Synchrotron pair halo and echo emission from blazars in the cosmic web: application to extreme TeV blazars

Abstract

High frequency peaked, high redshift blazars, are extreme in the sense that their spectrum is particularly hard and peaks at TeV energies. Standard leptonic scenarios require peculiar source parameters and/or a special setup in order to account for these observations. Electromagnetic cascades seeded by ultra-high energy cosmic rays (UHECR) in the intergalactic medium have also been invoked, assuming a very low intergalactic magnetic field (IGMF). Here we study the synchrotron emission of UHECR secondaries produced in blazars located in magnetised environments, and show that it can provide an alternative explanation to these challenged channels, for sources embedded in structured regions with magnetic field strengths of the order of 10 −7 G. To demonstrate this, we focus on three extreme blazars: 1ES 0229+200, RGB J0710+591, and 1ES 1218+304. We model the expected gamma-ray signal from these sources through a combination of numerical Monte Carlo simulations and solving the kinetic equations of the particles in our simulations, and explore the UHECR source and intergalactic medium parameter space to test the robustness of the emission. We show that the generated synchrotron-pair halo and echo flux at the peak energy is not sensitive to variations in the overall IGMF strength. This signal is unavoidable in contrast to the inverse Compton-pair halo and echo intensity, which is appealing in view of the large uncertainties on the IGMF in voids of large scale structure. It is also shown that the variability of blazar gamma-ray emission can be accommodated by the synchrotron emission of secondary products of UHE neutral beams if these are emitted by UHECR accelerators inside magnetised regions.

Open Questions in Cosmic-Ray Research at Ultrahigh Energies

“…AGN with jets pointing to the Earth, referred to as blazars, would be the natural candidates if UHECRs escape the sources beamed and do not suffer severe deflections (Dermer and Razzaque, 2010;Resconi et al, 2017;Rodrigues et al, 2018). The signature of UHECR acceleration could be detectable in the gamma-ray spectra of blazars (Gabici and Aharonian, 2005;Kotera et al, 2011;Murase et al, 2012;Prosekin et al, 2012;Aharonian et al, 2013;Takami et al, 2013;Oikonomou et al, 2014;Tavecchio, 2014). However, the present-day density of nearby blazars shown in Figure 10 suggests that blazars alone do not satisfy the number density constraint.…”

Can we observe neutrino flares in coincidence with explosive transients?

“…Such emissions are related to the acceleration of leptonic and/or hadronic particles within the source. The observed photon spectra is often modeled by synchrotron radiation of leptons; hadrons are less frequently invoked, although they also lead to consistent pictures in specific cases, for example, for some blazars where leptons alone fail to provide a satisfactory explanation to the data (e.g., Oikonomou et al 2014;Petropoulou et al 2016bPetropoulou et al , 2017c. A&A 603, A76 (2017) In the latter situation, the detection of neutrinos would be an unquestionable indicator of the acceleration of hadrons and of their interaction within the source environment.…”

An emerging population of BL Lacs with extreme properties: towards a class of EBL and cosmic magnetic field probes?

“…These features can be exploited to effectively constrain the intergalactic magnetic field (IGMF, e.g. Neronov & Vovk 2010;Tavecchio et al 2010Tavecchio et al , 2011Dolag et al 2009;Dermer et al 2011;Vovk et al 2012;Oikonomou et al 2014). Other properties shared by these sources are the extremely large ratio between the X-ray and the radio flux and the hardness of the X-ray continuum (Γ ∼ 2), locating the synchrotron peak in the medium-hard X-ray band.…”