Compton Observatory Science Report #132
Thursday, August 12, 1993 
Eric Chipman, Compton Observatory Science Support Center

Questions or comments can be sent to the Compton SSC.  Phone 301/286-7764,
e-mail SPAN GROSSC::CHIPMAN, Internet chipman@grossc.gsfc.nasa.gov.  

Science operations are continuing.  Analysis of the reboost OATS burn
test continues.

There were 3 maneuvers this week.  On Tuesday, August 10, there was a
maneuver to the attitude of period 229 (z-axis target = GAL 5+05, 
x-axis target= Coma).  The times were 1356Z to 1422Z, and the angle
of the maneuver was 70.45 degrees.  The HGA boom-equator angle was
37.88 degrees.

On August 11 and 12, a maneuver was performed to orient the spacrcraft
so that the bottom (-z-axis) would be in the direction of the Perseid
meteor shower radiant, to minimize any chance of a particle creating a
hole in the light shield for one of the anticoincidence domes on CGRO.
The maneuver, of 64.36 degrees, was from 1641Z to 1702Z on the 11th.
Then on the 12th, a maneuver back the period 229 attitude was done 
from 1401Z to 1421Z.  

BATSE
-----
The newly discovered pulsar, GRO J1008-57, is fading rapidly.  It reached its
peak flux during the time interval from July 19 to July 29.  At the present 
decay rate, it will no longer be observable by BATSE in another week or so.  
The BATSE collaborators at Caltech are performing timing analyses on the 
pulsed data obtained thus far.

Over 160 abstracts have been received for the Huntsville Gamma-ray Burst
Workshop, Oct. 20-22.  The program committee, chaired by K. Hurley, is 
arranging the program, which will be mailed on or about Sept.18.  The extreme
interest in the Workshop may result in fewer and/or shorter talks.

Beginning with this report, the BATSE burst and solar flare totals will
be reported less frequently.

COMPTEL
------- 
The COMPTEL instrument is performing well and continues routine
observations.  During the period of most intense activity predicted
for the Perseid meteor shower this week, with the GRO spacecraft in a
"shielded" orientation, the primary detector high voltages of COMPTEL
were switched off, as an added safety precaution.

Very preliminary analysis of the flight data received for the recent
target-of-opportunity observation of the x-ray pulsar GRO J1008-57
shows no strong indication of emission from this object in the COMPTEL
energy range at medium-energy gamma rays.

**********

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EGRET
-----
EGRET operations were normal this week.  Work continues on 
the Compton Symposium presentations.  Three papers are being
submitted to the burst conference in October.  Interactions
with guest investigators continues at a normal level.  We
are watching the instrument for any indication of an
abnormality due to the meteor shower.

Dr. Ramana Murthy will be joining the EGRET science team as
a guest on the first of September.  He will be a Senior
National Research Council Associate.

OSSE
----
OSSE operations are normal.

For viewing periods 229 and 229.5, the Z-axis target is the galactic plane
at 5 degrees longitude (PI team), and the X-axis target is the Coma Cluster
(GI:  Y Rephaeli).  The viewing strategy near the galactic center employs large
source and background offsets from the galactic plane to measure the extent
of the bulge in the diffuse continuum and annihilation radiation.  When neither
target is above the Earth's limb, engineering data are being collected to
improve our calibration of the neutron v. gamma-ray pulse-shape discrimination
efficiency.

Because the Sun is not available on the OSSE scan plane, the slewing response
to BATSE solar flare triggers is disabled.  There have been no slews to the
Sun since the last report.

During the brief reorientation for the Perseid meteor shower, OSSE's single
target was the radio pulsar PSR 1509-58.  The instrument configuration was
optimized to give maximum sensitivity to pulsed emission.

Data from viewing periods 1 and 2.0 have been delivered to the Compton Science
Support Center Archive.  These data have now been screened with the same
techniques as the subsequent viewing periods already delivered to the archive.

We issue the following science report on the extended Target of Opportunity
pointing at the recent supernova in M81.

--- Preliminary OSSE Results on SN 1993J ---

OSSE observed SN 1993J during three different intervals, approximately
9--15, 23--36, and 93--121 days after outburst. There is some evidence
for continuum emission below 200 keV in the first two of these periods.
Power-law fits yield intensities at 100 keV of (1.82+-0.39)e-3 photons cm-2 s-1
MeV-1 and (0.89+-0.35)e-3 photons cm-2 s-1 MeV-1, and photon indices of
-2.3+-0.5 and -2.2+-0.9,  respectively. There is no evidence for any
emission in the longer, more sensitive, third observation. These
measured continua are probably too bright to be entirely due to
Comptonized gamma-rays from radioactive 56Ni and 56Co alone. A
power-law of fixed photon index -1.2 fit to the first OSSE observation
extrapolates approximately to the total luminosity measured by ASCA
(Tanaka IAU Circ. 5753) from 1--10 keV one day earlier.

This emission cannot be unambiguously attributed to SN 1993J. Because
of the large OSSE field of view, SN 1993J cannot be separated from
other sources such as the nucleus of M81 or even M82. However, OSSE did
observe this region twice earlier, 597 and 443 days before SN 1993J and
no continuum emission was detected at either time. It might be possible
to attribute the OSSE emission to SN 1993J by correlating its variations
with other observations, e.g. ASCA or ROSAT, of SN 1993J. 

No evidence for line emission is seen in any observation.  In the third
OSSE observation of SN 1993J, the time when gamma-ray lines  were most
likely to be detectable, preliminary measured fluxes of 847 and 1238 keV
lines are (1.6+-2.1)e-05  photons cm-2 s-1 and (0.7+-1.9)e-05 photons
cm-2 s-1, respectively. These  limits do not include OSSE data stored
on-board GRO during periods of no real-time TDRS coverage, which will
probably add 20-30 percent additional data on SN 1993J.  Combining these
OSSE data with COMPTEL results will provide the  best overall search for
56Co line emission from SN 1993J.