[vsnet-grb-info 6046] GRB 080503: Slow-rise optical AG ?
GCN Circulars
gcncirc at capella.gsfc.nasa.gov
Tue May 6 12:50:23 JST 2008
TITLE: GCN CIRCULAR
NUMBER: 7682
SUBJECT: GRB 080503: Slow-rise optical AG ?
DATE: 08/05/06 03:50:15 GMT
FROM: Arnon Dar at Technion-Israel Inst. of Tech <arnon at physics.technion.ac.il>
Many long GRBs have optical light-curves which are initially slowly
rising. The optical light curve of the short hard burst (SHB) 080503 (Mao
et al. GCN 7665; Ukwatta et al. GCN 7673; Guidorzi et al. GCN 7674)
discovered by Perley et al. (GCNs 7678, 7680) may be their analog
slow-rise optical afterglow (AG) changing to a plateau phase and entering
the gradual transition to an asymptotic decline, which, in the CB model,
has roughly the form,
Fnu ~ [1/(t+t_b}^{\beta_O+1/2}] * nu^{-beta_O}
for a constant low-density SHB environment, or
Fnu ~ [1/ (t+t_b)^{beta_O+1}] * nu^{-beta_O}
for an isothermal sphere environment with a density propto 1/r^2.
t_b is the bending time of its canonical AG, and beta_O = Gamma - 1 ~ 1.1 is
the spectral index above the bend frequency. In the CB model, the initial
increase of the CB's radius, R^2 ~ t^2/(t^2+t_exp^2), where t_exp is the time
beyond which the CB's radius settles to its approximate constant value,
produces the initial rise of the synchrotron AG. The low density environments
of SHBs yields typically, t_b~ 10^5 s, and a long expansion time, t_exp, of the
CB's radius relative to its values in slow-rise optical AGs of long GRB such as
071010A (Covino et el. arXiv:0804.4367).
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