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We present new X-ray spectral data for the Seyfert 1 nucleus in NGC 4151observed with Chandra for 200 ks. A significant ACIS pileup is present,resulting in a non-linear count rate variation during the observation. Withpileup corrected spectral fitting, we are able to recover the spectralparameters and find consistency with those derived from unpiled events in theACIS readout streak and outer region from the bright nucleus. The absorptioncorrected 2-10 keV flux of the nucleus varied between 6E-11 and 1E-10 ergs^{-1} cm^{-2}. Similar to earlier Chandra studies of NGC 4151 at a historicallow state, the photon indices derived from the same absorbed power-law modelare \Gamma~0.7-0.9. However, we show that \Gamma is highly dependent on theadopted spectral models. Fitting the power-law continuum with a Comptonreflection component gives \Gamma~1.1. By including passage of non-uniformX-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 hardspectra from previous ASCA "long look" observation of NGC 4151 in the lowestflux state. The spectral variability during our observation can be interpretedas variations in intrinsic soft continuum flux relative to a Compton reflectioncomponent that is from distant cold material and constant on short time scale,or variations of partially covering absorber in the line of sight towards thenucleus. An ionized absorber model with ionization parameter \log\xi ~ 0.8-1.1can also fit the low-resolution ACIS spectra. If the partial covering model iscorrect, adopting a black hole mass M_{BH} ~ 4.6E+7 Msun we constrain thedistance of the obscuring cloud from the central black hole to be r<~9light-days, consistent with the size of broad emission line region of NGC 4151from optical reverberation mapping.

Revisit Short Term X-ray Spectral Variability of NGC 4151 with Chandra