An Unveiling Event in the Type 2 Active Galactic Nucleus NGC 4388:A Challenge for a Parsec-Scale Absorber

We present two Rossi X-Ray Timing Explorer ( RXTE ) Proportional Counter Array (PCA) observations of the type 2 Seyfert galaxy NGC 4388 caught in an unusual low X-ray absorption state. The observations were triggered by a detection in the 1.5-3 keV band of the RXTE all-sky monitor. NGC 4388 was found at a somewhat high continuum level [ f (2-10 keV) = 8 × 10 -11 ergs cm -2 s -1 ] and with a column density N H ~ 3 × 10 22 cm -2 , a factor of ~10 lower than normal. The second PCA observation, 4 hr later, gave N H < 2 × 10 21 cm -2 indicating, at the 3.1 σ level, variability so rapid it puts the absorber on a few 100 Schwarzschild radii scale, similar to the broad emission line region or smaller. This small scale creates difficulties for the parsec-scale obscuring torus paradigm of unified schemes for type 1 and type 2 active galactic nuclei.