Dynamics and High Energy Emission of the Flaring HST-1 Knot in the M 87 Jet
Abstract
Stimulated by recent observations of a giant radio-to-X-ray synchrotron flare from the HST-1 knot in the M 87 jet, as well as by a detection of a very high energy gamma-ray emission from M 87, we investigated the dynamics and multiwavelength emission of the HST-1 region. We study thermal pressure of the hot interstellar medium in M 87 and argue for a presence of a gaseous condensation in its central parts, linked to the observed central stellar cusp of the elliptical host. Interaction of the jet with its ambient medium characterized by the enhanced in this way thermal pressure is likely to result in formation of a converging reconfinement shock structure in the innermost parts of the M 87 jet. We show that for a realistic set of the outflow parameters, a stationary and flaring part of the HST-1 knot located ~100 pc away from the active center can be associated with the decelerated portion of the jet matter placed downstream of the point where the reconfinement shock reaches the jet axis. We discuss a possible scenario explaining a broad-band brightening of the HST-1 region. We show that assuming a previous epoch of the high central black hole activity resulting in ejection of excess particles and photons down along the jet, one may first expect a high-energy flare of HST-1 due to inverse-Comptonisation of the nuclear radiation, followed after a few years by an increase in its synchrotron continuum. If this is the case, then the recently observed order-of-magnitude increase in the knot luminosity in all spectral bands could be regarded as an unusual echo of the outburst that had happened previously in the highly relativistic active core of the M 87 radio galaxy. We show that very high energy gamma-ray fluxes expected in a framework of the proposed scenario are consistent with the observed ones.Keywords
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