{
  "bibcode": "1999GCN...323....1B",
  "body": "J. S. Bloom, S. R. Kulkarni and S. Djorgovski California Institute of\nTechnology; D. A. Frail, NRAO; T. S. Axelrod, J. R. Mould, B. P. Schmidt\nThe Australian National University report:\n\nWe have observed a number of Landolt Stars on May 11 under photometric\ncondition with the MSO 50inch. Based on these observations we report\nthe following magnitudes for the secondary stars discussed in GCN 316:\n\n        Offset from GRB  RA (J)        Dec (J)       R      V    sig\nStar 1  33.4\"E  43.9\"S   13:38:20.56  -80:30:31.9  15.09  15.58  0.03\nStar 2  59.8\"W  21.0\"N   13:37:43.48  -80:29:26.7  15.62  16.19  0.03\nStar 3  16.5\"W  36.7\"N   13:38:00.82  -80:29:11.5  16.50  16.99  0.03\n\nAt the time of these observations, the OT and these stars have very\nsimilar colors and  we have transformed both MachoR and MachoV to\nstandard R_C and V magnitudes from color transformations determined\nfrom the Landolt Standard stars. The corrections from VM->V and RM->R_C\nare less than 0.02 magnitudes in all cases, and are invariant in time,\nbecause the color of the OT is not varying significantly.\n\nStar 3 is the reference star used by Galama et al. (GCN 313) and we\nnote that our new determination is 0.7 mag brighter than the value\nassumed by Galama et al. and Axelrod et al. (GCN 315).  Our analysis\nreported below takes into account this new calibration.\n\nWe report the following new measurements:\n    DATE (UT)        V        Verr        R        Rerr   \n   May 11.508       20.11     0.09       19.67     0.07 \n   May 11.512       20.01     0.08       19.71     0.06\n   May 11.516       20.06     0.07       19.76     0.09\n\nAs noted earlier (GCN #318; GCN #320) analysis of the R-band light\ncurve shows steepening of the initially observed power law decay.  We\nmodel the flux in each band with the following analytic 4-parameter\nfunction:\n\n        F_nu(t) = [f_* x (t/t_*)^alpha1] x  [1 - exp(-J)]/J\nwhere\n        J(t,t_*,alpha1,alpha2) = (t/t_*)^(alpha1 - alpha2).\n\nThe principal virtue of this formulation is that the asymptotic\npower law indices are alpha1 (at early times) and alpha2 (at late\ntimes).\n\nWe have fitted this function to the MSO V and R band data of May 10 and\nMay 11 and the R and I band data reported in earlier GCNs.  Remarkably,\nthe flux in V, R, I-bands are all consistent with same set of shape\nparameters, differing only by the flux normalization f_*:\n\n        alpha1 =  -0.88 (+/- 0.02)\n        alpha2 =  -2.5  (+/- 0.3)\n        t_*    =   1.55 days (+/- 0.05)\n\nThe consistency of alpha1, alpha2 and t_* between the different bands\nmeans that the break is wide band. This is the first clear observation\nof a wide band break. A break due to electron cooling is strongly\nchromatic and the expected difference in asymptotic slopes would be\nsmall: alpha1 - alpha2 = 0.25.  The simplest interpretation is that we\nare seeing evidence for a spreading jet.  \n\nWe assume that the optical band is below the cooling frequency given\nthe relatively flat spectrum (see below). If so, application of\nstandard afterglow models (Sari, Piran & Halpern 1999; astroph/9903339)\nwe deduced from the measured alpha1 that the electron energy spectral\nindex is p=2.2.  In the simplest models for spreading jets (Rhoads\n1999, astroph/9903399; Sari, Piran, Halpern 1999), we expect alpha2 = p.\nThe observed value of alpha2 is consistent with this expectation.\nFuture observations of the flux will define alpha2 better and the\nconsistency between alpha2 and p can be tested more rigorously.\n\nHowever, as with the case of GRB 990123, the frequency spectral index\nis not consistent with theoretical expectations. After correcting for\nGalactic extinction (GCN 312) the spectral index in the optical (V,R)\nis nearly 0 on May 11.\"\n\nThis message can be cited.",
  "circularId": 323,
  "createdOn": 926572326000,
  "email": "srk@astro.caltech.edu",
  "subject": "GRB 990510: Broad Band Break",
  "submitter": "Shri Kulkarni at Caltech  <srk@astro.caltech.edu>",
  "eventId": "GRB 990510"
}