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CGRO Status Report for September 1995

 Compton Observatory Science Report #185, Thursday, September 7 1995
      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

                       Guest Investigator News

Cycle 5 is set to being on October 3, 1995! At this point a majority
of the Cycle-5 grants have been processed or are being processed at
present. If you have observations approved for Cycle-5, and have not
already done so, please check the observing plan and related
information on GRONEWS (or in the appendages to your acceptance
letter) to note when your observations will occur. If you have any
special requirements, please communicate them to the SSC or to the
appropriate CGRO Instrument Team. Changes to the observing plan due
to targets of opportunity or unforseen spacecraft problems will be
communicated to affected observers by the project scientist.

The GRO-SSC and the BATSE team have worked out data-product
definition issues and established a delivery schedule for BATSE
Occultation history data (for about the 25 brightest sources) and
Pulsar history data (about 15 sources). This follows extensive effort
by the BATSE team to minimize possible effects of systematic errors
inherent in the data. Data deliveries will begin in October of this
year and the archive is expected to be up to date (i.e. about one
year behind the observations) by about October 1996. The CGRO public
data archive is accessible via the WWW at
http://cossc.gsfc.nasa.gov/cossc/cossc.html or via anonymous ftp at
cossc.gsfc.nasa.gov, /pub/data.

The manuscript deadine for the 3'rd Compton Symposium is imminient -
October 1, 1995!

Related NASA News: The ASCA AO4 deadline for 1 yr of observations
commencing Dec 1995 is Sept 15. Further details can be found on the asca
gof WWW page http://heasarc.gsfc.nasa.gov/docs/asca/ascagof.html 

                         Instrument Reports


EGRET operations were normal this monthly period.  Delivery of the
final phase 3 data to the GRO SSC remains on schedule and delivery of
the phase 4 preliminary data to the GRO SSC is now running ahead of 
schedule. Interaction with guest investigators continues at a good
level. Preparations are well along for Cycle 5.  All of the relevant
information is in the computors, and we are ready to handle the cycle
5 data when it comes.

Results on the high-energy gamma-ray galactic and extragalactic
diffuse radiation determined from EGRET data were presented at the
International Cosmic Ray Conference in Rome.

On September 7, 1995, the pointing direction changes to the south
Galactic pole, a region that has not had much exposure, and on
September 20, 1995, the pointing direction moves to Galactic
coordinates (l=18o, b=4o).  This is the last pointing scheduled for
Cycle 4.


OSSE operations are normal.  The slewing response to BATSE burst
triggers has been enabled since 30 June.  OSSE slewed in response to
BATSE trigger #3772 on day 95/244 (TJD: 9961) at 00:25:45 UT and
mapped the area for 3 hours. 

Recent observations are listed in the following table.

View period    Dates               Target         (owner)
     427       22 Aug - 7 Sep      MGC +8-11-11   (PI team)
                                   NGC 3227       (GI. R. Cameron)

Upcoming observations in the planning stages (to close out cycle 4)
are listed in the following table.

View period    Dates               Target         (owner)
     428       7 Sept - 20 Sep     NGC 253        (GI: Bhattacharya)
                                Galactic plane/ 
                                   Vela region    (GI: Diehl)

     429       20 Sep -  3 Oct     AE AQR         (GI: Bookbinder)
                                   SCO X-1        (PI team)

Data up to viewing period 329 have been delivered to the Compton GRO
Science Support Center archive.

OSSE team members presented the following papers at the 24th
International  Cosmic Ray Conference:

"Implications of the OSSE Observations of the Diffuse Galactic Gamma 
Ray Continuum"  by J. Skibo et al.

"Diffuse Galactic Gamma Ray Emission"  by W.R. Purcell et al.

"OSSE Spectra of X-Ray Binary Pulsars"  by J.E. Grove et al.


The COMPTEL instrument is performing well and continues routine

The collaboration has presented a number of papers at the 24th
International Cosmic Ray Conference, currently underway in Rome.
Among the recent results presented were those related to the diffuse
gamma-ray emission from the Galaxy, including spectral-line emission
at 1.8 MeV, the extragalactic diffuse radiation observed at MeV
energies by COMPTEL, and recent results on sources such as the x-ray
binary Cyg X-3, millisecond pulsars, and gamma-ray bursts, as well as
a report on Bayseian analysis techniques applied to COMPTEL data.

Two new conferences organized in part by members of the collaboration
have just recently been announced.  The 2nd INTEGRAL Workshop on "The
Transparent Universe" will be held 16-20 September 1996, in St. Malo,
France. Further information can be obtained via the anonymous ftp
account, astro.estec.esa.nl/pub/integral_workshop, or by referring to
the following pages on the WWW, at URL
Also, a NATO ASI workshop on "The Many Faces of Neutron Stars" will
be held in the Aeolian Islands (Sicily) from 1 to 10 October 1996;
further details on this latter conference will be forthcoming soon.

A final reminder to attendees of the The 3rd Compton Symposium
recently held in Munich: contributions to the proceedings volume for
the symposium are due by October 1.

The past month has been a busy one for the BATSE occultation and
pulsar teams, with several announcements made in the IAU Circulars.
The following notice appeared in IAU Circular 6204:

     GRS 1915+105 

     B. A. Harmon, W. S. Paciesas, and G. J. Fishman, Marshall Space
     Flight Center, NASA, report, for the Compton Observatory BATSE
     Team: "The 20- to 100-keV hard-x-ray flux from GRS 1915+105,
     measured by BATSE, has been increasing gradually since late
     July. The source showed variability on a 1-day timescale; the
     average flux was about 150 mCrab for Aug. 5-8. This sustained
     level of activity is comparable to that seen in 1995 Jan.-Feb." 

The following notice appeared in IAU Circular 6205:


     B. A. Harmon, W. S. Paciesas, and G. J. Fishman, Marshall Space
     Flight Center, NASA, report for the Compton Observatory BATSE
     Team: "The radio-jet transient GRO J1655-40, which has been
     outburst since late July (IAUC 6196, 6201), reached a peak
     intensity in the band 20-100 keV of 650 +/- 30 mCrab on Aug. 1,
     decreasing to 80 +/- 20 mCrab as of Aug. 9." H. Inoue, F.
     Nagase, and Y. Ueda, on behalf of the ASCA team, communicate:
     "ASCA will observe this source during Aug. 15.45- 16.18 UT, to
     investigate jet activity. Simultaneous optical and radio
     observations are encouraged." 
The following notice appeared in IAU Circular 6207:

     GRO J1735-27 

     R. B. Wilson, S. N. Zhang, M. Scott, and B. A. Harmon, Marshall
     Space Flight Center, NASA; and T. Koh, D. Chakrabarty, B.
     Vaughan, and T. A. Prince, California Institute of Technology,
     report for the Compton Observatory BATSE team: "Pulsations from
     a previously- uncatalogued source were detected on July 29 in
     the galactic-center region by BATSE. The observed period history
     shows a clear orbital signature. The maximum pulsed intensity
     was observed on Aug. 13, with a measured barycentric pulse
     period of 4.45565 +/- 0.000025 s; the pulses continue to be
     observed as of Aug. 16, at energies from 20 to at least 70 keV.
     Using occultation imaging, we observed a source of about 100
     mCrab in the energy range 20-50 keV not present during July
     2-14. A preliminary location (90-percent confidence) is bounded
     by a quadrilateral with vertices (equinox 2000.0) at R.A. =
     17h36m, Decl. = -26o.9; 17h37m, -28o.0; 17h33m, -27o.3; 17h32m,
     -26o.1. Positions estimated independently from the pulsed and DC
     flux are consistent at a 68-percent confidence level. If the
     detected emissions are from the same source, the pulsed fraction
     is about 30 percent. The energy spectrum of the pulsed component
     on Aug. 5 was well fitted by a power-law model F(E) = C[E/(30
     keV)]E(-alpha), with C = (2.7 +/- 1.0) x 10E-4 photon cmE-2 sE-1
     keVE-1, and alpha = 3.5 +/- 1.0." 
The following notices appeared in IAU Circular 6209:


     S. N. Zhang, B. A. Harmon, W. S. Paciesas, and G. J. Fishman,
     Marshall Space Flight Center, NASA, report for the Compton
     Observatory BATSE Team: "The x-ray/radio transient source GRO
     J1655-40 has been undergoing another bright outburst in hard
     x-rays, immediately after the last outburst (IAUC 6196) reached
     a minimum flux level of about 60 mCrab (20-100 keV) on Aug. 9
     (IAUC 6205). The energy spectrum can be well fitted by a power
     law with significant flux detectable up to 200 keV. The peak
     flux of the current outburst was 600 +/- 70 mCrab (20-100 keV)
     on Aug. 17. The flux has since declined to about 250 +/- 50
     mCrab on Aug. 19. The photon spectral index changed from -2.1 on
     Aug. 10 to -2.5 on Aug. 17 and to -1.8 on Aug. 19, consistent
     with the spectral evolution seen previously from this source. We
     note that the 8- to 20-keV flux detected by WATCH/GRANAT
     continued to rise since Aug. 17 (see above). The current
     activity from July 22 appeared in phase with the previous three
     major outbursts with peak flux above 1 Crab, which were
     separated by 120 days." 

     GRS 1915+105 

     Zhang et al. report: "Hard x-ray activity from GRS 1915+105 has
     continued since a previous report (IAUC 6204). The flux in the
     band 20-100 keV peaked on Aug. 11 at 220 +/-25 mCrab, following
     the intense radio flare on Aug. 10 (IAUC 6204), then decreased
     rapidly until the source was undetectable on Aug. 14. We note
     that the 8- to 20-keV flux (see above) continued to increase
     after Aug. 11, suggesting a hard-to-soft transition in the
The following notice appeared in IAU Circular 6210:

     CYGNUS X-1 

     W. Paciesas, D. Crary, G. Fishman, B. A. Harmon, C. Kouveliotou,
     and S. N. Zhang, Marshall Space Flight Center, NASA, report for
     the Compton Observatory BATSE team: "Since about June 28, the
     blackhole-candidate binary system Cyg X-1 (= HDE 226868) has
     been in a state of unusually intense low-frequency noise. The
     power spectrum of the 20- to 100-keV emission between 0.005 and
     0.5 Hz shows the typical shape for Cyg X-1: flat at low
     frequencies and falling off roughly as 1/f at high frequencies.
     However, the total power in the low-frequency noise increased by
     a factor of about 3 between June 28 and July 30, while the break
     frequency dropped from about 0.08 to about 0.04 Hz, similar to
     the effect seen at lower energies by Belloni and Hasinger (1990,
     A.Ap. 227, L33). The increased noise is not correlated with
     source intensity. Individual Cyg X-1 flares with timescales of
     order 1-10 s have triggered the burst detection algorithm aboard
     BATSE > 30 times since June 28. Continuous BATSE monitoring has
     shown no similar level of activity in Cyg X-1 since 1991 Nov." 

The following notice has been submitted for inclusion in an IAU

     GRO J1750-27 and GRO J1735-27

     T. Koh, D. Chakrabarty, T. A. Prince, B. Vaughan, California
     Institute of Technology; and S. N. Zhang, M. Scott, M. H.
     Finger, and R. B. Wilson, Marshall Space Flight Center, report
     for the Compton GRO/BATSE team: "Further observations of the
     recently discovered 4.45 s transient hard X-ray pulsar in the
     Galactic center region (originally designated GRO J1735-27; see
     IAUC 6207) have yielded a refined position, based only on pulsed
     flux, which indicates that the pulsed and unpulsed emission
     reported in IAU 6207 come from different sources. The estimated
     90-percent-confidence error box for the pulsed source is
     approximately a quadrilateral of size 0.7 square degrees, with
     vertices RA = 17h47m, Dec = -27o.8; 17h50m, -28o.0; 17h52m,
     -26o.9; 17h49m, -26o.7 (equinox 2000.0). In accordance with the
     new position, we have redesignated the pulsed source as GRO
     J1750-27 and the unpulsed source as GRO J1735-27. The pulsed
     emission first detected on Jul 29 UT continues to be visible as
     of Aug 29. The barycentric pulse period for GRO J1750-27 on Aug
     29.5 was 4.45311 +/- 0.00003 s, with an approximate period
     derivative of -0.11 s yrE-1. Optical and soft X-ray observations
     of the error box are encouraged to further constrain the source
     position and identify the accreting companion. The unpulsed
     source (GRO J1735-27) reported in IAUC 6207 is not observed in
     data combined from Aug. 11 - 21, 1995, while pulses from GRO
     J1750-27 have been detected at a nearly constant level through
     Aug. 29. The position reported in IAUC 6207 remains valid for
     the unpulsed source GRO J1735-27.

GX 339-4 continues its outburst. The flux level in the 20-100 keV
band is about 200 mCrab. GRO J1719-24 = GRS 1716-249, which was in
outburst earlier this summer, began fading around Aug 11 and is no
longer detectable to BATSE.

The BATSE pulsed source monitor has detected the following sources in
the last month : Her X-1, Cen X-3, GRO J1750-27, 4U 1626-67, OAO
1657-415, GX 1+4, Vela X-1, and GX 301-2. 

The BATSE burst trigger is using the full LAD energy range (~20-1800
keV). As of September 2 BATSE has detected 1346 gamma-ray bursts out
of a total of 3669 on-board triggers in 1593 days of operation. There
have been 757 triggers due to solar flares, 9 due to SGR events, and
49 due to terrestrial gamma-ray flashes.

The BATSE data analysis team has developed a schedule to provide
overdue data products on discrete/steady (non-burst) objects to the
CGRO-SSC. These products include low- and medium-level data as well
as data on sources observed by earth occultation and pulsar analysis.
Details of these data products and their scheduled delivery may be
obtained from the SSC.

There are a limited number of extra copies of the Proceedings of the
2nd Huntsville GRB Workshop (AIP Proc. #307). A copy may be requested
from Ms. Susan Benefield: benefield@batse.msfc.nasa.gov.