Issue |
A&A
Volume 424, Number 2, September III 2004
|
|
---|---|---|
Page(s) | 477 - 484 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20041115 | |
Published online | 23 August 2004 |
The very early afterglow powered by ultra-relativistic mildly magnetized outflows
1
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008, PR China e-mail: yzfan@pmo.ac.cn
2
National Astronomical Observatories, Chinese Academy of Sciences, Beijing, 100012, PR China
Received:
19
April
2004
Accepted:
18
May
2004
In the Poynting Flux-dominated outflow (the initial
ratio of the electromagnetic energy flux to the particle energy
flux ) model for gamma-ray bursts, particularly the
γ-ray emission phase, nearly half of the internally
dissipated magnetic energy is converted into the γ-ray
energy emission and the rest is converted into the kinetic energy
of the outflow. Consequently, at the end of the γ-ray
burst, σ decreases significantly (
or even
smaller). We numerically investigate the very early reverse shock
emission powered by such mildly magnetized outflows interacting
with medium–uniform interstellar medium (ISM) or stellar wind (WIND). We show that for
and typical parameters
of gamma-ray bursts, both the ISM-ejecta interaction and the
WIND-ejecta interaction can power very strong optical emission
(
mag or even brighter).
Similar to the very early afterglow powered by the non-magnetized
ejecta interacting with the external medium, the main difference
between the ISM-ejecta interaction case and the WIND-ejecta
interaction case is that, before the reverse shock crosses the
ejecta, the R-band emission flux increases rapidly for the former,
but for the latter it increases only slightly.
At the very early stage, the ejecta are ultra-relativistic. Due to
the beaming effect, the random magnetic field generated in shocks
contained in the viewing area is axisymmetric, unless the line of
sight is very near the edge of ejecta. The formula
(where b is the ratio of the
ordered magnetic field strength to that of random one) has been
proposed to describe the net linear polarization of the
synchrotron radiation coming from the viewing area. For
, the ordered magnetic field dominates over the
random one generated in the reverse shock (As usual, we assume
that a fraction
of the thermal energy
of the reverse shock has been converted into the magnetic energy),
the high linear polarization is expected. We suggest that the
linear polarization detection of the early multi-wavelength
afterglow is required to see whether the outflows powering GRBs
are magnetized or not.
Key words: gamma-rays: bursts / magnetic fields / magnetohydrodynamics (MHD) / shock waves / relativity
© ESO, 2004
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