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IAUC 6530: GRO J1744-28; C/1995 O1

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                                                  Circular No. 6530
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/cfa/ps/cbat.html
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GRO J1744-28
     C. Kouveliotou, Universities Space Research Association (USRA)
and Marshall Space Flight Center (MSFC), NASA; K. J. Deal, G. A.
Richardson, and M. S. Briggs, University of Alabama in Huntsville
(UAH); G. J. Fishman, MSFC; J. van Paradijs, University of
Amsterdam and UAH, report:  "We have recorded a series of outbursts
starting on 1996 Dec. 2 that are consistent with the position of
GRO J1744-28, the bursting pulsar.  The bursts are detected between
20 and 100 keV, and on Dec. 2 they came in variable rates ranging
from every 300 to every 500 s, with a preliminary total number
estimated between 80 and 100 events.  After Dec. 3, the burst rate
dropped to 15-20 bursts/day, where it has remained.  The current
average error radius of the burst locations is about 4 deg.  The
burst fluence is currently about 2 x 10E-7 erg cmE-2.  This
indicates that the source is in outburst again.  Although the
source location is currently very close to the sun, we encourage
observations in other wavelengths whenever possible."
     M. H. Finger and C. R. Robinson, USRA; B. A. Harmon, MSFC; and
B. A. Vaughan, California Institute of Technology, report for the
Compton Observatory BATSE team:  "Persistent, hard x-ray emission
from GRO J1744-28 (IAUC 6272, 6284, 6285) is currently being
detected through both pulsation and earth occultation.  Pulsations
with a period near 0.467 s became detectable on Dec. 17, rising to
a 20- to 40-keV-rms-pulsed flux of 1.3(1) x 10E-9 erg cmE-1 sE-1
(170 mCrab) by 1997 Jan. 3.  On Jan. 1.0, the intrinsic pulse
frequency was 2.1408983(4) Hz, and the spin-up rate was 2.8(5) x
10E-12 Hz sE-1.  This assumes the following circular orbit
parameters:  P = 11.83665(14) days; epoch of longitude 90 deg = JD
2450126.9977(4) TDB; a sin i = 2.6371(5) light-s (determined using
data from the 1995 Dec.-1996 Apr. outburst).  Earth-occultation
monitoring of flux from GRO J1744-28 shows its intensity increasing
by a factor of about two during 1996 Dec. 25-1997 Jan. 6.  Although
source confusion prevents a precise flux estimate, subtracting a
25-percent background for unresolved galactic-center sources yields
a persistent flux for Jan. 3-6 in the energy bands 20-30, 30-40,
and 40-50 keV of 590, 470, and 290 mCrab, respectively."

     Visual m1 estimates with 10x50 binoculars:  1996 Dec. 14.70
UT, 4.2 (G. Glitscher, Darmstadt, Germany); 20.94, 4.2 (J. E.
Bortle, Stormville, NY); 25.71, 3.8 (R. J. Bouma, Groningen, The
Netherlands); 1997 Jan. 1.70, 3.3 (B. H. Granslo, near Oslo,
Norway); 6.72, 3.1 (Granslo).

                      (C) Copyright 1997 CBAT
1997 January 7                 (6530)            Daniel W. E. Green

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