TITLE: GCN GRB OBSERVATION REPORT NUMBER: 234 SUBJECT: Search for Potential Images of GRB 990123 DATE: 99/01/27 20:27:44 GMT FROM: Robert Rutledge at Caltech Search for Potential Images of GRB 990123 R. Rutledge and S. R. Kulkarni (CIT) note: Djorgovski et al. (GCN #216) have suggested that GRB 990123 is lensed by a foreground galaxy identified by Odewahn et al. (GCN #201) and presumed to be at redshift 0.21 or 0.28 (Hjorth et al. GCN #219). The basis of this argument is two fold: (1) the energetics of the GRB are reduced, as lensing would provide strong amplification, and (2) the foreground galaxy, due to its placement and likely mass, must result in some amount of lensing of a background object at the position of the optical transient. A consequence of this lensing hypothesis is image splitting. The same burst would arrive at different times, with the time difference proportional to the image separation (e.g., Turner et al. GCN #221). Motivated by these considerations we have looked into the BATSE catalog to see if there are GRBs in the general vicinity of the location of GRB 990123 (Piro et al, GCN #199) and with close to identical profile. The two profiles need not be exactly identical since microlensing combined with source expansion can lead to changes in profile. Within a 4-sigma error radius consistent with the GRB 990123 OT transient position, we find two double-peaked GRBs. In one of these (GRB 970627, BATSE Trigger #6279), the peaks are similar in separation and peak-width ratio to GRB 990123, although the peak intensity ratio is different by about 60+/-20%. In addition, there is excess emission in GRB 990123 following the two peaks, which is not observed from GRB 970627. However, based on the characteristics of intensity profiles, it is possible that GRB 970627 and GRB 990123 are lensed images of the same GRB event. We estimate that the chance probability of a similar profile GRB being consistent in position is about 2%, based on the identification of 8 similar GRB intensity profiles among the approximately 2000 GRBs in the BATSE catalog. If we include in this statistic GRBs with a more dissimilar intensity profile, the chance probability increases. We find 24 double-peaked GRBs (of 2000 in the BATSE Catalog) which are comparable in peak separation (15-25 seconds) but are still dissimilar to GRB 990123, resulting in a chance probability of 6.4%. A comparison between the light curves of these two GRBs is available at http://www.srl.caltech.edu/personnel/rutledge/0123/bursts.html. If GRB 970627 is indeed a lensed image of GRB 990123, then to explain the very long time delay between the two images (1.5 years), the positional splitting must be several arcseconds in size and would most certainly require multiple lenses or a cluster. This message may be cited.