Association of the IceCube neutrinos with CAZ blazar light curves

TL;DR

This work tests whether major optical flares in blazars are temporally linked to IceCube high-energy neutrinos by leveraging an expanded set of blazars (RFC$^$ and 4LAC) and high-cadence CAZ optical light curves. The analysis uses a spatio-temporal framework with a top-hat weighting scheme and three temporal metrics (BB95, BBHOP, and BB95-at-peak of BBHOP) to search for associations across 356 neutrinos and thousands of blazars, accounting for large $(\Omega)$ error regions and seasonal gaps. The key finding is a predominantly weak population-level correlation, with only a couple high-weight associations driving the signal; post-trial significance peaks at $\sim2.6\sigma$ in a single scenario, and simulations imply that at most $\lesssim8\%$ of cosmic neutrinos originate from blazars during major optical flares. The results also indicate that neutrino-associated blazars tend to be Doppler-boosted and X-ray bright, suggesting jet-related production, though the overall contribution of flares remains small, highlighting the need for multiwavelength, next-generation observations. Overall, the study constrains the role of optical blazar flares in IceCube neutrino production and demonstrates a robust framework for future multi-messenger investigations.

Abstract

Over the past decade, the IceCube Neutrino Observatory has detected a few hundreds of high-energy (HE) neutrinos from cosmic sources. Despite numerous studies searching for their origin, it is still not known which source populations emit them. A few confident individual associations exist with active galactic nuclei (AGN), mostly with blazars which are jetted AGN whose jet points in our direction. Nonetheless, on a population level, blazar-neutrino correlation strengths are rather weak. This could mean that blazars as a population do not emit HE neutrinos, or that the detection power of the tests is insufficient due to the strong atmospheric neutrino background. By assuming an increase in HE neutrino emission during major blazar flares, in our previous studies we leveraged the arrival time of the neutrinos to boost the detection power. In this paper we utilize the same principle while substantially increasing the number of blazars. We search for the spatio-temporal correlation of 356 IceCube HE neutrinos with major optical flares of 3225 radio- and 3814 $γ$-ray-selected blazars. We find that, despite the increase in data size, the number of confident spatio-temporal associations remains low and the overall correlation strengths weak. Two individual associations drive our strongest and the only $>$2$σ$ post-trial spatio-temporal correlation, occurring with the BL Lac objects of the radio-selected blazar sample. We estimate that $\lesssim$8\% of the detected cosmic neutrinos were emitted by blazars during major optical flares. As a complementary analysis, we compare the synchrotron peak frequency, redshift, Doppler factor, X-ray brightness, and optical variability of spatially neutrino-associated blazars to those of the general blazar population. We find that spatially neutrino-associated blazars of the tested samples have higher than average Doppler factor and X-ray brightness.

Figures (10)

• Figure 1: Sky distribution of the 356 updated IceCat1+ neutrinos and the 5880 blazars of the RFC$^\dagger$ and 4LAC samples. The black ellipses show the enlarged $\gtrsim$90%-likelihood error region of the neutrinos. The circles (symbol count: 2066), crosses (2655), and stars (1159) display blazars present only in RFC$^\dagger$, only in 4LAC, and in both RFC$^\dagger$ and 4LAC, respectively. The faded blue markings (4822) represent uncorrelated blazars, the orange (955) markings spatially associated blazars, the purple markings (7) spatio-temporally via BB95, the green markings (86) spatio-temporally via prominent BBHOP flares, and the red markings (10) spatio-temporally via both metrics which include, but are not limited to, the five associations arising from BB95 at the peak of BBHOP flares (see Sect. \ref{['sec_data_blz_BB95_and_BBHOP']}). In total, there are $7+10=17$ spatio-temporal associations using the BB95 metric and $86+10=96$ ones using the prominent BBHOP metric.
• Figure 2: Spatio-temporal correlation significance in Gaussian $\sigma$ against fraction of blazar-flare-emitted cosmic neutrinos in %. The blue box plots represent the distribution of the significances for 500 simulations. The running solid (dark blue) line denotes the mean of the significances. The dotted red and the dashed olive horizontal lines show the correlation significances between the updated IceCat1+ neutrinos and all blazars of the RFC$^\dagger$ and 4LAC samples, respectively, via the BB95 temporal metric (see Sect. \ref{['sec_results_frac_of_neutrinos_coming_from_blazars']}). The significance in each simulation is limited to a maximum of 3.9$\sigma$ (i.e., $p=0.0001$). In these simulations we used a light curve gap fraction of 50%.
• Figure 3: Light curve and sky map of the blazar CAZJ0211+1051 which is spatio-temporally associated with the neutrino IC131014A ($W_\mathrm{T}=0.665$). Plot (i) shows the entire CAZ light curve of the blazar along with the arrival time of the associated neutrino (shown using a solid, vertical, black line). The horizontal dotted green line shows the 75$^\mathrm{th}$ percentile flux density, and the horizontal dashed grey line the 95$^\mathrm{th}$ percentile. In the plot legend, "[C]V" refers to the CRTS data points (obtained without filter), "[K]V" to the V-band of KAIT, "[T]R" to R-band of Tuorla, "[A]o" to o-filter of ATLAS, "[A]c" to c-filter of ATLAS, "[Z]r" to r-filter of ZTF, "[Z]g" to g-filter of ZTF, and "[Z]i" to i-filter of ZTF. Plot (ii) gives the sky map centered around the main associated neutrino, whose enlarged error region edge is drawn using a solid black line and its published error region using a dashed black line. All other neutrino events are plotted in grey (enlarged error regions using dash-dotted lines and published ones using dotted lines). The location of the main blazar within the main associated neutrino is shown with an arrow. Similar to Fig. \ref{['fig_all_assoc_sky_map']}, blazars which are only in RFC$^\dagger$ are shown as circles, those only in 4LAC as crosses, and those in both RFC$^\dagger$ and 4LAC as stars. Likewise, all blazars which are uncorrelated to the main associated neutrino are marked in faded blue, in orange if only spatially associated, in grey if spatio-temporally associated via BB95, in green if spatio-temporally associated via prominent BBHOP flares, and in red if via both BB95 periods and prominent BBHOP flares simultaneously (not to be confused with the BB95 at the peak of a BBHOP metric described in Sect. \ref{['sec_data_blz_BB95_and_BBHOP']}). Plot (iii) shows a zoomed-in version of plot (i) around the arrival time of the associated neutrino. The zoom is demarcated in plot (i) using a pink box. The brown solid lines in the background show the BB, the grey vertical bands show the BB95, and the green solid lines forming a wedge show the prominent BBHOP flares (connecting the start of the flare to its peak and subsequently to its end). CAZJ0211+1051 is 0.17$^\circ$ outside of the published error region of IC131014A.
• Figure 4: Light curve and sky map of the blazar CAZJ0207+0950 which is spatio-temporally associated with the neutrino IC131014A ($W_\mathrm{T}=0.665$). For plot details, see description of Fig. \ref{['fig_CAZJ0211+1051_neut_MJD56579.909']}. We note that the y-axis in plot (i) is truncated to omit a few outliers. Spatially, CAZJ0207+0950 is 0.37$^\circ$ outside of the published error region of IC131014A.
• Figure 5: Light curve and sky map of the blazar CAZJ0509+0541 (TXS 0506+056) which is spatio-temporally associated with the neutrino IC170922A ($W_\mathrm{T}=0.631$). For plot details, see description of Fig. \ref{['fig_CAZJ0211+1051_neut_MJD56579.909']}. The grey dash-dotted vertical lines in plot (i) show spatial associations with a second (IC190317A with $W_\mathrm{T}=0.035$) and a third neutrino (IC220918A with $W_\mathrm{T}=0.089$).