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GCN Circular 2485

GRB 021203: A Normal GRB viewed far off axis?
2003-12-11T14:13:33Z (20 years ago)
Arnon Dar at Technion-Israel Inst. of Tech <>
In the "standard" fireball model, ordinary GRBs, XRFs and
low-inferred-luminosity GRBs, such as GRB 980425 are intrinsically
different. The low-luminosity GRB 031203 (GCN 2460, 2482) has thus
been interpreted as a case that "may bridge GRB 980425 ... and
cosmological GRBs" (GCN 2483).

In the cannonball model, XRFs and GRBs such as 980425 and 031203 
(GCN 2460) are ordinary GRBs, also viewed off-axis but from angles 
a few times larger than those of ordinary GRBs (astro-ph/0308248). 
GRB 031203 offers sensitive observational tests of this CB-model
unification of XRFs and ALL long-duration GRBs:

In the CB-model --and in reality-- both for long GRBs and XRFs, the
product of the peak energy Ep (E at max E^2 dn/dE) and the full
width at half maximum (FWHM) of the photon flux in a single pulse is
approximately 200 keV s, independent of redshift.

From the reported 30 s total duration of the single-pulse GRB 031203
(GCN 2460) and from the fairly universal "FRED" temporal shape of
GRB pulses, we estimate the FWHM to be ~ 10 s. Consequently, 
we predict Ep ~ 20  keV. Moreover, from the mean FWHM = 0.92 s of
long-duration GRB pulses, from their mean redshift z ~ 1, and from
the estimated z ~ 0.105 (GCN 2482) of GRB 031203, we conclude that
the value of the Doppler factor of its single pulse was approximately
20 times smaller than the mean value for long GRBs. In the CB model
this implies (astro-ph/0308248, astro-ph/0309294) a viewing angle 
approximately 4 times larger than the average for the bulk of the GRB
sample. The predicted equivalent isotropic energy of GRB 031203 
should then be ~ 2.5E49 erg. For a Hubble constant of 70 km/[s Mpc],
and a FWHM of ~ 10 s, this energy is indeed consistent with that
estimated from the reported peak energy flux of 1.3E-7 erg/[cm^2 s]
in the 20-200 keV interval (GCN 2460).

Due to a large Galactic extinction (E(B-V)=0.971; GCN 2463) in the
direction of GRB 031203, a standard-candle supernova akin to
SN1998bw placed at the GRB position will reach (around December 20)
a peak brightness R ~ 22  in the R band, if the extinction in the host
galaxy is negligible.

The radio afterglow may first decline for a couple of days (GCN 2483) and
then rebrighten, before declining again like t^{-1.33} at later times;
further steepening to the universal ~ t^{-2.1} as each specific radio
frequency crosses the CB-model's "injection bend" (astro-ph/0204474).

- Arnon Dar, Shlomo Dado and Alvaro De Rujula
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