Bayesian parameter study of the Seyfert-starburst composite galaxies NGC 1068 and NGC 7469

Abstract

Multimessenger observation of the Seyfert-starburst composite galaxies NGC 1068 and NGC 7469 indicate a characteristic feature in the radio band (the so-called mm-bump) as well as indication of high-energy neutrinos by the AGN corona. Moreover, also the starburst ring of these sources is bright in the radio and hence, a potential source of $γ$-rays and neutrinos. We aim to explain the non-thermal features of these two sources with our homogeneous steady-state Seyfert-starburst composite model, which we refined in this work. Hereby, we account for stochastic diffuse acceleration and energy losses within the corona and $γγ$-pair attenuation of the escaping $γ$-rays. Since the non-thermal features of Seyfert sources contribute only marginally to the electromagnetic spectrum, only few data points can be assigned to the starburst ring or the AGN corona. Hence, prior information on the physical parameters is incorporated within a Markov Chain Monte Carlo approach to avoid overfitting. Based on this Bayesian parameter study we show, that the non-thermal features of NGC 1068 can be explained well. Still a more detailed treatment of the spatial inhomogeneities in the central region of the AGN could further improve the fit results. This manifests itself even more clearly in the case of NGC 7469, where the mm-bump needs to emerge from a coronal size $R_{\rm c}>100\,\mathcal{R}_{\rm s}$, whereas (TeV-PeV)-neutrino emission requires $R_{\rm c}< 10\,\mathcal{R}_{\rm s}$. Similar to what has previously been shown in other wavebands, our analysis highlights that the spatial extension of the so-called AGN corona depends the considered energy of the messenger. Hence, it seems that there is not a unique edge of the corona and a substantial progress in the understanding of these phenomena is expected if future analysis account for these spatial inhomogeneities.

Figures (7)

• Figure 1: The best-fit model predictions of the CR electron (blue) and proton (red) distribution within the AGN corona (left) and the starburst-ring (right) of NGC 1068.
• Figure 2: The best-fit model prediction (and 100 alternative scenarios within 1$\sigma$ of the best-fit in transparent colored lines) of the photon ($\gamma$) and neutrino ($\nu$) SED of NGC 1068. The grey markers indicate characteristic neutrino flux values within the observed 68% confidence band that are used in the MCMC.
• Figure 3: The best-fit model predictions (and 100 alternative scenarios within 1$\sigma$ of the best-fit in transparent colored lines) of the photon ($\gamma$) and neutrino ($\nu$) SED of NGC 7469. The blue and red shaded bands at low energies refer to the uncertainty of the adopted extended gas distribution and its opacity with respect to free-free absorption. The grey marker indicate the characteristic neutrino flux value of the average energy sommani2025two of the two neutrinos within the confidence band that is used in the MCMC.
• Figure 4: The posterior distribution of the fit parameters for NGC 1068. The blue lines indicate the best-fit parameter values and the magenta curve illustrates the adopted prior distribution.
• Figure 5: The posterior distribution of the fit parameters for the starburst ring of NGC 7469. The blue lines indicate the best-fit parameter values and the magenta curve illustrates the adopted prior distribution.