IceCube has detected astrophysical neutrinos in the TeV-PeV range that show an isotropic angular distribution, and their dominant origin remains unidentified [1]. Active Galactic Nuclei (AGN) is one such promising class of candidate sources of very-high-energy (VHE) neutrinos. According to the unified model of AGN [2], these sources have a central supermassive black hole surrounded by an accretion disk and are sometimes accompanied by relativistic jets. Blazars are the AGN with the orientation of the jet lying close to our line of sight.
An IceCube study from 2016 searched for neutrino point sources from the second Fermi-LAT AGN catalog (2LAC) [3,4]. No significant excess of neutrinos was observed but upper limits were obtained for a contribution to the diffuse flux of cosmic neutrinos from blazars. However, a neutrino detected by IceCube on September 22, 2017, was found to be in temporal and spatial coincidence with a blazar, TXS 0506+056, that was in a flaring state [5]. This was the first evidence of a correlation between any astrophysical source and a high-energy neutrino. Another study conducted with 10 years of IceCube data, searched for point-like sources of neutrinos and found an excess of neutrinos from a Seyfert II galaxy, NGC 1068, at a significance level of 2.9𝜎 [6]. These observations provide evidence in favor of AGN as possible neutrino sources.
There are various models that explain the production mechanisms of high-energy neutrinos from AGN. Purely leptonic models can reproduce the spectral energy distribution (SED) of blazars without neutrino production, while purely hadronic models may fail to reproduce the SED completely [7]. According to some hybrid (leptohadronic) models, the TeV-PeV gamma rays produced alongside neutrinos can interact with the ambient photons in AGN and cascade down to hard X-rays (high energy X-rays 10 keV) or MeV gamma rays [8][9][10]. This creates a scenario where we can no longer observe the TeV-PeV gamma rays but possibly find signatures of hard X-rays and MeV gamma rays. Hard X-rays could therefore be an important probe into the emission of neutrinos through hadronic processes.
The sources in our analysis are selected from the Swift-BAT 70-Month hard X-ray Survey [11]. It is a collection of objects observed by the Burst Alert Telescope (BAT) hard X-ray detector onboard the Niel Gehrels Swift gamma-ray burst observatory for a period of 70 months [12]. There are 1171 sources detected in the >10 keV band which are associated with 1210 counterparts.
The BAT AGN Spectroscopic Survey, or the BASS catalog Data Release-1, is a selection of AGN sources from the 70-month BAT catalog [13]. It has 838 AGN sources. The BASS catalog is the most complete all-sky catalog of hard X-ray sources observed in the range of 14-195 keV and includes soft X-ray spectra in the range of 0.3-10 keV. Spectral parameters such as the column density of the sources have been determined by the analysis of the X-ray data, both hard and soft, along with the available optical spectroscopic data [14].
A skymap of the BASS sources used in the study is shown in figure 1a. Several hard X-ray sources in the BASS catalog also exhibit gamma-ray emission (see figure 1b). For two sources in the BASS catalog, there are no column density values since they do not have a soft X-ray observation in the 0.3-10 keV range. They have not been included in our sample. Hence, we have 836 sources in our sample. 105 of the sources in the catalog are classified as blazars and 731 as non-blazars.
The analysis involves a stacked search for point sources using a time-integrated unbinned maximumlikelihood method [16,17].
For this analysis, the IceCube dataset used is the gamma-ray follow-up (GFU) sample [18]. It solely contains muon neutrino tracks (the tracks are ascribed to 𝜈 𝜇 and ν𝜇 interactions and have 2021 JINST 16 C09013 typical angular resolutions of <1 • ) with an all-sky coverage that have been reprocessed offline for better sensitivity. 7.5 years of the GFU sample from the complete 86-string IceCube configuration between 2011-2018 have been used.
We plan to test different hypotheses with our analysis. Currently, we have only considered the hypothesis outlined below.
In this part, we use all 836 AGN to perform the analysis by stacking the sources according to two different Weighting Schemes:
1. Flux weights: it is proportional to the intrinsic flux of the sources in the range, 14-195 keV.
this is an unbiased weight that treats all sources in the catalog equally.
The sensitivity and the 5𝜎 discovery potential are computed to give a sense of the power of the analysis performed. The sensitivity is defined as the flux required so that 90% of injected signal trials have a value of Test Statistic (TS) higher than in 50% of the background-only trials. The 5𝜎 discovery potential is the flux required so that 50% of the time this flux will produce a result inconsistent with the background-only hypothesis at a significance of 5𝜎. Figure 2 shows that the analysis is more sensitive for harder spectra and, for flux weights as compared to equal weights, for e.g., the sensitivity (and discovery potential) flux at 𝛾 = 2.0 for flux weights is 4.06 × 10 -12 (1.15 × 10 -11 ) TeV -1 cm -2 s -1 and for equal weights is 5.81 × 10 -12 (1.42 × 10 -11 ) TeV -1 cm -2 s -1 .
In future work, we will perform parts II and III of the analysis as described below.
Hypothesis II. We will search for a correlation between the luminosity of the sources and their neutrino emission by performing analyses separately on Blazars and non-blazars using Flux weights. The categorization follows the classification presented in the Swift-BAT catalog.
https://www.bass-survey.com/.