Compton Observatory Science Report #131 Thursday, August 5, 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. There were two maneuvers performed on Tuesday, August 3, 1993, first to align the thrust vector with the orbital velocity vector for the B-side OATS calibration burns, and then to maneuver back to do science. The first maneuver times were 1345Z to 1405Z, through an angle of 58 degrees. The second maneuver times were 1947Z to 2009Z, through an eigenaxis angle of 70.47 degrees. The HGA boom axis with respect to the equator is now 11.8 degrees. The Z-axis target is NGC 6814, and the OSSE X-axis target is NGC 4507. The following was received from R. Wilson, Mission Ops. Manager August 3, 1993 The Compton GRO engineering test burns of the two B-side orbit adjust thrusters (OATs) were successfully completed on August 3, 1993. After performing a reaction-wheel attitude maneuver earlier in the day to align the thrust vector with the orbital velocity vector, both OATs were fired simultaneously for a period of one second while the spacecraft was in velocity control mode with the attitude control thrusters disabled. The spacecraft was then returned to wheel control and disturbance torques were removed. This test burn was then repeated. The objectives of these calibration burns were to verify the operation of the thrusters and to calibrate their performance by observing the induced disturbance torques. The calibration analysis is expected to provide information on the effective spacecraft center of gravity that will be used to evaluate orbit reboost options. These test burns were the first time that the OATs had been fired; previously, only the B-side ACTs had been fired. Preliminary conclusions pending detailed data analysis from these burns indicate that the OATs firings were nominal. Both burns gave similar data and looked identical. Pitch disturbance for both burns was within predicted levels. A slight disturbance in the roll axis is being analyzed. The center of gravity appears to be within predicted levels based on prelaunch data. The tests were successful and will provide key information to be used in performing the orbit reboost. >From Wilson/LaVigna BATSE ----- The pulsed flux from the newly discovered x-ray pulsar GRO J1008-57 has been declining since July 27th, and is now at 40% of its peak value. The outburst from the x-ray binary A0535+262, which began July 8th, is ending. The pulsed flux is expected to soon be undetectable. During viewing period 231 BATSE folded-on-board pulsar data will be collected for the Crab pulsar, PSR 0525+21, PSR 0540-693, PSR 0736-40, Her X-1 and PSR 0736-40. The pulsar hardware is also begin used to collect single sweep 31 ms resolution data for pulsar searches using LAD 0. As of August 1st, BATSE has detected 729 cosmic gamma-ray bursts out of a total of 2372 on-board triggers in 831 days of operation. There have been 663 triggers due to solar flares with emission above 60 keV. COMPTEL ------- The COMPTEL instrument is performing well and continues routine observations. During the period of the test burn of the spacecraft propulsion system earlier this week, the primary detector high voltages of COMPTEL were switched off, as a safety precaution. After the successful completion of the propulsion test, the instrument was reactivated without incident. Flight data has been received for the recent target-of-opportunity observation of the newly-discovered x-ray pulsar GRO J1008-57; an accelerated processing schedule is being implemented for these data. EGRET ----- The EGRET instrument continues to function well, and operations are normal. Two weeks ago the spark chamber readout system experienced a high rate of lock ups, although the loss of data was minimal due to an auto- matic reset procedure. The rate has now fallen to a very low level with only one occurrence in the past week. OSSE ---- OSSE operations are normal. The OSSE and BATSE teams together submitted the following IAU Circular regarding the location of the new X-ray pulsar. GRO J1008-57 J. E. Grove, R. L. Kroeger, B. Phlips, J. D. Kurfess, and W. N. Johnson, for the OSSE team; M. H. Finger, R. B. Wilson, B. A. Harmon, M. T. Stollberg, B. C. Rubin, and G. J. Fishman, for the BATSE team; and N. Gehrels, for the Compton Gamma-Ray Observatory, report: "An improved position for the recently discovered x-ray pulsar GRO J1008-57 (IAUC 5836) has been obtained by using OSSE scans of the region along with a re-evaluation of the BATSE Earth occultation times. The 90-percent confidence region is bounded by a box with the following corners: R.A. = 10h03m, Decl. = -58o25' (equinox 2000.0); 10h03m, -58o50'; 10h13m, -58o20'; 10h13m, -57o55'. The source flux observed by BATSE in the energy range 20-120 keV remained within 15 percent of the value reported on IAUC 5836 from July 19 to 28." Since the submission of the circular, the complete set of source-positioning data from OSSE has become available, and further analysis has allowed us to narrow the error box in declination. Note that the box boundaries are set entirely by statistical uncertainties and do not include an estimated 0.1 deg systematic uncertainty for OSSE. The right ascension of the boundaries was determined by BATSE. Corners of 90% confidence region, not including ~0.1 deg systematic error. R.A. = 10h03m, Decl. = -58.40d R.A. = 10h03m, Decl. = -58.65d R.A. = 10h13m, Decl. = -58.15d R.A. = 10h13m, Decl. = -57.90d For viewing period 231, the Z-axis target is NGC 6814 (PI team), and the X-axis target is NGC 4507 (GI: L Bassani). 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. Version 7.2 of the OSSE data analysis system, IGORE, has been delivered to the Compton GRO Science Support Center and has been demonstrated to work on the SSC's computers.