A scintillation spectrometer is included in each of the eight BATSE/GRO detector modules, resulting in all-sky coverage for gamma-ray bursts. The scientific motivation, design and capabilities of these spectrometers for performing spectral observations over a wide range of gamma-ray energies and burst intensities are described

Cline, T. L.

Fishman, G. J.

Matteson, J. L.

Meegan, C. A.

Paciesas, W.

Parnell, T. A.

Teegarden, B. J.

Wilson, R. B.

English

NASA Technical Reports Server

347 cx; 9.2-1s
GA)a_A-RAY BURST SPECTROSCOPY CAPABILITIES
OF THE BATSE/GRO EXPERIMENT
J. L. Matteson
Center for Astrophysics and Space Sciences
University of California, San Diego
La Jolla, California 92093 USA
G.3. Fishman, C.A. Meegan, T.A. Parnell and R.B. Wilson
Space Science Laboratory
RASA/Marshall Space Flight Center
Huntsville, Alabama 35812 USA
W. Paciesas
Physics Department
University of Alabama in Huntsville
Huntsville, Alabama 35899
T. Cline and B. Teegarden
Laboratory for High Energy Astrophysics
RASA/Goddard Space Flight Center
Greenbelt, Maryland 20771 USA
ABSTRACT
A scintillation spectrometer is included in each of the
eight BATSE/GRO detector modules, resulting in all-sky
coverage for gamma-ray bursts. The scientific motivation,
design and capabilities of these spectrometers for
performing spectral observations over a wide range of
gamma-ray energies and burst intensities are described.
i_. Introduction. In the past five years it has become clear on both
observational [1, 2] and theoretical [3] grounds that the spectra of gamma-
ray bursts are rich sources of information which is crucial to determining
the nature of the burst phenomenon. The observations have revealed various
spectral components, most of which must arise in separate regions. A long-
lived component in the x-ray band may be the afterglow of the cooling burst
plasma [4]. A gamma-ray component from tens to hundreds of keY, the
classical burst emission, has an exponential form [5,6] and appears to be
due to some thermal process with a temperature of -i09 K. Above -500 keV a
high energy gammm_-ray component often extends to at least 6 MeV with an
approximate power law form [7] and has been observed up to 30 MeV in one
case [8]. In the 25-70 keV band unresolved absorption lines have been
observed in -30 percent of the bursts and a few emission lines have also
been observed [5,6,9]. These have been interpreted as due to cyclotron
processes in terragauss fields, i.e. near a neutron star. At -400 keY,
broad, marginally resolved emission lines with "200 keV width have been
observed in "5 percent of the bursts [5,6] and are interpreted as red-
shifted pair annihilation radiation. All these lines' narrow widths
require a cool, T<2xI00K, region for their formaton. A narrow, -40 keV
wide, emission line observed in one burst at "740 keY is consistent with a
redshifted gamma-ray from the first excited state of s 6Fe [I0] and implies
a temperature of " 2x109K. All of the components vary on time scales
https://ntrs.nasa.gov/search.jsp?R=19850026641 2020-03-20T16:47:31+00:00Z
$48 oG 9.2-15
shorter than the observations' integration times, which range from 0.25
sec to 5 seconds. In general, correlations among the components'
variations have not been established. However, the annihilation line
appears to be correlated with intensity on a 0.25 sec time scale in one
well observed burst [11].
It is apparent that each region has its particular spectral
signature which, in principle, can be independently followed with time-
resolved spectroscopy over a broad energy band during a burst. Such
observations would result in a new level of understanding of the evolution
of the physical conditions of the various emission regions, their causal
relationships, and their relationships to the underlying energy source of
the burst. This is the primary motivation for the BATSE Spectroscopy
Detectors, which are the subject of this paper. The very short cooling
time scales near a neutron star, ,10 -6 sec, imply that the spectral
components are expected vary on the same time scale as the intensity. Thus
sensitive spectral observations are required on short time scales,
typically 0.1-1 sec, often as short as I0 msec and in one case, 5 March
1979, _0.2 msec.
2. Instrumentation. An overall description of the BATSE experiment is
presented in an accompanying paper [12]. Each of the eight BATSE modules
contains a 12.70 cmx 5.62 cm NaI(TI) Spectroscopy Detector, which has an
energy resolution of 7 percent at 662 keV and a He window that extends its
energy range down to 7 keV. (The effective area versus energy is shown in
the accompanying paper. ) The set of detectors has all-sky coverage and
burst data are taken taken from the four detectors which most directly
view a burst, resulting in an average sensitive detector area of ~ 500
cm 2 . 14 keV to i0 MeV energy losses are analyzed into 2752 linear channels
and then data compressed into 256 channels, preserving the
detectors'energy resolution. An average event conversion cycle requires ~
6_s, allowing a throughput of ~ 2xl0S/sec-det, which corresponds to a
burst flux of " 2x10-4erg/cm2-sec. Discriminators at 7 keV, 20 MeV and 40
MeV provide data in the 7-14 keV, 10-20 MeV, 20-40 MeV and _ 40 MeV bands.
Linear operation at high counting rates is assured by 1 ) high current PMT
bleeder strings which are zoner diode regulated and 2 ) a gated baseline
restorer in the analog electronics. It is planned to calibrate the
nonlinear light output of the NaI(TI ) scintillators below I00 keV with the
monochromatic x-ray beam at the Stanford Synchrotron Radiation
Laboratory. Additional calibrations up to " i0 MeV will use radioactive
sources and neutron capture gamma-rays.
The data are simultaneously stored in memories in two modes which
are enabled by the SATSE burst trigger and preserve the detector IDs. In
the HER (high energy resolut ion ) mode, 192 spectra, including
discriminator counts, are stored with a time resolution based on a
time-to-spill algorithm that allows integrations as short as 64 msec at
times of high rates and provides up to ~ I00 seconds of coverage with an
average integration time of " 1 sec. Higher time resolution, for a subset
of a burst' s gamma-rays, is provided by the TTE (time-tagged events ) mode,
which stores 65,536 pulse heights, each time tagged to 128 _s precision.
These data will provide a statistically significant spectrum measurement
for each " 200 counts during source limited observations, e.g. each meec
at a burst flux of " 5x10-5erg/cm2-sec. Part of the TTE memory acts as a
349
OG 9.2-15
pre-trigger buffer and will capture the leading edge of a burst that
preceeds the the burst trigger.
3. Backqround and Sensitivit 7. Since the detectors are unshielded, their
background is dominated by gamma-rays of cosmic, spacecraft and
atmospheric origin. Lines at 511 keV and several other energies due to
detector radioactivity will not significantly affect the sensitivity and
the 511 keV line will be used for energy calibration. The background rate
above 20 keVwill be ~ 2000 sec-1. A burst with <E> = I00 keV and a flux of
2xlO-6erg/cm2-sec would produce the same rate.
In order to estimate sensitivity, we assume a 5 second burst with
the spectrum, dN/dE = I/E exp(-E/250 keY) plus an extension above 300 key
as an E-2 power law. Line strengths are expressed in units of fractional
equivalent width, i.e. the usual equivalent width (keY) divided by the
line centroid energy (keY). Figure la shows the sensitivity to unresolved
lines. Data from various line observations [5,9, i0] are plotted. These all
have large equivalent widths and much weaker lines could be detected in
the more intense bursts. The time required to detect a typical, 20 percent
width line is shown in Figure lb. During the most intense flux periods it
TIME REQUIREDFOR A 5o" DETECTION
¢o= OF A 20% EQUIVALENTWIDTH LINE
5o" SENSITIVITY TO LINES
{5 SECONDINTEGRATION)
I0 i" ' ' '' ....ENERGyI' ' '' .... I / ...... I0IO'Serg/cm2 __
31 : I
"- KONUS
_- o_ _ ,s_./ , -
_" / z/
_ 0 em/cm / _ ,0"
iO-_l[ * I , L .... I _ , .... l , , , , .... I :0
I0 102 103 104 10 I0z 103 104
ENERGY(keY) ENERGY(keY)
(a) (b)
Figure 1. (a) - Sensitivity to spectral lines. (b) - Integration times
required for line detection. The observations will be background limited
at fluences ( fluxes ) below i0- 5erg/cm2 (2x10- serg/cmZ-sec ). The
anticipated observed frequency of bursts above the indicated strength is
given in parenthesis.
350
OG 9.2-15
will be possible to obtain cyclotron and annihilation line measurements
with few msec time resolution, while at low fluxes, i.e. ~ lO-6erg/cm 2-
sec, ~ I sec time resolution will be possible. The search for nuclear
lines in the ~ 1-10 MeV region will require longer integration times, from
~ 0.01 to 1 sec in the more intense bursts. Assuming 6 logarithmic energy
channels per decade, 50 measurements of the continuum spectrum would
require half of the time indicated in Figure lb.
!. Conclusion. The BATSE Spectroscopy Detectors have a powerful
combination of energy band, sensitivity, and energy and time resolution,
which represents a one to two orders of magnitude improvement over
previous instruments. We can anticipate an equally large increase in our
knowledge of gamma-ray bursts late in the decade when these detectors'
observations become available.
3. Acknowledqements. This work is supported at the University of
California, San Diego by NASA Contracts NAS8-35012 and NAS8-36081.
References
i. Teegarden, B.J., (1982), in Gamma Ray Transients and Related
Astrophysical Phenomena, Lingenfelter, R.E., Hudson, H.S., and Worrall,
D.M., eds. Am. Inst. of Physics, N.Y., p. 123.
2. Teegarden, B.J., (1984), in _ Enerqv Transients in Astrophysics,
Woosley, S.E., ed., Am. Inst. of Physics, NY, p. 352.
3. Lamb, D.Q., (1982), in_Ene__Transients in Astrophysics, Woosley,
S.E., ed., Am. Inst. of Physics, NY, p. 512.
4. Laros, J.G., e_t_tal., (1984), A__._., 28___66,681.
5. Mazets, E.P., e__ta_!l.,(1981), Natur_______ee,29 __O0,378.
5. Mazets, E.P., et a_!l., (1983), in Positron-Electron Pairs in
Astrophysics, Burns, M.L., Harding, A.K., and Ramaty, R., eds., Am. Inst.
of Physics, N.Y., p. 36.
7. Matz, S.M., e_t a_!l.,(1985), AD.J. (Letters), 28___88,L37.
8. Rieger, E. et al., (1982), in Accretinq Neutron Stars, Brinkman W. and
Trumpet, J., eds., Max-Planck Institute, Garching, p. 229.
9. Hueter, G.J., et a_!l., (1984), in _ Ene__ Transients in
Astrophysics, Woosley, S.E., ed., Am. Inst. of Physics, NY, p. 373.
10. Teegarden, B.J., and Cline, T.L., (1980), AD.J. (Letters) 23___66,L67.
ii. Barat, C., et a_!l.,(1984), AD.J. (Letters), 28__66,LII.
12. Fishman, G.J., et a_!l.,(1985), paper OG9.2-14, these proceedings.


Gamma-ray burst spectroscopy capabilities of the BATSE/GRO experiment

Gamma-ray burst spectroscopy capabilities of the BATSE/GRO experiment

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