Circular No. 6637
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
BMARSDEN@CFA.HARVARD.EDU or DGREEN@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/cfa/ps/cbat.html
Phone 617-495-7244/7440/7444 (for emergency use only)


GRB 970228
     F. Frontera, Universita di Ferrara and Istituto Tecnologie
e Studio Radiazioni Extraterrestri (ITESRE), Consiglio Nazionale
Ricerche, Bologna; J. Greiner, Astrophysikalisches Institut,
Potsdam; L. A. Antonelli, BeppoSAX, Rome; D. Dal Fiume and M.
Orlandini, ITESRE; T. Boller and W. Voges, Max-Planck-Institut fur
Extraterrestrische Physik, Garching; E. Costa, M. Feroci and L.
Piro, Istituto Astrofisica Spaziale, Frascati; and G. Zavattini,
Universita di Ferrara, communicate:  "A 34 280-s ROSAT HRI
observation centered at the BeppoSAX/WFC position of the gamma-ray
burst GRB 970228 (IAUC 6572), performed between Mar. 10.7875 and
13.32 UT, detected eight sources in the HRI field-of-view, of which
only one is in the 3' WFC error radius and within the 50" error box
of the BeppoSAX fading source SAX J0501.7+1146 (IAUC 6576).  The
position of the pointlike source, RX J050146+1146.8, has the
following coordinates:  R.A. = 5h01m46s.7, Decl. = +11o46'51"
(equinox 2000.0), with an error radius of 10", coinciding (within
2") with the optical transient (IAUC 6584).  Using only the 10351 s
of low background level, the source countrate is (1.0 +/- 0.3) x
10E-3 count/s.  Assuming a power-law spectrum with photon index
1.9, the corresponding flux in the band 0.1-2.4 keV is (3.8 +/-
1.2) x 10E-14 erg cmE-2 sE-1, uncorrected for any galactic
absorption."


GRO J1655-40
     E. Kuulkers, University of Oxford; T. Belloni, M. Mendez, M.
van der Klis, and R. Wijnands, Astronomical Institute 'Anton
Pannekoek', University of Amsterdam (UoA); and J. van Paradijs,
University of Alabama at Huntsville and UoA, report:  "Using RXTE
All Sky Monitor data, we discovered about twenty drops in the
2--12-keV intensity by about 25-95 percent during 90-s measurements
of the relativistic jet blackhole binary GRO J1655-40 in its
current outburst (IAUC 6393).  During these drops, the spectral
hardness increases.  Public RXTE PCA observations on Feb. 26.8-27.0
UT show two additional drops by about 92 percent with durations of
order 1 min.  A period of 2.621 +/- 0.001 days (1-sigma) is best-
fit with the occurrence of the ASM and PCA intensity drops.  This
is consistent with the optically determined period of the system
(Orosz and Bailyn 1997, Ap.J. 477, 876).  All drops occurred
between orbital phases 0.72 and 0.86.  This constitutes the first
orbital evidence of GRO J1655-40 in x-rays.  Similar orbital dips
have previously been seen in low-mass x-ray binary 'dip' sources.
Further monitoring at all wavelengths at these orbital phases is
encouraged."

                      (C) Copyright 1997 CBAT
1997 April 28                  (6637)            Daniel W. E. Green