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GCN Circular 14983

Subject
GRB 130702A: NOT spectroscopy and redshift of the nearby bright galaxy
Date
2013-07-04T13:10:00Z (11 years ago)
From
Giorgos Leloudas at Dark Cosmology Centre <giorgos@dark-cosmology.dk>
G. Leloudas (OKC, Stockholm and DARK/NBI), J. P. U.  Fynbo (DARK/NBI), S. Schulze (PUC, MCSS), D. Xu (DARK/NBI), D. Malesani (DARK/NBI), S. Geier (NOT, DARK/NBI), Z. Cano(U. Iceland), P. Jakobsson (U. Iceland) report on behalf of a larger collaboration:

We observed the field of GRB 130702A (Singer et al., GCN 14967; Cheung et al., GCN 14971; Collazzi et al., GCN 14972) with the Nordic Opitcal Telescope (NOT) equipped with ALFOSC. In addition to our photometric observations (reported in Schulze et al., GCN 14978), we obtained an optical spectrum, with a total exposure of 2 x 1800 s, using Grism #4 and covering the wavelength range 3750 - 9000 AA at a resolution of 17 AA.

The slit was oriented to cover both the transient source (also dubbed iPTF13bxl) that has been suggested to be the afterglow of GRB 130702A, and the bright SDSS galaxy located 7.6" South of the source (SDSS J142914.57+154619.3; Singer et al., GCN 14967). 

The galaxy is found to be at z = 0.145, based on Ca II H & K absorption and emission by [OII] and Halpha. The emission features are weak and we do not detect Hbeta or [OIII]. Overall, and combined with the SDSS colors, this galaxy shows very weak star formation activity. 

This redshift is consistent with the redshift of the transient source reported by Mulchaey et al. (ATel 5191) and the description given there. The trace of our NOT spectrum at the location of the transient is mostly featureless and we cannot detect the emission lines reported by Mulchaey et al., probably due to a lower S/N.

The common redshift suggests that the transient source iPTF13bxl is associated with the galaxy SDSS J142914.57+154619.3. If this transient is indeed the afterglow of GRB 130702A at z = 0.145, it implies an E_iso = (3.0 +- 1.0) * 10^50 erg in the 10-1000 keV (Collazzi et al., GCN 14972).

We note that this is an unusual environment for a long GRB: located at a large offset (19.1 kpc in projection) from a relatively passive galaxy. However, we cannot exclude that more sources in the field, including the fainter (r ~ 23 mag) object at 0.6" from the transient (SDSS J142914.75+154626.0; Singer et al., GCN 14967) might be at the same redshift. A possible confirmation of the GRB nature of this transient will be the emergence of an associated supernova. At z = 0.145, a supernova similar to SN 1998bw will peak at R ~ 19.8 mag about 20 days after the GRB, and will be easily detectable.
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