Spectral analysis of M87 jet

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 ~2mas to alpha_22-43GHz_~-2.5 at ~6mas.

In the KaVA observations, the spectral index remains unchanged until ~10mas, but this trend is unclear in the VLBA observations. 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 2-10mas (~900r_s_~4500r_s_ in the deprojected distance) has a range of B=(0.3-1.0G)(z/2mas)^-0.73^.

Extrapolating to the Event Horizon Telescope scale yields consistent results, suggesting that the majority of the magnetic flux of the jet near the black hole is preserved out to ~4500r_s_ without significant dissipation.