Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

Abstract

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.

The High Energy View of FR0 Radio Galaxies

“…This requires the accretion rate to be below a critical value, typically ∼0.01 (e.g., [108]) and in turn leads to a constraint on the average jet power. Assuming a rapidly spinning BH, the constraints on the gap size and accretion rate thus translate into a characteristic upper limit on the extractable VHE power of L V HE gap ∝ 2 × 10 46ṁ E M BH (h/r g ) [109] whereṁ E is the accretion rate scaled at the rate at the Eddington limit and r g is the gravitational radius. Assuming that the gap width in FR 0s is consistent with the gravitational radius as found from the VHE variability of M 87 (e.g., [110]) and assuming similar accretion rates to M 87 as supported by the results for CoreG [33] and for FR 0s [30], the expected VHE luminosities for FR 0s from the gap models are 10 42-43 erg s −1 .…”

Physics of Pair Producing Gaps in Black Hole Magnetospheres

“…The magnetospheric gap has been considered as possible candidate site for producing the rapid γ-ray variability seen from certain AGN, especially radio galaxies (e.g., Levinson & Rieger 2011;Aleksić et al 2014;Aharonian et al 2017;Katsoulakos & Rieger 2018). The most promising source seems to be M87 (Katsoulakos & Rieger 2018). It has a massive black hole with mass M ≈ 6.5 × 10 9 M (Event Horizon Telescope Collaboration et al 2019), and an estimated jet power L jet ∼ 10 44 erg s −1 (e.g., Broderick & Tchekhovskoy 2015).…”

Gamma-ray emission from the black hole's vicinity in AGN

“…This suggests that an electromagnetic force-free solution provides a reasonable approximation to the time-averaged structure of such a magnetosphere [3]. While the qualitative picture seems evident, the resulting non-thermal emission features (e.g., maximum particle energy, dominant emission mechanism, radiative window, gap power) will depend on the details of the magnetospheric set-up (boundary conditions), and different realisations of the electric field and potential are in principle conceivable (and actually encountered, see e.g., [8]) as motivated below.…”

“…The different gap potential noted above, however, result in different expectations for the maximum gap luminosity L gap n c V gap dE e /dt (with characteristic gap volume V gap ∝ r 2 g h) for a gap of height h. As we show in ref. [8] the maximum extractable gap power is in general proportional to the classical Blandford-Znajek jet power,…”

“…3.5. However, to ensure transparency to VHE photons the accretion flow needs to be radiatively inefficient (of ADAF-type) and this on average constrains inner accretion rates to satisfyṁ c 0.01, see e.g., [8] for details. When this is put in context, extractable gap powers are such as to allow accommodation of the gamma-ray emission seen from M 87, see Fig.…”