TXS 0506+056-like blazar sources and their role as possible neutrino emitters

TL;DR

The paper investigates TXS 0506+056-like blazars as potential neutrino emitters by selecting a TXS-like sample from 4LAC-DR2 using $\Gamma_\gamma$, $L_\gamma$, and $\nu_S$ constraints anchored to TXS values, and then focusing on three BL Lacs with detailed optical/UV spectroscopic data to estimate accretion rates. Public LAT and Swift data are analyzed to build quasi-simultaneous multiwavelength SEDs, which are modeled in a lepto-hadronic framework that includes external photon fields from the BLR and torus. The results indicate high-energy emission is predominantly leptonic, with a subdominant hadronic component; the predicted neutrino fluxes generally fall below current IceCube sensitivities, except potentially for PKS 0048-09 which could be detectable by CTAO or IceCube-Gen2 with favorable states. The study highlights the need for more extensive spectroscopic measurements to enlarge the TXS-like sample, refine accretion-rate estimates, and improve multi-messenger predictions for blazar neutrino emission.

Abstract

The interest in blazars as candidate neutrino emitters grew after the 3$σ$ evidence for a contemporaneous joint photon and neutrino emission from the flaring blazar TXS 0506+056 in 2017. Blazars, a class of extragalactic sources with relativistic jets pointing toward Earth, present a broadband emission interpretable via leptonic and hadronic processes, the latter relevant for proton acceleration and neutrino production. Several emission models have been developed to explain this multi-messenger observation, but the details of the neutrino production and the nature of TXS 0506+056 are not yet fully understood. In this work we investigate the properties of sources similar to TXS 0506+056. We select a sample of blazars from the Fermi 4LAC-DR2 catalog by constraining a number of key parameters in ranges centered on TXS 0506+056 values. We estimate their disk accretion efficiency and model their spectral energy distribution (SED) in terms of lepto-hadronic emission, gaining information respectively on the potential similarity of their environment with that of TXS 0506+056 and on their neutrino flux and detectability prospects at TeV energies. Our study shows the candidates' high energy emission to be dominated by leptonic processes. Part of them also show a high accretion rate, characteristic of FSRQs. For these sources, the very high energy (VHE) and neutrino fluxes appear undetectable by current and future instruments in an average emission state.

Figures (8)

• Figure 1: Histogram of the photon indexes of all sources in 4LAC-DR2 with known redshift. The legend reports: BL Lacs (bll), FSRQs (fsrq), blazar candidate of unknown type (bcu) and TXS 0506+056.
• Figure 2: Luminosity histogram of all sources in 4LAC-DR2 catalog with known redshift. The legend is the same as in Fig. \ref{['fig:PL_hist']}.
• Figure 3: Histogram of the synchrotron peak frequencies of all sources in 4LAC-DR2 catalog with known redshift. The legend is the same as in Fig. \ref{['fig:PL_hist']}.
• Figure 4: Multiwavelength lightcurve of the source PKS 0048-09. From top to bottom, the panels show: Fermi-LAT flux in 60-days bins, values assumed by the $\gamma-$ray spectral index in the same bins, Swift/XRT flux in daily bins, Swift/UVOT flux in daily bins. The shaded areas identify the boundaries of the low (orange) and high (blue) state periods chosen to build the SED.
• Figure 5: Multiwavelength lightcurve of the source GB6 J0114+1325. The reported information are the same as in Fig. \ref{['fig:pks_lc']}.