TITLE:   GCN CIRCULAR
NUMBER:  8685
SUBJECT: Modification of Swift/UVOT response to bright GRBs
DATE:    08/12/19 21:10:47 GMT
FROM:    Wayne Landsman at GSFC/SSAI  <wayne.b.landsman@nasa.gov>

W. Landsman (GSFC), P. Kuin (MSSL), F. Marshall (GSFC), M. Page (MSSL), 
P. Roming (PSU), N. Gehrels (GSFC) report on behalf of the Swift/UVOT team

The Swift/UVOT has modified its automated response to GRBs with a BAT 
trigger rate larger than 100 cps.   For these bright GRBs, a 50s UV 
grism exposure is now inserted following the first 150s finding chart in 
the white filter.      Roughly half of all Swift GRBs are sufficiently 
bright to trigger the use of the grism.

The UVOT grism covers the spectral range 1700 - 5500 A, and  can provide 
useful spectra for GRBs brighter than about V~13.     Further details 
are available in Kuin et al. (2008, astro-ph 0812/2943 ) who report 
their analysis of the UVOT grism spectrum of GRB081203A.

The grism zero-order spectrum in the 50s exposure can provide photometry 
for sources brighter than B-18.    The zero-order spectrum includes 
visible light and its brightness roughly  correlates with Johnson B 
magnitude, with a scatter of about 0.23 mag.