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 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.