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GCN Circular 1002

Subject
GRB 010222, Optical Observations
Date
2001-03-02T21:19:06Z (23 years ago)
From
Stephen Holland at U. of Notre Dame <sholland@nd.edu>
An Early Break in the Optical Light Curve of GRB 010222

Stephen Holland (Notre Dame)
Johan Fynbo (ESO)
Javier Gorosabel (DSRI)
Arne Henden (USNO)
Jens Hjorth (Copenhagen)
Brian Jensen (Copenhagen)
Holger Pedersen (Copenhagen)

     We have obtained deep Cousins R-band images of the optical
afterglow associated with GRB 010222 using the 2.5m Nordic Optical
Telescope (NOT) at La Palma and the USNO 1.0m telescope at Flagstaff
Station.  Preliminary magnitudes, using Rc = 17.175 for Stanek's
(2001a, GCN 970) Star "A", as calibrated by Henden (2001, GCN 987),
are:

    UT(middle)          Rc     err    Telescope
 -----------------------------------------------
  2001:02:23.0102     19.733  0.014      NOT
  2001:02:23.0171     19.707  0.013      NOT
  2001:02:23.0216     19.739  0.012      NOT
  2001:02:23.0844     19.829  0.011      NOT
  2001:02:23.0889     19.852  0.010      NOT
  2001:02:23.1463     19.933  0.009      NOT
  2001:02:23.1661     19.960  0.014      NOT
  2001:02:23.2116     20.031  0.009      NOT
  2001:02:23.2858     20.126  0.018      NOT
  2001:02:24.0701     20.950  0.024      NOT
  2001:02:24.1313     20.975  0.024      NOT
  2001:02:24.1358     21.012  0.025      NOT
  2001:02:24.1410     21.052  0.025      NOT
  2001:02:24.1579     21.006  0.023      NOT
  2001:02:24.1750     21.043  0.025      NOT
  2001:02:24.1863     21.000  0.023      NOT
  2001:02:24.2469     21.035  0.022      NOT
  2001:02:25.0534     21.536  0.057      NOT
  2001:02:25.0579     21.426  0.058      NOT
  2001:02:25.2535     21.638  0.044      NOT
  2001:02:25.2583     21.667  0.041      NOT
  2001:02:22.4874     18.39   0.05       USNO
  2001:02:22.5241     18.51   0.05       USNO
  2001:02:22.5236     18.67   0.06       USNO
  2001:02:22.5385     18.66   0.06       USNO
  2001:02:24.4632     21.23   0.06       USNO
 -----------------------------------------------

     In order to maintain as much uniformity as possible in the
photometry we first restrict ourselves to data taken with the NOT.
This data extends from 0.70 to 2.95 days after the burst and is
consistent with a single power law with a slope of -1.24 +/- 0.02 (Q <
99.954%).

     We supplemented this data with R-band photometry of GRB 010222
from Stanek et al. (2001a, GCN 970), Price et al. (2001, GCN 973),
Orosz (2001, GCN 976), Stanek et al. (2001b, GCN 983), Masetti et
al. (2001, GCN 985), Oksanen et al. (2001, GCN 990), Stanek & Falco
(2001, GCN 991), Velentini et al. (2001, GCN 992), Watanabe et
al. (2001, GCN 993), and Veillet (2001, GCN 1000).  All the photometry
was adjusted to the photometric zero points of Henden (2001a, GCN
987), except for that of Veillet (2001, GCN 1000), which was used "as
is".  The photometry was corrected for Galactic extinction assuming a
reddening of E(B-V) = 0.023 and an R-band extinction of A_R = 0.061.

     The complete data set is not consistent with a single power law
(Q < 0.001%).  Photometry earlier than 0.33 days after the burst is
weakly consistent with a single power law with a slope of -0.71 +/-
0.13 (Q = 77.969%), and the photometry later than 0.57 days after the
burst is consistent with a single power law with a slope of -1.24 +/-
0.02 (Q = 99.990%).  Fitting a broken power law (see Holland et al.,
2000, A&A, 364, 467 for the method) to the light curve yields a break
at 0.48 +/- 0.11 days (11.5 +/- 2.6 hours) after the burst, and slopes
of -0.72 +/- 0.13 before the break and -1.24 +/- 0.02 after the break
(Q = 99.991%).  The smooth function of Beuermann et al. (1999, A&A,
352, L26) with slopes of -0.67 +/- 0.34 before the break, -1.29 +/-
0.06 after the break, and an exponent of 1.8 +/- 4.4 provides a
slightly better fit (Q = 99.998%) to the data than the broken power
law does.  Our fits suggest that the break occurred rapidly between
0.33 and 0.57 days (8 and 14 hours) after the burst.  A plot of the
light curve and the two fits, along with a deep composite image of the
field containing the GRB, is available at
"http://www.nd.edu/~sholland/grb/grb010222/index.html".

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