{
  "bibcode": "2001GCN..1157....1O",
  "body": "K. Olsen, M. Brown, R. Schommer (NOAO), and C. Stubbs (U. Washington)\nreport:\n\nOn November 22 UT, we observed the OT associated with GRB011121\n(Wyrzykowski, Stanek, and Garnavich: GCN 1150) with the CTIO 0.9-m\ntelescope and UBVRI filters.  The bright OT is clearly detected in\neach band.  We performed aperture photometry on the transient through\n2\" apertures.  We calibrated the photometry with observations, taken\nat nearby time and airmass, of stars E5-c and E5-59Y from Graham 1982,\nPASP, 94, 244.  These stars have similar colors to the OT (E5-59Y: U-B\n= -0.02; E5-c: B-V = 0.89, V-R = 0.49, V-I = 0.96).  We find:\n\n  U = 20.7  +/- 0.2     at 08:19 UT\n  B = 20.86 +/- 0.05    at 06:59 UT\n  V = 20.00 +/- 0.05    at 07:10 UT\n  R = 19.25 +/- 0.05    at 06:35 UT\n  I = 18.66 +/- 0.05    at 06:48 UT\n\nwhere the errors reflect both random error and the estimated\nuncertainty in the absolute calibration.  These magnitudes have been\nlowered by 0.051, 0.019, 0.03, 0.015, and 0.013 magnitudes in UBVRI,\nrespectively, to account for the small difference in airmass at which\nthe OT and the standards were observed.  We assume \"typical\" CTIO\nextinction coefficients of 0.45 (U), 0.28 (B), 0.15 (V), 0.08 (R), and\n0.11 (I) magnitudes per airmass.  We have not attempted to adjust the\nmagnitudes for difference in color between the OT and the standards;\nwe estimate that such effects will be smaller then 0.01 magnitudes in\nBVRI, but perhaps as large as 0.05 magnitudes in U.\n\nComparing the magnitude of the OT in R to that of star \"A\" (R=17.4;\nWyrzykowski et al., GCN 1150) we find the R magnitude of the OT to be\n18.46 +/- 0.02 (internal error only).  There is thus a zero point\ndifference of 0.79 magnitudes between our photometry and that of\nWyrzykowski et al.\n\nAssuming E(B-V) = 0.5 (Schlegel et al. 1998) and adopting the\nextinction curve of Cardelli, Clayton, & Mathis (1989), we find\nextinctions of A_U = 2.43, A_B = 2.07, A_V = 1.55, A_R = 1.16, and A_I\n= 0.74.  The intrinsic colors of the OT are thus (U-B)=-0.52,\n(B-V)=0.34, (V-R)=0.36, and (V-I)=0.53.\n\nAstrometry of the object, tied to the USNO-A2 catalog, shows the OT to\nbe at 11 34 29.61 -76 01 41.5 (2000.0).  This position agrees with\nthat of Price et al. (GCN 1155), and is 2.\"4 further west than that of\nWyrzykowski et al.\n\nThis message may be cited.",
  "circularId": 1157,
  "createdOn": 1006577378000,
  "email": "kolsen@ctio.noao.edu",
  "subject": "GRB011121",
  "submitter": "Knut Olsen at CTIO  <kolsen@ctio.noao.edu>",
  "eventId": "GRB 011121"
}