Correlation Between Hard X-Ray and Cosmic Neutrino Sources: From Obscured AGN to Blazars

Abstract

The origin of high-energy astrophysical neutrinos remains a key open question in multimessenger astrophysics. A correlation between unabsorbed hard X-ray and high-energy neutrino luminosity has been reported in six active galactic nuclei with the highest individual IceCube significances, linking neutrino production to compact, photon-rich environments near supermassive black holes. We study whether the threshold-near IceCube excesses associated with seven NuSTAR-observed blazars are statistically consistent with that relation. Calibrating the $L_{\mathrm{hX}}$-$L_{\nu }$ relation on the six published sources via a Bayesian regression with errors on both axes, the slope is consistent with $\beta =1$ and the intrinsic scatter is $\sim 0.6$\,dex. All seven new blazars are posterior-predictively consistent with this calibration (${\chi }_7^2=1.58$, $p=0.98$) under the working hypothesis that the published IceCube ${\hat{n}}_s$ values reflect the signal. A null-injection test confirms that, at the present calibration sample size, the consistency test does not by itself adjudicate between signal and selected-background origins. A distance-free $L_{\mathrm{hX}}/L_{\nu }$ ratio diagnostic places both populations within the photohadronic prediction band, statistically indistinguishable. Two diagnostics that control the common $d_L^2$ distance bias, a redshift-partial rank correlation ($\tau |z=0.69$, $\sim 2.7\sigma$) and a flux-space permutation test on the 13-source joint sample ($p=6.3\times 10^{-4}$, $3.23\sigma$), indicate a residual $L_{\mathrm{hX}}$-$L_{\nu }$ association beyond the distance-induced trend. We interpret these results as a conditional consistency check; a detection-level statement requires either an enlarged calibration set or an X-ray-weighted IceCube stacking likelihood with internal data.

21 pages, 7 figures, 5 tables. Accepted for publication in PASP