{
  "bibcode": "2002GCN..1618....1M",
  "body": "N. Mirabal, J. P. Halpern (Columbia), R. Chornock, and A. V. Filippenko\n(UC Berkeley) report:\n\n\"Continued monitoring of the optical afterglow of GRB 021004 with the\nMDM 1.3m by E. K. Armstrong yields the following measurements:\n\n\t----------------------------\n\t      UT   \t  R     +/-\n\t----------------------------\n\tOct. 8\t07:05\t20.48\t0.03\n\tOct. 9\t04:21\t20.72\t0.04\n\t----------------------------\n\nHere we use the calibration of Henden (GCN #1583) in which the reference star\nhas R=15.52.  An updated decay curve accounting for revised calibrations of\nGCN reported data is at:\n\nhttp://www.astro.columbia.edu/~jules/021004.ps\n\nFurther examination of the optical spectrum reported by Chornock and Filippenko\n(GCN #1605) yields the following features in the highest redshift system \n(\"em\" refers to emission, \"bl\" denotes a broad blend of z=2.323 and 2.318):\n\n\t-------------------------------------------\n\tLine\t\tRest wavelength      z\n\t-------------------------------------------\n\tLy beta\t\t1025.7\t\t   2.323 bl\n\t\t\t\t\t   2.291\n\n\n\tLy alpha \t1215.7\t\t   2.328 em\n\t\t\t\t\t   2.319 bl\n\t\t\t\t\t   2.295\n\n\tSi IV\t\t1393.8,1402.8\t   2.323\n\t\t\t\t\t   2.318\n\t\t\t\t\t   2.293\n\n\tC IV\t\t1548.2,1550.8\t   2.323\n\t\t\t\t\t   2.317\n\t\t\t\t\t   2.293\n\n\tAl II\t\t1670.8\t\t   2.328\n\t-------------------------------------------\n\nThis spectrum was taken in five 1200 s exposures from Oct 8 10:14-14:05 UT.\nIn view of the 6 Angstrom resolution of this spectrum, we consider that the\nlowest redshift absorber of the three is a blend of two of the four systems that\nwere seen by Salamanca et al. (GCN #1611) in higher resolution data.  Also, we\ndetect one absorption line (Al II 1670) at the same redshift as the Ly alpha\nemission, z=2.328, which we therefore designate the redshift of the host galaxy.\nThe detection of this lone low-ionization absorber is plausible because of its large \noscillator strength, f=1.83.  Thus, all of the high-ionization absorbers are \nblueshifted from the host redshift, by up to 3,200 km/s.\n   \nThe decay curve shows significant deviations from a power law, which might be\nunderstood in terms of inhomogeneities in the ambient medium in which the afterglow \nexpands.  In view of the revised redshift of 2.328 and the multiple blueshifted\nH, C IV, and Si IV absorption lines, we hypothesize that the velocity span of\n3,200 km/s in these components is indicative of a clumpy wind from a massive hot star\nprogenitor, such as a Wolf-Rayet star.  This would avoid the difficulties of scenarios \ninvolving supernova ejecta or cosmic clustering, as explained in GCN #1611.  The absence\nof nitrogen lines and the presence of Ly alpha and Si IV weaker than C IV is consistent\nwith a WC star, in which the bulk of the wind is approximately equal parts He and C.\n\nWe assume a typical mass-loss rate of 10(-5) Msun/yr and a wind terminal velocity \nof 3,000 km/s.  Following Chevalier & Li (2000, ApJ, 536, 195), the predicted radius \nr_s of a spherical adiabatic blast wave in such a stellar wind at an observed time of\n1 day is 3.5x10(17) cm for an isotropic energy of 5.6x10(52) erg (Malesani et al.,\nGCN #1607), or 3.5x10(16) cm if there is a beaming correction of 1/100 to the energy.  \nChosing the larger value of r_s, the column density of C in the wind is at least\n3.6x10(15) cm(-2) between the shock and the deceleration radius of the wind, which\nis of order 5x10(17) cm or greater (Ramirez-Ruiz et al. 2000, MNRAS, 327, 829).  This\nwould be enough to account for the observed absorption at high velocities.  We note that \nfor a typical nu(-1) broad-band spectrum, C IV ions in the wind would absorb in the line \nbefore being photoionized.  Detailed photoionization models are needed to predict the \nevolution of the column densities.  A consequence of this model is that the high-velocity \ncomponents of C IV should decline after being photoionized, or as the shock overruns the \nwind in several days.  Continued spectroscopic monitoring of this bright afterglow is\nurged in order to search for this definitive signature of the massive star progenitor of \nGRBs.  Changes in the decay curve could also result when the afterglow reaches the \ntermination of the stellar wind with the ISM.\"",
  "circularId": 1618,
  "createdOn": 1034291739000,
  "email": "jules@astro.columbia.edu",
  "subject": "GRB 021004: WC wind?",
  "submitter": "Jules Halpern at Columbia U.  <jules@astro.columbia.edu>",
  "eventId": "GRB 021004"
}