The Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory (GRO) is a sensitive all-sky detector system. It consists of eight uncollimated detectors at the corners of the spacecraft which have a total energy range of 15 keV to 100 MeV. The primary objective of BATSE is the detection, location, and study of gamma ray bursts and other transient sources. The instrement also has considerable capability for the study of pulsars, solar flares, and other discrete high energy sources. The experiment is now in full operation, detecting about one gamma ray burst per day. A brief description of the on-orbit performance of BATSE is presented, along with examples of early results from some of the gamma ray bursts

Fishman, G. J.

Meegan, C. A.

Paciesas, W. S.

Pendleton, G. N.

Wilson, R. B.

English

NASA Technical Reports Server

N92-21877
The BATSE Experiment on the Compton Gamma Ray Observatory:
Status and Some Early Results
G.J. Fishman, C.A. Meegan, R.B. Wilson,
NASA Marshall Space Flight Center
W.S. Paciesas, G.N. Pendleton
University of Alabama in Huntsville
for the BATSE Science Team
Abstract
The Burst and Transient Source Experiment (BATSE) on
the Compton Gamma Ray Observatory is a sensitive
all-sky detector system. It consists of eight
uncollimated detectors at the corners of the spacecraft
which have a total energy range of 15 keY to i00 Mev.
The primary objective of BATSE is the detection,
location and study of gamma-ray bursts and other
transient sources. The instrument also has
considerable capability for the study of pulsars, solar
flares and other discrete high energy sources.
The experiment is now in full operation, detecting
about one gamma-ray burst per day. A brief description
of the on-orbit performance of BATSE is presented,
along with examples of early results from some of the
gamma-ray bursts observed.
Description and Scientific Objectives
The Burst and Transient Source Experiment (BATSE) consists of
eight uncollimated detector modules arranged on the corners of
the Compton Gamma Ray Observatory to provide the maximum
unobstructed view of the celestial sphere. Each detector module
contains a large-area detector (LAD), optimized for sensitivity
and directional response, and a spectroscopy detector (SD)
optimized for broad energy coverage and energy resolution. The
eight planes of the large-area detectors are parallel to the
faces of a regular octahedron. The three primary axes of the
octahedron are parallel to the three principal axes of the
spacecraft, as shown in figure i. This provides nearly uniform
sky coverage and produces an inherent symmetry in various aspects
of the data analysis.
26
https://ntrs.nasa.gov/search.jsp?R=19920012634 2020-03-17T13:17:10+00:00Z
-Y +Z
_. / _ _ -- _ 8ATSE {8)
" /-X
+Y
GAMMA RAY OBSERVATORY
,Z
-Z
BATSE DETECTOR ASPECT
Figure i. The location and aspect of the eight
BATSE detectors on the Compton
Gamma Ray Observatory.
Table 1
Material:
Frontal Area:
Thickness:
Energy Range:
Energy Resoh:
BATSE DETECTORS
LARGE AREA
NaI(Te)
2,025 cm 2
1.27 cm
30 - 1900 keV
27% @ 88 keV typ.
SPECTROSCOPY
NaI(Tt)
127 cm:
7.62 cm
15 keV - 110 MeV*
7.2% @ 662 keV typ.
*Detector gain dependent
The basic characteristics of the two types of detectors are given
in Table i. A more comprehensive description of the instrument
is given in several papers included in the first GRO Science
Workshop (Fishman et al 1989a,b; Paciesas et al 1989; Pendleton
27
et al 1989), and in a recent report (Horack 1991). A detailed
paper on instrumentation and calibration is currently in
preparation (Fishman et al 1992). The BATSE data system is a
versatile and complex system. Data from the detectors are sorted
on-board into 14 data types with different temporal and spectral
resolution. Some data types are continuously transmitted
according to a preprogrammed schedule, while other, higher time
resolution data are accumulated from on-board triggered gamma-ray
bursts. The data recorded from a burst require the duration of
the following orbit, about 93 minutes, to unload the experiment
burst memory. A summary of the characteristics of the data types
are contained in the references above.
BATSE is expected to provide the most sensitive observations of
gamma-ray bursts yet obtained. The more significant features of
BATSE include: the ability to obtain the locations of weak
bursts; the ability to detect spectral features and rapid
spectral variations in many bursts, and the ability to provide
burst locations to the scientific community quicker than
previously possible. The celestial distribution and intensity
distribution of hundreds of weak gamma-ray bursts, unobtainable
with previous instrumentation, may finally provide the distance
scale for gamma-ray bursts. BATSE will also be able to provide
long-term and frequent monitoring of hard x-ray and low energy
gamma-ray emission from the stronger sources, including
transients, pulsating sources and solar flares.
On-Orbit Performance
All eight detector modules have been operating well since the
experiment high voltage was turned on April 24, 1991.
Considerable progress has been made in the determination of the
on-orbit energy calibration, atmospheric scattering contributions
and the detector response matrices (Pendleton et al, 1992; these
proceedings). The background in the large area detectors above
30 key varies between approximately 3000 and 6000 counts/s per
detector during most portions of the orbit. Often, usually above
geographic latitudes of about 24 degrees, the spacecraft
encounters regions of enhanced background radiation due to
precipitating electrons. During these times, bremsstrahlung from
these electrons increases the background considerably and many
times produces false burst triggers (Horack and Fishman 1991).
The BATSE operations team has been successful in eliminating many
of these false triggers by pre-determining the regions of high
background and disabling the on-board burst trigger system at the
appropriate times. Other sources of false burst triggers include
28
solar flares and large increases in background radiation around
the region of the South Atlantic Anomaly, especially during
periods when the magnetosphere is disturbed.
During undisturbed times, the background radiation at energies
between 30 keY and i00 keV is dominated by the diffuse x-ray
background and discrete sources. Thus, the detector background
is sensitive to the ratio of the earth/sky in the field of view,
as well as which sources are in the field of view. At energies
above i00 keV, the primary source of background is due to cosmic
ray secondaries and is closely coupled to the geomagnetic
latitude. The background shows an increase of approximately a
factor of three twice per orbit at the high latitudes compared to
that near the equator. At these energies, the counting rate is
relatively insensitive to the detector aspect with respect to the
earth.
Gamma-ray Burst Observations
The criteria for the on-board trigger of a gamma-ray burst are
given in Table 2. During undisturbed times, the rate of
gamma-ray bursts has averaged about one per day. The rate of
false triggers increases to as much as six or eight per day
during times of high solar activity or when the earth's
magnetosphere is disturbed. During these times the apparent rate
of real gamma-ray bursts decreases due to the reduction of the
trigger efficiency, since the trigger threshold is increased
until the burst data are read out over the next orbit.
Table 2
BATSE-- Burst Trigger Criteria
• Trigger energy range : 60300 keV
• 3 timescales tested : 64, 256, 1024 ms
• 55 a increase above 17s background rate
• 2 or more detectors required to trigger
29
A few examples of the variety of gamma-ray bursts observed since
launch are shown in Figures 2,3, and 4, and discussed in the
captions. Detailed temporal and spectral analyses of gamma-ray
bursts are underway. The results of these analyses will be
presented in publications and presentations in the coming months.
40.000
30,0O0
"E 2o0oo
--1
O
o
I0.000
April 21,199t April 25, 1991 May 18, 1991
64 ms bins 64 ms bins 8 ms bins
12,000t
, l 8.000
0 4,000
40.000
30,000
20,000
10,000
0 5 10 -tO 0 20 40 60 80 100 04 08 t'2'
SECONDS
Figure 2. A sample of the wide variety of time
profiles observed by BATSE. Burst durations
have ranged from 8 ms to over 500 s. This figure
illustrates three of the many types of time
profiles that have been seen. The energy range of
counts in these plots is 60 keV to 300 keV.
3O
%U
32
28
24
2O
16
12
160
120
0
80
<#'£1
0,.
4O
' _ • [ • _ . 1 , ] "/ • I , ] 'r" I .
E • 60 keY
Res = 1 s Res = 64 ms
, I , I I I L I A l , I L I * I * I •
-40 -20 0 20 40
Res = 1 ms
. I . I _ L , l _ l , I , I , I , I , I •
-120 -80 -40 0 40
280 r. ,[_. .................................... :
2401
I
40 I :
-70 -60 -50 -40 -30
mS
Figure 3. The shortest gamma-ray burst seen thus far
(GRB 910711). The FWHM is 8 ms in this energy
range. The burst is shown on three different
time scales, with the time resolution indicated.
3]
GRB 910522
110010 r,lj|r,,llii,,iiririiT_+l,,,lliiiil,,ri
100010
g0001
80001
u
70001
in
c 60001
o
u
bJ
__ 50001
cr
40001
30000t
20000 I_
10001 - J J , , I l J , , I i I 11 _ I J J , I I , , i i
-50 0 50 100 150 200 250 300 350
SECONDS since burst
Figure 4. An unusual gamma-ray burst with a weak
precursor occurring 110 s before the main
part of the burst emission. Previous burst
experiments would have missed this precursor or
could not have determined that it was from the same
location as the main part of the burst.
Perhaps the most dramatic and unexpected observation made by
BATSE thus far is the isotropy of the distribution of gamma-ray
bursts. From recent burst experiments on other spacecraft, as
well as from the initial BATSE measurements of the frequency of
gamma-ray bursts verses their intensity, it is apparent that the
sources of gamma-ray bursts are spatially confined. The most
widely assumed source of the confinement was thought to be the
Galactic plane. The observed isotropy indicates that the
gamma-ray bursts are not confined to the Galactic plane, and in
32
fact, may not be due to Galactic objects. The explanation of
this observation is not presently known and is currently the
subject of intense speculation. Two exciting possibilities are
that: i) The gamma-ray bursts are coming from objects, probably
neutron stars, in an extremely large, and previously undetected,
spherical Galactic halo, or 2) They are coming from some unknown
objects at cosmological distances. These observations were
reported in an IAU Circular (Meegan et al 1991), and at a
gamma-ray burst Workshop in Huntsville in October. They will
also be published soon in Nature (Meegan et al 1991).
Other Early Results
The analysis of early BATSE observations of hard x-ray pulsars,
other discrete sources, and solar flares is proceeding well. The
high sensitivity and all-sky viewing of the BATSE detectors is
expected to provide new data on many of these objects. These GRO
Workshop Proceedings contain a sample of some of these early
results. The reader is referred to BATSE papers in these
proceedings with the following lead authors: R.B. Wilson, B.A.
Harmon, M. Finger, C. Kouveliotou, and R.A. Schwartz.
The BATSE experiment supports an active Guest Investigator
Program. Several of the GRO Phase One Guest Investigators have
been heavily involved with various aspects of BATSE data
analysis. We look forward to continued collaborations with guest
investigators in succeeding phases of the Compton Gamma Ray
Observatory Guest Investigator Program.
References
Fishman, G.J., C.A. Meegan, R.B. Wilson, W.S. Paciesas, T.A.
Parnell, R.W. Austin, J.R. Rehage, J.L. Matteson, B.J. Teegarden,
T.L. Cline, B.E. Schaefer, G.N. Pendleton, F.A. Berry, Jr., J.M.
Horack, S.D. Storey, M.N. Brock, and J.P. Lestrade"BATSE: The
Burst and Transient Source Experiment on the Gamma Ray
Observatory", GRO Science Workshop Proceedings, p.2-39, GSFC
(1989a).
33


The BATSE experiment on the Compton Gamma Ray Observatory: Status and some early results

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server