K. Wiersema (Leicester), A. Levan (Warwick), S. B. Cenko (NASA/GSFC)
and N. Tanvir (Leicester) report on behalf of a larger collaboration:
We acquired spectroscopy of two galaxies close to the afterglow position
of short GRB 130822A (Kocevski et al. GCN 15112), these galaxies have low chance
coincidence probability values (note that there are no sources found within the
XRT error circle, Cenko et al. GCN 15121).
These two galaxies, hereafter named G1 and G2, are located at approximate
positions (SDSS DR10):
A finder chart showing the locations of these galaxies (Gemini i' band;
Cenko et al. GCN 15121) can be found here:
We acquired spectroscopy with ACAM on the William Herschel Telescope
(directly following the photometry observations reported in Wiersema
et al. GCN 15119) and GMOS on Gemini-North (starting at 13:35 UT on
2 September 2013). In both cases the slit was aligned such that both
galaxies were covered simultaneously by the slit. The ACAM spectrum
covers the range 3500-9400, the Gemini spectrum the range 3850-6730 A.
Galaxy G1 shows strong absorption complexes (G band, Mg b triplet, Fe 4668,
Ca II) as well as a single, weak, emission line (H beta), at a common redshift z=0.154.
Galaxy G2 shows stronger emission lines of H alpha and H beta, as well as absorption
complexes (G band, Na I, Mg b triplet) at a common redshift z=0.045.
At these redshifts, the offset from the afterglow to galaxy G1 is 58 kiloparsec,
and to G2 72 kiloparsec.
If the brighter, but somewhat further offset, galaxy G2 is the host galaxy, it would
place GRB 130822A within the Advanced LIGO / Virgo horizon.