[vsnet-grb-info 10437] GRB 110328A / Swift J164449.3+573451: Keck/DEIMOS Optical Spectroscopy

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  11874
SUBJECT: GRB 110328A / Swift J164449.3+573451: Keck/DEIMOS Optical Spectroscopy
DATE:    11/04/04 04:27:16 GMT
FROM:    S. Bradley Cenko at Caltech  <cenko at srl.caltech.edu>

S. B. Cenko, D. A. Perley, J. S. Bloom (UC Berkeley), K. Hurley (SSL), J.
X. Prochaska, J. Brodie, N. Singh, J. Arnold, A. Romanowsky, J. C. Forbes
(UCO/Lick), and D. Forbes (Swinburne) report on behalf of a larger
collaboration:

We have obtained medium-resolution optical spectroscopy of the optical
counterpart (Cenko et al., GCN 11827) of the high-energy transient GRB
110328A / Swift J164449.3+573451 (Cummings et al., GCN 11823) with the
DEIMOS spectrograph mounted on the Keck II telescope.  Observations began
at 14:55 UT on 2011 Mar 31 and cover the wavelength range from 4500-9500
A.

We identify strong nebular emission features associated with [O II], [O
III], H-beta, H-alpha, [N II], and [S II] at a redshift consistent with
that reported by Levan et al. (GCN 11833) and Thoene et al. (GCN 11834).
Using a preliminary flux calibration, we find that the Balmer decrement
(H-alpha / H-beta) is only marginally larger than the value expected for
Case B recombination (consistent at the 2 sigma level).  This would
suggest that the observed red colors of the optical / NIR counterpart
(e.g., Morgan et al., GCN 11845; Levan et al., GCN 11846) are either due
to 1) an intrinsically red transient source, or 2) dust localized to the
source of the transient emission (which does not affect the bulk of the
ongoing star formation in the galaxy).

Constructing a diagnostic diagram (e.g., Baldwin, Phillips, and Terlevich
1981, PASP, 93, 5) based on the observed ratio of the narrow emission
lines, we find the source falls within the locus of star-forming
galaxies, and thus does not appear to exhibit any evidence for past
nuclear activity.  A plot of the diagnostic diagram of the host, showing
the empirical dividing line between star-forming galaxies and AGNs from
Kauffmann et al (2003, MNRAS, 341, 33- solid line), the theoretical
dividing lines from Kewley and Dopita (2002, ApJS, 142, 35 - dashed
lines), and the empirical dividing lines from Ho, Filippenko, and Sargent
(1997, ApJS, 112, 315), along with analogous measurements from the MPA-JHU
value-added SDSS database, can be found at:

   http://astro.berkeley.edu/~cenko/public/grb/GRB110328A/bpt.png