[vsnet-grb-info 2256] Photometric calibration of Swift/UVOT and Re-analysis of Swift/UVOT GRB 050922C data

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4095
SUBJECT: Photometric calibration of Swift/UVOT and Re-analysis of Swift/UVOT GRB 050922C data
DATE:    05/10/14 19:23:02 GMT
FROM:    Weidong Li at UC Berkeley KAIT/LOSS  <weidong at astron.berkeley.edu>

W. Li, S. Jha, A. V. Filippenko, J. S. Bloom, D. Pooley, R. J. Foley,
and D. A. Perley (University of California, Berkeley) report:

We would like to alert the community to the photometric calibration
of Swift/UVOT, for the U, B, and V filters, in an accepted paper
scheduled to be published in PASP in Jan. 2006 (available as
astro-ph/0505504, and significantly updated from its original
version). A simple, yet detailed, step-by-step recipe for
photometry is provided in Section 3.6 of the paper.

In particular, we have determined optimal aperture and sky regions,
and measured precise (+/- 0.01 mag) photometric zeropoints for the U,
B, and V filters, based on hundreds of observations of Landolt and
secondary standard stars.  We also introduce a theoretically
motivated single parameter (m_infinity) to correct for coincidence
loss; this parameter is determined for all three filters to +/- 0.02 mag.

With these calibration results, we have re-analyzed the Swift/UVOT
UBV data on GRB 050922C, tabulated below. The preliminary analysis
by Hunsberger et al. (GCN 4041), based on the preliminary in-orbit
zeropoints, is listed for comparison whenever possible:

Filter  Tmid(s) Exp(s)   Mag    Mag(err)           GCN 4041

V       160.5   99.8    14.68 +- 0.03              14.60 +- 0.10
V       289.9    9.8    15.29 +- 0.12
V       374.5    9.8    15.46 +- 0.13              15.49 +- 0.17
V       458.9    9.8    16.00 +- 0.16
V       543.5    9.8    15.94 +- 0.16              15.99 +- 0.21
V       627.9    9.8    15.89 +- 0.15
V       712.5    9.8    16.04 +- 0.17              16.01 +- 0.21
V      4543.5  899.8    17.68 +- 0.04              17.62 +- 0.11
V     15962.5  899.8    18.99 +- 0.10              18.89 +- 0.21
V     28443.5  899.8    20.26 +- 0.24             >19.95 (3sigma upper limit)
V     40821.5  721.3    20.09 +- 0.22
V     52450.0  805.9    20.82 +- 0.38
V     64023.0  806.7    20.47 +- 0.29

B       261.5    9.8    15.88 +- 0.09              15.88 +- 0.13
B       345.5    9.8    16.02 +- 0.10
B       430.5    9.8    16.23 +- 0.10              16.36 +- 0.15
B       514.5    9.8    16.31 +- 0.11
B       598.5    9.8    16.46 +- 0.12              16.45 +- 0.16
B       683.5    9.8    16.64 +- 0.13
B       767.0    8.4    16.72 +- 0.14              16.91 +- 0.20
B     11133.5  899.8    19.15 +- 0.05              19.12 +- 0.11
B     22648.5  899.8    20.20 +- 0.10              20.13 +- 0.21
B     39108.5  897.3    21.14 +- 0.25              20.70 +- 0.38
B     50681.5  899.8    21.24 +- 0.26              20.74 +- 0.39
B     62254.5  899.8    20.81 +- 0.20

U       246.5    9.8    15.34 +- 0.10              15.09 +- 0.19
U       331.5    9.8    15.29 +- 0.09
U       416.5    9.7    15.80 +- 0.12              15.69 +- 0.20
U       500.5    9.8    15.97 +- 0.13
U       584.5    9.8    15.92 +- 0.12              15.84 +- 0.21
U       669.5    9.8    15.94 +- 0.12
U       753.5    9.7    15.99 +- 0.13              16.00 +- 0.21
U     10226.5  899.8    18.55 +- 0.05              18.58 +- 0.18
U     21749.0  900.0    19.48 +- 0.08              19.13 +- 0.20
U     34939.5  507.6    20.17 +- 0.17              19.88 +- 0.31
U     46660.0  814.1    20.38 +- 0.15              19.89 +- 0.26
U     58236.5  807.0    20.78 +- 0.20
U     69598.0  348.5    20.63 +- 0.27

Note the significantly improved photometric precision, especially for
the long-exposure observations.  Our re-analyzed UVOT photometry for
GRB 050922C confirms the break in the light curve reported by Fynbo et
al. (GCN 4040), Covino et al. (GCN 4046), and Andreev et al. (GCN 4048).

A single power-law fit to the UVOT UBV data yields a decline index of
-0.87, but the fit is relatively poor (reduced chi^2 about 3.0).
Using a broken power-law fit (with parameters as in Li et al. 2003, PASP,
115, 844), we find t(break) = 2.7 +- 0.7 hours, alpha1 = -0.82 +- 0.06,
alpha2 = -1.16 +- 0.14, and s = 30.5 +- 5.0, with a reduced chi^2 = 1.21.
This fit can be seen at

http://astron.berkeley.edu/~weidong/grb050922c.lc1.gif .

If we instead force the break to be smoother (s = 1; e.g., Stanek et
al., 1999, ApJ, 522, L39), we derive t(break) = 4.3 +- 0.6 h, alpha1 =
-0.71 +- 0.08, and alpha2 = -1.37 +- 0.16, with a reduced chi^2 =
1.46.  This fit is shown at

http://astron.berkeley.edu/~weidong/grb050922c.lc2.gif .

In contrast, the UBV photometry reported in GCN 4041 can accommodate a
single power-law fit with a decline index of -0.84 (reduced chi^2 = 0.58).

We urge the community to make use of the calibration and photometric
recipe in astro-ph/0505504 for Swfit/UVOT U, B, and V data until even
better calibration results are available.