[AAVSO-HEN] GRB 090715B: further NOT optical observations

[grb at aavso.org]

The following is a GCN Circular circulated via the AAVSO network.
For an archive and more information on circulars visit:
http://lheawww.gsfc.nasa.gov/docs/gamcosray/legr/bacodine/gcn3_archive.html

TITLE:   GCN CIRCULAR
NUMBER:  9687
SUBJECT: GRB 090715B: further NOT optical observations
DATE:    09/07/18 15:31:46 GMT
FROM:    Daniele Malesani at Dark Cosmology Centre, Niels Bohr Inst  <malesani at astro.ku.dk>

D. Malesani (DARK/NBI), G. C. Cox (NOT), C. Mackay, D. King, T. Staley 
(IoA Cambridge), and P. Jakobsson (Univ. Iceland) report on behalf of a 
larger collaboration:

We observed again the field of GRB 090715B (Vetere et al., GCN 9668) 
using the NOT telescope equipped with the StanCam instrument. The 
optical afterglow (Vetere et al., GCN 9668; Smith et al., GCN 9670) is 
well detected in the V and R filters, and is still bright. Using the 
same reference star as in GCN 9671, we measure R ~ 21.3 on mean time 
July 17.94 UT (2.06 days after the GRB).

Assuming an unbroken power-law fading (Fnu propto t^-alpha), the 
corresponding decay index is alpha ~ 0.44, which is unusually flat at 
such late epoch (although not unprecedented). Our measurement is also 
consistent within the uncertainties with the broken power-law fit 
reported by Guidorzi et al. (GCN 9677).

DM thanks Carole G. Mundell and Cristiano Guidorzi for discussion.