GCN Circular 2685
Subject
GRB 040827: optical and NIR observations at the VLT
Event
Date
2004-09-01T10:23:31Z (21 years ago)
From
Daniele Malesani at SISSA-ISAS,Trieste,Italy <malesani@sissa.it>
Daniele Malesani, Paolo D'Avanzo, Andrea Melandri, Dino Fugazza, L.
Angelo Antonelli, Stefano Covino, Gianpiero Tagliaferri, Sergio Campana,
Guido Chincarini, on behalf of a larger collaboration, report:
We performed both NIR and optical observations of the field of GRB
040827 (Mereghetti et al., GCNs 2670, 2672) with the ESO-VLT. NIR
observations were performed on Aug 27.96 and 28.96 (11 and 35 hrs after
the GRB respectively).
We confirm the variability of source XMM2 (Gladders & Berger, GCN 2681;
Kaplan & Berger, GCN 2683; Tanvir et al., GCN 2684), which fades by 0.7
+- 0.2 mag in the K filter. During both observations the seeing was
similar (0.9" and 1.0"), therefore even if the source is extended (thus
making photometry more tricky) this variability is a real effect. JHK
colors of the object (on Aug 27) are consistent with a power law
spectral shape, with a spectral index beta ~ 1.4. Note however that a
significant contribution from the host may be present in our measurements.
Given the variability of XMM2 and its spatial coincidence with the X-ray
source detected by XMM (Rodriguez-Pascual & Juarez, GCN 2678; see also
http://xmm.vilspa.esa.es/external/xmm_news/items/grb040827/index.shtml
for a refined position), we also conclude that XMM2 is likely the
afterglow of GRB 040827 (Kaplan & Berger, GCN 2683; Tanvir et al., GCN
2684).
I-band observations were performed on Aug 28.99 and Aug 30.01. No
significant variation is seen between the two epochs (at the ~0.2 mag
level). The source has I ~ 22.6 in both nights. This suggests that the
emission is dominated by the host galaxy at these epochs, therefore
implying a very weak afterglow. Otherwise, the decay may have
experienced a plateaux phase.
A spectrum taken with the VLT on Aug 30.00) shows a weak featureless
continuum in the range 4600-8600 A. The lack of detection of any
emission lines does not allow us to determine a redshift, but it can put
some constraints on it. For example, the non-detection of [O III] 5006 A
(usually prominent in GRB host galaxies spectra) would imply z > 0.7.
The absence of Ly alpha emission also limits z < 2.7.
We acknowledge the big effort of the ESO observing staff at Paranal, in
particular Chris Lidman, Elena Mason, Claudio Melo, Nancy Ageorges,
Stephane Brillant and Andres Pino.
This message can be cited.