{
  "bibcode": "1999GCN...239....1K",
  "body": "S. R. Kulkarni (Caltech) and D. A. Frail (NRAO) report on behalf\nof the Caltech-NRAO-CARA gamma-ray burst collaboration the following.\n\n\"We have continued monitoring the error box of GRB 990123\n(GCN #202) with the Very Large Array (VLA). All our observations\nwere  conducted in the 8.46-GHz band. At the position of the\noptical transient we obtain the following fluxes:\n\n\nHere is a summary of all the X-band observations to date.\n\nJan 23:  <68 microJy  (GCN #200)\nJan 24:  260 microJy  (GCN #211)\nJan 26:  <78 microJy\nJan 27:  <50 microJy\nJan 28:  <50 microJy\n\nNote: The upper limits are indicated by \"<\" and are 2-sigma.\n\nTo summarize, the radio afterglow has been detected only once, on\nJanuary 24 1999. The detection was very secure being 8-sigma. The\nsource was seen in both intermediate frequency (IF) bands and in both\nsenses of polarization (R and L). In contrast, the average of the flux\nfor the period January 26-28 is <32 microJy.\n\nThis high degree of variability could be due to interstellar scattering\nand scintillation (ISS).  However, the factor of 10  variation in the\nflux density requires rather extreme ISS.  In addition, we expect the\nsource to be gradually rising with time. Thus the absence of the source\non three successive days is quite puzzling.\n\nDiscarding the ISS hypothesis, we have two choices. First, the radio\nemission detected on January 24 is some kind of precursor event to the\nmain afterglow. Typically, the radio afterglow in the 8.46-GHz band\nrises to the peak flux in about 10-20 days. We have little data about\nthe behavior of radio afterglow within a few days after the burst.\nThus it is difficult to accept or reject this hypothesis.\n\nThe alternative hypothesis is that the GRB is lensed (GCN #216).  At\nthe present time, there is no firm evidence for lensing. However,  the\nlensing idea is economical in energetics. Likewise, the lensing\nhypothesis offers a simpler alternative to the curious phenomenon\ndiscussed above.  As discussed by various people (S. Mao GCN#236, R.\nNarayan, pers.  comm.) a robust expectation of strong lensing is that\nwe should first see the faint image \"B1\" (in the terminology of GCN\n#236). In this framework, we identify the radio source of Jan 24th with\nthe radio afterglow of the B1 component.  If this interpretation is\ncorrect then the radio afterglow of the brighter components B2 and B3\nshould become visible in in the next few weeks. We do not possess\nsufficient astrometric accuracy to see if the reported (GCN #201)\noptical afterglow (which is presumably due to B1+B2) and the radio\nafterglow are offset on the sky.  Indeed, the excellent agreement (0.5\narcsec) between the positions of the optical and the radio afterglow\nplaces a limit of a delay of 30 days between components B1 and B2+B3.\nClearly, radio monitoring will refute or confirm this hypothesis.\n\nThis report is citeable.\"",
  "circularId": 239,
  "createdOn": 917583772000,
  "email": "srk@astro.caltech.edu",
  "subject": "GRB 990123: Continued Radio Observations",
  "submitter": "Shri Kulkarni at Caltech  <srk@astro.caltech.edu>",
  "eventId": "GRB 990123"
}