TITLE: GCN CIRCULAR NUMBER: 7889 SUBJECT: GRB 070809: Putative host galaxy and redshift DATE: 08/06/21 03:06:46 GMT FROM: Daniel Perley at U.C. Berkeley 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, GCN 6774). 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, GCN7140, 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) 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