High-frequency observations and spectrum of the jet in M 87

Abstract

This review collects the current fund of ground-based observations at v>10¹⁴ Hz which allows to determine the overall synchrotron spectrum of the jet in M 87 very accurately. Beyond knot D (at 3″ distance from the core), the spectrum is characterized by a straight power-law S _v ∼ v ^−0.66 which cuts off steeply at some frequency v _c The optically observed spectral variations along the jet are caused by changes in the cutoff frequency between v _c =4×10¹⁵ Hz and v _c =7 × 10¹⁴ Hz, while the power-law index between 10¹⁰ and 10¹⁴ Hz stays amazingly constant at α _PL =−0.66±0.02. We demonstrate that an ubiquitous particle distribution function N(E)∼E ^−2.32 which cuts off steeply at a maximum energy E _c =10⁶ m _e c ², together with local variations of the magnetic field strength, accounts well for both the observed variation of v _c and the apparent brightness distribution along the jet (i.e. the “knots”). The global particle spectrum might even be maintained into the inner lobes — albeit the eastern, “un-jetted” side seems to contain a steeper particle spectrum N(E) ∼ E ^−2.65. Thus, permanent particle re-acceleration seems required all along the jet and in the inner lobes.

Recent high-resolution observations with the HST (spatial scale ≲ 0″.1) seem to support this general behaviour of the kpc jet beyond knot D. However in the inner 300 pc from the core, rapidly evolving optical features with deviating synchrotron spectra are found. A careful analysis of their time-dependence (on time-scales comparable to the synchrotron loss-time τ _syn ⋍ 10 years) might eventually help to understand the nature of the particle acceleration process. The X-ray emission from the jet is most likely of synchrotron origin. It seems to arise from compact substructures where locally the maximum particle energy is boosted above its steady state value to E _c >5 × 10⁶ m _e c ².