GCN Circular 5079

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
Not found
 
 
), 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
Not found
 
 
) 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
Not found
 
 
).

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
Not found
 
 
).  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
Not found
 
 
) 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
Not found
 
 
), 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
Not found
 
 
; 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
Not found
 
 
;
Butler et al., GCNs 4165
Not found
 
 
 and 4170
Not found
 
 
).