A Major Radio Outburst in III Z[CLC]w[/CLC] 2 with an Extremely Inverted, Millimeter-peaked Spectrum

Abstract

III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio structure typical of a Seyfert galaxy, but also with an extremely variable, blazar-like radio core. We have now discovered a new radio flare in which the source has brightened more than 20-fold within less than 2 yr. A broadband radio spectrum between 1.4 and 666 GHz shows a textbook-like synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron spectral index ϩ2.5 at frequencies below 43 GHz and an optically thin spectral index Ϫ0.75 at frequencies above 43 GHz. The outburst spectrum can be well fitted by two homogenous, spherical components with equipartition sizes of 0.1 and 0.2 pc at 43 and 15 GHz and with magnetic fields of 0.4 and 1 G. VLBA observations at 43 GHz confirm this double structure and these sizes. Timescale arguments suggest that the emitting regions are shocks which are continuously accelerating particles. This could be explained by a frustrated jet scenario with very compact hot spots. Similar millimeter-peaked spectrum sources could have escaped our attention because of their low flux density at typical survey frequencies and their strong variability.

The extreme flare in III Zw 2:

“…Mpc −1 , q 0 = 0.5 as used in this paper). This detection was confirmed later (Kukula et al 1998;Falcke et al 1999), but no additional extended radio emission was found.…”

“…Earlier VLBI observations of the source have only shown an unresolved high-brightness temperature core (Falcke et al 1996b;Kellermann et al 1998), and Millimeter-VLBI observations by Falcke et al (1999) just barely resolved the source into two very compact components. A broadband radio spectrum showed a highly peaked spectrum that was well explained by a very compact source and synchrotron self-absorption.…”

“…An almost simultaneous broadband radio spectrum from 1.4 to 660 GHz during the increase in flux density in May 1998 was presented in Falcke et al (1999). The spectrum was highly inverted at centimeter wavelengths (α = +1.9±0.1) with a turnover frequency around 43 GHz.…”

“…In the last two epochs only 4 frequencies (1.4-15 GHz) were used because of a new outburst at high frequencies. The cross is the turnover frequency of the spectrum in May 1998, where the 1-43 GHz data is taken from Falcke et al (1999). The asterisks mark the epochs of our VLBA observations.…”

III Zw 2: Evolution of a Radio Galaxy in a Nutshell

“…A relativistic shock was proposed earlier by Falcke et al (1999) due to synchrotron cooling times of 14-50 days which are much shorter than the duration of the outburst. The ultracompact hotspots are pumped and powered by the jet and are responsible for the flux-density increase.…”