Probing the Central Black Hole in M87 With Gamma-Rays

Levinson

2018

Galaxies

Radio Galaxies have by now emerged as a new γ-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe the nature and physics of active galactic nuclei (AGN) in general. This review provides a compact summary of their observed characteristics at very high γ-ray energies (VHE; greater than 100 GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics.

Section: M87mentioning

confidence: 53%

Radio Galaxies at VHE Energies

Levinson

2018

Galaxies

“…The inferred jet seeds and inclinations are on average consistent with rather modest Doppler boosting D < ∼ a few (for review, see e.g. [64]). At VHE energies, M87 is known for its particularly interesting characteristics, including rapid day-scale variability (on flux doubling time scales ∆t obs ∼ 1 d) detected during active source states, and a relatively hard spectrum compatible with a power law of photons index Γ VHE 2.2 and extending from ∼ 300 GeV to beyond 10 TeV, e.g.…”

Section: M87mentioning

confidence: 68%

“…Similar to the case of Cen A, numerous models have been proposed to account for this, and the interested reader is directed to Refs. [64,70] for an overview and discussion of them. The observed rapid VHE variations in M87 are (still) the fastest compared to those seen at any other waveband.…”

Section: M87mentioning

confidence: 99%

Gamma-rays from non-blazar AGN

AIP Conference Proceedings

2017

Self Cite

Abstract. Non-blazar Active Galactic Nuclei (AGN) have emerged as a new γ-ray emitting source class on the extragalactic sky and started to deepen our understanding of the physical processes and the nature of AGN in general. The detection of Narrow Line Seyfert 1 galaxies in the Fermi-LAT energy regime, for example, offers important information for our understanding of jet formation and radio-loudness. Radio galaxies, on the other hand, have become particularly interesting at high (HE) and very high (VHE) gamma-ray energies. With their jets not directly pointing towards us (i.e. "misaligned"), they offer a unique tool to probe into the nature of the fundamental (and often "hidden") physical processes in AGN. This review highlights and discusses some of the observational and theoretical progress achieved in the gamma-ray regime during recent years, including the evidence for unexpected spectral hardening in Centaurus A and extreme short-term variability as seen in IC 310 and M87.

“…• , resulting in modest Doppler boosting of its jet emission and creating challenges for conventional jet models to account for the observed VHE characteristics (see e.g., Rieger & Aharonian 2012, for review and references). Gap-type emission models offer a promising alternative and different realisations have been proposed in the literature (e.g., Neronov & Aharonian 2007;Levinson & Rieger 2011;Vincent 2015;Broderick & Tchekhovskoy 2015;Ptitsyna & Neronov 2016).…”

Section: M87mentioning

confidence: 99%

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Katsoulakos

2018

ApJ

Self Cite

The rapidly variable, very high-energy (VHE) gamma-ray emission from Active Galactic Nuclei (AGN) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power L gap for different magnetospheric set-ups, and study its dependence on the accretion state of the source. L gap is found to be in general proportional to the Blandford-Znajek jet power L BZ and a sensitive function of gap sizeβ , where the power index β ≥ 1 is dependent on the respective gapsetup. The transparency of the black hole vicinity to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on L BZ . Similarly, rapid variability, if observed, may allow to constrain the gap size h ∼ c∆t. Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC310.