CGRO Status Report for November 1996

[SHRADER@grossc.gsfc.nasa.gov]

   Compton Gamma Ray Observatory Status Report #198
                 Friday November 8, 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

Cycle 6 officially began on October 15, 1996. There were several
minor changes to the Viewing Plan prior to the start of the
cycle, and an additional one very recently - refer to the CGRO
hompepage on the World Wide Web for the latest
(http://cossc.gsfc.nasa.gov). Cycle-6 budget requests have now
all been evaluated and are being processed. 

In a special arrangement between NASA and NRAO, the Greenbank
Interferometer (GBI) has been (or very nearly so) restored into
operational condition. Both the CGRO and RXTE projects have
contributed to support this effort and to operate the facility
for at least one year. The understanding between NASA and NRAO is
that the data obtained will be made available to CGRO and RXTE
Guest Investigators, and that individual Guest investigators on
either NASA facility will have input into how the GBI observing
program is defined. A steering committee, consisting of CGRO and
RXTE scientists, as well as several radio astronomers who are
highly familiar with the GBI facility, will consider specific
requests when constructing the GBI observing program. The
limiting sensitivity of the GBI is approximately 10-20 mJy 
(2-8GHz), and the program is likely to consist of nominal-daily
monitoring of 10-12 sources. Examples of CGRO targets include
galactic binaries such as Cyg X-3, GRO J1655-40, GRS 1915+105 and
LSI 61 303. Additionally, quasars such as PKS 1622-297 may be
possible as well. If you are currently participating in a CGRO
Cycle-6 investigation that could potentially benefit from this
program, please send e-mail with a brief scientific justification
to Chris Shrader (shrader@grossc.gsfc.nasa.gov). Please note:

-    the position of any proposed source (should be at  the level
     of 1 arcsec).
-    the proposed duration of GBI monitoring (typically not to
     exceed 1 month or so).
-    any time constraint for GBI monitoring (requests for
     simultaneous CGRO/GBI observations should be made well in
     advance to incorporate the proposed radio source in the GBI
     schedule.
*    Note that all the GBI data will be made publicly available
     promptly after they are obtained.



Instrument Team Reports


EGRET

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.

EGRET has now observed high-energy gamma rays from six
pulsars for certain and a seventh with high probability. 
For three of them, there are energy spectra as a function
of the phase in the pulse period which may be used to test
details of the theoretical models that exist.  In
addition, there is, for all of the pulsars seen by EGRET,
the information on the phase relative to the radio phase,
the knowledge of the energy flux observed, and a general
spectrum.  These results, together with other information,
allow the determination of the energy emitted as a
function of age, the fraction of the total energy going
into gamma radiation, and the spectral index of the gamma
rays as a function of age.  All of these characteristic
may be compared against the theoretical predictions to
choose between the models and make refinements to the
theory.

In order to conserve the remaining gas supply on EGRET for
observations determined to be of high priority by the
review committee, the high voltage for the spark chambers
was turned off on October 29, 1996 and will remain off
until December 10, 1996 when the 3C 279 observations
begin.


COMPTEL

The COMPTEL instrument is performing well and continues
routine observations. The collaboration looks forward to
further successful operation during the upcoming CGRO
Cycle 6.

The proceedings of the 3rd Compton Symposium, held in
Munich, are scheduled to appear imminently in an upcoming
issue of the Astronomy and Astrophysics Supplement Series. 
A new paper on the "COMPTEL detection of low-energy gamma
rays from the HVC complex M and A region?" by Blom et al.
has been accepted for publication in Astronomy and
Astrophysics.

A further delivery of COMPTEL Cycle 4 data products to the
COSSC public archive is currently in progress.  This will
include both low-level and high-level data products,
including standard-processing skymaps, for viewing periods
up to 420.

Members of the COMPTEL group at UNH were pleased to
recently co-host a symposium in honor of Bill Webber, in
recognition of 40 years of research in cosmic-ray
astrophysics; numerous luminaries were in attendance, at
the peak of the fall foliage season in New England.



OSSE


OSSE operations are currently normal.  The instrument is
working as designed, with all subsystems in complete and
full operation.

The slewing response to BATSE burst triggers continues to
be enabled. OSSE slewed in response to BATSE trigger #5623
(GRB961004) and viewed the position estimated on-board for
three hours.  Twenty-five of the 26 classical bursts in
the last month had 16 ms shield data available; 13 of
these produced significant responses in the shields. Only
5628 produced a significant central detector response (in
MLL), with a possible precursor. Largest signals were seen
from 5629, a long (30 s), chaotic event, and 5647, a short
(~0.2 s) event with peak flux of ~600 cts/16 ms.  The new
bursts are available on the WWW at:
http://www.astro.nwu.edu/astro/osse/bursts/newlist.html

The ApJ has recently accepted the paper, "Spallation of
Iron in Black Hole Accretion Flows" by J.G. Skibo. Papers
recently submitted to ApJ include "OSSE Observations of
the Soft Gamma Ray Continuum from Galactic Longitude l=95
deg" by Skibo et al. (ApJ Letters) and "Determining the
Hubble Constant from Coordinated Gamma Ray and Radio
Observations of MeV Blazars", by Skibo et al.  A number of
papers were also recently submitted to the proceedings of
the 2nd Integral Workshop in St. Malo: "OSSE Observations
of the Ultraluminous IR Galaxies Arp 220 and Mrk 273" by
Dermer et al.;  "Two Gamma-Ray Spectral Classes of Black
Hole Transients" by Grove et al.; "The Source and
Distribution of Galactic Positrons" by Purcell et al.;
"Spallation of Iron in Seyfert AGN" by Skibo; "The
Galactic Soft Gamma Ray Continuum" by Skibo; and "X-Rays
and Gamma-Rays from Accretion Flows onto Black Holes in
Seyferts and X-Ray Binaries" by Zdziarski et al.

Recent observations are listed in the following table.
View period     Dates        Target          (owner)
    531       3-15 Oct        Carina region  (D.Grabelsky)
                              Arp 220        (C. Dermer)
                              Mrk 841        (public)

    601.1    15-29 Oct        GRS 1915+105   (C. Robinson          
                                 & D.Smith)
                              NGC 4507       (G. Madejski)

    520.9  29 Oct - 12 Nov    PKS 0528+134   (W.N.Johnson)
                              IC 4329A       (J.E. Grove)

Low-level OSSE data products through viewing period 425
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.



BATSE

The following items were reported in IAU circular 6501:    
                  
                                                           
              
     GRO J1655-40                                          
                
     B. A. Harmon, C. R. Robinson, G. J. Fishman, S. N.
     Zhang, and W. S. Paciesas, report for the Compton
     Observatory BATSE Team: "Hard x-ray flux from the
     superluminal jet source GRO J1655-40 has been
     increasing since mid-October, and has approximately
     doubled in  intensity since Oct. 26, with fluxes near
     1.0 Crab on Oct. 30-31  (20-100 keV).  This activity
     follows an extended, bright, and highly-variable
     outburst (IAUC 6436) between June and Sept. Spectra
     obtained between Oct. 26 and 30 are adequately fit
     with a power law between 20 and 200 keV, with photon
     index changing from -2.6 to -2.9 (+/- 0.1) as the
     source brightened."      
                                                           
                
     SGR 1806-20  
   
     C. Kouveliotou, Universities Space Research
     Association; G. J. Fishman, C. A. Meegan, Marshall
     Space Flight Center, NASA; J. van Paradijs,
     University of Amsterdam and University of Alabama in
     Huntsville (UAH); M. S. Briggs and G. Richardson,
     UAH; and K. Hurley, University of California,
     Berkeley, report: "The BATSE has triggered on renewed
     activity from the soft gamma-ray repeater SGR 1806-20
     on Oct. 30.95220 and on Oct. 31.54094 and 31.97024
     UT. The fluence and peak flux of the first event are
     10E-7 erg cmE-2 and 10E-6 erg cmE-2 sE-1,
     respectively. The reactivation of the source has
     initiated several Target of Opportunity observations
     with other satellites and groundbased observatories;
     currently RXTE has detected several bursts from the
     source and is observing during Nov. 6. We have
     changed the BATSE trigger criteria to trigger on
     (weaker) bursts during the RXTE observations. We
     strongly encourage follow-up observations in other
     wavelengths."    
                           
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, GX 1+4, Vela X-1, 4U
1145-619,and GX 301-2.

On October 29, BATSE recorded the longest gamma-ray burst
of the mission in its energy band. The event consists of
several pulses over an interval of about23 minutes and
generated two burst triggers. The different pulses are
consistent with a common direction.

Since November 5, the burst trigger has been using rates
from channels 1+2 (20 - 100 keV), with thresholds of 5.5
sigma for 64ms and 256ms timescales and 7.0 sigma for the
1.024s timescale. As of November 7 BATSE has detected 1689
gamma-ray bursts out of a total of 5555 on-board triggers
in 2025 days of operation. There have been 768 triggers
due to solar flares, 15 due to SGR events, 63 due to
terrestrial gamma-ray flashes, and 1477 due to the
bursting pulsar GRO J1744-28.