Black hole lightning due to particle acceleration at subhorizon scales

Abstract

Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole.We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet

The extension of variability properties in gamma-ray bursts to blazars

“…Zhang & Zhang 2014). This scenario can also explain the ultra-fast variability (e.g., several minutes) as found in some blazars at TeV energies (e.g., Aharonian et al 2007;Albert et al 2007;Aleksic et al 2014). Sonbas et al (2014) found that the MTS becomes shorter as the gamma-ray luminosity (or Lorentz factor) increases in GRBs.…”

“…The typical minimum variability timescale (MTS) for blazars is around one day (e.g., Vovk & Neronov 2013), where ultra-fast variability as short as 3-5 min at TeV energies is also found in some blazars (e.g., Aharonian et al 2007;Albert et al 2007;Aleksic et al 2014). The typical MTS of GRBs is around 0.5 s with the shortest timescale on the order of 10 ms (e.g., MacLachlan et al 2013;, where the MTS is much shorter than the overall duration (e.g., T90).…”

Gamma rays from blazars

“…Moreover, also the observed VHE spectrum is quite peculiar, showing an unbroken hard (photon index close to 2) power law up to 10 TeV. An exciting possibility is that the fast variations flag the development of electromagnetic cascades triggered by pairs accelerated to the required TeV energies by unscreened electric fields in gaps of the black hole magnetosphere [47], a mechanism already proposed to work in other low-luminosity AGN, in particular the radiogalaxy M87 [58,59,60], and possibly related to the formation of the jet itself. Detailed calculations [61] show that the expected spectrum (originating from the IC scattering of the soft IR radiation from the radiatively inefficient accretion flow by the pairs in the cascade) can indeed FIGURE 2.…”

“…Vary rapid variations of the γ-ray flux, with inferred doubling time-scales down to few minutes, have been observed in several blazars (both BL Lacs and FSRQ), most notably PKS 2155-304 [43], Mkn 501 [44], BL Lac [45], PKS 1222+21 [46], IC 310 ( [47] although the precise classification of this source is still matter of debate) and, last but not least, 3C279 (at GeV energies by pointed observations with Fermi/LAT, [48]). Such small timescales directly imply, via the standard causality argument, very compact emission regions, with radii not exceeding r < ct var δ ≈ FIGURE 1.…”

Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

“…Magnetospheric gamma-ray emission has recently received a particular impetus in the context of the rapidly variable VHE emission detected from misaligned AGN, most prominently from the radio galaxies M87 (d 16 Mpc) and IC310 (d ∼ 80 Mpc) (e.g., Neronov & Aharonian 2007;Vincent 2010;Levinson & Rieger 2011;Aleksić et al 2014a;Broderick & Tchekhovskoy 2015;Ptitsyna & Neronov 2016;. This goes along with the recognition that r g /c, where r g = r s /2 = GM BH /c 2 is the gravitational radius of the black hole, provides a characteristic variability timescale of the emission in the frame of the galaxy, and that specific set-ups are needed to tap sufficient power on significantly shorter time scales (e.g., Barkov et al 2010;Giannios et al 2010;Aharonian et al 2017).…”