GRB 091109A
GCN Circular 10158
Subject
GRB 091109A: GROND and VLT observations
Date
2009-11-10T14:59:13Z (16 years ago)
From
Thomas Kruehler at MPE/MPI <kruehler@mpe.mpg.de>
P. Afonso, T. Kruehler, A. Rau, and J. Greiner (all MPE Garching) report
on behalf of the GROND team:
GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile), observed the field of GRB
091109A (Oates et al., GCN #10138) simultaneously in the g'r'i'z'JHK bands.
Observations were done under clear sky conditions, starting on Nov 10 at
00:07 UTC, 19.1 h after the burst and lasted for 1.8 hours.
In stacked images corresponding to a total integration time of 60 min in
JHK and 75 min in g'r'i'z', we detect the optical afterglow (Oates et
al., GCN #10138, Guidorzi et. al, GCN #10142) in the g'r'i'z' bands and
obtain the following preliminary magnitudes and upper limits (all in the
AB system):
g' = 24.5 +/- 0.1
r' = 23.6 +/- 0.1
i' = 23.4 +/- 0.1
z' = 23.3 +/- 0.2
J > 22.2
H > 21.8
K > 21.2
which were derived using the GROND zero points and 2MASS catalog field
stars as reference. No correction has been made for the expected
extinction corresponding to a reddening of E(B-V) = 0.03 mag in the
direction of the burst (Schlegel et al. 1998).
Associating the blue g'-r' color with Lyman-alpha absorption in the host
of the GRB, we obtain a photometric redshift of z = 3.5 +/- 0.4 assuming
no intrinsic dust.
In addition, we triggered VLT/FORS2 spectroscopy, that started Nov 10 at
02:23 UTC, 21.5 h post-burst. Two integrations of 1800 s each at a mean
airmass of 2.1 were obtained using the 600RI grism that covers the
wavelength range from 5300 to 8700 A. In the low S/N spectrum we find a
tentative ~60A EW broad absorption feature around a wavelength of 5700A.
If associated with Lyman-alpha absorption, the corresponding redshift of
~3.5 would be in agreement with the GROND photo-z.
We note, however, that all observations are also consistent with a host
galaxy at a redshift of z ~ 0.44. Given the absence of information about
a possible host galaxy contribution we cannot distinguish these two
possibilities at this point.
We thank A. Smette and P. Lynam at Paranal for excellent support.
GCN Circular 10246
Subject
GRB 091109A: GROND confirmation of redshift z = 3.5
Date
2009-12-04T17:32:12Z (16 years ago)
From
Paulo M. J. Afonso at MPE <pafonso@mpe.mpg.de>
GRB 091109A: GROND confirmation of redshift z = 3.5
P. Afonso, T. Kruehler, A. Rau, and J. Greiner (all MPE Garching) report
on behalf of the GROND team:
GROND (Greiner et al. 2008, PASP 120, 405), mounted at the 2.2m ESO/MPI
telescope at La Silla Observatory (Chile), reobserved the field of GRB
091109A (Oates et al., GCN #10138) simultaneously in the g'r'i'z'JHK
bands.
Observations started 2009-11-20 00:42, 10.8 days after the trigger and
lasted for an effective exposure of 75 min in g'r'i'z' and 60 min in JHK.
At the position of the optical afterglow (Guidorzi et al. GCN#10142,
Oates GCN#10143, Afonso et al., GCN #10158) we do not detect any source
down to a limiting magnitude of r' > 25.1.
Comparing with the brightness of r' = 23.6 +- 0.1 mag (AB) reported for
the 1st epoch (Afonso et al., GCN #10158), the last imaging clearly
shows a fading of more than 1.5 magnitudes. The 1st epoch was hence
dominated by afterglow light, favoring a redshift of around z = 3.5 +/-
0.4.
A thorough analysis of the VLT data (GCN # 10158) was limited by the
very low S/N of the optical spectrum and does not yield more precise
redshift constraints.
GCN Circular 10350
Subject
GRB091109A: VLT/FORS2 spectroscopic redshift
Date
2010-01-19T22:11:33Z (15 years ago)
From
Arne Rau at MPE <arau@mpe.mpg.de>
Arne Rau (MPE Garching), Johan Fynbo (DARK), and Jochen Greiner (MPE
Garching):
"We report on additional analysis of our VLT/FORS2 spectrum of the
optical afterglow of GRB 091109A (Oates et al., GCN #10138, Guidorzi et
al. GCN#10142, Oates GCN#10143, Afonso et al., GCN #10158 ).
A careful search revealed absorption lines at 6227, 6308, and 6321
Angstroem which we identify with SiII and the CIV doublet a common
redshift of z=3.076 +/- 0.002. Lyman-alpha absorption at this redshift
is located shortwards of the blue wavelength cut-off and the earlier
reported tentative detection at 5700A (Afonso et al., GCN #10158) is not
confirmed."