GCN Circular 7889

D. A. Perley, J. S. Bloom, M. Modjaz, A. A. Miller, J. Shiode, J. 
Brewer, D. Starr, and R. Kennedy (UC Berkeley) report:

GRB 070809 was a short-hard burst (Barthelmy et al., GCN 6788
Not found
 
 
) detected 
by Swift.  In our observations on 2007-08-10 and 2007-08-11 we detected 
a very faint optical afterglow candidate and presented evidence (at 
about the 3-sigma level) for fading between the two nights.  (GCNs 6739
Not found
 
 
, 
GCN 6774
Not found
 
 
).

On the night of 2008-02-10, we re-imaged the field using Keck I (+LRIS), 
again in R and g filters simultaneously, for a combined 2550s (R) and 
2820s (g).  We unambiguously confirm the fading behavior of the optical 
transient, with no detection in either filter to R > 25.0, g > 26.3.  We 
rule out the presence of a host galaxy coincident with the transient 
location to the same level.  (An color image of the field is posted to 
http://lyra.berkeley.edu/~dperley/070809/070809host.png.  A comparison 
between the early- and late-time imaging is available at 
http://lyra.berkeley.edu/~dperley/070809/070809compare.png)

Several other short bursts with no strictly coincident host galaxy have 
been found to be close in physical projection to relatively low-redshift 
galaxies (e.g. Bloom et al. 2006, 2007, Stratta et al. 2007, Troja et 
al. 2008).  This burst is not an exception, with an edge-on spiral 
galaxy centered at an offset of 5.6" to the northwest.  Photometry of 
this source gives magnitudes of R = 21.7 +/- 0.3, g = 22.7 +/- 0.2 (the 
relatively large uncertainties are due to the extended nature of the 
source and the variable background due to the presence of a nearby 
bright star).

On the night of 2008-06-07 we obtained 2x900s of spectroscopy of this 
source using Keck I (+LRIS), with a PA aligned with the orientation of 
the galaxy.  The galaxy is well contained within the 1 arcsec slit.  Two 
relatively bright emission lines are detected - one at 4542.0 A and one 
at 6100.1 A.  Associating these lines with [O II] and [O III], 
respectively, the redshift of this galaxy is z=0.2187.  No other 
emission lines are significantly detected.

The line flux of the [O II] doublet is ~2 x 10^-16 erg/s/cm^2 
(correcting for Galactic extinction of E(B-V) = 0.09), corresponding to 
an uncorrected star formation rate (Kewley et al. 2002) of ~0.15 
M_sun/year.  As the galaxy is edge-on this is likely to be well below 
the actual value.  The velocity dispersion of the galaxy along the slit 
axis is 110 +/- 20 km/s, which over the observed radius of 1.8" (=6.3 
kpc) gives a mass of 1.8 x 10^10 M_sun.  These values suggest a 
relatively small spiral galaxy.

While this galaxy is not particularly massive or luminous, the close 
proximity (20 kpc in projection at z=0.2187) and lack of a coincident 
host is strongly suggestive of association given previous short bursts. 
  However, we issue several caveats:

- Some short bursts have been shown to have secure hosts at z~1 (e.g. 
060801, 070429B and 070714B:  Berger et al. 2007, GCN 6836
Not found
 
 
, GCN7140
Not found
 
 
, 
Cenko et al. 2008.; see Berger 2008 for a review), and our limiting 
magnitudes do not rule out relatively underluminous galaxies at this 
redshift.

- One other probable galaxy is present somewhat closer to the afterglow, 
a very faint (R = 24.6, g = 25.7) source 2.3" away from the OT position. 
  The probability of chance association of the afterglow position with 
this source (if it is a galaxy) based on galaxy count/offset statistics 
is higher than the probability of association with the spiral (~10% vs. 
~5%), though they are comparable.  The source is at unknown redshift.

- It is possible (but unlikely, GCN 6788
Not found
 
 
) that this event is not a short 
burst, in which case a even higher-redshift origin would not be surprising.

At a redshift of z=0.2187, the isotropic energy release for this burst 
would be E_iso = 1.1 * 10^49 erg in the observed 15-150 keV band.


References:
Bloom et al. 2006 - ApJ 638,354
Bloom et al. 2007 - ApJ 654,878
Stratta et al. 2007 - A&A 474,827
Troja et al. 2008 - MNRAS 385L,10
Cenko et al. 2008 - arXiv 0802.0874
Berger et al. 2007 - ApJ 664,1000
Berger 2008 - arXiv 0805.0306