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Register a new userCosmic Neutrinos and the Cosmic-Ray Accelerator TXS 0506+056
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Francis Halzen
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IceCube discovered a flux of cosmic neutrinos originating in extragalactic sources with an energy density close to that in gamma rays and cosmic rays. A multimessenger campaign triggered by the coincident observation of a gamma-ray flare and a 290-TeV IceCube neutrino pinpointed the cosmic-ray accelerator TXS 0506+056. Subsequently, the IceCube archival data revealed a 3-month burst of 13 cosmic neutrinos in 2014-15 that dominates the neutrino flux of the source over the 9.5 years of observations. The original identification of the source as a blazar was puzzling because it requires a major accretion event onto the rotating supermassive black hole to accommodate the neutrino burst. Subsequent high-resolution radio images of the source with the VLBA brought to light a merger of two galaxies, revealed by the interaction of two jets entangled in the source. Recently, the blazar PKS 1502+106 was found in the direction of a 300-TeV neutrino alert, IC-190730. OVRA radio observations at 15 GHz indicate that the neutrino also coincides with the highest flux density of a flare that started five years ago. This matches the similar long-term outburst seen from TXS 0506+056 and may indicate merger activity. Also, the dominant hotspot in the 10-year IceCube neutrino sky map, NGC 1068 (Messier 77), is a Seyfert galaxy undergoing a major accretion event onto the black hole. A few-percent fraction of such special sources, now labeled as gamma-ray blazars, is sufficient to accommodate the diffuse cosmic neutrino flux observed by IceCube. While rapid progress seems likely, the observations also convincingly make the case for the construction of more and larger neutrino telescopes with better angular resolution.
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While blazars have long been one of the candidates in the search for the origin of ultra-high energy cosmic rays and astrophysical neutrinos, the BL Lac object TXS 0506+056 is the first extragalactic source that is correlated with some confidence with a high-energy neutrino event recorded with IceCube. At the time of the IceCube event, the source was found in a high state in gamma-rays with Fermi-LAT and MAGIC. We have explored in detail the parameter space of a lepto-hadronic radiative model, assuming a single emitting region inside the relativistic jet. We present the complete range of possible solutions for the physical conditions of the emitting region and its particle population. For each solution we compute the expected neutrino rate, and discuss the impact of this event on our general understanding of emission processes in blazars.
We present evidence that TXS 0506+056, the first plausible non-stellar neutrino source, despite appearances, is not a blazar of the BL Lac type but is instead a masquerading BL Lac, i.e., intrinsically a flat-spectrum radio quasar with hidden broad lines and a standard accretion disk. This re-classification is based on: (1) its radio and O II luminosities; (2) its emission line ratios; (3) its Eddington ratio. We also point out that the synchrotron peak frequency of TXS 0506+056 is more than two orders of magnitude larger than expected by the so-called blazar sequence, a scenario which has been assumed by some theoretical models predicting neutrino (and cosmic-ray) emission from blazars. Finally, we comment on the theoretical implications this re-classification has on the location of the $gamma$-ray emitting region and our understanding of neutrino emission in blazars.
The results of three different searches for neutrino candidates, associated with the IceCube-170922A event or from the direction of TXS 0506+056, by the ANTARES neutrino telescope are presented. The first search refers to the online follow-up of the IceCube alert; the second is based on the standard time-integrated method employed by the Collaboration to search for point-like neutrino sources; the third uses the information from the IceCube time-dependent analysis reporting a bursting activity centered on December 13, 2014, as input for an ANTARES time-dependent analysis. The online follow-up and the time-dependent analysis yield no events related to the source. The time-integrated study performed over a period from 2007 to 2017 fits 1.03 signal events, which corresponds to a p-value of 3.4% (not considering trial factors). Only for two other astrophysical objects in our candidate list, a smaller p-value had been found. When considering that 107 sources have been investigated, the post-trial p-value for TXS 0506+056 corresponds to 87%.
Motivated by the observation of a $>290$ TeV muon neutrino by IceCube, coincident with a $sim$6 month-long $gamma$-ray flare of the blazar TXS 0506+056, and an archival search which revealed $13 pm 5$ further, lower-energy neutrinos in the direction of the source in 2014-2015, we discuss the likely contribution of blazars to the diffuse high-energy neutrino intensity, the implications for neutrino emission from TXS 0506+056 based on multi-wavelength observations of the source, and a multi-zone model that allows for sufficient neutrino emission so as to reconcile the multi-wavelength cascade constraints with the neutrino emission seen by IceCube in the direction of TXS 0506+056.
A neutrino with energy of $sim$290 TeV, IceCube-170922A, was detected in coincidence with the BL Lac object TXS~0506+056 during enhanced gamma-ray activity, with chance coincidence being rejected at $sim 3sigma$ level. We monitored the object in the very-high-energy (VHE) band with the MAGIC telescopes for $sim$41 hours from 1.3 to 40.4 days after the neutrino detection. Day-timescale variability is clearly resolved. We interpret the quasi-simultaneous neutrino and broadband electromagnetic observations with a novel one-zone lepto-hadronic model, based on interactions of electrons and protons co-accelerated in the jet with external photons originating from a slow-moving plasma sheath surrounding the faster jet spine. We can reproduce the multiwavelength spectra of TXS 0506+056 with neutrino rate and energy compatible with IceCube-170922A, and with plausible values for the jet power of $sim 10^{45} - 4 times 10^{46} {rm erg s^{-1}}$. The steep spectrum observed by MAGIC is concordant with internal $gammagamma$ absorption above a few tens of GeV entailed by photohadronic production of a $sim$290 TeV neutrino, corroborating a genuine connection between the multi-messenger signals. In contrast to previous predictions of predominantly hadronic emission from neutrino sources, the gamma-rays can be mostly ascribed to inverse Compton up-scattering of external photons by accelerated electrons. The X-ray and VHE bands provide crucial constraints on the emission from both accelerated electrons and protons. We infer that the maximum energy of protons in the jet co-moving frame can be in the range $sim 10^{14}$ to $10^{18}$ eV.
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