{
  "bibcode": "2000GCN...889....1R",
  "body": "E. Rol, P. Vreeswijk (U. of Amsterdam), R. Strom (U. of Amsterdam,\nASTRON), E. Richards (Hubble fellow at UAH), R. Wijers (SUNY),\nL. Kaper (U. of Amsterdam) C. Kouveliotou (MSFC/USRA) report on behalf\nof a larger collaboration:\n\nWe observed the WFC error circle of GRB001109 (Gandolfi, GCNs #878,\n#879) with the Westerbork Synthesis Radio Telescope at 4.8 GHz, on\nNov. 11.6, 13.7 and 19.7 UT. The flux density detection limits for\nthese three epochs are 0.043 mJy, 0.048 mJy and 0.059 mJy,\nrespectively. Within the NFI error circle (Amati et al., IAUC #7519),\nwe do not detect any sources down to a 4 sigma limit for the first two\nepochs, and 5 sigma for the last epoch.\n\nAssuming a low value for the spectral slope of an extra-galactic radio\nsource of -0.5, the reported VLA 8.4 GHz flux density of 0.24(3) mJy\n(Taylor et al., GCN #880), implies a 4.8 GHz flux of about 0.32 mJy on\nNov. 9.8 UT. Then this source has decreased by at least a factor of\nabout two in flux density (from 0.32 mJy to below our 4 sigma limit of\n0.17 mJy on Nov. 11.6) between 0.4 days and 2.2 days after the\nburst. If the radio source is an extragalactic object of constant\nflux, then our measurement requires its spectral index to be +0.6 or\nsteeper, which is well outside the usual range -1 to -0.5 for\nextragalactic sources.\n\nTherefore, it is more likely that the source has actually faded,\nmaking this source the probable counterpart of GRB001109. The radio\nbehavior is then reminiscent of the short radio flare counterparts,\nlike those of GRB970828 and GRB990123. Assuming a radio spectral index\nduring the fading phase of -0.8, the temporal decay has to be faster\nthan t**-0.5.  This is consistent both with a reverse-shock and\nforward-shock origin of the radio emission.\n\nFurther radio measurements are required to exclude or confirm that \nthe radio source is a variable extragalactic object of some kind.\n\nWe thank the WSRT staff for their assistance.",
  "circularId": 889,
  "createdOn": 974847563000,
  "email": "pmv@astro.uva.nl",
  "subject": "WSRT 4.8 GHz observations of GRB001109",
  "submitter": "Paul Vreeswijk at U of Amsterdam  <pmv@astro.uva.nl>",
  "eventId": "GRB 001109"
}