Falcke et al. 1999: Millimeter Outburst in IIIZw2
A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum
Geoffrey C. Bower1,2,
Andrew R. Lobanov1,
Thomas P. Krichbaum1,
Alok R. Patnaik1,
Margo F. Aller3,
Hugh D. Aller3,
Melvyn C.H. Wright5,
1Max-Planck-Institut für Radioastronomie, Auf den Hügel 69, D-53121 Bonn, Germany
2Current address: NRAO, P.O. Box O, Socorro, NM 87801-0387
3Astronomy Department, University of Michigan, Ann Arbor, MI 48109-1090
4Metsähovi Radio Research Station, Metsahovintie, SF-02540 Kylm\"al\"a, Finland
5Radio Astronomy Laboratory, University of California, Berkeley, CA 94720
6National Radio Astronomy Observatory, P.O. Box 2, Green Bank, WV 24944, U.S.A.
Astrophysical Journal Letters, Vol. 514, p. L17 (1999)
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 where the source has brightened more than twenty-fold
within less than two years. A broad-band 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 +2.5 at
frequencies below 43 GHz and an optically thin spectral index -0.75
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
Gauss. VLBA observations at 43 GHz confirm this double structure and
these sizes. Time scale 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 hotspots.
Similar millimeter-peaked spectrum (MPS) sources could have escaped
our attention because of their low flux density at typical survey
frequencies and their stronger variability.
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