Antonio de Ugarte Postigo (DARK/NBI), Paolo Goldoni (APC/Univ. Paris 7
and SAp/CEA), Bo Milvang-Jensen, Daniele Malesani, Martin Sparre, Johan
P.U. Fynbo, Giorgos Leloudas (DARK/NBI), Stefano Covino (INAF/OABr),
Hector Flores (GEPI/Obs. de Paris), Valerio D'Elia (ASI/ASDC and
INAF/Roma), Andrew Levan (Univ. Warwick), report on behalf of the
X-shooter GTO GRB collaboration:
We observed the afterglow of GRB 101219B (Gelbord et al., GCN 11473)
with the X-shooter spectrograph mounted on the ESO-VLT UT2. Observations
were taken on two epochs, 2010 Dec 20.17 and 2011 Jan 5.09 UT (11.6 hr
and 16.4 days after the burst, respectively). Exposure times were 80 and
120 min, respectively.
In the first-epoch spectrum, the afterglow is detected over the full
spectral range from about 3200 to 22,000 AA. A careful analysis provides
a tentative redshift z=0.5519 based on detection of weak Mg II and Mg I.
No other clear features can be found at the same redshift either in
absorption or emission.
The second-epoch spectrum was taken 16.4 days after the GRB, which
corresponds to 10.6 days in the rest frame at z=0.55. The continuum is
again detected, over the wavelength range 3500 to 22,000 AA, and is
substantially redder. No narrow emission lines can be distinguished, but
broad SN-like undulations are apparent, particularly around 8000 AA. We
carried out a comparison of our spectrum with those of SN 1998bw (Patat
et al. 2001, ApJ, 555, 900) transformed at various redshifts. We find a
convincing match at z~0.55 after comparing with a spectrum taken on 1998
May 4 (i.e., 12 days after the GRB). The observed flux is consistent
assuming a roughly equal contribution from a SN as bright as SN 1998bw
and an underlying host galaxy. The match of the spectral shape,
brightness, and spectral epoch together strenghten both the redshift
determination and the existence of a SN associated with GRB 101219B.
Our findings are in good agreement with the GROND observations reported
by Olivares et al. (GCN 11578).
Further observations are planned. We acknowledge the assistence of the
ESO observing staff at Paranal, in particular Dimitri Gadotti, Leonel
Rivas, Thomas Rivinius, Maja Vuckovic, and Thomas Szeifert.