info for Cycle-5 Proposers (Instrument Status Report)

[SHRADER@GROSSC.GSFC.NASA.GOV]

Dear Colleague,

This messages gives updates on hardware status and new capabilities for the 
four instruments on the Compton Observatory.  For more information, please 
contact the instrument teams or feel free to contact me (gehrels@lheavx
.gsfc.nasa.gov) for references.

Sincerely,
Neil Gehrels
Compton Project Scientist


INSTRUMENT STATUS AND NEW CAPABILITIES


BATSE


The BATSE flight instrumentation continues to work extremely well.  There 
have been no hardware failures or degradation in the detectors since launch.  
All components, including photomultiplier tubes, power supplies  and the on-
board computer continue to have nominal performance.  The on-board software 
has undergone two minor modifications since launch, primarily to optimize 
the data types and to perform on-board data storage and delayed readout to 
overcome the loss of the GRO tape recorders.  A 1 kilobit/sec data stream is 
being utilized during periods of data gaps.  This helps to recover about 
half of the data that would otherwise be lost and is especially valuable for 
occultation analysis.

The BATSE all-sky hard x-ray source monitoring capabilities have been 
continually improved since launch.  Hard x-ray/gamma-ray source detection 
and monitoring have become more reliable and quantitative.  The results of 
these analyses are available to the scientific community quicker than 
previously.  This is especially important for follow-up observations by 
other, more sensitive instruments and observations at other wavelengths.  
Unusual events of broad interest are often submitted as IAU Circulars.  An 
occultation transform imaging technique has been developed which has been 
extremely valuable in locating and separating sources.  Typical 
sensitivities for continuous and pulsed sources can be derived from recent 
BATSE publications (e.g. 2nd Compton Symposium and the X-ray Binaries 
Conference, Univ. of Md.).

Gamma-ray burst detection and analysis has also improved considerably since 
launch.  The development of new spectral analysis software has now been 
completed and the first deliveries have been made.  Burst trigger criteria 
have been changed several times since Sept. 1994 in order to better study 
other transient phenomena such as the BATSE-discovered atmospheric gamma-ray 
flashes.  Perhaps the most significant change since launch for the study of 
gamma-ray bursts has been the development of the BAtse COordinate 
DIstribution NEtwork (BACODINE) system by S. Barthelmy/GSFC, in conjunction 
with the BATSE investigators.  By intercepting the BATSE data stream in 
real-time, this system can recognize a burst, derive its approximate 
location, and distribute this information in a matter of seconds of its 
occurrence.  This permits the search for a candidate burst counterpart by 
wide-field instruments sooner than ever before possible.  The gamma-ray 
burst location software has been considerably improved; location accuracies 
of about 2 degrees (for strong bursts) are expected, using the new software.



COMPTEL


COMPTEL operations are routine.  The instrument is stable and performing 
well at 93% of its maximum sensitivity in the energy range from 750 keV to 
30 MeV .  Individuals requiring additional detail should contact the COMPTEL 
instrument manager, John Macri, at UNH.  Telephone: (603)862-2793.  
Internet: jmacri@unh.edu.

In September 94 the GSFC and UNH/COMPTEL teams implemented systems for 
automated FTP transfer of selected COMPTEL data sets from the BACODINE BATSE 
real-time burst monitor computer to analysis systems at UNH.  With this 
method the locations of selected bursts are reported in 15 to 30 minutes.  
The previous best was 100 minutes.  For additional details contact Marc 
Kippen.  Telephone: (603)862-4378.  Internet: rmkippen@unh.edu.



EGRET


The EGRET instrument, as the others on the Compton Gamma Ray Observatory,
was designed for a two year lifetime with an effective factor of two
contingency.  At the present time just a few months before the fourth year
in orbit, the status of the EGRET instrument is reasonably good and the
consumable, the spark chamber gas, is expected to last significantly past
the four year period as will be described in the second paragraph.  There
has been only one failure, that being one of the sixteen lower phototubes
in the coincidence system.  Since it is on the periphery, the loss in
sensitivity is slightly less that it would be if it were in the middle.
The loss in sensitivity, about 6%, is included in the figures and software
provided to the guest investigators; so there is no need for a guest
investigator to take the matter into account further.  There has been a
slight degradation in the spark chamber system in the form of a decreased
efficiency in a few decks;  this change has little or no influence on the
final results, and any quantitative effect is included in the normalization
factors provided routinely to the guest investigators.

Regarding the gas life, it is now expected that the present full solid
angle operation mode will continue through the end of cycle 4 with the
exception of a few viewing periods where it does not seem warranted.  There
should be enough gas after that to continue to operate through cycle 5 and
probably significantly beyond that in a limited mode which is described
fully in the Cycle 5 NASA Research Announcement in the appendix concerned
with the EGRET instrument.  Basically, the recommendation is that most
observations should use only one or occasionally two or three of the nine
basic EGRET instrument telescope modes.  Further, EGRET should be shut off
if there is no significant observation for a given pointing.  This approach
allows examining a source, several sources close together, or a band of
radiation, but not looking at the full large solid angle.



OSSE


The OSSE instrument continues to operate normally, with all detector
sub-systems performing as designed.  Since launch, OSSE flight software has
been  modified several times to improve flight operations and add 
flexibility.  Most importantly, the On-board Data Storage (ODS) process was 
implemented in Phase 2 (following the failure of the on-board tape 
recorders) to ensure >95% coverage for the primary source and background 
spectral data for each orbit.  ODS accumulates spectral data during times 
when TDRSS contacts are not available, data that would otherwise be lost. 
The one area of concern in flight operations is the status of the motor 
drive for detector #1.  On three occasions, the drive has failed to step in 
response to the motor drive positioning process that, in normal operation, 
moves the detectors every two minutes.  In all cases, recovery from the 
anomaly was made without difficulty.  The cause is still under 
investigation. It should be noted that each OSSE detector has a redundant 
drive motor system in case of hard failure of the primary drive. 

In coordination with the BATSE team, a new capability to OSSE's response to 
cosmic gamma-ray bursts has recently been added.  The burst trigger signal 
generated by BATSE now encodes the results of an on-board calculation of the 
approximate position of a cosmic gamma-ray burst.  If the burst strength and 
position meet a specified set of criteria, the encoded scan angle is sent to 
OSSE, which then enters a special burst mapping mode.  The criteria are 
currently (1) count rate greater than 40 counts per 64 ms in the BATSE 100-
300 keV band, (2) azimuth from the OSSE scan plane less than 11 degrees, and 
(3) OSSE scan angle within -45 to +135 degrees from the spacecraft's +Z 
axis.  The OSSE detectors map the region within +-10 degrees of the 
estimated burst position for a period of 12 hours to search for persistent 
or delayed low-level emission from the burst source.  Both the mapping area 
and the duration of the response are user-definable.  If, in the meantime, 
an improved position becomes available via the BACODINE network, e.g. from a 
detection by COMPTEL, the OSSE mapping strategy can be updated appropriately 
within about one hour with a ground command.