PKS 2332$-$017 and PMN J1916$-$1519: Candidate Blazar Counterparts to Two High-energy Neutrino Events

Neutrino astronomy has great potential of providing us with more and different information for various celestial objects in the Universe, and the IceCube (Aartsen et al., 2017), a neutrino observatory built at the South Pole, is one of the large facilities leading the field in exploring the neutrino sky. Since 2013, IceCube has been detecting high-energy neutrino events that more likely have an astrophysical origin (Aartsen et al., 2013), and it was in 2017 that the first convincing association was established. The case was between a $\sim$0.3 PeV neutrino event, named IC-170922A, and a flaring blazar TXS 0506+056, resulting from the multimessenger campaign (IceCube Collaboration et al., 2018a, b). Following this first association case, the neutrino emission from the nearby Seyfert galaxy NGC 1068 at a significance of 4.2$\sigma$ and that from the Galactic plane at a 4.5$\sigma$ significance level have also been reportedly revealed (IceCube Collaboration et al., 2022; Icecube Collaboration et al., 2023).

Blazars are a sub-type of Active Galactic Nuclei (AGNs) having their jets pointing close to our line of sight. They have been considered as possible sources of high-energy neutrinos (e.g., Mannheim 1995; Mücke et al. 2003; Murase et al. 2018). They are generally categorized as two types, Flat Spectrum Radio Quasars (FSRQs) and BL Lac objects (BL Lacs), based on the observed strengths of their optical emission lines (Urry & Padovani, 1995; Scarpa & Falomo, 1997). Electrons accelerated in the blazars’ jets emit strongly via the synchrotron process in the radio to X-ray bands. Thus there are also three sub-classes used to describe their synchrotron emissions and spectral energy distributions (SEDs) based on the frequency values of the synchrotron peak $\nu^{\rm syn}_{\rm pk}$ (Abdo et al., 2010): low synchrotron peaked (LSP), intermediate synchrotron peaked (ISP), and high synchrotron peaked (HSP). Protons should also be accelerated in the jets, and the high-energy neutrino emission may originate from the photopion processes, as the result of the interaction between relativistic protons and low-energy photons; charged and neutral pions are produced in the interaction, which subsequently decay and produce neutrinos and high-energy photons.

Given the neutrino detection from TXS 0506+056, efforts have been made to find more cases. A few blazars have been reported as the possible neutrino sources. For instance, there are two LSP BL Lacs, GB6 J1040+0617 (Garrappa et al., 2019) and PKS 2254+074 (Ji & Wang, 2024), two ISP BL Lacs, MG3 J225517+2409 (Franckowiak et al., 2020) and PKS 0735+178 (Sahakyan et al., 2023), an HSP BL Lac, 3HSP J095507.9+355101 (Giommi et al., 2020), and three LSP FSRQs, PKS B1424$-$418 (Kadler et al., 2016), GB6 J2113+1121 (Liao et al., 2022), and NVSS J171822+423948 (Jiang et al., 2024). The criteria for the identifications of these blazar sources follow the case of TXS 0506+056: each of them was positionally coincident with a high-energy neutrino event and at the arrival time of the neutrino, the blazar was having a $\gamma$-ray flare (or an X-ray flare in the case of 3HSP J095507.9+355101).

Recently, the updated IceCube Event Catalog of Alert Tracks (IceCat-1) has been released, which contains more than 100 neutrino track-like events likely of astrophysical origin (with energy $\geq$ 100 TeV) detected from 2011 to 2023 (Abbasi et al., 2023). In IceCat-1, based on a quantity called “signalness”, the neutrino events having a significant probability of being an astrophysical neutrino are classified as Gold alerts, and they have an average signalness above 50% (Abbasi et al., 2023). We went through the events in IceCat-1 and found two possible association cases. The high-energy neutrino events are IC-130127A and IC-131204A, both belonging to the Gold alerts. Based on the general criteria currently considered in the association studies, the neutrinos possibly came respectively from two blazars, PKS 2332$-$017 and PMN J1916$-$1519. We report the association studies here. Throughout this work, we used the following cosmological parameters, $H_{0}$ = 67.7 km s^-1 Mpc^-1, $\Omega_{m}$ = 0.32, and $\Omega_{\Lambda}$ = 0.68 (Planck Collaboration et al., 2020).

We went through the Gold events in IceCat-1, and found two blazars, PKS 2332$-$017 and PMN J1916$-$1519, as the possible neutrino sources, in addition to the previously reported neutrino blazars. The two sources are in positional coincidence respectively with IC-130127A and IC-131204A and had a flare temporally coincident with the respective arrival times of the two neutrinos. For PKS 2332$-$017, significant flux variations at optical and MIR bands accompanying the flare were observed, which were also seen in nearly all previously reported neutrino blazars. For PMN J1916$-$1519, due to the lack of multiband flux measurements over the 4-month flare, we do not have the information. Although it is not possible to determine the emission hardening during the flares for the two sources because of their weak emission during the quiescent state, we have found high-energy photons, with energy as high as $\sim$10 GeV, during the flares, which may be considered as evidence for emission hardening (Ji & Wang, 2024).

PKS 2332$-$017 is an FSRQ at redshift $z$ = 1.18 (Wills & Lynds, 1978). It is classified as an LSP in the Data Release 3 (DR3) of the Fourth Catalog of Active Galactic Nuclei detected by the Fermi LAT (4LAC-DR3; Ajello et al. 2022), with the peak frequency $\nu^{\rm syn}_{\rm pk}\approx 2.52\times 10^{12}$ Hz (in 4LAC-DR3) or $\approx 6.31^{+6.29}_{-3.15}\times 10^{12}$ Hz given by Firmamento³³3https://firmamento.hosting.nyu.edu/data_access (Tripathi et al., 2024). It is thus similar to several other reported neutrino blazars, PKS B1424$-$418, GB6 J2113+1121, and NVSS J171822+423948, which are all LSP FSRQs. Its long-term $\gamma$-ray luminosity and spectral index $\alpha$ were $3.7\times 10^{46}$ erg s^-1 and 2.36, and during the neutrino-arriving flare (MJD 55582.66–57022.66), the values were $7.7\times 10^{46}$ erg s^-1 and 2.47, respectively. These values are typical for an FSRQ. Its SED is shown in the left panel of Figure 4.

PMN J1916$-$1519 only has a photometric redshift $z$=0.968 (Foschini et al., 2022). It is also an LSP blazar, as its $\nu^{\rm syn}_{\rm pk}\approx 6.24\times 10^{12}$ Hz (in 4LAC-DR3) or $\approx 6.31^{+3.69}_{-2.33}\times 10^{12}$ Hz given by Firmamento. Assuming the redshift, the $\gamma$-ray luminosities are estimated to be $2.0\times 10^{46}$ erg s^-1 in its long-term 16 year of Fermi LAT data and $9.4\times 10^{46}$ erg s^-1 in its 4-month flare. BL Lacs and FSRQs can be relatively well separated in the plot of $\gamma$-ray luminosity versus photon index (e.g., Chen 2018). Following the criterion line to separate BL Lacs and FSRQs from Chen 2018, PMN J1916$-$1519 may be classified as an FSRQ. Its SED is shown in the right panel of Figure 4.

The $\gamma$-ray flare of PMN J1916$-$1519 in temporal coincidence with IC-131204A was short, only $\sim$4 month. Such short-flare cases were seen before in other neutrino blazars. PKS 0735+178 had a $\sim$3-week $\gamma$-ray flare temporally coincident with the arrival time of IC-211208A (Sahakyan et al., 2023), GB6 J1040+0617 had a $\sim$3-month $\gamma$-ray flare (IC-141209A; Garrappa et al. 2019), MG3 J225517+2409 showed a $\sim$5 month one (IC-100608A; Franckowiak et al. 2020), and TXS 0506+056 had a $\sim$ 6-month one (IC-170922A; IceCube Collaboration et al. 2018a). PMN J1916$-$1519 would be unique as it only had such a short flare over 16 year of the Fermi LAT observation time. According to the simulation results presented in Capel et al. 2022, the chance coincidence probabilities between $\gamma$-ray flares of blazars and neutrino alerts are high, maybe $\sim$3.8%–12.7%, but when certain source flux and flare variation amplitude are considered, the probability could be lowered to $<$0.1% (i.e., $>$3$\sigma$ significance for the source association). PMN J1916$-$1519 could be such an example as its flare variation amplitude reached $\sim$5.0.

The redshift and $\gamma$-ray luminosity of PKS 2332$-$017 are comparable with the values of previously identified neutrino blazars. For example, their redshift range is 0.19–2.68 and luminosity range is $\sim$ 10⁴⁴–10⁴⁸ erg s^-1 (Ji & Wang, 2024). The neutrino flux may be estimated from the observed $\gamma$-ray flux (see Giommi et al. 2020 and Jiang et al. 2024), using the relationship between them given by (Murase et al., 2018),

Here, $Y_{IC}$ represents the Compton-Y parameter, expected typically $Y_{IC}$ $\leq 1$ (Murase et al., 2018), and the $\gamma$-ray luminosity term $\epsilon_{\gamma}L_{\epsilon_{\gamma}}\sim$ $\mathcal{L}$${}_{\gamma}/\ln(500\,{\rm GeV}/100\,{\rm MeV})$ $\approx$ $9.1\times 10^{45}$ erg s^-1 (for 4 yr) and $4.4\times 10^{45}$ erg s^-1 (for 16 yr; see Giommi et al. 2020 for details); $\mathcal{L}$_γ is the observed $\gamma$-ray luminosity for the source. Then the average muon neutrino luminosities (i.e., $1/3{\epsilon_{\nu}}{L_{\epsilon_{\nu}}}$) would be $3.5\times 10^{45}$ erg s^-1 (4 yr) and $1.7\times 10^{45}$ erg s^-1 (16 yr). On the other hand, the number of muon (and antimuon) neutrinos, $N_{\nu_{\mu}}$, detected by IceCube during time interval $\Delta{T}$ at declination $\delta$ may be estimated from

where $\epsilon_{\nu_{\mu,\text{min}}}$ and $\epsilon_{\nu_{\mu,\text{max}}}$ are the minimum and maximum neutrino energy, respectively, and $\phi_{\nu_{\mu}}$ is the differential energy flux of muon neutrinos. Using the muon neutrino flux estimated from Eq. 1 above and assuming a power-law neutrino energy spectrum with an index of $-$2 from 80 TeV to 8 PeV (Oikonomou et al., 2021), we find from Eq. 2 the expected numbers of neutrinos to be 0.09 ($\Delta{T}=$ 4 yr) and 0.18 ($\Delta{T}=$ 16 yr) for the effective area $A_{\rm eff}\approx 23$ m² (GFU_Gold, see Abbasi et al. 2023). In other words, the Poisson probabilities to detect one neutrino are $\sim$0.08 and $\sim$0.15, respectively. Alternatively, we may estimate the neutrino flux from Eq. 2 by setting $N_{\nu_{\mu}}=1$ (given the neutrino event), and the resulting energy fluxes for two $\Delta T$ values are shown in Figure 4.

Similarly for PMN J1916$-$1519, where we used the photometric redshift, we estimated its $\gamma$-ray luminosity term $\epsilon_{\gamma}L_{\epsilon_{\gamma}}\approx$ $1.1\times 10^{46}$ erg s^-1 (for 4 month) and $2.3\times 10^{45}$ erg s^-1 (for 16 yr), the average muon neutrino luminosities $\sim 4.2\times 10^{45}$ erg s^-1 (4 month) and $\sim 8.9\times 10^{44}$ erg s^-1 (16 yr). The expected numbers of neutrinos would be $\approx 5\times 10^{-4}$ (for 4 month) and $\approx 5\times 10^{-3}$ (for 16 yr), suggesting low neutrino detection probabilities of $<$1% for PMN J1916$-$1519. The low detection probabilities, including those estimated in 3HSP J095507.9+355101 (Giommi et al., 2020), NVSS J171822+423948 (Jiang et al., 2024), and PKS 2254+074 (Ji & Wang, 2024), are actually consistent with the results from studies of SEDs with one-zone lepto-hadronic models (e.g., Keivani et al. 2018; Petropoulou et al. 2020b). The extremely low probability in the flare case of PMN J1916$-$1519 was also seen in the SED modeling of 3HSP J095507.9+355101, as Petropoulou et al. (2020a) found a neutrino-detection probability of 0.06% for the X-ray flare of the blazar.

Finally, the plot of 0.1–100 GeV flux versus photon index of $\gamma$-ray blazars (Figure 5), which was previously shown in Liao et al. (2022), is updated by us. Five more sources, PKS 0735+178 (Sahakyan et al., 2023), NVSS J171822+423948 (Jiang et al., 2024), and PKS 2254+074 (Ji & Wang, 2024), plus two sources in this work, are added in the plot (but we do not include 3HSP J095507.9+355101 since its temporal coincidence was between an X-ray flare and a neutrino). Now there are 10 flaring blazar candidates in total. Their average fluxes, given by the long-term Fermi LAT data, are in a wide range of from $4\times 10^{-9}$ to $5\times 10^{-7}$ ph s^-1 cm^-2. However, as seen in the two sources of this work, one/two flares can dominate the fluxes. In other words, the emission activity of such blazars, with PMN J1916$-$1519 as an extreme case, is mainly contributed by flaring events. Thus as already shown by the case of TXS 0506+056, it is very likely that the neutrino emission in blazars is closely related to the flares. This is also supported, for example, by the blazar population studies (Capel et al., 2022).