Abstract
Abstract The broad high-energy spectral component in blazars is usually attributed to various inverse Compton scattering processes in the relativistic jet, but has not been clearly identified in most cases due to degeneracies in physical models. AP Librae, a low-synchrotron-peaking BL Lac object (LBL) detected in 2015 by H.E.S.S. at very high energies (VHE; >0.5 TeV), has an extremely broad high-energy spectrum, covering ∼9 decades in energy. Standard synchrotron self-Compton models generally fail to reproduce the VHE emission, which has led to the suggestion that it might arise not from the blazar core, but on kiloparsec scales from inverse Compton (IC) scattering of cosmic microwave background (CMB) photons by a still-relativistic jet (IC/CMB). IC/CMB models for the TeV emission of AP Librae in prior works have implied a high level of infrared emission from the kiloparsec-scale jet. With newly obtained Hubble Space Telescope (HST) imaging, we obtain a deep upper limit on the kiloparsec-scale jet emission at 1.6 μm, well below the expected level. High-resolution Atacama Large Millimeter/submillimeter Array imaging in bands 3–9 reveals a residual dust-disk signature after core subtraction, with a clearly thermal spectrum, and an extent (∼500 pc) that matches with a nonjet residual emission seen after point-spread function subtraction in our 1.6 μm HST imaging. We find that the unusually broad GeV and VHE emission in AP Librae can be reproduced through the combined IC scattering of photons from the CMB and the dust disk, respectively, by electrons in both the blazar core and subkiloparsec jet.
Highlights
Radio-loud active galactic nuclei (AGN) are characterized by relativistic jets of fully ionized plasma moving very near the speed of light (e.g., Urry & Padovani 1995; Blandford et al 2019)
Standard synchrotron self-Compton models generally fail to reproduce the very high energies (VHE) emission, which has led to the suggestion that it might arise not from the blazar core, but on kiloparsec scales from inverse Compton scattering of cosmic microwave background (CMB) photons by a still-relativistic jet (IC/CMB)
We find that the unusually broad GeV and VHE emission in AP Librae can be reproduced through the combined IC scattering of photons from the CMB and the dust disk, respectively, by electrons in both the blazar core and sub-kpc jet
Summary
Radio-loud active galactic nuclei (AGN) are characterized by relativistic jets of fully ionized plasma moving very near the speed of light (e.g., Urry & Padovani 1995; Blandford et al 2019). Even before the VHE detection of the source by H.E.S.S., Tavecchio et al (2010) noted this difficulty when attempting to fit a one-zone SSC model to Fermi /LAT observations Their model could not fit both the X-ray and the GeV bands simultaneously, underestimating the X-rays by ∼ an order of magnitude to fit the GeV or vice versa. Several efforts were made to move beyond a simple SSC model for the VHE emission Both Zacharias & Wagner (2016) and Sanchez et al (2015) suggested that inverse Compton scattering off cosmic microwave background photons in the extended jet (the IC/CMB mechanism, first proposed by Tavecchio et al 2000, for another source) could explain the high-energy Fermi/LAT and H.E.S.S. emission, the synchrotron spextrum of the large-scale jet was severely unconstrained in these works.
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