Recent active galactic nucleus ( AGN ) and quasar surveys have revealed a population showing rapid AGN luminosity variability by a factor of \sim 10 .
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 _ { 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 _ { 2 - 10 ~ { } keV } / { erg } ~ { } { s } ^ { -1 } ) < 41.2 , corresponding to \log ( L _ { \mathrm { bol } } / { erg } ~ { } { s } ^ { -1 } ) < 42.5 , indicating an inactive central engine .
Even if a putative torus model with N _ { H } \sim 1.5 \times 10 ^ { 24 } cm ^ { -2 } is assumed , the strong upper-bound still holds with \log ( L _ { 2 - 10 ~ { } keV } / { erg } ~ { } { s } ^ { -1 } ) < 41.8 or \log ( L _ { \mathrm { bol } } / { erg } ~ { } { 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 .