Compton Gamma Ray Observatory Status Report #204 Monday May 12, 1997 Questions or comments can be sent to Chris Shrader at the CGRO-SSC. Phone: 301/286-8434 e-mail: shrader@grossc.gsfc.nasa.gov Guest Investigator News The Fourth Compton Symposium was held in Williamsburg VA last month. Nearly 300 scientists from the US and abroad participated, and a wide range of exciting new science was presented. The meeting included several press events, one of which, involving a new source of spatially extended 511-keV line emission in the galactic center region, led to prominent coverage by several major newspapers and news magazines. Some additional details are presented in the OSSE instrument team report below. The proceedings will be published through the American Institute of Physics. Release is targeted for the Fall 1997 time frame. Cycle-7 proposals are due one month from today. The Cycle-7 NASA Research Announcement (NRA) is available on the World Wide Web through Goddard (http://cossc.gsfc.nasa.gov) or NASA Headquarters (http://www.hq.nasa.gov/office/oss). Paper copies are also available by contacting the CGRO-SSC (barnes@grossc.gsfc.nasa,gov). Proposal forms must be prepared electronically and submitted BOTH electronically and as hard copy. You must accomplish this using the Remote Proposal Submission (RPS) system, which is accessible through the Web, or e-mail (end a black e-mail message to rps@legacy.gsfc.nasa.gov for instructions). Address questions about RPS to Paul Barrett (barrett@compass.gsfc.nasa.gov). Proposals will be evaluated by peer review in August, and an observing time line will be constructed in late August or September after which selections will be announced. Direct any questions about the Cycle-7 NRA, the proposal submission procedure or the Cycle-7 program to Chris Shrader (shrader@grossc.gsfc.nasa.gov). Instrument Team Reports EGRET EGRET participated in the Target of Opportunity observation of MRK 501 that was seen flaring by ground-based high energy detectors. Since no one had proposed for this source, the photon data was made public on April 18 when the final processing was completed. The EGRET team participated in the 4th Compton Symposium by offering several posters and talks. Invited review papers are also in preparation for the Proceedings. The EGRET spark chamber is disabled for the viewing periods until August except for burst triggered intervals. The TASC however is active. Deliveries of archived and GI data are on schedule. COMPTEL The COMPTEL instrument is performing well and continues routine observations. Since the recent reboost of the CGRO spacecraft, the COMPTEL operations group has noted an increase in event count-rates, occasionally reaching the available telemetry limit for the highest-priority events ("Gamma-1"). The elevated count-rates are presumably due to the increased radiation environment at the newer, higher orbital altitudes. This rate increase will be closely followed and studied in the coming weeks, but no action is planned prior to the circularization of the CGRO orbit in June. An accelerated processing is currently underway of the data associated with the recent target-of-opportunity observation of MRK 501. These results from COMPTEL will be posted on the WWW pages of the collaboration as soon as available. The collaboration extends compliments to the organizers of the recent 4th Compton Symposium in Williamsburg for a job well done, and for a most enjoyable meeting. On a more sober note, serious preparations are now underway within the team for the CGRO Cycle 7 proposal round. OSSE OSSE operations are currently normal. The instrument is working as designed, with all subsystems in complete and full operation. Members of the OSSE team were part of the local organizing committee for the 4th Compton Symposium held in Williamsburg, VA, April 28-30. We were very pleased with the large turnout and the tremendous number of interesting scientific results presented by the large high-energy astrophysics community. The highlight of the OSSE results, new maps of the 511 keV positron radiation, was presented in an oral session and at a press conference held at the Symposium. These maps show a significant asymmetric enhancement of emission which appears to extend up to ~10 degrees above the galactic center region. While the exact morphology of the enhancement is not yet known, several possible production and ejection mechanisms have been suggested. These include enhanced supernova activity in the galactic center region during the past 10^5-10^6 years, pair jets originating from an accreting black hole, or a gamma ray burst-like event in the galactic center region. The enhancement may not originate near the galactic center region, however, but rather represent a site of more local origin. The 511 keV mapping results are also important because they demonstrate a new and useful mapping capability for the OSSE instrument. The mapping team working on these results included William Purcell from Northwestern University, Robert Kinzer, James Kurfess and Jeffery Skibo from the Naval Research Laboratory, David Dixon from the University of California, Riverside, Lingxiang Cheng and Marvin Leventhal from the University of Maryland, David Smith from the University of California at Berkeley, Jack Tueller from the Goddard Space Flight Center, and Michael Saunders from Stanford University. The scientific results have been submitted to the Astrophysical Journal. Public interest in the story has been very high. So far, the "antimatter cloud" story has appeared in over 30 U.S. publications, including Time, Newsweek, the Washington Post, the Los Angeles Times, and a front-page story in the New York Times, as well as several television news reports and numerous international publications. For more details and summaries of the press coverage, see the Web site Bursts, flares, and transients through 3 May 1997 have been processed. There were 36 classical gamma-ray burst triggers in the one-month period since the last burst summary, of which six were slew triggers. Three occurred while slewing was disabled around the reboost. Of the three remaining bursts, two were well-located on the scan plane (compared to the final ground-based position); both were fairly far off the scan plane, but 6167 was still seen at the 36% response level. Two other events (6190, 6214) were observed by OSSE in the FOV near the time of the burst, at 43% and 6% responses. Further analysis of these events is in progress. Two new IPN locations were near the scan plane but not in the FOV. The new bursts are available on the burst web site . OSSE data on GRB 970228, along with pointers to GRB 970228 data and preprints on the web can be found at Recent observations are listed in the following table. 617.8 9-15 Apr Mrk 501 (public TOO) Cas A (public) 618 15 Apr - 6 May Carina region (D. Grabelsky) NGC 4507 (G. Madejski) NGC 918 (public) 619 6-28 May Cir X-1 (N. Brandt) PSR 1509-58 (S. Matz) PKS 0521-365 (public) Low-level OSSE data products through viewing period 513 are awaiting delivery to the Compton GRO Science Support Center archive. High-level data products through viewing period 220 have been delivered. In addition, by special request all subsequent public Cyg X-1 data sets, both low and high level, have been delivered. Public data and high-level products from the Mrk 501 TOO should be ready for delivery to the archive within a few weeks. Refer to the CGRO-SSC page on the WWW , or contact Tom Bridgman (bridgman@grossc.gsfc.nasa.gov) for more information. BATSE The following has been submitted as an IAU cirular: C.R. Robinson, S.N. Zhang, M.L. McCollough (USRA/MSFC), B.A. Harmon (NASA/MSFC), S. Dieters, W.S. Paciesas (UAH), M. Tavani (Columbia and IFCTR, Milan), R. P. Fender (Sussex), G.G. Pooley (Cambridge), E. Waltman (NRL), I.F. Mirabel (Saclay), R.M. Hjellming, M. Rupen, F. Ghigo (NRAO), L.F. Rodriguez (UNAM) and K. Ebisawa (USRA/GSFC), report: Following recent flaring activity at 2.25 and 8.3 GHz, observed by the Green Bank Interferometer, and hard X-ray activity, observed by CGRO/BATSE, we observed the superluminal jet source GRS1915+105 from 21:31 on 7 May to 00:44 on 8 May with the RXTE/PCA. Four repeating outburst cycles, of period near 2000 seconds, were observed from the source. The gross structure of the light curve can be described as a high intensity spike, reaching a height of 3200 counts/second, followed by lower intensity oscillations, a dip to a minimum level, and finally a gradual recovery after which the entire cycle repeats. The hardness ratio (defined above and below 11 keV) is inversely correlated with intensity. BATSE monitoring of GRS 1915+105 before and during these observations indicates a persistent hard component with intensity of 0.10-0.13 photons/cm2/second in the 20-100 keV band. Preliminary analysis of data since mid-April indicates that the energy spectrum can be adequately fit with a power law of photon index about -3. The day-to-day variability of the source increases roughly in conjunction with the first GBI flare on April 25-26. Additional RXTE observations and a coordinated IR/VLA/VLBA observation is scheduled for 13-16 hours UT on May 15. A light curve from part of our RXTE observations, future observing plans and additional information on galactic superluminal sources will be available from the WWW site at http://www.batse.msfc.nasa.gov/multiwave. Interested observers are encouraged to visit this site and to send information on their observations to multiwave@bbking.msfc.nasa.gov. During the last month the following pulsed sources have been detected by the BATSE pulsed source monitor: Her X-1, Cen X-3, 4U 1626-67, OAO 1657-415, EXO 2030+375,GX 1+4, Vela X-1, 4U 1145-619,and GX 301-2. The burst trigger is currently using rates from channels 2+3 (50 - 300 keV), with thresholds of 5.5 sigma for all timescales. As of May 8 BATSE has detected 1817 gamma-ray bursts out of a total of 6120 on-board triggers in 2207 days of operation. There have been 782 triggers due to solar flares, 42 due to SGR events, 65 due to terrestrial gamma-ray flashes, and 1800 due to the bursting pulsar GRO J1744-28. The current burst catalog is now available from the BATSE Web page (http://www.batse.msfc.nasa.gov/data/grb/catalog/). Currently available are the basic table, which contains the locations, the peak flux and fluence table, and a table giving trigger criteria changes. Others tables, such as V/Vmax and duration, will be added as they become available. The tables will be kept up to date, but the data are subject to change.