TITLE: GCN CIRCULAR NUMBER: 6188 SUBJECT: GRB060901: Candidate optical afterglow DATE: 07/03/10 16:22:10 GMT FROM: Klaas Wiersema at GRACE/U of Amsterdam K. Wiersema (University of Amsterdam), E. Rol (University of Leicester) and C. C. Thoene (DARK Cosmology Centre) report: We re-analysed the data we took of INTEGRAL GRB 060901, as reported earlier in GCN 5501, supplemented with late time observations. We obtained three epochs of imaging in R band using DFOSC on the Danish 1.5m telescope at La Silla. The first two epochs were reported in GCN5501, and we re-observed the GRB position on September 8, midpoint 6.3 days after the burst. The first two epochs are close together in time, and suffered from bad seeing (~1.6 arcsec) and proximity to the bright Moon. The third epoch, while much shorter in exposure time and at full Moon, reaches a deeper magnitude limit than epoch 2 due to good seeing (~1 arcsec). The field is very crowded and the XRT error circle (Racusin, GCN 5496) partly overlaps with a bright star (present in the USNO-B catalogue with number 0833-0546379, possibly a blend of multiple stars), so we performed PSF matched image subtraction (Alard 2000) on the three epochs. A subtraction of the first two epochs does not yield a candidate afterglow (Wiersema & Thoene, GCN5501), but a subtraction of the first and third epoch reveals a clear, fading, residual located in the XRT error circle, at position: RA: 19:08:37.9 DEC: -06:38:07.4 With conservative astrometric errors of 0.4 arcsec. There are no other variable sources in the XRT error circle, but the source is located in the wing of the much brighter USNO star. We tentatively identify the variable object as the optical afterglow of GRB 060901. Using artificial stars with the subtractions, we find the OT candidate to be already below the (3-sigma) limiting magnitude of the second epoch image, and are able to derive a magnitude for the first epoch (midpoint 0.26 days after burst). of R ~ 22.6 +/- 0.5. We note that the Galactic extinction in this direction is considerable, at A_R ~1.8 mag. An image of the three epochs, the subtraction and the XRT error circle can be found on: http://staff.science.uva.nl/~kwrsema/grb060901/