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GCN Circular 17321

GRB 150101B/Swift J123205.1-105602, deep VLT observations
2015-01-20T15:31:06Z (9 years ago)
Andrew Levan at U.of Leicester <>
A.J. Levan (U. Warwick), J. Hjorth (DARK/NBI), N.R. Tanvir, K. Wiersema (U. Leicester), A.J. van der Horst (George Washington University) report on behalf of a larger collaboration:

"We obtained further deep optical observations of the field of GRB 150101B/Swift J123205.1-105602 (Cummings; GCN17267, 17268) using the VLT and FORS2, on the 19 & 20 Jan 2015. At this epoch 1200s of observations were taken in both g, R and I, in dark time with seeing between 0.6-0.8". 

Within these deep images we locate a faint counterpart to the radio source identified by van der Horst et al. (GCN 17286) and Fong et al. (GCN 17288). It appears to be a marginally extended source with R~25 (calibrated against USNO), and to be relatively blue given its non-detection in the IR. However, its persistent radio emission implies it is likely not related to the outburst.

We do not see any object at the location of the putative optical counterpart identified in Magellan imaging by Fong et al. (GCN 17271), whose location is consistent with the presence of an X-ray source with L_X ~ 1e43 ergs/s in Chandra observations (Troja et al. GCN 17289). Given our detection of the possible radio counterpart above, we conclude that these images are deeper than those reported previously by Fong et al. (GCN 17271, GCN 17288). Although the filters are slightly different, this may suggest optical fading of this source, which would strengthen its association with GRB 150101B. 

We further note that this position clearly lies on the stellar field of the galaxy 2MASX J12320498-1056010. Such luminous, non-nuclear X-ray sources are extremely rare, and while a background AGN is possible, the small impact parameter makes this unlikely within the limited BAT error circle. The luminosity is much higher than for most short-GRB afterglows at this epoch, and there is apparently little evidence for X-ray variability (although this is complicated by the presence of the nearby AGN which is a factor ~4 brighter, Troja et al. GCN 17289). Hence, while we can not yet make firm claims about the relationship of the Magellan and Chandra source to GRB 150101B/ Swift J123205.1-105602, it remains a plausible candidate. "
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