[AAVSO-HEN] GRB 070721A: optical observations

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TITLE:   GCN CIRCULAR
NUMBER:  6674
SUBJECT: GRB 070721A: optical observations
DATE:    07/07/27 17:14:46 GMT
FROM:    Daniele Malesani at Niels Bohr Inst,Dark Cosmology Center  <malesani at astro.ku.dk>

D. Malesani, J.P.U. Fynbo, P.M. Vreeswijk (DARK), I. Ilyn (AIP), report 
on behalf of a larger collaboration:

We observed the field of GRB 070721A (Ziaeepour et al., GCN 6639) in two 
different epochs, using the NOT and VLT telescopes. Observations were 
carried out 0.78 and 1.98 days after the GRB. A bright star (R~17.3) is 
close to the XRT position (Evans & Ziaeepour, GCN 6640). Inside the XRT 
error circle, we detect a single source at the coordinates (J2000, 
against USNO-B1):

  RA = 00:12:39.13
  Dec = -28:33:00.9

This source does not vary between the two epochs to within 0.2 mag, and 
is pointlike under seeing conditions of 0.7". We measure R~22.9 using 
VLT instrumental zeropoints (~0.1 mag error). No other sources are seen 
inside the XRT error circle down to the limits reported below.

Mean UT       T-T0 (d)  t_exp (s) Instrument  Rlim
--------------------------------------------------
Jul 22.20065  0.78320   4x600     NOT+StanCam 24.0
Jul 23.39637  1.97892   2x180     VLT+FORS2   25.5

T0 = trigger time = Jul 21.41745 UT.

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