Compton Gamma Ray Observatory Status Report #205 Tuesday, July 8, 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 Note: These reports will now be distributed on a bi-monthly (rather than a monthly) basis. Guest Investigator News: A total of 238 proposals were received in response to the Cycle-7 NRA, a slight increase over last years total. The proposals will be evaluated by peer-review committees next month, and the Cycle-7 observation timeline will be constructed late next month or in early September - after which all notification letters and budget solicitations will be distributed. Cycle 7 will start on November 12, 1997. The CGRO Users Committee met last month. Topics of discussion included the optimal use of diminishing project resources over the FY 99 - FY 00 timeframe, and possible changes to the current Guest Investigator program. The proprietary data-rights period will be reduced to 4 months for Cycle 8. The remaining EGRET gas supply is lasting longer than earlier estimates had predicted - it is currently anticipated that there will be up to 20-25 weeks of observing time available for Cycle 8 . The total Cycle-8 grants budget is expected to be approximately half of the Cycle-7 budget. Substantial savings are expected to be realized through increased levels of automation to the operations. A reminder - 4th Compton Symposium proceedings contributions are due - refer to the 4th Compton Symposium WWW site, accessible through the CGRO-SSC at http://cossc.gsfc.nasa.gov. BATSE: Burst Science: The BATSE team has recently implemented a new "BATSE Rapid Burst Response" system to quickly (within 20 min) compute and distribute GRB locations accurate to ~2 deg. The system has been used 18 times in the past three months and the locations of three of these events (GRB 970603, GRB 970616 and GRB 970704) were scanned within 3-4 hours with the RXTE-PCA. In the case of GRB 970616, a weak variable x-ray source was detected and localized to ~0.1 deg, initiating a large search for radio/optical afterglow counterparts (see IAUC 6683, 6687, 6688, 6690, 6691). "Gamma-Ray Burst Arrival Time Localizations: Simultaneous Observations by Mars Observer, Compton Gamma Ray Observatory, and Ulysses" by J. G. Laros et al. appeared in Astrophysical Journal Supplement (110, 157) in May. This paper discusses localization of nine bursts that were detected by Mars Observer, Ulysses and BATSE. "The Identification of Two Different Spectral Types of Pulses in Gamma-Ray Bursts" by G. N. Pendleton et al., will be published in Astrophysical Journal November 1, 1997, Vol. 489. Recently we have discovered two types of pulses in gamma-ray bursts that exhibit significantly different luminosities. Emission pulses in GRBs that show a marked lack of fluence above 300 keV appear to be 10 times less luminous than emission pulses with high energy flux. Some bursts show both types of pulses indicating that one source object is capable of producing both types of emission. A preprint is available at http://www.batse.msfc.nasa.gov/publications/. 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 July 5, BATSE has detected 1865 gamma-ray bursts out of a total of 6189 on-board triggers in 2265 days of operation. There have been 782 triggers due to solar flares, 43 due to SGR events, 65 due to terrestrial gamma-ray flashes, and 1800 due to the bursting pulsar GRO J1744-28. Occultation Studies: The BATSE Earth Occultation Team has been active in the past couple of months with several papers and presentations, mainly on black hole systems and other transients. Most noteworthy is a paper by Zhang, Cui and Chen, "Black Hole Spin in X-Ray Binaries: Observational Consequences" appearing in Astrophysical Journal (482, L155) last month about spin in black holes. This paper presents a calculation that connects the spin of the black hole with the appearance of a soft excess. The authors suggest that the recently discovered superluminal jet sources may contain a rapidly spinning black hole with momentum vector in the same direction as that of the accretion disk. Also Zhang et al. provide an explanation for spectral transitions in the archetypical black hole candidate Cygnus X-1. Several papers have been submitted by team members to the 4th Compton Symposium Proceedings which include new data on various sources from BATSE such as GX 339-4, Cyg X-1, Cyg X-3, GRO J2048+42, Aql X-1 and scanning of the galactic plane for weak black hole transients. Recently, the team has installed a galactic plane scan (to +/- 20 degrees latitude) which allows searches for weak hard X-ray sources. This has been in place since March of this year. Software constructed by the CFA Harvard and BATSE Teams are being used in this effort. So far we have not found any new sources, but consistent detections of sources during outburst such as the pulsar GS 1843+00 and the burster GRS 1826-238 show that the sensitivity of the technique is on the order of 50 mCrab in the 20-50 keV band over a three week period. On May 1-3, a workshop was held at Goddard Space Flight Center on the the two microquasar sources GRO J1655-40 and GRS 1915+105. Also some discussion of other objects such as Cyg X-3 took place. This workshop was organized by J. Swank of the RXTE Team and B. A. Harmon of the BATSE CGRO Team. The workshop brought together many observers and theorists with a common interest in these objects. Although no proceedings were planned for this workshop, a partial list of the scheduled speakers can be found at http://www.batse.msfc.nasa.gov/information/mqw/. An IAU Circular appeared May 9 on GRS 1915+105 (Robinson et al.) describing the peculiar cyclic behavior of the source as seen by the RXTE/PCA. The source showed a series of flares and dips which repeated approximately every 2000 seconds. This followed a trigger from radio and X-ray flaring as seen by the Green Bank Interferometer and BATSE, respectively. A light curve from part of the RXTE observations, observing plans, and additional information on galactic superluminal sources are 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." The scheduled public ToO for Mrk 501 resulted in the discovery of hard x-ray/low energy gamma-ray flux on April 12,13 and 16 at a level of 150-200 mCrab. The BATSE occultation history has been delivered to the COSSC, and is now available via the cossc WWW site. Accreting Pulsar Studies: Several papers have been submitted for the 4th Compton Symposium proceedings covering BATSE observations of accreting pulsars. This include a review of Compton observatory observations, five papers on the bursting pulsar GRO J1744-28, and papers on Her X-1, GRO J2058+42, EXO 2030+375, 4U 1538-52, and GX 1+4. Preprints can be obtained from http://www.batse.msfc.nasa.gov/data/pulsars/. "Observations of Accreting Pulsars", by L. Bildsten et al. has been accepted for publication in the Astrophysical Journal Supplement. This paper reviews the first five years of BATSE observations of accreting pulsars, which have provided a continuous record of pulsed flux and frequency for the eight bright persistent accreting pulsars, and flux-limited observations of more than 60 outbursts from 13 transient accreting pulsars. The BATSE observations, because of there continuity and uniformity over a long baseline, have provided the first detailed picture of the long-term behavior of the persistent sources. Several of these show an unexpected "torque switching" behavior, with intervals of fairly constant spin-up alternating with intervals of fairly constant spin-down. Flux limited monitoring of transient accreting pulsars (mainly Be/X-ray binaries) has lead to the discovery of five new sources. These observation also provide new clues about the outburst mechanism. They show that most outbursts are not isolated events but occur during an interval of source activity with outbursts regularly occurring each orbit. The giant outbursts, during which disk accretion is known to occur, are found to start at the same orbital phase as smaller outbursts. Preprints can by obtained from the web page http://www.batse.msfc.nasa.gov/data/pulsar/. "On the Correlation of Torque and Luminosity in GX 1+4" by D. Chakrabarty et al. appeared in Astrophysical Journal (481, L101) on June 1. This study uses five years of BATSE observations of GX 1+4, which is a pulsar accreting from a red giant. The observations show that during spin-down, changes in torque are anti-correlated with the 20-60 keV pulsed flux. This contradicts standard magnetic disk accretion theory, but would be expected if the spin-down was due to the formation of a retrograde rotating accretion disk. "Observations of a Long-Term Spin-Up Trend in 4U 1538-52" by B. C. Rubin et al. has been accepted for publication in the Astrophysical Journal. This paper uses BATSE observations of the 530 s period accreting pulsar to show that the source, which historically was observed to be spinning down, is now spinning-up. The power spectrum of torque fluctuations determined from the data are consistent with white noise, as is expected from a source accreting from a stellar wind. The observed long-term trends may be part of the resulting random-walk in frequency. Contact Dr. Rubin at rubin@crab.riken.go.jp for preprints. During the last two months (May 9 - July 5) 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,and GX 301-2. COMPTEL During the recent second phase of reboost maneuvers of the CGRO spacecraft COMPTEL science operations were suspended and the instrument placed in a low-power "safe" mode. After completion of the CGRO reboost COMPTEL was powered up without incident and has now resumed normal scientific observations. The collaboration extends compliments to the entire Flight Operations Team at the GSFC for a job well done. Since the recent CGRO reboost, 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. A working group has been formed within the team to study this effect and to consider posssible instrument retuning options in order to minimize any scientific impact of the elevated background count-rates. An accelerated processing of the data associated with the recent target-of-opportunity observation of MRK 501 (VP 617.8) has not shown any evidence of significant emission from this source at MeV energies, down to timescales of much less than a day. These upper limits clearly indicate the need for a spectral break to join the reported detections of this source in the TeV and hard x-ray bands. These results from COMPTEL will be posted shortly on the WWW pages of the collaboration, and the relevant datasets will be publicly available. A similar accelerated data processing is currently underway for the recent target-of-opportunity observations of 3C 279 (VP 621.5). Research reports were presented by members of the collaboration at the recent meeting of the Solar Physics Division of the AAS in Bozeman, Montana: A report on long-duration gamma-ray emission from solar flares was presented by Rank et al. These investigators presented their latest results on extended gamma-ray emission (on timescales of several hours) from three strong flares observed by COMPTEL on 9, 11, and 15 June 1991. The long-term behaviour measured in gamma rays indicates that the large-scale magnetic structure of the emitting region remains quite stable during the active period. These data also favor the continuous acceleration of particles over long-term trapping in solar magnetic loops as the more likely explanation of the observed extended high-energy emission. Ryan et al. presented their latest results on a search with COMPTEL for quiescent gamma-ray emission from the Sun. Several models have been put forward that postulate that gamma-ray emission (e.g., from microflares) might be detectable during periods when the Sun is normally considered to be "quiescent." If detected, such emission during periods of minimal solar activity would provide new information on small-scale energy releases on the Sun, and may be relevant to the general problem of the heating of the solar corona. A number of team presentations on further recent research results from COMPTEL will take place at the upcoming 25th International Cosmic Ray Conference in Durban, South Africa. Finally, the COMPTEL TdF group notes with pleasure that the 1997 Tour de France is underway! EGRET The coordinated target of opportunity observation of 3C 279 between EGRET and ISO was completed. A preliminary analysis of the EGRET data shows that the source is at moderately low level at energies above 100 MeV. Except for the one week of this observation, the EGRET spark chamber has been disabled since April 15, 1997. It will resume operation on August 8. In the past two months, two papers have been accepted for publication: "EGRET High-Energy Gamma-Ray Observations of the Moon and Quiet Sun" by D. Thompson, D. Bertsch, D. Morris, and R. Mukherjee will appear in the JGR, and "Observations of High Energy Gamma-Ray Emission from Blazars: an Update" by R. Mukherjee and the EGRET Team will appear in ApJ. The EGRET Team participated in the Fourth Compton Symposium and contributed over 20 papers, 6 of which were invited reviews. Seventeen proposals were submitted in response to the GRO Phase 7 NASA NRA by members of the EGRET Team. Finally, an invited paper on diffuse gamma-ray emission and two contributed papers were given at the American Astronomical Society meeting in Winston Salem, NC in June. OSSE: OSSE operations are currently normal. The instrument is working as designed, with all subsystems in complete and full operation. There has been no need yet to activate any of the redundant systems. Several papers have recently been accepted by or submitted to the Astrophysical Journal. Preprints of the accepted papers will shortly be available from the OSSE Web site . "OSSE Mapping of Galactic 511 keV Positron Annihilation Line Emission" by W. R. Purcell et al. has been accepted for publication by the ApJ. 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. For more details and summaries of press coverage of this result, see the Web site . "Annihilation Fountain in the Galactic Center Region", by C.D. Dermer and J.G. Skibo has been accepted for publication by the ApJ. A model is presented for the recently discovered annihilation feature above the Galactic center region. The model invokes a starburst episode within the inner few hundred pc of our galaxy that drives hot pair-laden gas into the halo. Positrons lose energy and annihilate as they are convected upward with the gas flow. High-latitude annihilation patterns and fluxes are calculated in accord with the observations. "Accelerated Particle Composition and Energetics and Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare", R.J. Murphy et al. has been accepted for publication by the ApJ. The paper describes new developments in our understanding of solar flares based on OSSE's observation of the 1991 June 4 flare. Key findings are: evidence for extended nuclear emission both before and after the impulsive phase; energy in accelerated ions exceeds the energy in electrons; inferrence of a high accelerated alpha particle to proton ratio (~0.5) compared with an assumed photospheric ambient He/H ratio of ~0.1; evidence for a change in the ambient composition of material impacted by flare accelerated particles as the flare progresses; and an upper limit on 3He abundance in the photosphere. "A Method for Determining the Photospheric 4He Abundance", by G.J. Share and R.J. Murphy to be published in July ApJ Letters. The paper suggests using high quality CGRO solar flare data and new calculations to obtain better measurements of the photospheric 3He/H ratio. This ratio can be obtained by high sensitivity measurements of the neutron capture line at 2.223 MeV. The 4He/H ratio in the photosphere can then be obtained by multiplying the derived 3He/H ratio by the 4He/3He ratio observed in the solar wind after correction for fractionation (i.e. the loss of 4He relative to 3He in the acceleration process). This will provide a "direct" measurment of the helium composition of the Sun; to date the abundance has only been obtained indirectly from stellar evolution calculations or helioseismology, both of which are model dependent. "A Multiwavelength Investigation of the Relationship Between 2CG135+1 and LSI+61 303" by M.S. Strickman et al. has been submitted to the ApJ. The results of a multiwavelength observing campaign for 2CG135+1, highlighted by a series of CGRO/OSSE observations, are reported. OSSE saw only weak emission from the region, with no particular correlation or anticorrelation to the periodic radio flares from LSI+61 303, the Be binary associated by EGRET with 2CG135+1. We observed some evidence that the OSSE and possibly COMPTEL emission may be from the nearby QSO0241+622. "Gamma-Ray Spectral States of Galactic Black Hole Candidates" by J.E. Grove et al. has been submitted to the ApJ. It summarizes OSSE spectra from seven transient BHCs. Two gamma-ray spectral states are evident: the "breaking" gamma-ray state, which appears to be associated with the X-ray low, hard state, shows a spectrum that is cut off near 100 keV; while the "power-law" gamma-ray state, which appears to be associated with the X-ray high, soft state, shows a spectrum that is a fairly soft power law with no break detected. Likely emission mechanisms for the two states are considered. Recent observations are listed in the following table. 619.2 14-20 May GRS 1915+105 (R. Kroeger) M31 (public) 619.4 20-28 May Cir X-1 (N. Brandt) PSR 1509-58 (S. Matz) PKS 0521-365 (public) 619.5 28 May - 4 Jun reboost engineering test data 619.7 4-10 Jun Cir X-1 (N. Brandt) PSR 1509-58 (S. Matz) PKS 0521-365 (public) 620 10-17 Jun Gal Plane 16 (R. Kinzer) PKS 2155-304 (public) NGC 7172 (public) 621.5 17-24 Jun 3C279 (public TOO) Crab Pulsar (J. Cordes) OJ 287 (public) 622 24 Jun - (15 Jul) NGC 3516 (R. Svensson) Mrk 85 (public) Bursts, flares, and transients through 1 July 1997 have been processed. There were 50 classical gamma-ray burst triggers in the two-month period since the last burst summary. There was one slew trigger in this period, for trigger 6262 on June 9. First accumulation began 78 s after the trigger; however the large cross-scan offset to the best position reduces the OSSE response to ~16% of on-axis. OSSE data will be examined for any long-term post-burst flux. We see a weak signal in our shields from the SAX event GRB 970508 and have a good time history in our longer time scale shield data for GRB 970616 (which had an XTE counterpart). Positions for both events were outside the FOV of our central detectors. The new bursts are available on the burst web site . Low-level OSSE data products through viewing period 516.5 and high-level products through the end of Phase 2 are awaiting delivery to the Compton GRO Science Support Center archive. In addition, by special request all subsequent public Cyg X-1 data sets, both low and high level, have been delivered, as have public data and high-level products from the Mrk 501 TOO (vp 617.8). Refer to the GROSSC page on the WWW , or contact Tom Bridgman (bridgman@grossc.gsfc.nasa.gov) for more information.