[Paper Review] Dissecting the broadband emission from γ-ray blazar PKS 0735+178 in search of neutrinos

Raj Prince, Saikat Das+3 more|arXiv (Cornell University)|Jan 16, 2023

Astrophysics and Cosmic Phenomena44 references4 citations

TL;DR

This study models the broadband spectral energy distribution (SED) of the γ-ray blazar PKS 0735+178 during a multiwavelength flare, incorporating photohadronic (pγ) interactions in a one-zone lepto-hadronic model with an external photon field to explain a potential neutrino association. It predicts a neutrino event rate of 0.12 per 100 days in IceCube, constrained by X-ray emission from secondary electrons, and finds a plausible correlation between X-ray and neutrino fluxes despite low overall flux levels.

ABSTRACT

The origin of the diffuse flux of TeV-PeV astrophysical neutrinos is still unknown. The $γ$-ray blazar PKS 0735+178, located outside the 90\% localization region at 2.2 deg from the best-fit IC-211208A event, was found to be flaring across all wavebands. In addition to leptonic synchrotron (SYN) and synchrotron self-Compton (SSC) emission, we invoke photohadronic ($pγ$) interactions inside the jet to model the spectral energy distribution (SED) and neutrino emission. We analyze the 100 days $γ$-ray and X-ray data and 10 days around the neutrino event is chosen to generate the broadband SED. The temporal light curve indicates that the source was in a high state in optical, UV, $γ$-ray, and X-ray frequencies during the neutrino detection epoch. In the one-zone lepto-hadronic model, the SSC photons do not provide enough seed photons for $pγ$ interactions to explain the neutrino event. However, including an external photon field yields a neutrino event rate of 0.12 in 100 days, for the IceCube detector, using physically motivated values of the magnetic field, an external photon field peaking at optical wavelength, and other jet parameters. The radiation from secondary electrons at X-ray energies severely constrains the neutrino flux to a lower value than found in previous studies. Moreover, the flux of high-energy $γ$-rays at GeV energies from the decay of neutral pions is subdominant at the high-energy peak of the SED, suggesting a higher correlation of neutrinos flux with X-ray flux is plausible.

Motivation & Objective

• To investigate the origin of the IceCube neutrino event IC-211208A, located 2.2° from PKS 0735+178, by modeling its broadband emission across γ-ray, X-ray, UV, and optical bands.
• To determine whether photohadronic (pγ) interactions in the jet can explain the observed neutrino event, given the lack of sufficient seed photons in the standard one-zone SSC model.
• To constrain the neutrino flux by including radiation from secondary electrons produced in pγ interactions, particularly at X-ray energies.
• To assess the feasibility of the blazar as a source of the diffuse high-energy neutrino background, considering jet power and external photon field requirements.

Proposed method

• A one-zone lepto-hadronic model is used to fit the broadband SED of PKS 0735+178 during a 100-day flare, incorporating synchrotron (SYN), synchrotron self-Compton (SSC), and external Compton (EC) emission components.
• Photohadronic (pγ) interactions are included as the primary mechanism for high-energy neutrino production, with seed photons provided by an external blackbody field peaking at optical wavelengths.
• The model incorporates radiation from secondary electrons produced in pγ interactions, which significantly affects the X-ray band and constrains the allowed neutrino flux.
• Jet parameters such as magnetic field, size, bulk Lorentz factor, and proton injection spectrum are constrained using observational data and physical consistency checks.
• The neutrino event rate is calculated for the IceCube detector using the derived neutrino flux, assuming a 100-day observation window around the IC-211208A event.
• The model is tested against constraints from the X-ray flux, which limits the maximum allowed neutrino flux due to secondary electron emission.

Experimental results

Research questions

• RQ1 Can photohadronic (pγ) interactions in the jet of PKS 0735+178 explain the IceCube neutrino event IC-211208A, given the lack of sufficient seed photons in the SSC component?
• RQ2 What is the required external photon field (in terms of luminosity and peak energy) to enable pγ interactions and produce a detectable neutrino flux?
• RQ3 How do secondary electron emissions in the X-ray band constrain the maximum allowed neutrino flux in the model?
• RQ4 Is the predicted neutrino flux from PKS 0735+178 consistent with the observed IceCube sensitivity and background levels?
• RQ5 What is the predicted correlation between X-ray and neutrino fluxes in this model, and how does it compare to observational data?

Key findings

• The inclusion of an external photon field peaking at optical wavelengths (8.3 eV) enables pγ interactions, yielding a neutrino event rate of 0.12 per 100 days in IceCube, which is consistent with the observed neutrino event rate.
• The radiation from secondary electrons in pγ interactions produces X-ray emission that severely constrains the allowed neutrino flux, reducing it below previous estimates.
• The required jet power (6.3×10⁴⁷ erg/s) exceeds the Eddington luminosity (1.2×10⁴⁶ erg/s) for a black hole mass of 10⁸.⁸ M☉, indicating extreme physical conditions.
• The high-energy γ-ray emission from neutral pion decay is subdominant at the SED's high-energy peak, suggesting a stronger correlation between X-ray and neutrino fluxes than between γ-ray and neutrino fluxes.
• The model predicts a plausible neutrino flux that is still below IceCube's detection sensitivity, but consistent with the observed event rate and multiwavelength constraints.
• The study confirms that X-ray flux is a more reliable proxy for neutrino emission than γ-ray flux in this scenario, due to secondary electron contributions.