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."