The central parsecs of M87: jet emission and an elusive accretion disc

TL;DR

This study presents the first simultaneous multi-wavelength spectral energy distribution of M87's core, revealing jet dominance and challenging existing accretion and jet power models, suggesting variability or black hole spin as possible energy sources.

Contribution

It provides the first simultaneous SED of M87's core at 0.4 arcsec resolution, analyzing jet emission and constraining accretion rates and jet power, with implications for black hole spin energy extraction.

Findings

Jet emission explains the core's SED across ten orders of magnitude.

Inferred jet power is lower than previously reported in literature.

Accretion rate is too low to account for jet kinetic power, implying variability or spin energy.

Abstract

We present the first simultaneous spectral energy distribution (SED) of M87 core at a scale of 0.4 arcsec ($\sim 32\, \rm{pc}$) across the electromagnetic spectrum. Two separate, quiescent, and active states are sampled that are characterized by a similar featureless SED of power-law form, and that are thus remarkably different from that of a canonical active galactic nuclei (AGN) or a radiatively inefficient accretion source. We show that the emission from a jet gives an excellent representation of the core of M87 core covering ten orders of magnitude in frequency for both the active and the quiescent phases. The inferred total jet power is, however, one to two orders of magnitude lower than the jet mechanical power reported in the literature. The maximum luminosity of a thin accretion disc allowed by the data yields an accretion rate of $< 6 \times 10^{-5}\, \rm{M_\odot \, yr^{-1}}$, assuming 10% efficiency. This power suffices to explain M87 radiative luminosity at the jet-frame, it is however two to three order of magnitude below that required to account for the jet's kinetic power. The simplest explanation is variability, which requires the core power of M87 to have been two to three orders of magnitude higher in the last 200 yr. Alternatively, an extra source of power may derive from black hole spin. Based on the strict upper limit on the accretion rate, such spin power extraction requires an efficiency an order of magnitude higher than predicted from magnetohydrodynamic simulations, currently in the few hundred per cent range.