The Neil Gehrels Swift Observatory

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Zero Points

Updated July 7, 2011

The in-orbit photometric zero points have been updated as described in Breeveld et al. (2011, in press) and presented at the GRB 2010 conference. Also see the UVOT calibration document of 2010-09-16 on the UVOT Calibration Documents Page, which is part of the Swift CALDB for detailed discussion of the photometric zero points. The current (as of 2010-09-17) photometric zero points for UVOT are 

Filter	Zero Point	Error	

V		17.89		0.01
B		19.11		0.02
U		18.34		0.02
UVW1	17.44		0.03
UVM2	16.85		0.03
UVW2	17.38		0.03
White	20.29		0.04

These zero point convert instrumental magnitudes measured in a circular aperture with a radius of 5.0 arcseconds into a UVOT system magnitude. To convert UVOT system ubv magnitudes to Johnson UBV magnitudes apply the color transformation equations described in the "Color Correction" document available at the Swift UVOT Calibration Documents page, which is part of the Swift CALDB Web site.

The photometric zero points for converting UVOT count rates to the AB magnitude system of Oke (1974, ApJS, 27, 21) are given below. The AB-Vega column is the value to be added to a Vega magnitude to get the corresponding AB magnitude. 

Filter	Zero Point	Error	  AB-Vega

V		17.88		0.01        -0.01
B		18.98		0.02        -0.13
U		19.36		0.02        +1.02
UVW1	18.95		0.03        +1.51
UVM2	18.54		0.03        +1.69
UVW2	19.11		0.03        +1.73
White	21.09		0.04        +0.80

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