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IAUC 7099: GRB 990123

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                                                  Circular No. 7099
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)
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Phone 617-495-7244/7440/7444 (for emergency use only)


GRB 990123
     J. Heise, Space Research Organization of the Netherlands,
Utrecht; C. DeLibero, M. R. Daniele, and G. Scotti, BeppoSAX
Operation Control Center, Telespazio, Rome; D. Ricci and M.
Capalbi, BeppoSAX Science Data Center, Rome; L. A. Antonelli,
Osservatorio Astronomico, Rome; and E. Costa, Istituto di
Astrofisica Spaziale, CNR, Rome, report:  "The BeppoSAX Wide Field
Camera (WFC) error box of GRB 990123 (IAUC 7095) was observed from
Jan. 23.65 UT (6 hr after the gamma-ray trigger) until about Jan.
24.75, using the Narrow Field Instruments of BeppoSAX.  A bright,
previously unknown x-ray source designated as 1SAX J1525.5+4446,
was detected by the LECS and MECS units at R.A. = 15h25m31s, Decl.
= +44o46'.3 (equinox 2000.0), with an error-circle radius of 50".
The new source is within the WFC error circle and at a distance of
22" from the optical transient reported on IAUC 7094.  The
intensity of the source at 2-10 keV in the MECS at the start of the
observation is 0.14 count/s, corresponding to 1.1 x 10E-11 erg
cmE-2 sE-1.  The source fades by an order of magnitude in 24 hr,
consistent with a decay in time with a power-law index of -1.35.
The average featureless spectrum has a photon power-law index of
about 2, with an interstellar absorption column of 1.2 x 10E21
cmE-2, above the galactic absorption of 2.1 x 10E20 cmE-2 expected
in that direction.  We conclude that we have observed the x-ray
afterglow of GRB 990123.  It is the brightest of all gamma-ray-
burst x-ray afterglows observed until now."
     A. Maury, Observatoire de la Cote d'Azur, Caussols; and M.
Boer and S. Chaty, Centre d'Etude Spatiale des Rayonnements,
Toulouse, France, report:  "We clearly detected the optical
candidate for GRB 990123 and its presumed host galaxy with the
Observatoire de la Cote d'Azur 0.9-m Schmidt telescope, in a series
of 300-s unfiltered frames.  The unfiltered magnitudes (mean
uncertainty 0.4 mag; USNO A2.0 reference stars) correspond
approximately to R magnitudes, given the detector response:  Jan.
24.03703 UT, 19.6; 24.05241, 19.6; 24.05723, 20.0; 24.06188, 19.8;
24.06653, 19.7; 24.07119, 19.4; 24.07584, 20.4; 24.08049, 19.9;
24.08515, 20.1; 24.09445, 20.3; 24.09910, 20.2; 24.11306, 19.7;
24.11773, 19.8; 24.12237, 20.2; 24.12702, 20.5; 24.13167, 20.3;
24.13633, 20.5; 24.14098, 20.2; 24.14563, 20.0; 24.15028, 20.1;
24.15494, 20.4; 24.15959, 20.4; 24.16424, 20.0; 24.16890, 20.1;
24.17820, 20.2; 24.18285, 20.1; 24.20107, 20.4.  The resulting
decay slope, 1.35, is in good agreement with the slope mentioned by
Bloom et al. (GCN 208), given the uncertainties."

                      (C) Copyright 1999 CBAT
1999 January 26                (7099)            Daniel W. E. Green

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