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GCN Circular 243

GRB 990123: New Constraints on Possible Foreground Galaxies
1999-02-01T15:21:54Z (25 years ago)
George Djorgovski at Caltech/Palomar <>
GRB 990123:  New Constraints on Possible Foreground Galaxies

S. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, C. Koresko, R. R. Gal, 
S. C. Odewahn (Caltech), M. A. Malkan, I. S. McLean (UCLA), H. I. Teplitz
(GSFC), D. Koerner (U. Penn.), D. Kirkpatrick (IPAC), and D. A. Frail (NRAO), 
report on behalf of the Caltech-UC-NRAO-CARA GRB collaboration:

We performed a new differential astrometry between the Palomar discovery 
images of the optical transient (OT) associated with GRB 990123 (GCN 201, 
GCN 205) and the DPOSS F (red) plate containing the field.  The results 
indicate that the apparent foreground galaxy near the OT position (GCN 201, 
GCN 206, GCN 207) is offset by 2.4 arcsec to the N and 1.0 arcsec to the E 
of the OT, with the estimated errors of 0.13 arcsec (systematic) + 0.3 arcsec 
(random) in each coordinate.  As we noted before, at R ~ 21.5 mag this 
object corresponds to approximately 2-sigma detection on the plate scan.

We detect no objects down to a limiting magnitude K ~ 23 in this location
in the stack of deep K-band images obtained at the Keck-I telescope on 24 
and 27 January 1999 UT (GCN 240).  The implied limiting color is so blue 
that we can exclude even the most actively star-forming galaxies.  We thus
conclude that the sky survey detection was spurious.  This is in a complete
agreement with the findings by Yadigaroglu et al. (GCN 242) in the R band.

This leaves open the question of the identification of galaxies responsible
for the absorption systems at z = 0.286 and z = 0.210 reported by Hjorth
et al. (GCN 219), as well as the z = 1.60 absorption system itself (Kelson
et al., IAUC 7096; and GCN 219).

Using the best-fit R-band light curve power-law slope alpha = -1.13 +/- 0.03
(GCN 208, GCN 240) normalized to the K-band detection from 24 January 1999,
we estimate the maximum allowed contribution to the observed K-band light
in our measurements from 27 and 28 January 1999 UT from any underlying galaxy
(either the host and/or the foreground absorbers).  We obtain K > 22 mag for
any such objects.

However, in our best-seeing (FWHM ~ 0.5 arcsec) images, from 27 January 1999 
UT, we detect a faint, galaxy-like extension to the N of the OT, with an 
apparent center about 0.5 arcsec from the OT itself.  Its rough estimated 
magnitude is in the range of K ~ 22 to 23 mag, in agreement with the limits
derived from the light curve analysis.

Pending an independent confirmation of this detection, this object may be
either a foreground dwarf galaxy responsible for one of the absorption
systems reported by Hjorth et al., or a highly luminous host of the GRB
itself (possibly responsible for the absorption system at z=1.6).

While there is no clear observational evidence for a strong gravitational 
lensing of this burst, the possibility remains open, and the presence of 
foreground galaxies along the line of sight suggests that some lensing 
magnification must be taking place (regardless of the extraordinary apparent 
energetics of this burst).  Forthcoming observations from the Keck and the 
HST should clarify the situation.

This report is citeable.
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