Spectral analysis of a parsec-scale jet in M87: Observational constraint on the magnetic field strengths in the jet

Abstract

Because of its proximity and the large size of its black hole, M87 is one of the best targets for studying the launching mechanism of active galactic nucleus jets. Currently, magnetic fields are considered to be an essential factor in the launching and accelerating of the jet. However, current observational estimates of the magnetic field strength of the M87 jet are limited to the innermost part of the jet or to HST-1. No attempt has yet been made to measure the magnetic field strength in between. We aim to infer the magnetic field strength of the M87 jet out to a distance of several thousand $r_s$ by tracking the distance-dependent changes in the synchrotron spectrum of the jet from high-resolution very long baseline interferometry observations. In order to obtain high-quality spectral index maps, quasi-simultaneous observations at 22 and 43 GHz were conducted using the KVN and VERA Array (KaVA) and the VLBA. We compared the spectral index distributions obtained from the observations with a model and placed limits on the magnetic field strengths as a function of distance. The overall spectral morphology is broadly consistent over the course of these observations. The observed synchrotron spectrum rapidly steepens from ${\alpha }_{22-43GHz}$ ~ -0.7 at ~ 2 mas to ${\alpha }_{22-43GHz}$ ~ -2.5 at ~ 6 mas. A spectral index model in which nonthermal electron injections inside the jet decrease with distance can adequately reproduce the observed trend. This suggests the magnetic field strength of the jet at a distance of 2 - 10 mas (~ 900 $r_s$ - ~ 4500 $r_s$ in the deprojected distance) has a range of $B=(0.3-1.0G)(z/2mas{)}^{-0.73}$. Extrapolating to the EHT scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ~ 4500 $r_s$ without significant dissipation.

16 pages, 9 figures. Accepted for publication in Astronomy & Astrophysics

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Cited by in corpus (12)

• Properties of the jet in M87 revealed by its helical structure imaged with the VLBA at 8 and 15 GHz
• Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode
• M87: a cosmic laboratory for deciphering black hole accretion and jet formation
• On the possible core shift break in relativistic jets
• Hillas meets Eddington: The case for blazars as ultra-high-energy neutrino sources
• The Subparsec-scale Structure and Evolution of Centaurus A. III. A Multi-Epoch Spectral And Polarimetric VLBA Study
• VLBI Detection of an Active Radio Source Potentially Driving 100-kpc Scale Emission in the Ultraluminous Infrared Galaxy IRAS F01004$-$2237
• Constraints on High-Frequency Gravitational Waves from Graviton-Photon Conversion in the M87 Galaxy
• Transverse Oscillations and Wave Propagation in the Magnetically Dominated M87 Jet
• Millimeter VLBI constraints on the central magnetic field and symmetric jet production in the twin-jet galaxy NGC 1052
• Spatially resolved spectral properties of M87* on event horizon scales
• The twin-jet system in the FRII radio galaxy 3C 452: A sub-parsec scale VLBI study