TITLE: GCN GRB OBSERVATION REPORT NUMBER: 996 SUBJECT: Sub-millimeter Excess/GRB 010222 DATE: 01/02/27 13:28:18 GMT FROM: Shri Kulkarni at Caltech S. R. Kulkarni (Caltech), D. A. Frail (NRAO), G. Moriarty-Schieven (JAC), F. Bertoldi (MPIfR), F. Walter (OVRO), D. Shepherd (NRAO), R. Sari (Caltech), D. E. Reichart (Caltech) and S. G. Djorgovski (Caltech) report: Observations of the afterglow of GRB 010222 have continued with the SCUBA sub-millimeter continuum bolometer array on the James Clark Millimetre Telescope (JCMT). The first observation, reported in GCN#971 by Fich et al. was begun 5.6 hrs after the burst on Feb 22 at 13:03 UT. Two additional observations were made beginning on Feb 23.46 UT and Feb. 24.68 UT, or 27.7 hrs and 56.8 hrs after the burst. The source had a 350 GHz flux density of 4.2+/-1.2 mJy, 3.6+/-0.9 mJy and 4.2+/-1.3 mJy on these three days, respectively. In contrast to the bright sub-millimeter emission, the source is weak or undetectable at millimeter wavelengths. F. Bertoldi has conducted observations with the Max-Planck Millimeter Bolometer (MAMBO) array (220 GHz) on the IRAM 30-m telescope on the nights of February 23 and the 24. The data reduction is in progress. However, the preliminary results are 1.4+/-0.5 mJy and 1.2+/-0.3 mJy, respectively. F. Walter and D. Shepherd have conducted observations at the OVRO Interferometer (98 GHz). The observations of Feb 23.81 yield an upper limit of -0.3 +/-0.8 mJy. [At the time of the submission we recieved GCN 995 which reports similar upper limits from PdBI but for the epoch Feb 24.94-25.11 UT]. The relative constancy of the 350 GHz flux density and the steep spectrum between 220 GHz and 350 GHz (spectral slope>+2.4) cannot be reconciled with standard afterglow models. Free-free or synchrotron self-absorption in the millimeter band is unlikely because the source is detectable at centimeter wavelengths (GCN#968). Thus we conclude that the sub-millimeter flux originates as a distinct emission component, separate from the main afterglow emission (which dominates the centimeter fluxes). The simplest hypothesis is that the the sub-millimeter flux arises from the host galaxy. Indeed, the sub-millimeter flux of the host (assumed to be at redshift of 1.467; GCN 965, GCN 989) is typical of star-forming galaxies selected in SCUBA surveys. The star formation rate estimated in the usual manner (Carilli & Yun, ApJ, L13, 1999) is 500 Msun/yr -- typical of the sample of dusty, high redshift starburst galaxies (Smail et al. 2000; astro-ph/000823). This inference is entirely consistent with models in which GRBs are related to massive stars. There is considerable diversity in the morphology and star-formation rates of star-forming galaxies and this also appears to be the case for GRB selected host galaxies. If the relation between GRBs and starformation holds strictly then GRBs, thanks to the immense dust penetrating power of gamma-rays, could be used to select a sample of star-formation galaxies with no bias towards dust. The additional advantage of this sample is that the afterglow offers the opportunity to measure the redshift of the host galaxy (as is the case here). There are other possible explanations for the sub-millimetric excess such as reprocessing of the burst (flash) by the ambient dust or a very strong reverse shock. We are investigating these possibilities but they appear to be less likely or contrived. In the host hypothesis, we expect the sub-millimeter emission to be constant and we also expect a contribution (about 100 microJy) at 1.4 GHz (from synchrotron emission). Continued observations at radio wavelengths are urgently required both to test the constancy of the sub-millimeter flux and also to monitor the afterglow emission.