{
  "bibcode": "1999GCN...286....1S",
  "body": "Bradley E. Schaefer and Eric H. McKenzie (Yale University) report:\n\n\"We have obtained 139 photometric observations in B, V, and I of the late\ntime light curve for SN1998bw associated with GRB980425.  These\ndemonstrate a strikingly linear decline in magnitude versus time.  \n\nOur data was taken with the Yale 1.0-m telescope on Cerro Tololo between\n27 June and 28 October 1998.  We used standard IRAF reduction and the\ncomparison stars of Galama et al.\n(http://www.astro.uva.nl/~titus/grb980425/grb980425chart.html).  During\nthe time interval of our observations, the light from the underlying\ngalaxy (ESO 184-G82) was insignificant in our photometry.\n\t\nOur first and last data nights had measured photometry as follows:\n  JD2450992.9   B=16.68+-0.03, V=15.79+-0.02, I=15.16+-0.03\n  JD2451115.6   B=18.35+-0.06, V=18.09+-0.05, I=17.33+-0.06\nBetween these two nights, all our photometry is perfectly consistent with\nan exactly linear decline in magnitudes (hence an exponential decline in\nluminosity).  Our measured uniform decline rates are as follows:\n  B   0.0141+-0.0002 mag/day for equivalent half-life of 53.4+-0.8 days\n  V   0.0184+-0.0003 mag/day for equivalent half-life of 40.9+-0.7 days\n  I   0.0181+-0.0003 mag/day for equivalent half-life of 41.6+-0.7 days\n\nThe observed exponential decline is in contradiction to the theoretical\npredictions of Iwamoto et al. (1998, Nature, 395, 672) and Iwamoto (1999,\nApJ, astro-ph/9810400), which claims that the decline will be as a power\nlaw.  The observed exponential decline has a rate similar to that expected\nfrom the decay of radioactive cobalt as modified by the effects due to the\nexpansion of the shell.  So it is reasonable to conclude that the\nlate-time light curve of SN1998bw is being powered by radioactive cobalt.\nThis then implies that the underlying explosion mechanism must create\nlarge masses of radioactive cobalt.\n\nThe light curve of SN1998bw is significantly different from all\npreviously known supernovae.  For a comparison with Type Ia events, the\ndecline rate of SN1998bw is the same in B but not V and I as for Type Ia\nevents, while SN1998bw does not show the bump in the I band light curve\nfrom 20-50 days after peak.  A comparison with Type Ic events is difficult\nsince their lights curves are not well defined, yet the late time decline\nrate of Type Ic events is substantially smaller than for SN1998bw.\n\t\nOur observations of SN1998bw are continuing with the Yale 1.0-m\ntelescope.  However, since the source has come out from behind the Sun,\nthe light from the galaxy  provides an increasingly significant\nobstacle to accurate photometry.  Perhaps future accurate photometry must\nawait the complete fading of SN1998bw to allow for subtraction of the\ngalaxy light.\"",
  "circularId": 286,
  "createdOn": 922830033000,
  "email": "schaefer@grb2.physics.yale.edu",
  "subject": "GRB980425 Optical observations of exponential decline at late time",
  "submitter": "Brad Schaefer at Yale U  <schaefer@grb2.physics.yale.edu>",
  "eventId": "GRB 980425"
}