Revisit Short Term X-ray Spectral Variability of NGC 4151 with Chandra
  We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151
observed with Chandra for 200 ks. A significant ACIS pileup is present,
resulting in a non-linear count rate variation during the observation. With
pileup corrected spectral fitting, we are able to recover the spectral
parameters and find consistency with those derived from unpiled events in the
ACIS readout streak and outer region from the bright nucleus. The absorption
corrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 erg
s^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historical
low state, the photon indices derived from the same absorbed power-law model
are \Gamma~0.7-0.9. However, we show that \Gamma is highly dependent on the
adopted spectral models. Fitting the power-law continuum with a Compton
reflection component gives \Gamma~1.1. By including passage of non-uniform
X-ray obscuring clouds, we can reproduce the apparent flat spectral states with
\Gamma~1.7, typical for Seyfert 1 AGNs. The same model also fits the hard
spectra from previous ASCA "long look" observation of NGC 4151 in the lowest
flux state. The spectral variability during our observation can be interpreted
as variations in intrinsic soft continuum flux relative to a Compton reflection
component that is from distant cold material and constant on short time scale,
or variations of partially covering absorber in the line of sight towards the
nucleus. An ionized absorber model with ionization parameter \log\xi ~ 0.8-1.1
can also fit the low-resolution ACIS spectra. If the partial covering model is
correct, adopting a black hole mass M_{BH} ~ 4.6E+7 Msun we constrain the
distance of the obscuring cloud from the central black hole to be r<~9
light-days, consistent with the size of broad emission line region of NGC 4151
from optical reverberation mapping.
