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GCN Circular 9648

Subject
GRB 090709A: evidence for a very red afterglow at low z
Date
2009-07-10T12:02:42Z (15 years ago)
From
Cristiano Guidorzi at Ferrara U,Italy <guidorzi@fe.infn.it>
C. Guidorzi (U. Ferrara), A. Melandri, C.G. Mundell, D. Bersier,
Z. Cano, S. Kobayashi, I.A. Steele, R.J. Smith (Liverpool JMU),
and A. Gomboc (U. Ljubljana) report, on behalf of a large collaboration:

We re-analysed the data promptly acquired with the 2-m Faulkes
Telescope North in the R and i' filters (Cano et al. GCN 9630)
of the Swift GRB 090709A (Morris et al. GCN 9625).
 From the stacked images, we marginally detect a source in the
r' frame at the position of the NIR afterglow found by PAIRITEL
(Morgan et al. GCN 9635), identified as source 'B' by Subaru
(Aoki et al. GCN 9634) and possibly detected in the r' and z' bands
(Cenko et al. GCN 9646). The same object is not detected in the
i' band frame (although with a shorter total exposure).

Start time        End time        Exposure      Filter    mag
(min)             (min)           (s)
-------------------------------------------------------------------
2.88             55.02           720            R        22.7 +/- 0.5
7.55             47.05           520            i'       > 20.6
-------------------------------------------------------------------

Calibration was performed against the I and R2 values of the USNOB1
nearby star at 19:19:44.575, +60:43:27.44 assuming I=14.68 and R2=15.70.

We built a tentative SED at the PAIRITEL observation mid time of 438 s
(Morgan et al. GCN 9635), combining the P60 values in the z' filter
(Cenko et al. GCN 9646) and the assuming for the r' filter an
intermediate value comprising both the P60 and our own value.
This value as well as our i' band upper limits were backextrapolated
to the chosen time assuming a range of power-law decay indices, from
0.3 to 1. The SED was finally corrected for the Galactic reddening
of E(B-V)=0.09. We found that the SED can be fitted with a simple
power-law with spectral index beta=3.4 +/- 0.8 (90%cl) and no
evidence for a Lyman drop.
We therefore support the nature of a very red afterglow at low z
(Butler et al. GCN 9639) and possibly associated with a non-GRB
progenitor as suggested by the quasi-periodic oscillations found
in the gamma-ray profile (Markwardt et al. GCN 9645; Golenetskii
et al. GCN 9647).

Our stacked R band frame is available at the following link (red
and green circles show the Swift-XRT and PAIRITEL circles, respectively):

http://www.fe.infn.it/u/guidorzi/GRB090709A/090709A_R.jpg
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