2019ApJ...883L..13I

Recent active galactic nucleus (AGN) and quasar surveys have revealed a population showing rapid AGN luminosity variability by a factor of ∼10. Here we present the most drastic AGN luminosity decline by a factor of >=10³ constrained by a NuSTAR X-ray observation of the nearby galaxy Arp 187, which is a promising "dead" quasar whose current activity seems quiet but whose past activity of L_bol∼10⁴⁶ erg s^–1 is still observable at a large scale by its light echo. The obtained upper bound of the X-ray luminosity is log(L_2–10keV/ergs^–1)< 41.2, corresponding to log(L_bol/ergs^–1)< 42.5, indicating an inactive central engine. Even if a putative torus model with N_H∼1.5×10²⁴ cm^–2 is assumed, the strong upper bound still holds with log(L_2–10keV/ergs^–1)< 41.8 or log(L_bol/ergs^–1)< 43.1. Given the expected size of the narrow-line region, this luminosity decrease by a factor of >=10³ must have occurred within <=10⁴ yr. This extremely rapid luminosity/accretion shutdown is puzzling, and it requires one burst-like accretion mechanism producing a clear outer boundary for an accretion disk. We raise two possible scenarios realizing such an accretion mechanism: a mass accretion (1) by the tidal disruption of a molecular cloud and/or (2) by the gas depletion as a result of vigorous nuclear star formation after rapid mass inflow to the central engine.