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Neutrino Emissions from Tidal Disruption Remnants

We study high-energy neutrino emissions from tidal disruption remnants (TDRs) around supermassive black holes. The neutrinos are produced by the decay of charged pions originating in ultrarelativistic protons that are accelerated there. In the standard theory of tidal disruption events (TDEs), there are four distinct phases from debris circularization of stellar debris to super- and sub-Eddington to radiatively inefficient accretion flows (RIAFs). In addition, we consider the magnetically arrested disk (MAD) state in both the super-Eddington accretion and RIAF phases. We find that there are three promising cases to produce neutrino emissions: the super-Eddington accretion phase of the MAD state and the RIAF phases of both the non-MAD and MAD states. In the super-Eddington MAD state, the enhanced magnetic field makes it possible to accelerate the protons to $E_{p,max}~0.35 PeV (M_bh/10^{7.7}M_\odot)^{41/48}$ with the other given appropriate parameters. The neutrino energy is then $E_{ν,pk}~67 TeV (M_bh/10^{7.7}M_\odot)^{41/48}$ at the peak of the energy spectrum. For $M_bh\gtrsim10^{7.7} M_\odot$, the neutrino light curve is proportional to $t^{-65/24}$, while it follows the standard $t^{-5/3}$ decay rate for $M_bh<10^{7.7} M_\odot$. In both cases, the large luminosity and characteristic light curves diagnose the super-Eddington MAD state in TDEs. In the RIAF phase of the non-MAD state, we find $E_{p, max}~0.45 PeV (M_bh/10^7M_\odot)^{5/3}$ and $E_{ν,pk}~0.35 PeV (M_bh/10^7M_\odot)^{5/3}$, and its light curve is proportional to $t^{-10/3}$. This indicates that one can identify whether the existing RIAFs are the TDE origin or not. TDRs are potentially a population of hidden neutrino sources invisible in gamma rays.