GCN Circular 996
Subject
Sub-millimeter Excess/GRB 010222
Date
2001-02-27T13:28:18Z (24 years ago)
From
Shri Kulkarni at Caltech <srk@astro.caltech.edu>
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.