GCN Circular 1556

A. Gal-Yam (Tel Aviv University) reports:

"We have extracted from the USNO plate archive
(http://ftp.nofs.navy.mil/data/fchpix/) two blue plates, covering the
location of the variable object possibly connected with XRF 020903,
reported by Soderberg et al (GCN 1554
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 ). Inspecting these plates we find
the following:

1) A plate obtained on UT 1977.6 reveals an object, coincident with the
variable source reported by Soderberg et al. The B-band magnitude of this
object appears to be about one magnitude fainter than that of the nearby
galaxy noted as "G2" by these authors.

2) Comparison of the 1977 plate with the (much poorer) POSSI blue plate
obtained on 1954.6 may suggest a variation of the flux ratio between the
source coincident with the Soderberg et al. variable, and the galaxy G2,
with G2 being brighter on the 1977 plate, but possibly fainter than the
source reported by Soderberg et al., in 1954. This may hint that this
source was variable in the past. We stress, however, that the 1954 data
are quite poor, so past variability can not be ascertained at this stage.

GIF Cutouts from both plates, showing the area around the Soderberg et al.
variable, can be obtained from ftp://wise3.tau.ac.il/pub/avishay/xrf.
These can be compared with the Palomar data shown by Soderberg at
http://www.astro.caltech.edu/~ams/XrF.html.

The source detected on the 1977 plate may be hard to interpret as a
possible "host" galaxy for the Soderberg et al. transient, even if we
disregard its possible past variability. Soderberg et al. report that this
source is not detected on POSSII red plates, and estimate a magnitude of
R=22 from their OT+SN+host model. This fact can be reconciled with the
1977 B-band data, showing this object only about 1 magnitude fainter than
G2 (which has R=17 mag), if this object has a very blue B-R color.
However, this appears to be in contradiction with the Magellan spectrum
presented by Soderberg et al., which is flat, or possibly even red, in
B-R.

An alternative explanation fitting both the archival analysis and the
entire data set presented by Soderberg et al. and Berger et al (GCN 1555
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 )
is that this source is a variable radio-loud AGN. We have recently shown
(Gal-Yam et al. 2002, PASP, 114, 587) that such objects can appear to be
very similar to optical transients associated with GRBs. Further
inspection of archival data, as well as further observations (B-band
especially), can distinguish between an OT+SN model, and an AGN, according
to their variability properties.

If an AGN, the source discovered by Soderberg et al. may still be related
to XRF 020903. However, in that case, a SN component no longer seems to be
required.

Until this issue is clarified, we urge some caution in interpreting the
Soderberg et al. data set as evidence for a connection between XRF 020903
and a SN.

This work has made use of the USNOFS Image and Catalogue Archive operated
by the United States Naval Observatory, Flagstaff Station
(http://www.nofs.navy.mil/data/fchpix/)."