GCN Circular 1554

A.M. Soderberg, P.A. Price, D.W. Fox, S.R. Kulkarni, S.G. Djorgovski, E.
Berger and F. Harrison, S. Yost (Caltech); M. Hamuy and S. Shectman (OCIW);
E. Armstrong, C. Espaillat N. Mirabal and J. Halpern (Columbia); and
J. Kemp (Joint Astronomy Center and Columbia) report:

We imaged the entire error-box of XRF 020903 with the Palomar 200-inch
telescope + Large Format Camera (LFC) on Sep 4.3 UT (epoch 1) and Sep 10.3
UT (epoch 2) in Steidel R-band.  Visual comparison of these images with
the Digitised Sky Survey and with each other did not reveal any obvious
transient.  Recently, we undertook a more detailed analysis, namely
PSF-matched image subtraction of these two epochs, and found
an optical transient (OT) located at coordinates:

	  RA: 22:48:42.34   Dec: -20:46:09.3   J2000

with a bright elliptical galaxy 4 arcsec SE (hereafter G2).

The source is also present in images from the MSO 50-inch telescope
(GCN #1533
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 ) when the second epoch LFC image is subtracted.

Upon discovery of the OT, we immediately pursued further observations,
thereby obtaining a third epoch of imaging with the MDM 1.3-meter
telescope on Sep 28.3 UT in the R-band.  Initial photometry demonstrates
that the object has re-brightened by ~ 0.3-0.4 mag between epochs 2 and
3.  We propose that this optical rebrightening is from an associated
supernova, peaking between ~7-24 days after the initial XRF trigger.

Spectroscopic observations by M. Hamuy and S. Shectman for the source were
also obtained with the Magellan 6.5m telescopes (Baade+LDSS2, Clay+B&C) on
Sep 28.1 UT.  We detect narrow emission lines from an underlying host
galaxy (hereafter G1) are (Halpha, Hbeta, Hgamma, [O III], [Ne III],
[O II]) at a mean redshift of z = 0.25 +/- 0.01.  Subtraction of the
emission lines reveals a continuum that is consistent with a SN-like
transient near maximum.  In particular, the spectrum reveals
deficit of emission at rest wavelengths < 4000A, as would be expected
from a SN.

Spectra of G2 (R ~ 17 mag) reveal it to be an elliptical galaxy at
z = 0.23 and therefore is not associated with the optical transient.

If the OT is indeed associated with XRF020903, it is the first known
optical afterglow of an XRF, and may be the most nearby cosmological high
energy transient known yet.  Parenthetically we note that microlensing
from G2 may result in sharp variations of the light curve (see
Garnavich, Loeb and Stanek, 2000). We encourage further monitoring of
this OT/SN.

A web page summarizing the discovery and subsequent observations can be
found at:
	http://www.astro.caltech.edu/~ams/XrF.html

This message may be cited.

[GCN OPS NOTE (29Sep02):  Three names were added to the author list.]