{
  "bibcode": "2005GCN..4095....1L",
  "body": "W. Li, S. Jha, A. V. Filippenko, J. S. Bloom, D. Pooley, R. J. Foley,\nand D. A. Perley (University of California, Berkeley) report:\n\nWe would like to alert the community to the photometric calibration\nof Swift/UVOT, for the U, B, and V filters, in an accepted paper\nscheduled to be published in PASP in Jan. 2006 (available as\nastro-ph/0505504, and significantly updated from its original\nversion). A simple, yet detailed, step-by-step recipe for\nphotometry is provided in Section 3.6 of the paper.\n\nIn particular, we have determined optimal aperture and sky regions,\nand measured precise (+/- 0.01 mag) photometric zeropoints for the U,\nB, and V filters, based on hundreds of observations of Landolt and\nsecondary standard stars.  We also introduce a theoretically\nmotivated single parameter (m_infinity) to correct for coincidence\nloss; this parameter is determined for all three filters to +/- 0.02 mag.\n\nWith these calibration results, we have re-analyzed the Swift/UVOT\nUBV data on GRB 050922C, tabulated below. The preliminary analysis\nby Hunsberger et al. (GCN 4041), based on the preliminary in-orbit\nzeropoints, is listed for comparison whenever possible:\n\nFilter  Tmid(s) Exp(s)   Mag    Mag(err)           GCN 4041\n\nV       160.5   99.8    14.68 +- 0.03              14.60 +- 0.10\nV       289.9    9.8    15.29 +- 0.12\nV       374.5    9.8    15.46 +- 0.13              15.49 +- 0.17\nV       458.9    9.8    16.00 +- 0.16\nV       543.5    9.8    15.94 +- 0.16              15.99 +- 0.21\nV       627.9    9.8    15.89 +- 0.15\nV       712.5    9.8    16.04 +- 0.17              16.01 +- 0.21\nV      4543.5  899.8    17.68 +- 0.04              17.62 +- 0.11\nV     15962.5  899.8    18.99 +- 0.10              18.89 +- 0.21\nV     28443.5  899.8    20.26 +- 0.24             >19.95 (3sigma upper limit)\nV     40821.5  721.3    20.09 +- 0.22\nV     52450.0  805.9    20.82 +- 0.38\nV     64023.0  806.7    20.47 +- 0.29\n\nB       261.5    9.8    15.88 +- 0.09              15.88 +- 0.13\nB       345.5    9.8    16.02 +- 0.10\nB       430.5    9.8    16.23 +- 0.10              16.36 +- 0.15\nB       514.5    9.8    16.31 +- 0.11\nB       598.5    9.8    16.46 +- 0.12              16.45 +- 0.16\nB       683.5    9.8    16.64 +- 0.13\nB       767.0    8.4    16.72 +- 0.14              16.91 +- 0.20\nB     11133.5  899.8    19.15 +- 0.05              19.12 +- 0.11\nB     22648.5  899.8    20.20 +- 0.10              20.13 +- 0.21\nB     39108.5  897.3    21.14 +- 0.25              20.70 +- 0.38\nB     50681.5  899.8    21.24 +- 0.26              20.74 +- 0.39\nB     62254.5  899.8    20.81 +- 0.20\n\nU       246.5    9.8    15.34 +- 0.10              15.09 +- 0.19\nU       331.5    9.8    15.29 +- 0.09\nU       416.5    9.7    15.80 +- 0.12              15.69 +- 0.20\nU       500.5    9.8    15.97 +- 0.13\nU       584.5    9.8    15.92 +- 0.12              15.84 +- 0.21\nU       669.5    9.8    15.94 +- 0.12\nU       753.5    9.7    15.99 +- 0.13              16.00 +- 0.21\nU     10226.5  899.8    18.55 +- 0.05              18.58 +- 0.18\nU     21749.0  900.0    19.48 +- 0.08              19.13 +- 0.20\nU     34939.5  507.6    20.17 +- 0.17              19.88 +- 0.31\nU     46660.0  814.1    20.38 +- 0.15              19.89 +- 0.26\nU     58236.5  807.0    20.78 +- 0.20\nU     69598.0  348.5    20.63 +- 0.27\n\nNote the significantly improved photometric precision, especially for\nthe long-exposure observations.  Our re-analyzed UVOT photometry for\nGRB 050922C confirms the break in the light curve reported by Fynbo et\nal. (GCN 4040), Covino et al. (GCN 4046), and Andreev et al. (GCN 4048).\n\nA single power-law fit to the UVOT UBV data yields a decline index of\n-0.87, but the fit is relatively poor (reduced chi^2 about 3.0).\nUsing a broken power-law fit (with parameters as in Li et al. 2003, PASP,\n115, 844), we find t(break) = 2.7 +- 0.7 hours, alpha1 = -0.82 +- 0.06,\nalpha2 = -1.16 +- 0.14, and s = 30.5 +- 5.0, with a reduced chi^2 = 1.21.\nThis fit can be seen at\n\nhttp://astron.berkeley.edu/~weidong/grb050922c.lc1.gif .\n\nIf we instead force the break to be smoother (s = 1; e.g., Stanek et\nal., 1999, ApJ, 522, L39), we derive t(break) = 4.3 +- 0.6 h, alpha1 =\n-0.71 +- 0.08, and alpha2 = -1.37 +- 0.16, with a reduced chi^2 =\n1.46.  This fit is shown at\n\nhttp://astron.berkeley.edu/~weidong/grb050922c.lc2.gif .\n\nIn contrast, the UBV photometry reported in GCN 4041 can accommodate a\nsingle power-law fit with a decline index of -0.84 (reduced chi^2 = 0.58).\n\nWe urge the community to make use of the calibration and photometric\nrecipe in astro-ph/0505504 for Swfit/UVOT U, B, and V data until even\nbetter calibration results are available.",
  "circularId": 4095,
  "createdOn": 1129317782000,
  "email": "weidong@astron.berkeley.edu",
  "subject": "Photometric calibration of Swift/UVOT and Re-analysis of Swift/UVOT GRB 050922C data",
  "submitter": "Weidong Li at UC Berkeley KAIT/LOSS  <weidong@astron.berkeley.edu>",
  "eventId": "GRB 050922C"
}