CGRO Biweekly Status Report

[SHRADER@GROSSC.GSFC.NASA.GOV]

  Compton Observatory Science Report #168, Tuesday October 18, 1994
      Chris Shrader, Compton Observatory Science Support Center

         Questions or comments can be sent to the CGRO SSC.
          Phone: 301/286-8434
          e-mail:   NSI_DECnet: GROSSC::SHRADER 
          Internet: shrader@grossc.gsfc.nasa.gov
                                                                    

                          Spacecraft Status

 The SAA prediction tables uplinked to the spacecraft on Saturday
morning, October 8 had errors that resulted in several SAA exit
commands being executed prematurely. The instruments are designed to
take full SAA passes with their high voltages turned on, so this was
not a serious problem. However, there were false BATSE gamma-ray
burst triggers due to SAA particles. Unfortunately, these went out on
the BACODINE network and may have resulted in astronomers attempting
observations of non-existent bursts, a situation for which the
project my apologizes. The source of the problem was traced to
changes to the shape of the SAA exclusion zone in response to false
BATSE triggers uplinked several weeks ago. The BATSE burst trigger
criteria had been modified to allow improved studies of atmospheric
gamma-ray flashes and by the gradual expansion of the SAA region as
solar minimum approaches. We now have a stable situation with an
acceptable SAA box and accurate predictions and software fixes are
underway. 


                         Instrument Reports

EGRET

EGRET operations were normal this biweekly period except for the
matter discussed in the next paragraph. Delivery of data to the CGRO-
SSC remains on schedule. Interaction with guest investigators remains
at a good level. Over the last several months, EGRET has been
receiving about 82% of the possible data that it could have received
if there had been no tape recorder failure, indicating that the
Compton Observatory is continuing to receive good coverage.

A change was made in the SAA boundary two weeks ago to alleviate
false triggers that BATSE was experiencing, ad to reduce the regions
of high background for OSSE and COMPTEL. The new definition was found
to be incompatible with the existing software in Flight Dynamics. A
second minor adjustment in the region definition was made beginning
last Saturday to correct the problem until the software could be
modified. A new error then occurred. The processing system was
confused when the spacecraft crossed the prime meridian.
Consequently, SAA passages were terminated prematurely at 0-degrees
longitude. All of the instruments, including EGRET, were turned on at
that location resulting in high count rates in EGRET and EGRET
triggers. Fortunately, the intense region of the SAA was not
penetrated. The problem is now believed to be corrected.


OSSE

OSSE operations have returned to normal. Because an error in the
GRO-wide definition of the boundary of the SAA activated OSSE during
periods of high particle flux, OSSE automatically entered a safe-hold
mode for one orbit on each of five nights. We commanded the
instrument back into science mode each time without incident. In
addition, we deactivated the on-board slewing to bursts to prevent
accidental triggering. The error in the SAA definition has been
fixed, and we expect to reactive the slewing shortly.

In viewing period 401 (4-18 Oct), the Z-axis target is PSR 2334+61
(Guest Investigator S. Sturner), and the X-axis target is M51 (Guest
Investigator D. Bhattacharya).

Two interesting OSSE results have recently been accepted for
publication and will be distributed as preprints shortly. The average
OSSE and Ginga Seyfert spectrum (Zdziarski et al.) and the detection
of a (presumably cyclotron) absorption line at 110 keV from A0535+26
(Grove et al.) will appear in ApJL.

Data from viewing period 227 were delivered to the Compton GRO
Science Support Center archive this week. The targets were SN1993J
and PSR 0740-28.


COMPTEL

The COMPTEL instrument is performing well and continues routine
observations. The collaboration toasted the successful completion of
CGRO Phase 3 observations, and looks forward to continued productive
operation during the Cycle 4 observing cycle.

A paper discussing a possible repeating source of gamma-ray bursts at
MeV energies has recently been accepted for publication in Astronomy
and Astrophysics (Letters). In "COMPTEL detection of two
spatially-coincident gamma-ray bursts," Kippen et al. present
evidence, based on COMPTEL and IPN localizations, and supported by
Monte Carlo simulations, that GRB 930704 and GRB 940301 may have
arisen from a single source. The random probability of the occurrence
of two such bursts from unrelated sources is estimated to be on the
order of ~1.5 %.

A second announcement has recently been released regarding the
upcoming 29th ESLAB Symposium "Towards the Source of Gamma-Ray
Bursts" to be held on 25-27 April 1995 at the European Space and
Technology Centre ESTEC in Noordwijk, The Netherlands. In this second
circular, the organizers at the Space Science Department of ESA issue
a call for papers and provide information concerning the submission
of abstracts and the preparation of contributions for publication in
the proceedings. The deadline for the receipt of abstracts is 13
January 1995. Further information can be found at the following URL
on the WWW, "http://astro.estec.esa.nl/", or via anonymous ftp to
estsa2.estec.esa.nl within the directory /pub/ESLAB29.

BATSE

The x-ray transient GRO J1719-24 = GRS 1716-249 which was first seen
in November 1993 appears to again be in outburst, with low flux
levels being detected at least since early September. Its intensity
has gradually increased to about 150 mCrab in the 20-100 keV band.

The following sources were detected by the BATSE pulsed source
monitor in the past two weeks: Her X-1, Cen X-3, 4U 1626-67, 2S
1417-624, OAO 1657-415, EXO 2030+375, GX 1+4, Vela X-1, and GX 301-2.
The outburst of 2S 1417-624 which began 50 days ago is still
continuing. The pulse frequency increased steadily until September
30, and then fell for five days, probably indicating periastron
passage, and has since continued to increase. The current e-folding
time is 25 years!

Since the BATSE burst trigger criteria was change to use channels 3
and 4 rather then 2 and 3, there has been two triggers due to
atmospheric gamma-ray flashes.

As of October 13th, BATSE has detected 1134 cosmic gamma-ray bursts
out of a total of 3135 on-board triggers in 1269 days of operation.
There have been 740 triggers due to solar flares with emission above
60 keV.