TITLE: GCN CIRCULAR NUMBER: 6165 SUBJECT: GRB 070125, deep late-time optical observation DATE: 07/03/01 22:47:13 GMT FROM: Peter Garnavich at U of Notre Dame 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) 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) 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). 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.