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The TeV emission of 3C273: inverse Compton radiation from shear-accelerated high-energy electrons in the large-scale jet?

F. Tavecchio

TL;DR

The paper addresses the origin of the VERITAS-detected TeV emission from the quasar 3C273, challenging explanations tied to the inner pc-scale jet and proposing that inverse Compton emission from ultra-high-energy electrons accelerated by shear in the kpc-scale jet can account for the VHE component. It employs a two-population electron model resolved via a leaky-box Fokker-Planck treatment of shear acceleration, predicting $\gamma$ up to $\sim 10^8$ and TeV radiation primarily from IC scattering of the CMB and local synchrotron photons, with the knots A and B providing the integrated emission. The study reproduces the knots' SEDs and derives a jet power of $P_J \approx 3 \times 10^{46}$ erg s$^{-1}$, while linking the observed $t_{dec} \sim 3$ years to compact emission regions formed downstream of recollimation shocks; the results have implications for large-scale jet dynamics and future CTA testing of variability and spatial scales. This work highlights shear acceleration as a viable mechanism for high-energy emission in large-scale quasar jets and informs interpretation of VHE observations in similar systems.

Abstract

The VERITAS Collaboration recently reported the detection of very-high-energy (VHE) gamma-ray emission from the prototypical radio quasar 3C273. The temporal and the spectral properties of this component do not appear compatible with the extrapolation of the beamed blazar-like emission of the inner, pc-scale jet. We explore the possibility that the VHE component is produced in the jet at kpc scale through the inverse Compton emission of a population of ultra-high energy electrons (with Lorentz factor $γ\sim 10^8$). In the model these electrons are accelerated through the shear acceleration mechanism, and they also account for the still puzzling X-ray emission of knots detected by {\it Chandra} in the large-scale jets of several powerful quasars (including 3C273). In our scenario the VHE component can be interpreted as the integrated emission from the two brightest knots of the 3C273 jet. We speculate that the decay of the emission on the timescale of $\sim 3$ years could be accounted for by the scenario if the VHE radiation is produced in some compact regions in the downstream flow of a recollimation shock.

The TeV emission of 3C273: inverse Compton radiation from shear-accelerated high-energy electrons in the large-scale jet?

TL;DR

The paper addresses the origin of the VERITAS-detected TeV emission from the quasar 3C273, challenging explanations tied to the inner pc-scale jet and proposing that inverse Compton emission from ultra-high-energy electrons accelerated by shear in the kpc-scale jet can account for the VHE component. It employs a two-population electron model resolved via a leaky-box Fokker-Planck treatment of shear acceleration, predicting up to and TeV radiation primarily from IC scattering of the CMB and local synchrotron photons, with the knots A and B providing the integrated emission. The study reproduces the knots' SEDs and derives a jet power of erg s, while linking the observed years to compact emission regions formed downstream of recollimation shocks; the results have implications for large-scale jet dynamics and future CTA testing of variability and spatial scales. This work highlights shear acceleration as a viable mechanism for high-energy emission in large-scale quasar jets and informs interpretation of VHE observations in similar systems.

Abstract

The VERITAS Collaboration recently reported the detection of very-high-energy (VHE) gamma-ray emission from the prototypical radio quasar 3C273. The temporal and the spectral properties of this component do not appear compatible with the extrapolation of the beamed blazar-like emission of the inner, pc-scale jet. We explore the possibility that the VHE component is produced in the jet at kpc scale through the inverse Compton emission of a population of ultra-high energy electrons (with Lorentz factor ). In the model these electrons are accelerated through the shear acceleration mechanism, and they also account for the still puzzling X-ray emission of knots detected by {\it Chandra} in the large-scale jets of several powerful quasars (including 3C273). In our scenario the VHE component can be interpreted as the integrated emission from the two brightest knots of the 3C273 jet. We speculate that the decay of the emission on the timescale of years could be accounted for by the scenario if the VHE radiation is produced in some compact regions in the downstream flow of a recollimation shock.

Paper Structure

This paper contains 4 sections, 5 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. The model
  3. Results
  4. Discussion

Figures (3)

  • Figure 1: SED (circles) of knot A (upper panel) and B (lower panel) of the jet of 3C273 (from Sambruna01: we also report the X-ray spectral slopes derived in He23). Light blue datapoints report the (EBL-deabsorbed) VERITAS spectrum. The lines show the result of the model for the electrons accelerated at the shock (black) and at the shear (blue). Solid: synchrotron; dotted IC from synchrotron; dashed: IC/CMB. The red and the green lines report the total emission.
  • Figure 2: Historical SED data for the 3C273 core (gray) with the VERITAS spectrum (light blue) and the SED of knot A and B. Red, green and black lines show the emission from knot A, B and the total, respectively.
  • Figure 3: Timescales (in the source frame) relevant for the shear acceleration process as a function of the particle Lorentz factor, for knot A. Solid lines show the acceleration (red) and escape (light blue) timescales. We report (in blue) the curves showing separately the cooling time for synchrotron, IC on the synchrotron photons and IC/CMB (dashed, dotted and long-dashed lines, respectively). The vertical orange dashed line shows the Lorentz factor above which the mean free path exceeds the width of the shear layer, halting the acceleration process. The horizontal dashed green line indicates the light crossing time of the shear layer.