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IAUC 6480: GRB 960720; C/1996 R1

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                                                  Circular No. 6480
Central Bureau for Astronomical Telegrams
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
Phone 617-495-7244/7440/7444 (for emergency use only)

GRB 960720
     L. Piro, E. Costa, M. Feroci, and M. Cinti, Istituto
Astrofisica Spaziale, CNR, Frascati; F. Frontera, D. Dal Fiume, L.
Nicastro, M. Orlandini, and G. Pizzichini, Istituto Tecnologie e
Studio delle Radiazioni Extraterrestri, CNR, Bologna; J. Heise and
R. Jager, Space Research Organization of the Netherlands, Utrecht;
F. Fiore, P. Giommi, A. Matteuzzi, and H. Muller, Beppo-SAX
Scientific Data Center, Rome; A. Parmar, Space Science Department,
ESTEC; B. Sacco, Istituto Fisica Cosmica ed Applicazioni
Informatica, Palermo; S. Molendi, Istituto di Fisica Cosmica e
Tecnologie Relative, Milano; and R. C. Butler, Agenzia Spaziale
Italiana, Rome, report:  "BeppoSAX observed the error box of GRB
960720 (IAUC 6467) during Sept. 3.715-4.708 UT for a total of
46 000 s with the MECS telescope and 26 000 s with the LECS
telescope, and discovered a source in both telescopes within the
WFC error box at R.A. = 17h29m26s.0, Decl. = +49o04'35" (equinox
2000.0), about 2'.9 off the center of the Beppo-SAX WFC error box
and about 1'.6 off the ROSAT source (IAUC 6469).  We estimate about
1' for the systematic error in position reconstruction.  BeppoSAX
and ROSAT positions are consistent within systematic and
statistical errors.  The LECS (0.1-2 keV) and MECS (1.5-10 keV)
countrates are 0.0010 +/- 0.0002 and 0.0021 +/- 0.0004 count/s,
respectively.  Taking into account point-spread-function
corrections and the instrumental response, we derive (based on
preliminary calibrations) the following fluxes:  F(0.1-2 keV) =
(1.5 +/- 0.6) 10E-13 erg cmE-2 sE-1 and F(2-10 keV) = (2.5 +/- 0.5)
10E-13 erg cmE-2 sE-1.  The LECS flux is approximately a factor of
three greater than the ROSAT flux, assuming a power-law spectrum
with an energy index of 1.3-1.5.  The LECS/MECS countrate ratio is
consistent with no significant absorption in addition to the
galactic one (2.6 x 10E20 cmE-2).  No other source brighter than
about 1 x 10E-13 erg cmE-2 sE-1 is found within the 10' WFC error
box.  At least two other sources of flux between 1 and 3 x 10E-13
erg cmE-2 sE-1 lie at about 15'-20' from the center of the WFC
error box.  Assuming a value of (log N - log S) of active galactic
nuclei from GINGA background fluctuations and recent ASCA data, we
estimate the probability of detecting a source with a flux of 2E-13
erg cmE-2 sE-1 within a 5' radius as about 10 percent."

     Total visual magnitude estimates:  Sept. 9.88 UT, 11.8 (R. J.
Bouma, Groningen, The Netherlands, 0.25-m reflector); 14.83, 11.7
(Bouma); 18.20, 11.4 (J. M. Trigo, Castellon, Spain, 0.17-m

                      (C) Copyright 1996 CBAT
1996 September 30              (6480)            Daniel W. E. Green

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