GCN Circular 13267
Subject
GRB 120422A: Additional Gemini observations and Keck/LRIS spectroscopy
Date
2012-04-27T16:45:33Z (12 years ago)
From
Daniel Perley at Caltech <dperley@astro.caltech.edu>
D. A. Perley (Caltech), A. Cucchiara (UCO/Lick), S. B. Cenko (UC
Berkeley), A. J. Levan (Warwick), S. R. Kulkarni, S. Ben-Ami, and Y. Cao
(Caltech) report:
We conducted a second epoch of imaging at the position of GRB 120422A
(Troja et al., GCN 13243) with GMOS-N on Gemini-North starting at 05:42
UT on 2012-04-26 in each of the griz filters. Seeing conditions were
excellent (0.7"), cleanly resolving the transient previously identified
by Cucchiara et al. (GCN 12345) from its putative host galaxy. A curved
bridge of emission connects the transient with the host, suggesting it
occurred either in a spiral arm (however no counter-arm is visible on
the far side of the galaxy) or within an interacting companion. An
image is posted to:
http://www.astro.caltech.edu/~dperley/gcn/120422a/120422a_gmos.png
The transient has clearly faded since the last observations, and is now
at r = 22.05 +/- 0.09 mag, i = 22.31 +/- 0.07 mag (using a 0.7" radius
aperture and calibrating relative to SDSS standards, with the
uncertainties due almost entirely to the systematics of the color
comparison). This would indicate a relatively blue color. However, the
(nearly contemporaneous) g-band flux lies approximately 1 magnitude
below a simple extrapolation of the r-i color, indicating that the SED
peaks in or near the r-band range.
As previously noted by Schulze et al. (GCN 13257) on the basis of the
blue UVOT color, this type of SED is very unusual for traditional GRB
afterglows (which normally are power-laws in the optical band) but
similar properties have been seen at early times in GRBs 060218 (e.g.
Campana et al., Nature 442:1008), as well as GRBs 100316D (Starling et
al. 2011, MNRAS 411:2792), and 101225 (Th�ne et al. 2011, Nature
480:72). We suggest that the apparent color evolution of the peak
wavelength of the transient from the UV to the optical and NIR is likely
to continue in subsequent days, and encourage multi-band photometric
follow-up as well as spectroscopy.
Additionally, on the night of 2012-04-27 UT we acquired 30 minutes of
spectroscopy using LRIS on the Keck 10m telescope. A preliminary flux
calibration of the spectrum shows a similar signature as evident in the
photometry, with a pronounced, smooth peak around 6500 Angstroms. No
obvious absorption or emission features are evident except for the
narrow emission lines from the underlying host, previously mentioned by
Tanvir et al. (GCN 13251) and Schulze et al. (GCN 13257). In
particular, we do not yet recognize any broad supernova signatures.
Further observations are planned.
