GCN Circular 6165

P. Garnavich (Notre Dame), X. Fan, L. Jiang (U Ariz), X. Dai (Ohio State),
O. Kuhn (LBTO), N. Bouche, P. Buschkamp (MPE), P. Smith,
P. Milne, J. Bechtold (U Ariz), K. Z. Stanek, J. Prieto (Ohio State),
R. M. Wagner (LBTO/OSU), J. Rhoads (Ariz State), J. Hill (LBTO/UAz),
A. Baruffolo, C. DeSantis, E. Diolaiti, A. DiPaola,  J. Farinato,
A. Fontana,  S. Gallozzi, F. Gasparo, E. Giallongo, A. Grazian,
F. Pasian, F. Pedichini, R. Ragazzoni, R. Smareglia, R. Speziali,
V. Testa, E. Vernet (LBC Team/INAF) reprt:


The Large Binocular Telescope (LBT) imaged the position of the GRB 070125
afterglow (Cenko & Fox, GCN 6028
Loading...
 
 ) with the LBC-blue CCD camera
(http//lbc.mporzio.astro.it) and 8.4-m SX mirror on 2007 February 21.1 (UT).
Ten dithered, 200 second exposures were obtained with the Sloan r filter
in 1.3" seeing. After combining the images a faint source is detected at
the position of the afterglow. Using SDSS stars in the field and
1.4" apertures, we estimate the brightness of the source
at r=26.3+/-0.3 mag. Since the source may be dominated by the host galaxy,
this observation represents a lower-limit on the magnitude of the
afterglow 26.8 days after the GRB.

The last reported detection of the GRB afterglow was R=21.07 at 4.04 days
(Mirabal, Halpern & Thorstensen, GCN 6096
Loading...
 
 ) and it was decaying with an
index of 1.6. Converting our observation to Johnson-Cousins R-band
(assuming beta=-1.0 so B-V=0.34) we find that after the light curve break
the power-law decay index was equal to, or greater than 2.5.

This confirms the sharp steepening in the light curve four days after the
GRB and the jet opening angle reported by Mirabal et al. (GCN 6096
Loading...
 
 ). We
expect that a supernova at z=1.5 and similar to SN 1998bw (Galama et al.
1998, Nature, 395, 670) would have r~28 mag at the time of our observation
so would not contribute significantly to the observed flux.

The LBT image is available at:
http://www.nd.edu/~pgarnavi/grb070125/LBT_070221.jpg

The X-ray light curve is generally consistent with the optical. Both show
a break at late time and a steep decay. The x-ray light curve actually has
a strong detection at an age of 10 days suggesting a steeper post-break
decay and possibly a wider opening angle than the optical limits.

It has been suggested that the existence of jet breaks in Swift x-ray
light curves are rare (e.g. Burrows & Racusin 2007, astro-ph/0702633),
but here is a good example occurring at a time later than expected. This
suggests the lack of Swift jet breaks is a selection effect and that many
occur beyond the Swift sensitivity limit.

The X-ray/optical light curve comparison is available at:
http://www.astronomy.ohio-state.edu/~xinyu/grb/070125.jpg


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 provides access to The
University of Notre Dame, University of Minnesota and University of
Virginia.

This message may be cited.