[vsnet-grb-info 4886] Updated Swift/UVOT calibration

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  6614
SUBJECT: Updated Swift/UVOT calibration
DATE:    07/07/09 12:34:36 GMT
FROM:    Alice Breeveld at MSSL-UCL  <aad at mssl.ucl.ac.uk>

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.