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) 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,
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
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
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, GCN7140,
Cenko et al. 2008.; see Berger 2008 for a review), and our limiting
magnitudes do not rule out relatively underluminous galaxies at this
- 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) 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.
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