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 x 10(-11) ergs cm(-2) s(-1)] and with a column density N-H similar to 3 x 10(22) cm(-2), a factor of similar to10 lower than normal. The second PCA observation, 4 hr later, gave N-H < 2 x 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.