TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5079 SUBJECT: GRB 060306: Further Analysis of SOAR and APO NIR Observations DATE: 06/05/06 03:53:15 GMT FROM: Don Lamb at U.Chicago GRB 060306: Further Analysis of SOAR and APO NIR Observations D. Q. Lamb (Chicago), M. Nysewander (UNC), F. Hearty (Colorado), H.-W. Chen (Chicago), and D. E. Reichart (UNC) report on behalf of the FUN GRB Collaboration: We have further analyzed the Ks-band images taken with the ARC 3.5-meter telescope using NIC-FPS, beginning 1.6 hours after the burst, and the Ks-band images taken with the SOAR 4.1-meter telescope using OSIRIS, beginning 24 hours after the burst. From the ARC image, we measure a limiting magnitude of Ks = 19.5 (3 sigma) and a magnitude of Ks = 17.78 ± 0.13 mag for the source that is in the XRT error circle and in the DSS (Price et al., GCN 4854), using six 2MASS objects (five of "A" quality and one of "C" quality; Skrutskie et al. 2006, AJ, 131, 1163) in the NIC-FPS image for calibration and an aperture radius of 1.6 arcsec, as dictated by a curve of growth analysis of the source. The magnitude of Ks = 18.3 ± 0.1 originally reported by Chen et al. (GCN 4861) corresponded to an aperture radius of 1.0 arcsec, and is therefore fainter. From the SOAR image, we measure a limiting magnitude of Ks = 18.4 (3 sigma) and a magnitude of Ks = 17.53 ± 0.20 for the source, using the four 2MASS objects (one of "A" quality and three of "C" quality) in the OSIRIS image for calibration and an aperture radius of 1.6 arcsec, as dictated by a curve of growth analysis of the source. This result is consistent with that reported by Nysewander et al. (GCN 4857). The two results are consistent with a constant brightness source, and PSF-matched image subtraction provides no indication that the source brightened between the two epochs. The source appears to be marginally extended in the ARC image, and may be the host galaxy (Price et al., GCN 4854). Spectroscopy of the source is therefore encouraged. However, even if the source is the host galaxy, or a chance-coincidence foreground star (~1% probability), PSF-matched image subtraction of the Ks-band images does not detect the afterglow at 1.6 hours after the burst to the limiting magnitude [Ks = 18.4 (3 sigma)] of the SOAR image. If one assumes minimal extinction in the Ks-band and extrapolates the unabsorbed X-ray flux (Page et al., GCN 4850) to 1.6 hours after the burst, the resulting Ks-band to X-ray spectral index is shallower than -0.48 (3 sigma), using the limiting magnitude of the SOAR image (which is appropriate if the location of the burst is coincident with the faint source in the XRT error circle), and shallower than -0.36 (3 sigma), using the limiting magnitude of the ARC image (which is appropriate if the location of the burst is not coincident with the faint source in the XRT error circle). This suggests one or more of the following possibilities: 1. The redshift of the burst is greater than z ~ 17, which is highly unlikely. 2. The cooling break is blueward of the Ks-band at 1.6 hours after the burst. Given that the X-ray temporal index is ~-0.6 (Page et al., GCN 4850), the Ks-band to X-ray spectral index would be ~-0.7 (e.g., Sari, Piran, and Narayan 1998, ApJ, 497, L17) if the cooling break were instead redward of the Ks band. 3. The afterglow is highly extinguished. Given that the source-frame N_H ~ 4 x 1021 cm^-2 (1+z)2.6 (Page et al., GCN 4850; Morrison and McCammon 1983, ApJ, 270, 119), a redshift of only z ~ 1 is needed in order to produce 3 - 4 mag of extinction in the observer-frame Ks-band, provided that dust destruction by the burst and/or its afterglow is minimal. Furthermore, a redshift of z ~ 1 yields source-frame values of N_H that are consistent with giant molecular cloud environments. Finally, we point out that the properties of the afterglow of this burst are reminiscent of those of GRB 051022 (Doty et al., GCN 4145; Butler et al., GCNs 4165 and 4170).