Abstract
We review and discuss recent results on the search for correlations between astrophysical neutrinos and γ-ray-detected sources, with many extragalactic studies reporting potential associations with different types of blazars. We investigate possible dependencies on blazar sub-classes by using the largest catalogues and all the multi-frequency data available. Through the study of similarities and differences in these sources we conclude that blazars come in two distinct flavours: LBLs and IHBLs (low-energy-peaked and intermediate-high-energy-peaked objects). These are distinguished by widely different properties such as the overall spectral energy distribution shape, jet speed, cosmological evolution, broad-band spectral variability, and optical polarisation properties. Although blazars of all types have been proposed as neutrino sources, evidence is accumulating in favour of IHBLs being the counterparts of astrophysical neutrinos. If this is indeed the case, we argue that the peculiar observational properties of IHBLs may be indirectly related to proton acceleration to very high energies.
Highlights
The discovery of ultra-high energy cosmic rays (UHECRs) established the existence of powerful cosmic accelerators capable of reaching energies millions of times larger than those that can be achieved by the best accelerators on Earth
Based on the empirical evidence described above, which reveals strong similarities between IBLs and HBLs properties, and large differences from LBLs, we suggest that blazars come only in two main flavours: LBLs, and IBLs plus HBLs combined, which we propose to collectively call IHBLs
Neutrino astronomy is still in a nascent phase, a status characterised by consolidated results on the detection of neutrinos of astrophysical origin, and by the lack of indisputable evidence about the nature of their electromagnetic counterparts
Summary
The discovery of ultra-high energy cosmic rays (UHECRs) established the existence of powerful cosmic accelerators capable of reaching energies millions of times larger than those that can be achieved by the best accelerators on Earth (see ref. [1] and references therein). [6,7,8] and references therein) In this early phase of multi-messenger astronomy, where the available instrumentation is limited in precision and sensitivity, unambiguous associations would greatly benefit from the detection of high-energy neutrinos together with enhanced activity in some parts of the electromagnetic spectrum of the astrophysical counterpart [9,10]. This condition, requires that hadronic-related electromagnetic flares are strong enough to outshine the non-thermal radiation generated via different mechanisms, e.g., from accelerated electrons radiating in magnetic fields in the so-called leptonic scenarios [11,12,13]. This so-called “neutrino flare” occurred when TXS 0506+056 was in a period of low γ-ray activity, suggesting that the relationship between astrophysical neutrinos and γ-ray emission is not straightforward
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