Long-term millimeter VLBI monitoring of M87 with KVN at milliarcsecond resolution: nuclear spectrum

Abstract

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M87 at $\lesssim 0.8\mathrm{mas}\sim 110R_s$ spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index $\alpha$ of $\gtrsim -0.37$ ($S\propto {\nu }^{+\alpha }$) between 22GHz and 129GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15GHz and taking the Event Horizon Telescope (EHT) 230GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15GHz and 129GHz, which could naturally connect the 230GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency $\gtrsim 100$GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at $>$100GHz) and the relatively weakly magnetized (more extended, optically thin at $<$100GHz) plasma components are colocated in the footprint of the relativistic jet.

arXiv comment5 pages, 5 figures, accepted for A&A letters

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Cited by in corpus (19)

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• Investigating the connection between gamma-ray activity and relativistic jet in 3C273 during 2015-2019
• The dynamics of the parsec-scale jet in the neutrino blazar PKS 0735+178
• Impact of non-thermal particles on the spectral and structural properties of M87
• Spatially resolved spectral properties of M87* on event horizon scales