TITLE: GCN CIRCULAR NUMBER: 15319 SUBJECT: GRB 131004A: MOSFIRE NIR imaging DATE: 13/10/08 09:31:30 GMT FROM: Daniel Perley at Caltech D. A. Perley (Caltech) reports: I imaged the location of GRB 131004A (GCN 15303) in Ks-band with MOSFIRE on the Keck I telescope on the night of 2013-10-07 for a total integration time of 291 seconds under good seeing conditions but some cloud cover. The midpoint of the integration was 07:21 UT, 2.40 days after the GRB. At the location of the optical transient reported by Xu et al. (GCN 15304), no source is detected to a limiting magnitude of Ks > 20.6. The nearest detected object to this location is a faint (Ks = 19.7), compact source approximately 1.4 arcsec to the west, at RA=19:44:26.99, dec=-02:57:30.15. Given the moderate redshift of z=0.717 (Chornock et al., GCN 15307), these results suggest either that the host of this event is a relatively low-mass galaxy or that the GRB exploded outside the bulk of the stellar light of its host. The >1.5 sec duration and significant lag reported in the BAT refined analysis (Lien et al., GCN 15316) may suggest the former interpretation is more likely. I thank D. Malesani for providing the NOT image to verity the location of the optical transient.