study of the height distribution and anisotropy of solar X rays. A twin instrument was to be placed on the NASA ISPM spacecraft, in

order to provide and stereoscopic II.

very long baseline observations observations of solar X rays.

of cosmic gamma ray bursts,

INSTRUMENT DESCRIPTION

Figure 1 shows the engineering model of the experiment. The detector package, which will be mounted on the magnetometer boom, is in two parts, each with two detectors. The two parts are separated by about 20 cm for thermal

203

204

K. Hurley

Fig. 1 Engineering model of the ISPM cosmic gamma ray/solar X ray experiment. At left, two hemispherical CsI(Tl) crystals and PMT assemblies with associated electronics. At right, two Silicon surface barrier detectors in a radiatively cooled mounting structure. 2 x control. One both part contains harrier 0.5(i.e., cm 500pm thick; detectors two are Silicon orientedsurface parallel to thedetectors spin axis towards the earth). Both detectors measure solar X—rays in the 5—15 key energy range, in 4 channels. They operate in the temperature range _350 ~O _550 C, which is reached by passive cooling. The other part consists of two photomultiplier and crystal assemblies, aligned parallel and antiparallel to the spin axis of the spacecraft. Each assembly consists of a 1 mm thick by 5 cm diameter C5I(Tl) hemispherical crystal optically coupled to a curved photocathode photomultiplier. Both detectors operate in the range 15-150 key, with 32 channel pulse height analysis. In the absence of a gamma ray burst, the hard X—ray detectors operate in a low time resolution waiting mode. A sudden increase in the counting rate triggers the storage of high time resolution data (up to 8 ms) into memories which are read out slowly into the telemetry stream. The absolute arrival time of the event at the spacecraft can be determined on the ground to about 5 ms. The sensitivity of the detectors to a gamma ray burst depends on the background and the burst spectra, and the sampling time (which can be changed by command) used by the experiment to detect and increase in the counting ra~e. Typically, the detector should trigger on a flux of about 2x106 erg! cm s, and 50 or more events/year should be detectable at this level. III. LOCALIZATION ACCURACY The accuracy to which a gamma ray burst may be localized by arrival time analysis depends on the timing accuracy for cross correlating the time histories observed on various spacecraft, and on the distance between the spacecraft, via the relation cos 9 = c At/D. Here 9 is the angle between the arrival direction of the burst and the line between the two spacecraft, ~t is the difference in arrival times at the two spacecraft, c is the speed of light, and D is the distance between the two spacecraft. The sources of error in the determination of 9 are: 1) errors in the knowledge of the spacecraft positions, 2) errors in the knowledge of the absolute time of arrival of the event at a spacecraft, and 3) statistical errors in the determination of ~t which arise from the cross correlation of the two time histories. Except for special cases, e.g. very fast rise time events like 1979 March 5 C 1,2,3] , the first two errors are usually negligable, contributing several ms, while the statistical errors are perhaps an order of magnitude greater than this. The statistical errors are related to the absence or presence of fine time structure in the burst time history, which allows two time histo—

The I.S.P.M. and Gamma—Ray Burst Localization

205

ries to be “lined up” accurately. Surprisingly, for this reason, the accuracy of localization is only weakly related to the total intensity of the event. Some very weak events have been extremely accurately localized ~ 4~ , because the event time history displayed a single, short spike, last— mt 50—100 ins: the total number of photons, and thus the intensity, may be quite small, but the concentration of these photons in a single, short spike makes the cross correlation exceptionally accurate. 2 as a function of Figure 2 shows the localization accuracy in spacecraft separation in light seconds. Two arcminutes curves are given: the upper

10O\\\~I

-

CURRENT fl4TERMA1IONA1~\ NETW)RK :VENERA 13,14 \ PVO. SEE. PROGNQZ~.,.k

Fig.2 Localization accuracy as a function of spacecraft sepa— ration. The curves are based on values actually obtained by the international network.

\~

\ 1

~ VENERAS

-

—

NEAR EARtH S/C.

NASA,ESA~SPM. 2VENERAS

LU I-

z C) .~‘

z

0.1-

-

NI

1979 MARCH S

-~

0 -J LL. 0 4: LU

~

<

.01 10

a

I

I

111111

102

I

I

III!)

iO~

SPACECRAFT SEPARATION, LIGHT

I

I

11111

10’

SECONDS

curve is based on average values of error box areas and spacecraft separations for the international network which operated from 1978—1980 and 1982— present [5] . The localization accuracy here is limited by the statistical errors in the cross correlation which were, on the average, 50 ms or more. The lower curve is based on the 1979 March 5 event: here, the cross correlation accuracy is on the order of 0.25 ms, and the errors in the spacecraft position and absolute time calibration dominate. It is precisely for this type of event that the instruments must be designed, since experience has

206

K. Hurley

shown that they yield the most information about the possible counterparts in the optical, radio, and soft X-ray ranges. The cancellation of the NASA ISPM spacecraft has prompted a re-evaluation of the localization accuracy. As Figure 2 indicates, the two spacecraft Solar Polar Mission, in conjunction with near earth and Venera—type missions, would have provided an order of magnitude reduction in the error box areas. However, as the figure shows, the loss of one spacecraft need not have dramatic consequences. If a network exists which consists of the ESA ISPM, a near— earth spacecraft (DISCO and/or Prognoz), Venera, and possibly SIS, the average interspacecraft distance is reduced by about 30%, and the error box area increases by a factor of about 2. The improvement over the current localization capability represents a factor of about 5, however, which is still quite significant. Many of the solar objectives of this experiment can also be accomplished with this spacecraft configuration. IV. CONCLUSION The loss of the NASA ISPM, although disappointing, need not have catastrophic consequences for the cosmic gamma—ray/solar X—ray experiment. Most of the objectives can be met if at least some of the missions in the planning stage (SIS, DISCO, Prognoz, Venera) are carried out. The improvement in localization accuracy will be a dramatic one which, taken with the long mission lifetime, should provide a wealth of new data on gamma ray bursts. V. REFERENCES 1. 2. 3. 4. 5.

Evans, W.E. et Cline, T.L. et Cline, T.L. et Laros, J.G. et Hurley, K., in AlP Conference

al., ~ J. Lett. 237, L7, 1980 al., ~. J. Lett. 237, Ll, 1980 al., Ap. J. Lett. 255, L45, 1982 al., Ap. J. Lett. 245, L65, 1981 Gamma Ray Transients and Related Astrophysical Phenomena, Proceedings No. 77, AlP, New York, 1982, 85