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CGRO Status Report for September 1996

   Compton Gamma Ray Observatory Status Report #196
                 Friday September 6, 1996

      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

All notification letters for Cycle-6 proposals were mailed out last month. The
second-phase (budget) review will be completed this month after all of the budget
requests are received. Cycle-6 observations will begin on October 29, 1996. The
cycle-6 timeline and information such as off-axis target summaries and program
titles and abstracts are available on the CGRO SSC WWW site:


EGRET operations were normal this monthly period.  Delivery of the final phase
4 data to the GRO SSC is on schedule, and delivery of the phase 5 preliminary
data to the GRO SSC is also on schedule.  Interaction with guest investigators
continues at a good level.

As most of you are aware, the study of the extragalactic diffuse radiation is a very
difficult problem requiring careful analysis to separate the Galactic component,
and is best performed after most of the data are available.  Several preliminary
reports have been given, but a final paper is now in preparation and should be
submitted shortly.  We can say in advance that the analysis confirms earlier work
of a power law spectrum in energy, and it extends this spectrum to 50 GeV.

Currently the EGRET telescope is pointed at GRO J0004+73 and will be until
October 3, 1996.


OSSE operations are normal.  The instrument is working as designed, with all
subsystems in complete and full operation.  The slewing response to BATSE burst
triggers was enabled on 96/233.  

We have recently added new functionality to the OSSE flight software to collect
"burst-regulated spectrum multiscale" data:  in response to a BATSE burst trigger,
high time-resolution spectra are accumulated from a single OSSE shield segment. 
In the standard configuration, 64-channel spectra covering 200 keV - 8 MeV are
accumulated every 128 ms for a total of 8.192 sec. The energy range and
sampling rate are user-definable parameters.  In response to the recent increase in
the rate of terrestrial gamma-ray flashes, we are currently collecting 64-channel
spectra over 200 keV - 2 MeV every 16 ms for 1.024 sec.

OSSE requested and was granted a target-of-opportunity observation of GRO
J1655-40 following an extended outburst monitored by BATSE. Preliminary
analysis of six days of OSSE observations near the peak of the recent flare
indicate that the source is similar in  intensity to previous outbursts.  The
spectrum is well described by a power law (photon index -2.73 to -3.00) from 50
to 600 keV.  There is no evidence of line emission.  Flaring with time scales on
the order of 4-6 days have been characteristic of this extended outburst.   By the
completion of this viewing period in a few days, OSSE is likely to have data from
one complete flare cycle.  

Recent observations are listed in the following table:

View period      Dates             Target    (owner)

    527        13-20 Aug           Crab Pulsar    (PI team)
                          IC 4329A      (PI team)
                    NGC 2992       (public)

    528        20-27 Aug           IC 443         (GI:C.Dermer)
                                        IC 4329A  (PI team)

    529.5   27 Aug - 6 Sep         GRO J1655-40 (PI team)
                                        NGC 2992       (public)

Low-level OSSE data products through viewing period 420 and high-level data
products through viewing period 220 have been delivered 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.  Refer to
the CGRO-SSC page on the WWW (http://cossc.gsfc.nasa.gov), or contact Tom
Bridgman (bridgman@grossc.gsfc.nasa.gov) for more information.


The COMPTEL instrument is performing well and continues routine operations.
COMPTEL data associated with the current target-of-opportunity observations of
the transient black-hole candidate GRO J1655-40 are presently undergoing
accelerated processing.

A major delivery of COMPTEL Cycle 4 data products is anticipated later this
month to the CGRO public archive at the COSSC.

A number of papers by COMPTEL team members and guest investigators will be
presented later this month at the 2nd INTEGRAL Workshop in St. Malo, France.
Among the topics to be covered will be gamma-ray line spectroscopy, all-sky
mapping at MeV energies, and the latest results on emission from the Orion
region, pulsars, and other sources.


Follows is the BATSE contribution to the September monthly report:

The following was reported in IAU circular 6447:                       
     CYGNUS X-1                                                             
     S. N. Zhang, B. A. Harmon, and W. S. Paciesas report for the Compton
     Gamma Ray Observatory BATSE team: "The blackhole candidate Cyg
     X-1 is experiencing a transition from a soft state back to its normal hard
     state. It went into the soft state in mid-May, following a sudden x-ray flux
     increase in the band 1.3-12 keV (IAUC 6404) and a rapid hard-x-ray-flux
     decrease in the band 20-200 keV (IAUC 6405). Since July 26, its 20- to
     200-keV flux increased from about 200 to about 600 mCrab (as of Aug.
     7). Its energy spectrum also evolved from a steep power law (photon index
     about -2.5), without obvious break up to about 200 keV, to a harder power
     law  (index about -2.0) with a break at about 100 keV, quite similar to its
     hard-state spectrum (typically with index -1.8 and with a break at 100-150
     keV). If its current trend of hard-x-ray-flux increase continues, Cyg X-1
     will be back to its normal hard state in about a week."                                  
The following was reported in IAU circular 6450:                       
     4U 0115+634                                                            
     M. Scott and M. H. Finger, Universities Space Research Association; R.
     B. Wilson, Marshall Space Flight Center, NASA; and T. A. Prince and B.
     Vaughan, California Institute of Technology,  report for the Compton
     GRO Observatory BATSE pulsar team: "Pulsed hard x-ray flux is
     currently being detected from the transient Be/x-ray pulsar 4U 0115+634.
     The source was detected on Aug. 10, when the rms pulsed flux (20-50
     keV) was approximately 25 mCrab.  This is the first detection by BATSE
     of this source since a pair of outbursts that occurred during the interval
     1995 Nov. 18-1996 Jan. 9 (IAUC 6266). On 1996 Aug. 12, the rms pulsed
     flux (20-50 keV)  was approximately 30 mCrab. The pulse frequencies
     observed from  Aug. 10 to 12 are consistent with a neutron-star spin rate
     of  0.27666712 +/- 0.00000022 Hz and no detectable spin-up, using the
     binary orbital ephemeris of Rappaport et al. (1978, Ap.J. 211, L1) with the
     epoch of periastron passage updated to 1996 Aug. 5.5 UT."  
This was corrected in IAUC 6451 as follows:                                                            
     4U 0115+634                                                            
     Corrigendum.On IAUC 6450, line 13, for  1996 Aug. 5.5 UT."    read
     1996 Aug. 5.583 TDB."                                            
The following was reported in IAU circular 6462:                       
     CYGNUS X-1                                                             
     S. N. Zhang, B. A. Harmon, W. S. Paciesas, C. R. Robinson, D. M. Scott,
     and W. Yu, on behalf of the BATSE/CGRO team; and R.  Remillard and
     W. Cui, on behalf of the ASM/RXTE team, report: "The blackhole
     candidate Cyg X-1 is returning to the hard state,  following the recent hard
     x-ray flux increase (IAUC 6447). Hard x- ray (20-200 keV) and soft x-ray
     (1.3-12 keV) fluxes are very close  to the values before the
     hard-to-soft-state transition in mid-May 1995 (IAUC 6405). Spectral
     hardening has also been observed in both bands. The ASM (1.3-12 keV)
     and BATSE (20-200 keV) long-term  lightcurves are anticorrelated, with
     the total 1.3- to 200-keV  luminosity remaining approximately unchanged
     within 15 percent  during the entire state transition from mid-May to late
     August 1996.  The BATSE and ASM soft-state power-density spectra
     (PDS) of Cyg X-1 (roughly 10E-7 to 10E-4 Hz), which can be
     approximately represented by a power law with an index of approximately
     -1.0, are apparently  a low-frequency extension of the same soft-state PDS
     detected by  PCA/RXTE above 10E-3 Hz (Cui et al., submitted to Ap.J.
     Lett.). Flaring episodes separated by approximately 45-50 days and of 
     duration 5-30 days were observed superimposed on the spectral  transition
     in both ASM and BATSE data. (The mid-1991 to early-1996 BATSE
     lightcurve for Cyg X-1 does not show evidence of any  significant
     periodicity near 50 days.) A possible 15-day delay of  the flares in the
     ASM band with respect to the flares in the BATSE  band was also
     detected from the cross correlation between ASM and BATSE
The following was reported in IAU circular 6468:                       
     SMC X-1                                                                
     S. N. Zhang, C. R. Robinson, R. B. Wilson, B. A. Harmon, and D. M.
     Scott, on behalf of the BATSE/CGRO team; and R. Remillard, on behalf
     of the ASM/RXTE team, report: "Quasiperiodic flux variations have been
     observed from the 0.7-s pulsar SMC X-1 with a period around 60 days, in
     the energy bands 1.3-12 keV (ASM/RXTE) and 20-100 keV
     (BATSE/CGRO), confirming the HEAO-1 results in 1977-1978 (Gruber
     and Rothschild 1984, Ap.J. 283, 546). A sequence of five outbursts
     between early Oct. 1995 and mid-Aug. 1996 is apparent in the
     BATSE/CGRO lightcurve, of which the last three coincide with those
     observed from ASM/RXTE (the first two of the three ASM  outbursts
     have been reported by Levine et al. 1996, Ap.J. 469, L33). The peak flux
     is about 30 mCrab in both bands. No significant outbursts were observed
     in the BATSE data between Apr. 1991 and Oct. 1995, though a weak level
     of emission at about 10-20 mCrab was sometimes detected. Eclipses with
     a duration of about 20 percent of the orbital period of 3.9 days are
     observed in both data sets. The current outburst was detected about Aug.
During the last month the following pulsed sources have been detected by the
BATSE pulsed source monitor: Her X-1, 4U 0115+634,Cen X-3, 4U 1626-67,
2S1417-624, OAO 1657-415, GRO J2058+42, GX 1+4, Vela X-1, and GX 301-2.

Since August 29, the burst trigger has been using rates from channels 2+3  (50 -
300 keV), with thresholds of 5.5 sigma for all three timescales. As of September 1
BATSE has detected 1638 gamma-ray bursts out of a total of 5486 on-board
triggers in 1958 days of operation. There have been 768 triggers due to solar
flares, 10 due to SGR events, 62 due to terrestrial gamma-ray flashes, and 1477
due to the bursting pulsar GRO J1744-28.