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A Seyfert galaxy as a hidden counterpart to a neutrino-associated blazar

Emma Kun, Santiago del Palacio, Imre Bartos, Francis Halzen, Julia Becker Tjus, Peter L. Biermann, Anna Franckowiak, Claudio Ricci

TL;DR

This study tests the proposed linear link between hard X-ray and high-energy neutrino emission across AGN by combining NuSTAR hard X-ray observations of PKS 1424+240 with IceCube data and the nearby Seyfert NGC 5610. PKS 1424+240 alone appears underluminous in hard X-rays for its neutrino flux, but treating the IceCube hotspot as a composite source via a dual-source attribution proportional to hard X-ray flux reconciles both AGN with the LhX–Lnup relation. The results support a multimessenger scenario in which hadronic cascades near the black hole core produce both high-energy neutrinos and cascaded hard X-rays, and they imply that some IceCube hotspots may be blends of multiple AGN within the angular resolution. Extending the sample to eight AGN (three blazars, five Seyferts) shows the correlation remains robust after controlling for redshift, highlighting the potential of hard X-ray measurements to trace environments of efficient pγ interactions and underscoring the need for higher-resolution neutrino data to resolve source confusion.

Abstract

The origin and production mechanisms of high-energy astrophysical neutrinos remain open questions in multimessenger astronomy. Previous studies have hinted at a possible linear correlation between the hard X-ray and high-energy neutrino emission in active galactic nuclei. New \textit{NuSTAR} observations, first presented here, reveal that blazar PKS 1424+240, located within a prominent IceCube neutrino hotspot, is far fainter in hard X-rays than expected from this trend. Motivated by this apparent ambiguity, we identify the nearby Seyfert galaxy NGC 5610, also coincident with the hotspot, whose unabsorbed hard X-ray flux exceeds that of PKS 1424+240 by about an order of magnitude. When the local IceCube neutrino flux is apportioned between the two AGN in proportion to their hard X-ray emission, both align with the previously suggested X-ray-neutrino correlation. This suggests that certain IceCube hotspots may be unresolved blends of multiple AGN, and supports a multimessenger scenario in which high-energy neutrinos and hard X-rays originate from the same hadronic interactions, with the X-ray emission produced through cascade reprocessing.

A Seyfert galaxy as a hidden counterpart to a neutrino-associated blazar

TL;DR

This study tests the proposed linear link between hard X-ray and high-energy neutrino emission across AGN by combining NuSTAR hard X-ray observations of PKS 1424+240 with IceCube data and the nearby Seyfert NGC 5610. PKS 1424+240 alone appears underluminous in hard X-rays for its neutrino flux, but treating the IceCube hotspot as a composite source via a dual-source attribution proportional to hard X-ray flux reconciles both AGN with the LhX–Lnup relation. The results support a multimessenger scenario in which hadronic cascades near the black hole core produce both high-energy neutrinos and cascaded hard X-rays, and they imply that some IceCube hotspots may be blends of multiple AGN within the angular resolution. Extending the sample to eight AGN (three blazars, five Seyferts) shows the correlation remains robust after controlling for redshift, highlighting the potential of hard X-ray measurements to trace environments of efficient pγ interactions and underscoring the need for higher-resolution neutrino data to resolve source confusion.

Abstract

The origin and production mechanisms of high-energy astrophysical neutrinos remain open questions in multimessenger astronomy. Previous studies have hinted at a possible linear correlation between the hard X-ray and high-energy neutrino emission in active galactic nuclei. New \textit{NuSTAR} observations, first presented here, reveal that blazar PKS 1424+240, located within a prominent IceCube neutrino hotspot, is far fainter in hard X-rays than expected from this trend. Motivated by this apparent ambiguity, we identify the nearby Seyfert galaxy NGC 5610, also coincident with the hotspot, whose unabsorbed hard X-ray flux exceeds that of PKS 1424+240 by about an order of magnitude. When the local IceCube neutrino flux is apportioned between the two AGN in proportion to their hard X-ray emission, both align with the previously suggested X-ray-neutrino correlation. This suggests that certain IceCube hotspots may be unresolved blends of multiple AGN, and supports a multimessenger scenario in which high-energy neutrinos and hard X-rays originate from the same hadronic interactions, with the X-ray emission produced through cascade reprocessing.
Paper Structure (4 sections, 1 equation, 4 figures)

This paper contains 4 sections, 1 equation, 4 figures.

Table of Contents

  1. Introduction
  2. Results
  3. Discussion
  4. Methods

Figures (4)

  • Figure 1: Reconstructed sky map of the scan for point sources in a 15 degrees radius about PKS 1424+240 with free spectral index, using Hammer-Aitoff projection (seasons IC86 I-II, full detector data between 2011 and 2020; ngc1068_2022_dataset). The fiducial sky positions of PKS 1424+240 and NGC 5610 are marked with a light blue cross and a yellow plus, respectively.
  • Figure 2: Correlation between unabsorbed hard X-ray and neutrino luminosities for radio-quiet Seyferts (red) and radio-loud blazars (blue). NGC 5610 and PKS 1424+240 are shown with updated luminosities under the dual-source scenario. These two additional sources are highlighted with filled markers and boldface in the legend. The dashed line corresponds to the fitted line, whose equation is shown on the top. For sources marked with $^*$, the neutrino luminosity comes from limiting IceCube MLE fluxes.
  • Figure 3: NuSTAR-FPMB intensity image in the 3--20 keV band. Source and background extraction regions are shown.
  • Figure 4: NuSTAR X-ray spectrum of PKS 1424+240 (top panel) and residuals of the fit (bottom panel). Black and red symbols correspond to FPMA and FPMB, respectively. The fitted model is a power law.