Astrophysical Neutrinos and Blazars

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.

A multi-messenger study of the blazar PKS 0735+178: a new major neutrino source candidate

“…This makes the SED of PKS 0735+178 that of a IBL/HBL blazar very similar to that of other likely bright neutrino emitters, namely TXS0506+056, PKS1424+240 and GB6J2542+6129 (see Fig. 4 of Giommi & Padovani 2021).…”

“…We analysed all the X-ray data from the X-Ray Telescope (XRT) using Swift_xrtproc, a tool developed within the Open Universe initiative (Giommi et al 2020a) that automatically performs a complete data reduction using the HEAsoftV6.29 software and generates high-level data products (assuming power law and log parabola spectral models and Galactic absorption), spectral and imaging analysis results, using the XSPEC V12.12.0 and XIMAGE V4.5.1 packages. Both data collected in single snapshots and over entire XRT observations 2 are processed -see Giommi et al (2021) for details.…”

“…The discovery of a flux of very high-energy (VHE; > 100 GeV) neutrinos of astrophysical origin by the IceCube South Pole observatory 1 (Aartsen et al 2013(Aartsen et al , 2020 and the first reliable association of Ice-Cube neutrinos with a cosmic source, the blazars TXS0506+056 (Ice-Cube Collaboration 2018; Padovani et al 2018), paved the way for the beginning of (extra-galactic) neutrino astronomy. Recent works reporting hints, at various level of significance, of several other possible associations between IceCube neutrinos and blazars have been published (Aartsen et al 2020;Plavin et al 2020;Giommi et al 2020b;IceCube Collaboration 2021a) strengthening the connection between VHE neutrinos and blazars (see Giommi & Padovani 2021, for a recent review).…”

“…Falomo et al 2014); the SED classification originally divides blazars into low-(LBLs), intermediate-(IBLs) and high-energy (HBLs) peaked sources (Padovani & Giommi 1995) or LSP, ISP and HSP (Abdo et al 2010). These definitions have recently been refined into LBLs and intermediate-high-energy-peaked objects (IHBLs) depending on whether the peak frequency of the radio to X-ray SED (𝜈 peak ) is located below or above ∼ 10 13.5 Hz (Giommi & Padovani 2021).…”

PKS 1424+240: yet another masquerading BL Lac object as a possible IceCube neutrino source

“…IceCube Collaboration et al ( 2021) have performed a search for flare neutrino emission in the 10 years of IceCube data and reported a cumulative time-dependent neutrino excess in the northern hemisphere at the 3 𝜎 level associated with only four sources: a radio galaxy, M87, two blazars, TXS 0506+056 and GB6 J1542+6129, and the Seyfert 2 galaxy NGC 1068, these last three sources being also associated with neutrinos by Aartsen et al (2020). GB6 J1542+6129, an IBL (Giommi & Padovani 2021), has then appeared twice in two years in a list of 3 − 3.3 𝜎 IceCube associations and is therefore also worth investigating. The redshift of this object is not available: all we know is that 0.34 ≤ 𝑧 ≤ 1.76 (Shaw et al 2013), which translates into 𝑃 1.4GHz = 10 26.4±0.7 W Hz −1 and 𝐿 𝛾 /𝐿 Edd = 10 0.3±0.9 (for 𝑀 BH = 6.3 × 10 8 𝑀 , i.e.…”