CGRO Biweekly Status Report

[SHRADER@GROSSC.GSFC.NASA.GOV]

  Compton Observatory Science Report #163, Friday August 5, 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 Observatory, scientific instruments and all spacecraft
subsystems continue to function nearly flawlessly.


                 Science Support Center Activities

There were changes made to the Phase-3 Timeline made to accommodate
the target of opportunity observations of the X-ray nova in
Scorpius described in the BATSE report below. The modified viewing
periods are summarized as follows:

VP    start   z-axis        GRO z-axis     GRO z-axis     GRO x-axis
              target        ra      dec    long   lat     ra     dec
----------------------------------------------------------------------
336    8/01  GAL 088-47    349.67   9.72   88.37 -46.83  106.56  69.25
336.7  8/04  GROJ1655-40   258.77 -38.85  348.25  -0.18  152.62 -19.05
337    8/10  PKS0528+134    87.70   0.98  205.00 -13.00  178.49  39.05


The Cycle-4 timeline has now been completed! It is now available on
GRONEWS, and printed copies are being mailed out to Guest
Investigators. At this point, all proposers to the Cycle-4 Guest
Investigator Program whose programs did not involve any Z-axis or
OSSE pointings have been notified of the peer-review evaluations of
their proposals. The remaining notification letters and evaluation
forms will be mailed out early next week.


                        Instrument Reports

EGRET

EGRET operations and data processing continue at a normal pace.

Interaction with guest investigators remains at a good level.

 
OSSE

OSSE operations are normal.

We are in the final stages of on-board testing of a new gamma-ray
burst response that will slew the OSSE detectors rapidly to view
bursts near the scan plane.  BATSE will calculate burst locations
on board and, if a burst is within 11 degrees of OSSE's scan plane,
encode the scan angle in a signal to OSSE.  The four OSSE detectors
will drive to that location and map the region for 12 hours,
looking for persistent low-level emission. The time from burst
onset to arrival of the detectors will be ~10-100 sec, depending
on the location, and will be typically ~30 sec.  The expected event
rate is of order one per month.  Testing should be completed within
a few days.

In viewing period 335.5 (25 July - 1 Aug), the Z-axis target was
PSR J0437-47 (Guest Investigator M. Bailes) and the X-axis target
was Geminga (PI team). When both targets were occulted, OSSE
observed SMC X-1.

In viewing period 336.0 (1-4 Aug), the Z-axis target was NGC 7469
(Guest Investigator R. Cameron), and the X-axis target was Mrk 3
(Guest Investigator R. Mushotzky).  When both targets were
occulted, we tested a fast-scanning strategy for localizing point
sources.

In the Target of Opportunity viewing period 336.5 (4-10 Aug), the
Z-axis target is the new transient source discovered by BATSE, GRO
J1655-40 (PI team), and the X-axis targets are MCG-5-23-16 and PSR
0656+14 (Guest Investigator J. Cordes).

Data from viewing periods 217 and 218 were delivered to the Compton
GRO Science Support Center archive this week. The targets during
period 217 were Cen A, GRO J1317-44, SN 1987A, and PSR 0540-693. 
The targets during period 218 were SN 1993J and NGC 4151.


COMPTEL 

The COMPTEL instrument is performing well and continues routine
observations.

Members of the COMPTEL collaboration are attending a team meeting
at ESTEC at the time of this release.


BATSE

The bright X-ray nova discovered by BATSE (see IAU Circular 6046
appended to this report) continues to be about 1.0 Crab, 20-100
keV.  The initial rise to near its peak flux occurred within about
1 day, beginning near TJD 9560.8. X-rays from the source are seen
in the low energy spectroscopy detectors by Earth occultation at a
level of about 1/3 Sco X-1 at 10 keV, consistent with a power-law
extrapolation from the LAD data.  Occultation transform imaging has
provided a better and quicker localization of this source than that
from previous x-ray transients.

The BATSE-OSSE on-board burst elevation signal is in the final
phase of testing with real data and appears to be working well. 
About 7 elevation signals from real triggers have been successfully
sent to OSSE for test purposes. Initial estimates of location error
is about 8 degrees.

There have been 1069 gamma-ray bursts, as of 2 August out of a
total of 3012 triggers.


Excerpt from IAU Circ. No.6046:

X-RAY NOVA IN SCORPIUS
     S. N. Zhang, C. A. Wilson, B. A. Harmon, G. J. Fishman, R. B.
Wilson, W. S. Paciesas, M. Scott, and B. C. Rubin report for the
Compton Gamma Ray Observatory BATSE team:  "An x-ray nova has been
observed at R.A. = 16h55m, Decl. = -40o.5 (equinox 2000.0; error
radius 0o.3).  The source (GRO J1655-40) first appeared in data
from July 27 and has increased to its present level of about 1.1
Crab (20-100 keV) on Aug. 1. Significant flux is observed to 200
keV.  A power law with a spectral index of -3.15 +/- 0.07 fits the
data well from 20 to 200 keV. Pulsed emission is not seen in the
period range from 3 to 300 s at a limiting sensitivity of about 10
percent of the steady flux. No previously-cataloged x-ray source
is known at this location. It has also been determined that the
nearby x-ray pulsar OAO 1657-415, currently observed by BATSE, is
not responsible for the observed emission."