[vsnet-grb-info 4443] GRB 070311, deep optical photometry

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6219
SUBJECT: GRB 070311, deep optical photometry
DATE:    07/03/20 19:00:21 GMT
FROM:    Peter Garnavich at U of Notre Dame  <pgarnavi at nd.edu>

X. Dai (Ohio State), P. Garnavich (Notre Dame), R. Pogge (Ohio State),
J. Hill (LBTO/UAz), X. Fan (U Ariz), J. Prieto, K. Z. Stanek (Ohio State),
R. M. Wagner (LBTO/OSU), J. Rhoads (Ariz State), E. Egami, J. Bechtold,
S. Herbert-Fort (UAz) report:

The Large Binocular Telescope (LBT) imaged the position of the GRB 070311
afterglow (Mereghetti et al, GCN 6189; Covino et al. GCN 6190) with the
LBC-blue CCD camera (http//lbc.mporzio.astro.it) and 8.4-m SX mirror on
2007 March 17.14 (UT). Ten dithered, 200 second exposures were obtained
with the Sloan-r filter in 0.65" seeing. After combining the images, the
afterglow is clearly detected. No obvious GRB host galaxy is visible. The
field is filled with clouds of faint emission due to the low Galactic
latitude of the region.

The comparison star used by Halpern & Armstrong (GCN 6195; star "A"
with USNO-B1.0 R2=16.67 mag) is heavily saturated in the LBT data,
so we obtained short exposure images of the field with LBC-blue to
calibrate a fainter star ("B") at 5:50:10.341, 03:23:02.58 (J2000).
The difference in brightness between star A and B is 2.62+/-0.01 mag.
in Sloan-r and this is confirmed with R-band images taken at MDM
(E. Armstrong, private communication). From point-spread-function fitting
photometry, the afterglow is 5.44+/-0.06 mag fainter than comparison
star B or at a brightness of R2=16.67+2.62+5.44= 24.73+/-0.06 mag.

The brightness of the afterglow 6.0 days after the GRB suggests that it
has continued a very steep decline after its optical flare
(Halpern & Armstrong GCN 6203). The afterglow has not resumed the shallow
decay it followed on the first day after the GRB. A late flare
occurring just before the break resembles GRB 000301c (Garnavich, Loeb &
Stanek 2000, ApJ, 544, 11; Rhoads & Fruchter 2001, ApJ 546, 117)
and GRB 060526 (Dai et al. 2006, astro-ph/0609269).

The light curve of 070311 is available at:
http://www.astronomy.ohio-state.edu/~xinyu/grb/070311.jpg
and uses photometry from GCNs 6198 (Wren et al. 2007), 6195
(Halpern & Armstrong 2007), 6199 (Halpern & Armstrong 2007), 6203 (Halpern
& Armstrong 2007), 6204 (Greco et al. 2007), 6206 (Kann et al. 2007), 6208
(Halpern & Armstrong 2007).

The LBT image is available at:
http://www.nd.edu/~pgarnavi/grb070311/LBT_grb070311.jpg
and a wide-field view is at:
http://www.nd.edu/~pgarnavi/grb070311/LBT_grb070311_clr.jpg

We thank Eve Armstrong for providing MDM images to calibrate
fainter comparison stars.

The LBT is an international collaboration between institutions in the
U.S.A., Italy and Germany. LBT Corporation partners are The Universities
of Arizona; Italy's Istituto Nazionale di Astrofisica; Germany's LBT
Beteiligungsgesellschaft representing the Max-Planck Society, the
Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State
University and The Research Corporation, which represents The
University of Notre Dame, University of Minnesota and University of
Virginia.

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