TITLE:   GCN CIRCULAR
NUMBER:  14686
SUBJECT: GRB 130427A / SN 2013cq: Hubble Space Telescope Observations
DATE:    13/05/20 23:08:33 GMT
FROM:    Andrew S. Fruchter at STScI  <fruchter@stsci.edu>

A.J. Levan (U. Warwick), A.S. Fruchter, J. Graham (STScI), N.R. Tanvir (U. Leicester), Jens Hjorth, Johan Fynbo (Dark Cosmology Centre, Copenhagen), D. Perley (Caltech), S.B. Cenko (U.C. Berkeley), E. Pian (Trieste), Z. Cano (U. Iceland) A. Pe'er (Cork), R. Hounsell (STScI), K. Mishra (ARIES, India), C. Kouveliotou (MSFC) report:

We observed the optical/NIR counterpart of GRB 130427A (Maselli et al. GCN 14448) with the Hubble Space Telescope beginning at 02:23 UT on 20 May 2013. The afterglow is well detected in our multi-band observations in the UV (F336W), optical (F606W) and NIR (F160W) and is offset approximately 0.8" from the optical centroid of its host. The host itself also contains additional star forming complexes including a bright UV source approximately 0.25" from the GRB position.

In the three bands we measure preliminary magnitudes of the afterglow + supernova of

F336W=23.10 +/- 0.02
F606W=21.85 +/- 0.02
F160W=21.34 +/- 0.03

These magnitudes show significant curvature in the optical likely due to the underlying supernova SN 2013cq (de Ugarte Postigo CBET 3529; Xu et al. GCN 14597).    If the optical light were entirely dominated by supernova emission the absolute magnitude at z=0.34 would be M_B~ -19.1 at 17 rest-frame days post burst. However, SNe are weaker UV and IR emitters and so under the naive assumption that the UV and IR bands are dominated by power-law afterglow emission with minimal supernova contribution the inferred magnitude of the supernova in the V-band (rest frame B-band) is V~23. This corresponds to an absolute magnitude of M_B ~ -17.9, approximately a magnitude fainter than the B-band peak of SN 1998bw (which occurred at a comparable epoch of 15 days post burst).     However, the SN could contribute as much as one half of the flux we are seeing in the NIR and UV and there may be substantial host emission underneath the object in the optical and UV.   Thus the SN magnitude should be considered very approximate.  
Images of the field are posted at

http://www.star.le.ac.uk/~anl/GRB130427A

We thank the staff of STScI for their work in rapidly scheduling these observations.