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GCN Circular 1785

Subject
VLT spectroscopy of the probable host of GRB 021211
Date
2003-01-03T21:07:37Z (21 years ago)
From
Paul Vreeswijk at ESO <pvreeswi@eso.org>
Paul Vreeswijk (ESO), Andrew Fruchter (STScI), Jens Hjorth (University
of Copenhagen) and Chryssa Kouveliotou (MSFC) report for the GRACE
collaboration:

On the night of 29/30 December 2002, the field of GRB 021211 was
re-observed in spectroscopic mode with FORS2 at unit-4 of ESO's VLT at
Paranal, Chile. The total exposure time was 30 min in the 300V grism,
covering approximately 4000-9300A with a resolution of about 12A. The
slit, with a width of 1", was positioned to cover the position of the
probable host galaxy, as well as the galaxy 1.5" to the North-East
(see Fruchter et al., GCN 1781). The seeing during the observations
was ~0.7".

Preliminary reduction of these spectra confirm that the redshift of
the nearby galaxy is z=0.800+-0.001 (see Vreeswijk et al., GCN
1767). Due to a reference star also being in the slit, there is no
ambiguity in the identification of this galaxy as the source of these
emission lines (again [OII] and [OIII]).

At the position of the probable host, we find one clear emission line
at 7476A with an observed equivalent width of 66 +- 8 A (Poisson error
only). The continuum level just redward of the line is similar to that
on the blue side, suggesting that the line is not Lyman alpha. The
most likely identification is [OII] 3727A at a redshift of z=1.006. At
this redshift, other prominent emission lines such as Hbeta and [OIII]
5007A fall outside our spectral coverage. If the line were Halpha, we
would have expected to see [OII] and [OIII] in the blue part of the
spectra, which we do not detect. An alternative identification of the
emission line is [OIII] 5007A, which would put the host at z=0.493. 
However, in this case we would have expected to also detect [OII]
(although it would be very close to a bright sky line), assuming that
its flux would be more than about half the flux of [OIII] 5007A.

The [OII] at z=1.006 identification is strengthened by flux
calibration of the spectra, which shows a drop of about 1 magnitude
between the 8000-9000A continuum and that blueward of the emission
line. This is consistent with the red HST V-I colour reported by
Fruchter et al., and with the possibility that this colour is caused
by the 4000A break.

We are grateful for the assistence of the staff at Paranal, in
particular Thomas Szeifert and Elena Mason.
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