B. Light Curves for the "Standard" Adiabatic Synchrotron Model
In Section VC2 I discussed the
instantaneous synchrotron spectrum. The light curve that corresponds to
this spectrum depends simply on the variation of the
F, max and
the break frequencies as a function of the observer time
[236,
376].
This in turn depends on the variation
of the physical quantities along the shock front. For simplicity I
approximate here the BM solution as a spherical homogeneous shell
in which the physical conditions are determined by the shock jump
between the shell and the surrounding matter. Like in
Section VC2 the calculation is divided to
two cases: fast cooling and slow cooling.
Sari et al. [376] estimate the observed emission as a series of power law segments in time and in frequency 6:
![]() |
(85) |
that are separated by break frequencies, across which the
exponents of these power laws change: the cooling frequency,
c, the typical
synchrotron frequency
m and the self
absorption frequency
sa. To estimate the
rates one plugs the expressions for
and R as a
function of the observer time (Eq. 78), using for a homogenous external
matter k = 0:
![]() |
(86) |
to the expressions of the cooling frequency,
c, the
typical synchrotron frequency
m and the self
absorption frequency
sa (Eqs. 26) and to
the expression of the maximal flux (Eq. 29 for slow cooling and Eq.
26 for fast cooling). Note that the numerical
factors in the above expressions arise from an exact integration
over the BM profile. This procedure results in:
![]() |
(87) |
A nice feature of this light curve is that the peak flux is constant and does not vary with time [259] as it moves to lower and lower frequencies.
At sufficiently early times
c <
m, i.e. fast cooling,
while at late times
c >
m, i.e., slow
cooling. The transition between the two occurs when
c =
m. This
corresponds (for adiabatic evolution) to:
![]() |
(88) |
Additionally one can translate Eqs. 87 to the time in
which a given break frequency passes a given band. Consider a
fixed frequency =
15 1015
Hz. There are two critical
times, tc and tm, when the break
frequencies,
c and
m, cross the
observed frequency
:
![]() |
(89) |
In the Rayleigh-Jeans part of the black body radiation
I =
kT(2
2 /
c2) so that
F
kT
2. Therefore, in
the part of the synchrotron
spectrum that is optically thick to synchrotron self absorption, we have
F
kTeff
2. For slow cooling
kTeff ~
m
me c2 = const. throughout the
whole shell of shocked fluid behind the shock, and therefore
F
2 below
sa where the
optical depth to synchrotron self absorption equals one,
as = 1. For fast
cooling,
as we go down in frequency, the optical depth to synchrotron self
absorption first equals unity due to absorption over the whole
shell of shocked fluid behind the shock, most of which is at the
back of the shell and has kTeff ~
c.
The observer is located in front of the shock, and the radiation that
escapes and reaches the observer is from
~ 1. As
decreases
below
sa the
location where
~ 1 moves from
the back of the shell toward the front of the shell, where the
electrons suffered less cooling so that
kTeff(
= 1)
-5/8. Consequently
F
11/8. At a
certain frequency
~ 1 at the location behind
the shock where electrons with
m start to
cool significantly. Below this frequency,
(
ac), even though
~ 1 closer and closer to
the shock with decreasing
, the effective temperature at
that location is constant: kTeff ~
m
me c2 = const., and therefore
F
2 for
<
ac, while
F
11/8 for
ac <
<
sa.
Overall the expression for the self absorption frequency depends
on the cooling regime. It divides to two cases, denoted
sa
and
ac, for fast
cooling and both expression are different from the slow cooling
[143].
For fast cooling:
![]() |
(90) |
![]() |
(91) |
For slow cooling:
![]() |
(92) |
For a given frequency either
t0 > tm > tc
(which is typical for high frequencies) or
t0 < tm < tc
(which is typical for low
frequencies). The results are summarized in two tables
I and II describing
and
for fast
and slow cooling. The different light curves
are depicted in Fig. 23.
![]() |
![]() |
|
![]() ![]() |
1 | 2 |
![]() ![]() ![]() |
1/6 | 1/3 |
![]() ![]() ![]() |
-1/4 | -1/2 |
![]() ![]() |
-(3p-2)/4 | - p/2 =
(2![]() |
![]() |
![]() |
|
![]() ![]() |
1/2 | 2 |
![]() ![]() ![]() |
1/2 | 1/3 |
![]() ![]() ![]() |
-3(p-1)/4 | - (p - 1) / 2 =
2![]() |
![]() ![]() |
-(3p-2)/4 | - p / 2 =
(2![]() |
![]() |
Figure 23. The different
possible broad band synchrotron spectra from a
relativistic blast wave, that accelerates the electrons to a power
law distribution of energies. The thin solid line shows the
asymptotic power law segments, and their points of
intersection, where the break frequencies,
|
These results are valid only for p > 2 (and for
max,
the maximal electron energy, much higher than
min).
If p < 2 then
max
plays a critical role. The resulting
temporal and spectral indices for slow cooling with 1 < p <
2 are
given by Dai and Cheng
[64]
and by Bhattacharya
[28]
and summarized in table III below. For
completeness I include in this table also the cases of
propagation into a wind (see Section VIIE)
and a jet break (see Section VIIH).
ISM | wind | Jet | |
![]() ![]() |
(17p-26)/16(p-1) | (13p-18)/18(p-1) | 3(p-2)/4(p-1) |
![]() ![]() ![]() |
(p+1)/8(p-1) | 5(2-p)/12(p-1) | (8-5p)/6(p-1) |
![]() ![]() ![]() |
-3(p+2)/16 | -(p+8)/8 | -(p+6)/4 |
![]() ![]() |
-(3p+10)/16 | -(p+6)/8 | -(p+6)/4 |
The simple solution, that is based on a homogeneous shell approximation, can be modified by using the full BM solution and integrating over the entire volume of shocked fluid [140]. Following [271] I discuss Section VIIG1 a simple way to perform this integration. The detailed integration yields a smoother spectrum and light curve near the break frequencies, but the asymptotic slopes, away from the break frequencies and the transition times, remain the same as in the simpler theory. Granot and Sari [144] describe a detailed numerical analysis of the smooth afterglow spectrum including a smooth approximation for the spectrum over the transition regions (see also [148]). They also describe additional cases of ordering of the typical frequencies which were not considered earlier.
A final note on this "standard" model is that it assumes adiabaticity. However, in reality a fraction of the energy is lost and this influences over a long run the hydrodynamic behavior. This could be easily corrected by an integration of the energy losses and an addition a variable energy to Eq. 74, followed by the rest of the procedure described above [290].
6 The following notation appeared in the
astro-ph version of
[376].
Later during the proofs that author realized that
is used often in
astrophysics to denote a spectral index and in the Ap. J. version of
[376]
the notations have been changed to
F
t-
-
. However, in
the meantime the astro-ph notation became generally accepted. I
use these notations here.
Back.