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On the Origin of High Energy Neutrinos from NGC 1068: the Role of Non-Thermal Coronal Activity

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On the Origin of High Energy Neutrinos from NGC 1068: the Role of Non-Thermal Coronal Activity Draft version February 20, 2020 Typeset using LATEX twocolumn style in AASTeX63 On the Origin of High Energy Neutrinos from NGC 1068: The Role of Non-Thermal Coronal Activity Yoshiyuki Inoue,1, 2 Dmitry Khangulyan,3 and Akihiro Doi4, 5 1Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS), RIKEN, 2-1 Hirosawa, Saitama 351-0198, Japan 2Kavli Institute for the Physics and Mathematics of the Universe (WPI), UTIAS, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan 3Department of Physics, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan 4Institute of Space and Astronautical Science JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan 5Department of Space and Astronautical Science, The Graduate University for Advanced Studies (SOKENDAI),3-1-1 Yoshinodai, Chuou-ku, Sagamihara, Kanagawa 252-5210, Japan (Dated: February 20, 2020) ABSTRACT NGC 1068, a nearby type-2 Seyfert galaxy, is reported as the hottest neutrino spot in the 10-year survey data of IceCube. Although there are several different possibilities for the generation of high- energy neutrinos in astrophysical sources, feasible scenarios allowing such emission in NGC 1068 have not yet been firmly defined. We show that the flux level of GeV and neutrino emission observed from NGC 1068 implies that the neutrino emission can be produced only in the vicinity of the supermassive black hole in the center of the galaxy. The coronal parameters, such as magnetic field strength and corona size, making this emission possible are consistent with the spectral excess registered in the millimeter range. The suggested model and relevant physical parameters are similar to those revealed for several nearby Seyferts. Due to the internal gamma-ray attenuation, the suggested scenario cannot be verified by observations of NGC 1068 in the GeV and TeV gamma-ray energy bands. However, the optical depth is expected to become negligible for MeV gamma rays, thus future observations in this band will be able to prove our model. Keywords: accretion, accretion disks | black hole physics | galaxies: active | galaxies: Seyfert | acceleration of particles | neutrinos 1. INTRODUCTION Production of very-high-energy (VHE) neutrinos is IceCube registered first astrophysical neutrinos with accompanied by emission of gamma-rays and the lu- energies of TeV{PeV several years ago (Aartsen et al. minosity of that component exceeds the neutrino one. 2013). However, their origin remains uncertain. Re- NGC 1068 is known as a gamma-ray emitter (Lenain cently, a nearby Seyfert galaxy NGC 1068, which ap- et al. 2010; Ajello et al. 2017; The Fermi-LAT collab- peared as the hottest spot in all-sky 10-year survey oration 2019). However, the reported neutrino flux is data of IceCube, was reported to be a neutrino source higher than the GeV gamma-ray flux (IceCube Collabo- with 2.9-σ confidence level (IceCube Collaboration et al. ration et al. 2019). Thus, it requires a significant atten- uation of GeV gamma-rays. Unless one adopts a very arXiv:1909.02239v4 [astro-ph.HE] 19 Feb 2020 2019). Thus, studying possible neutrino production mechanisms in NGC 1068 is a timely task that may exotic spectrum of emitting particles, this implies a pres- ence of enough dense X-ray target photons, X 1 keV. provide a key clue for unveiling the origin of the cos- ∼ mic diffuse neutrino background flux. The gamma-ray optical depth τ depends on the X-ray luminosity of this component LX and the size of the production regions R: [email protected] −1 σγγ −1 −1 5 X LX Rs τ X LX R 10 ; (1) [email protected] ≈ 4πc ' 1 keV LEdd R [email protected] where LEdd is the Eddington luminosity and Rs is the Schwarzschild radius. Although this estimate is not sen- sitive to the mass of the source, it suggests that such a 2 dense X-ray target can exist only in the vicinity of com- data (Antonucci & Barvainis 1988; Barvainis et al. 1996; pact objects. Doi & Inoue 2016; Behar et al. 2018). A large number, 103, of stellar-mass black hole Recently, Inoue & Doi(2018) reported the detection (or neutron star) systems,N ∼ i.e., X-ray binaries, can pro- of non-thermal coronal radio synchrotron emission from duce the neutrinos with the flux level required by the two nearby Seyferts, IC 4329A and NGC 985, utiliz- IceCube detection without violating the above crite- ing the Atacama Large Millimeter/submillimeter Array ria. Their typical luminosity distribution in a galaxy (ALMA), which enabled multi-band observations with −1:6 is represented by a power-law as dN=dLX LX high enough angular resolution to exclude the galactic (Swartz et al. 2011). This implies that the dominant/ contamination. These observations provided the first contributor is the higher luminosity systems. How- determination of the key physical parameters of the ever, NGC 1068 hosts only three X-ray binaries with corona: magnetic field strength and its size. Given 39 −1 LX 10 erg s (Swartz et al. 2011), thus their num- the coronal parameters constrained by X-ray and ra- ber≥ is not sufficient at least by several orders of magni- dio observations, the non-thermal acceleration process tude. The only remaining candidate is the supermassive there should be capable of boosting particle energy to black holes (SMBHs) at the center of the galaxy. High the very-high-energy band resulting in the generation accretion rate, implied by the intrinsic X-ray luminosity of high-energy gamma-ray and neutrino emission (Inoue −2 of LX 10 LEdd (Bauer et al. 2015; Marinucci et al. et al. 2019). 2016),∼ does not allow an effective particle acceleration Very Long Baseline Array (VLBA) radio observations in the central black hole magnetosphere. Thus, Emis- detected a homogeneous emission at the cm-bands from sion of very-high-energy neutrinos from the vicinity of the central parsec of NGC 1068. The reported flux is SMBHs indicates the operation of efficient non-thermal attributed to the free-free emission (Gallimore et al. particle acceleration in the active galactic nuclei (AGNs) 2004). This spectrum is not consistent with the fluxes coronae. observed at higher frequencies, mm-bands, which shows NGC 1068 is a type-2 Seyfert galaxy, which is a class a spectral excess (see the Sec.\Observational Proper- of AGNs, emitting intense electromagnetic radiation in ties"). Because of the spectral shape the excess com- a broad range of frequencies. The intrinsic X-ray emis- ponent should be produced by non-thermal particles lo- sion is generated through Comptonization of accretion calized in a much more compact region, with the size disk photons in hot plasma above the disk, namely in similar to the one of the corona. corona (e.g., Katz 1976; Bisnovatyi-Kogan & Blinnikov In this Letter, we study the constraints on the non- 1977; Pozdniakov et al. 1977; Galeev et al. 1979; Taka- thermal particles in the corona of NGC 1068 imposed by hara 1979). Typical size of the AGN corona is about the observations in the radio and gamma-ray bands. We & 10Rs. If high-energy particles are accelerated in the check if the reported flux of VHE neutrino is consistent corona, the nucleus of NGC 1068 is a plausible candi- with the revealed properties of the corona non-thermal date for the observed neutrinos, which is consistent with particles. We also discuss the reason why NGC 1068 a number of previous studies (see e.g., Begelman et al. appears as the hottest spot among other Seyfert galaxies 1990; Stecker et al. 1992; Kalashev et al. 2015; Inoue based on the corona scenario. et al. 2019; Murase et al. 2019). There has been no clear observational evidence for the non-thermal coronal ac- 2. OBSERVATIONAL PROPERTIES tivity (Lin et al. 1993; Madejski et al. 1995). However, as NGC 1068 is one of the nearest and the best-studied several corona parameters, e.g., magnetic field strength Seyfert 2 galaxies in broadband (see, e.g., Pasetto et al. and corona size, remain highly uncertain, one cannot 2019, for details), where the central engine is supposed rule out the presence of high-energy particles in AGN to be blocked by the dusty torus. It locates at a distance corona. These parameters have a critical impact on the of 14 Mpc (100 70 pc, Tully 1988). expected non-thermal flux level. The∼ mass of the∼ central black hole is still uncertain. 7 The coronal synchrotron emission is a key for dissolv- It is estimated as 1 10 M from the measurement ing this problem as it reflects the non-thermal coronal of the rotational motion∼ × of a water maser disk (Green- activity directly and allows determining corona mag- hill et al. 1996; Hur´e 2002; Lodato & Bertin 2003). netic properties (e.g., Di Matteo et al. 1997; Inoue & However, the rotation curve is non-Keplerian (Lodato Doi 2014). By combining the radio and X-ray spectra, & Bertin 2003). The SMBH mass is also estimated as 7 8 one can also determine the size of coronae. A charac- 7 10 M and 1 10 M from the polarized broad teristic feature of the coronal synchrotron emission is Balmer∼ × emission line∼ and× the neutral FeKα line, respec- a spectral excess in the millimeter-band, so-called mm- tively (Minezaki & Matsushita 2015). In this study, we 7 excess. However, previous observations had presented adopt 5 10 M as the mass of the central SMBH of inconclusive evidence of such excess in the radio spectra NGC 1068.× of several Seyfert galaxies. A key observational chal- In centimeter radio observations, the jets are promi- lenge in registering the excess is the contamination by nent and extend for several kpcs in both directions.
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