{
  "bibcode": "1999GCN...354....1F",
  "body": "TITLE: Late-time HST Observations of GRB 990123\nSubject: HST/STIS Observations of GRB 990123\n\nA. Fruchter (STScI), S. Thorsett (Princeton), and E. Pian (ITESRE)\nreport for the HST GRB collaboration:\n\nThe field of GRB 990123 was reobserved by HST with the STIS CCD in open\nfilter (50CCD) mode during two orbits on 23 March 1999, or 59.5 days\nafter the GRB.  The total exposure time was 5040s.  The optical\ntransient was visible, but had declined by 2.1 +/- 0.2 mags from that\nobserved on 8 February 1999 by HST (Fruchter et al. 1999, Kulkarni et\nal 1999), to V = 27.55 +/- 0.2 mags.\n\nBoth of the HST observations of the OT of GRB 990123 fall well below\nthe t^{-1.09} power-law behavior of the OT during the first three days\n(Fruchter et al. 1999).  The 8 February observation lies more than 2\nmags below the continuation of such a power-law.   Additionally, a\npower-law fit of the flux density of the OT finds between the two\nHST observations finds t^{-1.52 +/- 0.15}; this slope is nearly 3 sigma \nsteeper than that found through day 3, but is also noticeably shallower than\nthat predicted from a break in the power-law due to either our now\nobserving the edge of a collimated outflow (Meszaros and Rees 1999) or\nthe sideways expansion of such an outflow (Kulkarni et al. 1999, Rhoads\n1999).\n\nThe HST images from both epochs will be made available in gif format \nat http://www.stsci.edu/~fruchter/GRB/990123.  The reduced HST data \nfrom the first epoch are also available in fits format at this\nsite.   The reduced data of the second observation will be made\navailable later, in accordance with the HST GRB Collaboration\npolicy of making reduced data publicly available upon publication \nin a refereed journal.\n\nReferences:\n\nFruchter, A. et al., 1999, Ap. J. (Letters), 519, L13.\nKulkarni, S. et al., 1999, Nature, 398, 389.\nMeszaros, P. and Rees, M., 1999, MNRAS (submitted), astro-ph/9902367.\nRhoads, J., 1999, Ap. J. (submitted), astro-ph/990399.",
  "circularId": 354,
  "createdOn": 930041381000,
  "email": "fruchter@stsci.edu",
  "subject": "Late-time HST/STIS Observation of GRB 990123",
  "submitter": "Andrew S. Fruchter at STScI  <fruchter@stsci.edu>",
  "eventId": "GRB 990123"
}