GCN Circular 4863

E. W. Guenther, S. Klose, Thueringer Landessternwarte Tautenburg,
P. Vreeswijk, University of Chile/ESO,
E. Pian, INAF-OA Trieste, and
J. Greiner, MPE Garching, on behalf of the GRACE collaboration

report:


ESO's VLT-Kueyen (UT 2) observed SN 2006aj (GRB 060218) around the
time of maximum light on March 3/4, 2006. Observations were performed
using the Ultraviolet and Visual Echelle Spectrograph (UVES) at a
spectral resolution of 46 000. The signal-to-noise ratio of the
spectrum (2100 sec exposure time) is about 30 per resolution element,
which is sufficient to measure the equivalent width (EW) of the Na I D
lines along the line of sight.

For the Na I D2 component (lambda 5889.95) produced in our Galaxy we
find EW = (0.321 +/- 0.008) Angstrom. Using the empirical relation
between the equivalent width of the 5889.95 line and the interstellar
reddening (Munari & Zwitter A&A 318, 269, 1997), this corresponds to
a Galactic reddening of E(B-V) = (0.127 +/- 0.005) mag. Assuming a
ratio of total-to-selective extinction of R_V = 3.1, we obtain a
Galactic visual extinction along the line of sight of A_V = (0.39 +/-
0.02) mag. This is slightly less than what follows from the COBE
maps (Schlegel, Finkbeiner, & Davis 1998).

In the GRB host galaxy we identify two redshift systems at a
heliocentric velocity of 10008.1 km/s and 10032.3 km/s (Na I D2) with
the following equivalent widths in the observer frame: system I:
EW(D2) = (0.084 +/- 0.008) Angstrom, system II: EW(D2) = (0.072 +/-
0.008) Angstrom. Assuming that the aforementioned empirical relation
is also representative for the interstellar medium in the GRB host
galaxy, and correcting EW for a factor of 1/(1+z) for the host frame, we
arrive at a combined reddening of E(B-V) = (0.042 +/- 0.003) mag. If
again R_V = 3.1, we obtain a host extinction along the line of sight
of A_V = (0.13 +/- 0.01) mag.

We finally note that the tiny error bars should not be overinterpreted.
They just include the measurement errors but not the systematic error
of the method itself, which we cannot quantify.

We thank the ESO staff, in particular Dominique Naef, for performing
the observations and Alain Smette, ESO, for valuable comments.


This message may be quoted.