TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2638 SUBJECT: XRF040701: Optical Observations at ESO DATE: 04/08/09 18:15:33 GMT FROM: Elena Pian at IASF,CNR,Bologna E. Pian, J. Fynbo , U. G. Joergensen, A. Levan, N. Masetti, E. Palazzi, F. Patat, E. Rol, J. Hjorth, B. Lindgren Jensen, P. Mazzali, C. Kouveliotou, K. Nomoto, E. Cappellaro, on behalf of a larger collaboration, report: The field of source CXOU J204816.1-401108 (Fox et al. GCN 2626), detected in the error box of XRF040701 (Barraud et al., GCN 2620), was imaged with the ESO Danish 1.5m telescope at La Silla on 2004 Jul 03,15,16,17,19,23,24, Aug 4,5, and with the VLT and FORS1 on 2004 Jul 23.16, 23.33, 30.35 UT in the R band. The VLT imaging of Jul 23 was done under excellent seeing conditions (0.6"). Both R-band photometry and PSF-matched image subtraction of the galaxy at z = 0.2146 underlying CXOU J204816.1-401108 (Kelson et al. GCN 2627) rule out flux variations larger than a few hundredths of a magnitude. Considering that we determine a lower limit of R~17.9 for the galaxy brightness, an event similar to supernova 1998bw would produce an observed variation of about 6% from explosion to maximum light. Therefore, our data appear marginally to exclude a supernova similar to 1998bw (see also Berger et al. GCN 2631), but not a weaker one, like SN2002ap or SN1994I. A 2hr VLT+FORS1+300V spectrum of the galaxy taken on Jul 23 does not reveal conspicuous supernova features. The 2 galaxies located at ~4" West and ~6" South of the galaxy underlying CXOU J204816.1-401108 have redshifts 0.215 and 0.17, respectively. This message can be cited.