Long-term millimeter VLBI monitoring of M 87 with KVN at milliarcsecond resolution: nuclear spectrum

Abstract

We study the centimeter-to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at 0.8 mas ∼ 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 α of −0.37 (S ∝ ν +α ) between 22 GHz and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 GHz and 129 GHz, which could naturally connect the 230 GHz 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 100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at >100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.

Relativistic Jets from AGN Viewed at Highest Angular Resolution

“…The M87 jet beyond the horizon scales has intensively been investigated over the last years with VLBI at 86 GHz and lower frequencies (Figure 1 top right). Recent high-sensitivity array (HSA) or GMVA 86 GHz images of M87 have resolved a wide opening angle jet base within ≤10-100 R s scales with a clear limb-brightening structure [43,44], which is consistent with magnetically-driven jet simulations/models with a spinning BH [45][46][47][48]. At similar scales VLBA polarimetric images at 43 GHz indicates the presence of hellically-wrapped B-fields [49], which is also suggestive of magnetic scenarios.…”

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

“…The total bandwidth of the observations at each frequency band is 64 MHz and the typical beam sizes of the observations are 6.1 × 3.1 mas at 22 GHz, 2.8 × 1.6 mas at 43 GHz, 1.5 × 0.8 mas at 86 GHz, and 1.1 × 0.5 mas at 129 GHz. Details of the scheduling, observations, data reduction including frequency phase transfer technique, analysis (i.e., imaging and model-fitting), and early results for M87 are shown in Lee et al (2016) and Kim et al (2018a). Despite the limited coverage of baselines and capability of the array to image the extended jet structure in M87, the flux density of the compact core can be rather reliably measured (Kim et al 2018a).…”

The Spectra of IceCube Neutrino (SIN) candidate sources -- II. Source Characterisation

“…2 shows 𝐿 𝛾 vs. 𝑃 1.4GHz for our sources (black circles). We also include the time-averaged very long baseline array (VLBA) core power at 15 GHz for M87 (Kim et al 2018), which, given its substantially flat radio spectrum between 15 − 129 GHz, should be representative of its 1.4 GHz core power as well. Two more sources get classified as masquerading BL Lacs thanks to their 𝑃 1.4GHz > 10 26 W Hz −1 , namely 5BZB J2227+0037 and CRATES J232625+011147.…”

“…Chang et al 2019). The radio power for M87, which is labelled, is derived from a time-averaged VLBA core flux at 15 GHz (Kim et al 2018). The average value for TXS 0506+056 (magenta point: Padovani et al 2019), which also belongs to our sample, is also shown.…”