TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2656 SUBJECT: XRF040812: possible afterglow second Chandra observation DATE: 04/08/25 08:53:50 GMT FROM: Sergio Campana at INAF-OAB S. Campana and A. Moretti (INAF-Osservatorio astronomico di Brera) report on behalf of a larger team: "We analyzed the second Chandra ACIS-S observation of the XRF040812 (Gotz et al. 2004, GCN 2640). We carried out spectral analysis of our previous afterglow candidate (CXOJ162602.3-444349, Campana & Moretti GCN 2649, see also Patel et al. GCN 2648, 2655). The source is still very hard with a power law photon index of 0.9+/-0.6 (fixing the column density to the Galactic value of 5.3x10^21 cm-2 and using Cash statistics in the XSPEC fit). The spectrum is slightly softer than in the first observation (Campana & Moretti GCN 2649). The unabsorbed 0.5-10 keV source flux is 9.30x10^-14 erg cm-2 s-1 a factor of ~3 lower than in the previous Chandra observation. A constant temporal evolution is not consistent with the data at a ~4.5 sigma level. A power law decay describes better the temporal evolution with a time decay as t^-1.4(+/-0.6) (90% confidence level, note that this is different from the Patel et al. GCN 2655, since we fitted the flux evolution and not the counts evolution)." This message can be cited.