Blazar PKS 0446+11 -- Neutrino connection study using a lepto-hadronic model

TL;DR

This study analyzes PKS 0446+11 around the IceCube neutrino IC240105A, combining Fermi-LAT, Swift-XRT/UVOT, and archival data to construct a broadband SED and light curves. The results favor a leptonic emission scenario with external Compton scattering from the broad-line region and dusty torus, while a hadronic, neutrino-producing component is disfavored by the observed SED and by neutrino upper-limit constraints. The predicted neutrino flux in the hadronic scenario is several orders of magnitude below IceCube/KM3NeT detectability within the examined window, suggesting no strong neutrino association for this epoch. The work highlights the potential for time-offset neutrino production in blazars, the importance of high-quality MeV–X-ray observations, and the need for continued multi-messenger monitoring to clarify the role of hadronic processes in high-redshift blazars and their contribution to the diffuse neutrino background.

Abstract

We present a multi-wavelength study of a blazar PKS 0446+11, motivated by its spatial association with the neutrino event IC240105A detected by the IceCube Neutrino Observatory on 2024 January 5. The source is located 0.4 degrees from the best-fit neutrino direction and satisfies selection criteria for VLBI-selected, radio-bright AGN that have been identified as highly probable neutrino associations. PKS 0446+11 exhibited a major gamma-ray flare in November 2023, reaching approximately 18x its 4FGL-DR4 catalog average. Around the neutrino epoch, PKS 0446+11 remained in an elevated state, with the gamma-ray flux more than six times above its catalog level, the X-ray flux an order of magnitude above the archival measurements, and the optical-UV emission also enhanced. We used Fermi-LAT, Swift-XRT/UVOT, and archival multi-wavelength data to construct multi-wavelength light curves and spectral energy distributions (SEDs). SED modeling shows that the emission is best described by a leptonic scenario, with synchrotron emission at low energies and external Compton scattering of broad-line region and dusty torus photons dominating the X-ray - gamma-ray output. A lepto-hadronic model fails to adequately reproduce the observed SED, although hadronic cascades can broadly account for the X-ray and gamma-ray spectral coverage at lower flux levels. We compute the expected neutrino flux for the hadronic scenario and compare it to the IceCube 90% upper limit. Our results highlight the importance of continued multi-wavelength and neutrino monitoring to better understand the physical conditions under which this blazar may serve as neutrino source.

Figures (2)

• Figure 1: Multi-wavelength light curve of PKS 0446+11 spanning over MJD 60222–60405. The red dashed vertical line marks the detection time of the IceCube neutrino event IC240105A. The time interval bounded by the two grey dashed lines (MJD 60315.80–60351) denotes the period selected for broadband SED modeling The $\gamma$-ray fluxes are shown in units of 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$ , the Swift-XRT fluxes in 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$, the Swift-UVOT fluxes flux in 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$.
• Figure 2: Broadband SED of PKS 0446+11 constructed using multiwavelength observations from RATAN-600 and MOJAVE (radio, red and blue), Swift-UVOT (optical/UV, magenta), Swift-XRT (X-ray, orange), and Fermi-LAT ($\gamma$-ray, blue), with archival data shown in grey. The SED is modeled with a one-zone leptonic emission component (black dashed line) and a sub-dominant hadronic cascade contribution (green dotted line), as described in § 5. The red downward arrow indicates the 90% C.L. IceCube muon-neutrino flux upper limit (ATel 16414), and the orange dashed curve represents the model-predicted neutrino spectrum.