NuSTAR Discovery of Dead Quasar Engine in Arp 187

Recent active galactic nucleus (AGN) and quasar surveys have revealed a population showing rapid AGN luminosity variability by a factor of $\sim10$. Here we present the most drastic AGN luminosity decline by a factor of $\gtrsim 10^{3}$ 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 $L_\mathrm{bol} \sim 10^{46}$ erg s$^{-1}$ is still observable at a large scale by its light echo. The obtained upper bound of the X-ray luminosity is $\log (L_{\rm 2-10 keV}/{\rm erg} {\rm s}^{-1}) < 41.2$, corresponding to $\log (L_\mathrm{bol}/{\rm erg} {\rm s}^{-1}) < 42.5$, indicating an inactive central engine. Even if a putative torus model with $N_\mathrm{H} \sim 1.5 \times 10^{24}$ cm$^{-2}$ is assumed, the strong upper-bound still holds with $\log (L_{\rm 2-10 keV}/{\rm erg} {\rm s}^{-1}) < 41.8$ or $\log (L_\mathrm{bol}/{\rm erg} {\rm s}^{-1}) < 43.1$. Given the expected size of the narrow line region, this luminosity decrease by a factor of $\gtrsim 10^3$ must have occurred within $\lesssim 10^4$ 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 starformation after rapid mass inflow to the central engine.