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

Abstract

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~3×10²² cm^-2, a factor of ~10 lower than normal. The second PCA observation, 4 hr later, gave N_H<2×10²¹ 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.