{
  "bibcode": "1999GCN...216....1D",
  "body": "GRB 990123: Possible Gravitationally Lensed Burst?\n\nS. G. Djorgovski, S. R. Kulkarni, J. S. Bloom, S. C. Odewahn, R. R. Gal \n(CIT), and D. A. Frail (NRAO) note on behalf of the Caltech-CARA-NRAO \ncollaboration:\n\nThe lower limit to redshift of GRB 990123 (Kelson et al., IAUC, 7096), \nz =< 1.61, implies a luminosity distance D >= 3.73e28 cm, assuming a\nstandard Friedmann cosmology with H_0 = 65 km/s/Mpc, Omega_0 = 0.2, and\nLambda_0 = 0 (larger if Lambda_0 > 0).\n\nAssuming the gamma-ray fluence reported by Feroci et al. (IAUC 7095), the\nimplied isotropic gamma-ray energy of this burst would be > 2.3e54 erg,\nhigher than most (or all) currently viable theoretical models can easily\naccomodate.  The absolute magnitude of the optical counterpart at maximum \nlight (Akerlof and McKay, GCN Circ. 205) would be M =< -36.5, i.e.,\n> 2e16 L_sun, a remarkable number itself.\n\nThe apparent host galaxy of this burst, coincident with the optical\ntransient (Odewahn et al., GCN Circ. 201 and IAUC 7094) detected on DPOSS\n(digital POSS-II) images calibrated with new CCD photometry in the field,\nhas a Gunn r magnitude 21.9 +- 0.4, i.e., R ~ 21.5 +- 0.5 (cf. Gal et al.,\nGCN Circ. 213).  Its probable redshift is likely to be approximately 0.2 to\n0.3, and (unless an active nucleus is present, for which there is no\nspectroscopic evidence so far) almost certainly not at z = 1.61, the \nabsorption line redshift measured by Kelson et al.\n\nWe therefore suggest that this may be the first documented case of a \ngravitationally lensed GRB, magnified by at least an order of magnitude by \na foreground galaxy detected in DPOSS (possibly including some microlensing).  \nThis hypothesis naturally accounts for all of the data so far: the large \nfluence of the burst, its enormous implied gamma-ray luminosity, its \nremarkably high peak optical brightness and implied luminosity (and makes \nit consistent with previous non-detections of other bursts by the ROTSE and \nLOTIS experiments, with m_lim ~ 14), and the presence of an apparent \nforeground galaxy positionally coincident with the optical transient.\n\nThe conclusive test of this hypothesis will be a direct measurement of the \ngalaxy's redshift, once the OT fades sufficiently.  If microlensing is\ninvolved, a signature may be seen in the shape of the OT light curve.\nPossible image splitting may be detectable, and high-resolution imaging\nin radio, near-IR, and optical is urged.\n\nIn any case, the early detection by the ROTSE team (GCN Circ. 205), even if\ncorrected for a putative large gravitational lensing magnification factor,\nsuggests that routine early detections of GRBs in the visible light are\nwithin reach, perhaps at ~ 15 mag level.\n\nThis message may be cited.",
  "circularId": 216,
  "createdOn": 917229431000,
  "email": "george@oracle.caltech.edu",
  "subject": "GRB 990123: Possible Gravitationally Lensed Burst?",
  "submitter": "George Djorgovski at Caltech/Palomar  <george@oracle.caltech.edu>",
  "eventId": "GRB 990123"
}