Ja. Stevens et al., THE EVOLUTION OF THE CENTIMETER-SUBMILLIMETER SPECTRUM OF 3C-345 DURING OUTBURST, The Astrophysical journal, 466(1), 1996, pp. 158-168
Monitoring data between 375 and 4.8 GHz are presented for the blazar 3
C 345 between 1989 and 1995, during which period the source hared. The
multifrequency Light curves are used to construct snapshot spectra th
at span the synchrotron seif-absorption turnover frequency. After subt
raction of the underlying quiescent level, the flare spectrum is isola
ted and followed during the outburst. The evolution of the turnover fr
equency (nu(m)) with turnover flux (S-m) is examined and compared with
the predictions of the Marscher & Gear shocked-jet model. We find tha
t the flare spectrum is well fitted by a homogeneous synchrotron curve
for at least 2 yr after the initial rise in flux. The high-frequency
optically thin section of this spectrum is observed to steepen with ti
me. The extent of this steepening suggests that little reacceleration
is occurring in the shock. We find that a power law is a remarkably go
od approximation to the observed trend of S-m with nu(m). The turn ove
r moves smoothly toward lower frequency with time, as expected from an
emitting region that is expanding. Furthermore, this movement is obse
rved to slow down with time, as predicted by the model. Initially, the
turnover flux rises and then decays with a power-law index of similar
to 1.0. This decay is interrupted by a second rise, which also decays
with an index of similar to 1.0. It is found that the initial rise co
rresponds to the expansion phase, with the jet bending toward the line
of sight rather than to the Compton or synchrotron phases of the mode
l. Furthermore, the decays can only be reconciled with the model if th
e jet is assumed to be nonadiabatic or if it curves away from the line
of sight during this period. The implied magnetic held orientation is
parallel to the shock front, as expected from compression of the unde
rlying field. This finding is supported by the total polarization data
that suggest that the outbursts depolarize the total emission, which,
during quiescent periods, is significantly polarized in a direction a
pproximately perpendicular (B-parallel to) to the jet axis.