Abstract

Recent active galactic nucleus (AGN) and quasar surveys have revealed a population showing rapid AGN luminosity variability by a factor of similar to 10. Here we present the most drastic AGN luminosity decline by a factor of greater than or similar to 10(3) 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 similar to 10(46) 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(L2-10 keV/erg s(-1)) < 41.2, corresponding to log(L-bol/erg s(-1)) < 42.5, indicating an inactive central engine. Even if a putative torus model with N-H similar to 1.5 x 10(24) cm(-2) is assumed, the strong upper bound still holds with log(L2-10 keV/erg s(-1)) < 41.8 or log(L-bol/erg s(-1)) < 43.1. Given the expected size of the narrow-line region, this luminosity decrease by a factor of greater than or similar to 10(3) must have occurred within less than or similar to 10(4) 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.