A Multimessenger Picture of the Flaring Blazar TXS 0506+056: Implications for High-energy Neutrino Emission and Cosmic-Ray Acceleration

Abstract

Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only ∼3σ high-energy neutrino source association to date, offers a potential breakthrough in our understanding of highenergy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar's particle acceleration processes and multimessenger (electromagnetic and high-energy neutrino) emissions. Accounting properly for electromagnetic cascades in the emission region, we find a physically-consistent picture only within a hybrid leptonic scenario, with γ-rays produced by external inverse-Compton processes and highenergy neutrinos via a radiatively-subdominant hadronic component. We derive robust constraints on the blazar's neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100 keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultra-high-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models, in which γ-rays and high-energy neutrinos are produced in a single emission region. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.

Proton Synchrotron Gamma-Rays and the Energy Crisis in Blazars

“…Figure 4 shows the peak neutrino energy and peak ν µ + ν µ energy flux derived from equation (2). Our results are in line with predictions made for individual sources, i.e., ∼ 0.1 − 1 EeV neutrinos with fluxes much lower than in γ-rays (e.g., Dimitrakoudis et al 2014;Keivani et al 2018). There are a few blazars that are potentially interesting neutrino sources (close to IceCube's discovery potential), with LSP blazar 4FGLJ2148.6+0652 being the best example.…”

“…In addition, the derived magnetic field strengths in the emission region imply either large amplification of the jet's magnetic field or sub-pc γ-ray production sites, well within the BLR and in tension with recent results. The expected neutrino emission (for all sources in our sample) peaks at ∼ 0.1 − 10 EeV, i.e., at much higher energies than the multi-TeV neutrinos associated with TXS 0506+056 (see also Keivani et al 2018). Meanwhile, the typical peak neutrino fluxes are ∼ 10 −4 times lower than the peak γ-ray fluxes.…”

Multi-epoch Modeling of TXS 0506+056 and Implications for Long-term High-energy Neutrino Emission

“…In addition, only small changes in the properties of the electron energy spectrum at injection (i.e., γ e,b , s e,2 , γ e,max ) are needed to account for the spectral variability in the X-ray band. The strongest upper limits on the neutrino flux are derived for epochs where X-ray measurements (with small uncertainties) are available, in agreement with previous studies (Keivani et al 2018;Murase et al 2018). Unfortunately, during the period of the reported neutrino flare by IceCube, there are no X-ray measurements available.…”

“…The parameters of the best-fit power-law model (with their 1σ statistical errors) are presented in Table 3 and the 0.3-10 keV fluxes are displayed in Figure 1 (middle panel). The photon index Γ derived by the fit is consistent with the values reported by Keivani et al (2018), who analyzed individual Swift-XRT observations after the IceCube-170922A detection as part of the source monitoring program (see Figure 2 of the reference and Table 1 therein). By performing a simultaneous fit to Swift-XRT and NuSTAR data, Keivani et al (2018) found that the broadband X-ray spectrum of TXS 0506+056 is best fitted by a sum of two power-law components with best-fit photon indices of 2.37 ± 0.05 (Swift-XRT band) and 1.68 ± 0.14 for the hard part of the spectrum.…”

“…After having determined the parameter values needed to explain the multi-epoch SEDs in terms of synchrotron and inverse Compton emissions from primary electrons, we can compute the maximal proton luminosity L (max) p by requiring that any proton-induced emission does not overshoot the X-ray and/or gamma-ray data (Keivani et al 2018;Gao et al 2019). In turn, this translates into an upper limit on the blazar's neutrino flux for each epoch.…”