Spectral observation of nine recent cosmic gamma-ray bursts are reported. The average photon number spectra of all nine events are shown to be consistent with a 150-keV exponential from 100 keV to about 400 keV, and a power law of index -2.5 from 400 keV to 1100 keV. The observations also indicate an event rate of 16 in 1972 and 1973, or 8 + or - 2 per year, higher than the 5 + or - 1 per year initially reported. This corresponds to an approximately 40-percent lower effective intensity threshold, attained by using more sensitive detectors in multiple-satellite coincidence

Cline, T. L.

Desai, U. D.

English

NASA Technical Reports Server

X661-74-296
PREPRINT
NASA Bj j 77k'
OBSERVATIONS OF COSMIC
GAMMA-RAY BURSTS WITH IMP-7:
EVIDENCE FOR A SINGLE SPECTRUM
T. L. CLINE
U. D. DESAI
(NASA-TM-X-7077 8 )- OBSERVATIONS OF COSmIC N75-11888
GAMMA-RAY BURSTS WITH IMP-7: EVIDENCE
FOR A SINGLE SPECTRUM (NASA) 14 p HC
$3.25 CSCL 03B Unclas
G3/93_ 02763
SEPTEMBER 1974 "
e'-' " .g I.-
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
https://ntrs.nasa.gov/search.jsp?R=19750003816 2020-03-23T02:57:48+00:00Z
"This paper presents the views of the
author(s), and does not necessarily re-
flect the views of the Goddard Space
Flight Center, or NASA."
To the best of our knowledge, neither NASA nor the Goddard Space
Flight Center has any published or stated views on gamma ray bursts.
If this situation should change in the future, every effort will be
made to distribute these views to our mailing list.
For information concerning availability
of this document contact:
Technical Information Division, Code 250
Goddard Space Flight Center
Greenbelt, Maryland 20771
(Telephone 301-982-4488)
Observations of Cosmic Gamma-Ray Bursts with IMP-7:
Evidence for a Single Spectrum*
T. L. Cline and U. D. Desai
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771
ABSTRACT
Spectral observations of nine recent cosmic gamma-ray bursts are
reported. The average photon number spectra of all nine events are
each consistent with a 150-keV exponential from 100 keV to about 400 keV,
and a power law of index -2.5 from 400 keV to 1100 keV. The observation
also indicate an event rate of 16 in 1972 and 1973, or 8 +2 per year,
higher than the 5 + 1 per year initially reported. This corresponds to
an approximately 40-percent lower effective intensity threshold, attainec
by using more sensitive detectors in multiple-satellite coincidence.
*Accepted for publication in the Astrophysical Journal (Letters).
-2-
I. INTRODUCTION
We report spectral observations of nine of the eleven known cosmic
gamma-ray bursts (GRB) detected from late 1972 to the end of 1973.
These events, seven of which are previously unreported, were observed
during the fourteen months following the launch of the IMP-7 satellite.
All nine events were foundto have event-average photon number spectra
from 100 to 1100 keV that are statistically consistent with each having the
same shape. In two of these events, departures from a good fit can
be reconciled as instrumental, due to effects of an eclipsing scan mode.
The hypothesis of a single >100-keV spectrum for all gamma-ray bursts,
when averaged over each event duration, was inferred from IMP-6 obser-
vations of the intensity maxima of five earlier events (Cline et al.,
1973), and .is hereby considerably strengthened. In addition to the
mutual consistency of these five IMP-6 and nine IMP-7 events, all other
independently reported GRB events are also consistent with the
same event-average spectral shape; these include an Apollo-16 event
(Metzger et al., 1974), and several OSO events (Wheaton et al., 1973,
and Palumbo et al., 1974).
The observed lack of variability of the event-average spectrum,
despite the indication of some spectral variation with time during one
of these events (Imhof et al., 1974), suggests that the production
mechanism is tied to more rigidly fixed source parameters than most
celestial or solar hard x-ray phenomena. Other characteristics of GRB
events, suggested by these observations, include a>400-keV spectral tail
with power-law index -2.5, a rate of occurrence of observed events up
-3-
from 5 + 1 yr - 1 to 8 + 2 yr-1, with a 40jo increase in experiment sensi-
tivity, and a hint of clustering of event occurrence in time.
II. INSTRUMENTATION
The IMP-7 satellite was placed into an approximately circular earth
orbit with a k 200,000-km apogee in October 1972, where interplanetary
data are obtained, free from interference by encounters with trapped
radiation, on a nearly continuous basis. Data collection for our elec-
tron and gamma-ray detector was initiated on 14 October 1972, and this
instrument is still operating properly. The IMP-7 detector's instru-
mental configuration and spectral analysis scheme are identical to
those on IMP-6 (Cline et al., 1973), but the spin rates, data collection
times and duty cycles differ. The consequent dissimilarities between
the IMP-7 and IMP-6 data collection modes warrant some discussion here,
since the IMP-7 detector is more effectively omnidirectional than IMP-6,
and thereby provides less biased and also less ambiguously interpreted
spectra.
The IMP-7 gamma-ray data were collected in two modes; six of the
nine GRB events detected (see Table 1) were observed in the mode which
consists of the continuous accumulation of data for 40.9 seconds into a
14-channel differential analyzer. During this time there are approxi-
mately thirty rotations of the satellite of about 1.3 seconds each. The
time duration of each of these six events, due to their brevity at ener-
gies over 100 keV, happened to lie entirely within a single 40.9-second
summation. The celestial directions of origin of nearly all of the IMP-7
events are unknown, so that the effective collection areas and aspects
-4-
are not calculable. However, the fact that each event has a >100-keV
duration long compared with the IMP-7 spin period (Klebesadel, pri.
comm.) means that each IMP-7 spectral accumulation takes place as the
satellite rotates through the incident flux at least several times
during the duration of each burst. Since the detector has a better
than semi-omnidirectional view angle, a fair representation of the
average spectrum of each event is obtained if the spectral character
is constant or changes slowly during an event. This collection mode is
in contrast to that on IMP-6, in which the spin period was about 10.5
seconds, long compared to the duration of a typical intensity variation;
also, on ISP-o6, the individual spectral storage times were much shorter,
averaging 2 or 3 seconds, and the data collection had a 0.5 duty cycle,
accumulating onlyduring the sunward half of each satellite rotation.
Consequently, the several-second wide intensity maxima of the GRB
events were asynchonously observed on IMP-6 at differing and usually
unknown relative azimuths. By contrast, these six events observed with
IMP-7 were effectively averaged over in a manner relatively independent
of source direction.
The other three of the IMP-7 events were observed in an alternate
40.9-second collection mode, which, as on IMP-6, consisted of storage
during only the half of each rotation in which the satellite faced the
sun, but which, unlike IMP-6, summed the data for about 30 spins. Thus,
depending on the azimuth direction of each of these three GRB sources
relative to the sun, their spectra could be obtained in an eclipsing mannei
thereby introducing attenuation effects at the lower energies.
-5-
In all nine events, independent of azimuth, some spectral distortion
must be present due to the fact that the rotation of the satellite system-
atically introduces a somewhat wider effective sensitive angle for high-
energy photons than for lower-energy quanta. On the average, however,
this effect will be the same for all of the IMP-7 events having spectral
time constants long compared with one spin period, and will not
alter the conclusion that these events are spectrally similar.
III. OBSERVATIONS
Table 1 lists the known gamma-ray burst events from the launch of
IMP-7 to the end of 1973, giving the satellite observations in the
order of detection and confirmation. Work is still in progress to search
for events in all the available data, and it is possible that other
events during this time interval may yet be found. The total number of
known bursts in calendar years 1972 and 1973 is now sixteen (Table 1 and
Strong et al., 1974), increasing the earlier rate of detection from
four or five per year (Klebesadel et al., 1973) to nearly double that
number. This increase in efficiency is due to the lowering of the
effective detection threshold by requiring only one Vela trigger in
coincidence with any IMP or other satellite observation, such as SAS-II
(C. Fichtel, to be published) or Apollo-16 (Metzger et al., 1974). It
should be noted that all GRB events that have been spectrally observed
are of relatively long duration and are structured in their time
histories; none of the very fast, ; 100-millisecond, events detected
by the Vela satellites as comprising about one-tenth of the total popu-
lation (Strong et al., 1974) have been observed spectrally. These
-6-
very fast events are sufficiently different in their time histories from
the slow events that their other characteristics, and even origin mech-
anism, may also be distinct.
The energy windows of the 14-channel analyzer on IMP-7 for all
nine events were approximately the same as those used in one of the two gain
modes on IMP-6, in which the 30 June 1971, 17 January 1972 and 28 March
1972 events were observed (Cline et al., 1973); the data were accordingly
treated in same manner regarding the corrections for efficiency and energy-
dependent geometric factors. The resulting differential photon number spectra
of all nine of these IMP-7 events are plotted in Figure 1. Minor statis-
tical differences are seen, but the similarity is striking, considering
the usual variations in the spectra of most astrophysical or solar x-ray
phenomena. A comparison spectrum was arbitrarily created by the construct
of a 150-keV exponential tangent to a power-law of index -2.5 at the
higher-energy end. This spectrum is shown as a dashed curve, plotted for
comparison against each event spectrum in Figure 1. Normalization was
chosen to provide the best visual fit in each case. It is noted that
two of the three eclipsing scan-mode events are deficient at the low-
energy end; this is exactly what one would expect from attenuation in the
satellite material if the sources were near or below the detector horizon.
Thus, the conclusion that these nine events are consistent with having
the same spectrum is easily reached by inspection. Since they were all
obtained with the same detector, the absolute calibration would not effect
this conclusion, but comparison with events observed with other detectors
would of course be subject to relative calibration errors.
-7-
The IMP-6 events were previously shown to be consistent with 150-keV
exponentials, when sampled through their intensity maxima (Cline et al.,
1973); their lack of an obvious power-law tail may result from the more
intensive spectral corrections that were, of necessity, applied to the
IMP-6 data due to electronic imperfections that did not exist on IMP-7.
Not only the various IMP results, but all spectral observations (including
Palumbo et al., 1974, Wheaton et al., 1973, Imhof et al., 1974 and
Metzger et al., 1974) of sixteen different event-average spectra are mut-
ually consistent. The GRB event of 27 April 1972, detected with Apollo
16(Metzger et al., 1974) and one Vela (Strong et al., 1974) was measured
with greater differential accuracy than any other, and yet agrees in
detail with our spectral model, including the existence and shape of
the power-law tail. Finally, the event of 18 December 1972 has been
found to have a certain amount of spectral variation in time (Imhof et
al., 1974); yet this event is also one which, on the average, was
observedwiLth IMP-7 to fit the standard spectrum. The equivalent event-
average spectrum cannot be produced from the results of Imhof et al.
since their observations cover only a portion of the burst duration
The fact that at least one of the IMP events spectrally varies thus
suggests that variations may be present in the other events, yet with
a pattern that reproduces the same average result.
-8-
IV. DISCUSSION
The similarity of all known GRB event spectra should provide infor-
mation regarding the nature of the production mechanism. It certainly impli
that the parameters of the source process should have narrow limits as
to their variability. Thus, a model such as stellar collapse, for
example, might be more likely than that of stellar superflares, since
it would seem that collapse would occur in a situation where more
critical limits are attained. On a different scale, the iron-grain
model (Grindley and Fazio, 1974) may also be capable of producing a
relatively fixed gamma-ray spectrum, but in that case it is not irmmed-
iately apparent why the fast-time spectral samples would not be repre-
sentative of the average spectrum. Extensive theoretical investigation
of the models proposed may provide arguments as to their compatibility
with this observation; the cooling blackbody model (Ramaty and Cohen,
1973), for example, is one in which the instantaneous spectra undergo
extreme variability, but the average spectrum of the bursts is a constant.
Another implication of these results is the existance of a hard
or high-energy x-ray tail on the photon number spectrum. [If the earlier
indication of a pure 150-keV exponential (Cline et al., 1973) were
extrapolated, there would be very little flux above a few MeV. ] This
picture of a power law with index -2.5 above a few hundred keV thus
allows for the possibility of observing a measurable flux in the
> several-MeV range. Of course, considering the lack of a large number
of observations of the soft x-ray component, one may prefer to view
steeper than -2.
the spectrum as composed of two power laws, of index % -1 and one of-indexA
In this interpretation, the spectral bend always occurs at the same energy.
-9-
Another result is that the occurrence rate of GRB observations is
increasing as the data from a variety of satellites are more extensively
examined for coincident event detection. However, comparing the
IMP and Vela measurements to provide event confirmation has lowered
the integral size spectrum by only 40 percent,assuming a -1.5 power
law. Thus, considerably greater sensitivity will be required to improve
the measurement of the size distribution, sufficient to establish any
structural details, as would occur if the source objects are all within
our galaxy. Finally, it is worth mentioning that the evidence hints at
clustering of event occurrence; e.g., there are now six occasions of
event-to-event intervals of less than 4 days, including two occasions
of two bursts on the same day. An analysis of compatibility with a
Poisson distribution in time intervals gives a result some three standard
deviations away from randomicity. Thus, it cannot be established whether
multiple GRB events clustered within a few days interval, are produced
by the same source object.
ACKNOWLEDGEMENTS
We wish to thank R. Klebasedel and C. Fichtel for their assistance
in identifying events that were indicated as candidates in Vela, SAS,
or IMP records. We are grateful to W. Anderson, C. Dickman, H. Domchick,
F. Perazzoli and B. Wells for their help in data processing and to
G. Porreca, H. Costlow and C. Thomas, whose efforts made the successful
launch of the IMP-7 instrument possible.
-10-
Table 1
KNOWN COSMIC GAMMA RAY-EVENTS OCCURRING AFTER THE LAUNCH OF IMP-7
DATE SECS KNOWN IMP-7 ANALYSIS IMP-7
YEAR MN. DY. (UT) SATELLITE OBS. TIME (SECS) MODE*
1972 11 01 68206 Velas+  (no data**) 1800
1972 12 18 73695 Velas + , IMP-7 73634-73675 3600
1972-076B***
1973 03 02 84478 Velas+ + , SAS-2 + +, 84474-84515 3600
IMP-7
1973 05 07 29072 Velas+  (no telemetry) ---
1973 06 06 25648 IMP-7 SAS-2+ + +, 25638-25679 3600
Vela
1973 06 06 67634 SAS-2+++,IMP-7, 67611-67652 3600
Vela+ +
1973 06 10# 75582 Velas+,IMP-7 75551-75592 3600
1973 07 21 32118 Velas++,IMP-7 32096-32137 1800
1973 07 25 61677 Velas++,IMP-7 61636-61677 1800
1973 12 17 29348 IMP-7,Vela +  29343-29384 3600
1973 12 23 02387 IMP-7,Vela++  02385-02426 1800
* 1800 mode consists of solar hemisphere turn-on only of 0.65
second of each 1.3 second spin; see text.
** It is likely that this event was missed by IMP-7 due to its
unusual 0.1-second brevity (Strong et al., 1974) and to the
0.65-second gaps in the data collection cycle in the 1800
scan mode of IMP-7 (See text).
*** Imhof et al. (1974), using the 1972-076B satellite, found the
direction of this event to be approximately r.a. = 70* to 120*
and decl. = 0O to 200, along or up to 20 degrees from the
ecliptic plane, in which the IMP-7 instrument rotates.
-11-
+(Strong et al., 1974).
++ (R. Klebasedel, private communication).
+++(C. Fichtel, private communication).
#Directional information (Strong et al., 1974) and some solar activity
suggest that a flare origin here cannot be completely ruled out.
-12-
REFERENCES
Cline, T. L., Desai, U. D., Klebesadel, R. W. and Strong, I. B. 1973,
Ap. J. (Letters), 185, L1.
Grindlay, J. E., and Fazio, G. G. 1974, A. J. (Letters), 187, L93.
Imhof, W. L., Nakano, G. H., Johnson, R. G., Kilner, J. R., Reagan, J. B.,
Klebesadel, R. W., and Strong, I. B. 1974, Ap. J. (Letters), 191, L7.
Klebesadel, R. W., Strong, I. B., and Olson, R. A. 1973, Ap. J. (Letters)
182, L85.
Metzger, A. E., Parker, R. H., Gilman, D., Peterson, L. E., and Trombka,
J. I., submitted to Ap. J. (Letters).
Palumbo, G. G. C., Pizzichini, G., and Vespignani, G. R. 1974, 4p. J.
(Letters), 189, L9.
Ramaty, R., and Cohen, J. 1973, presented at Los Alamos Conf. on Transient
Cosmic Gamma- and X-ray Sources, September 20-21, 1973 (Reprint LA-5505-(
Strong, I. B., Klebesadel, R. W., and Olson, R. A. 1974, Ap. J.
(Letters), 188, L1.
Wheaton, W. A., Ulmer, M. P., Baity, W. A., Datlowe, D. W., Elcan, M. J.
Peterson, L. E., Klebesadel, R. W., Strong, I. B., Cline, T. L.,
and Desai, U. D. 1973, Ap. J. (Letters), 185, L57.
-13-
FIGURE CAPTION
Figure 1. The nine event spectra of gamma-ray bursts observed with
IMP-7. The three at the right were taken in the eclipsing scan mode,
in which, depending on the azimuths of the source objects, the spectra
could be attenuated at the low-energy ends due to absorption of photons
by spacecraft material. Such an effect appears to have occurred in two
of these observations, if one assumes that all event spectra fit the
same typigal shape. A comparison spectrum is shown for each event by
a dashed curve, which was arbitrarily created by the combination of a
150-keV exponential and tangent power law of index -2.5. This curve
appears to provide a reasonable fit in all nine events, taking into
account the eclipsing effect discussed. (The daggers in all cases
represent statistical errors, with only a small contribution of systematic
errors from the uncertainty in the amount of background subtraction.)
ECLIPSING
OMNIDIRECTIONAL (3600) SCAN MODE (1800) SCAN MODE
50 12/18/72 -- 6/6/73 2 - 7/21/73
73634-73675 - 67611-67652 32096-3213720 5 - I aSECONDS, U.T. - SECONDS, U.T. SECONDS, U.T.
10 - 2 .5 "
5=.I
S"4_ 75551-75592 - 7/25/73
-- 2/73 7/2I./ .
r 20 72 SECONDSUT 2- 61636-61677
o 10 84515 SEC.,U.T. I SECONDSU.T.
5 .5 .5
o 2 .
F:
z 5 5- 12/17/73 5 12/23/73
3 6/6/75 29343- 02385-02426
J28 2 2 63 7
- 25638- - 29384 - SECONDS, U.T.
10 . 25679 SEC., I F SEC.,IU.T. I
5 .5 .5:
2 - 'I' o2- -. 2
..
.02 I I ,. 2 2 3 2 0235 .0 40 300 600 900 O 300 600 900 0 300 600 90T
E (keV) E (keV) E y (keV)


Observations of cosmic gamma-ray bursts with IMP-7: Evidence for a single spectrum

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server