TITLE: GCN CIRCULAR NUMBER: 6614 SUBJECT: Updated Swift/UVOT calibration DATE: 07/07/09 12:34:36 GMT FROM: Alice Breeveld at MSSL-UCL A. Breeveld (MSSL/UCL) reports on behalf of the Swift/UVOT team: A new, improved calibration for all the UVOT broadband filters (u, b, v, uvw1, uvm2, uvw2 and white) has been released by the HEASARC. The new calibration includes updated zero points, coincidence loss formula, filter effective areas, colour transformations and count rate to flux transformations. The latest CALDB files and documentation can be downloaded from: http://heasarc.gsfc.nasa.gov/docs/heasarc/caldb/swift/ For filters u, b and v the zero points are consistent with the previous zero points within the previous quoted error, but the uv and white zero points have changed more significantly, by up to 0.5 mag (white). Please be aware that from now on the UVOT magnitudes given in GCNs sent out by the Swift UVOT team will be calculated using these zero points. The quoted magnitudes are in the UVOT instrumental system (lower case u, b, v) and can be converted to Johnson U, B, V magnitudes using the colour transformations. The differences between UVOT magnitudes and Johnson are most significant in the u filter but are generally small.