  Since the launch of the Einstein X-ray Observatory in the 1970s, a number of
broad absorption features have been reported in the X-ray spectra of BL Lac
objects. These features are often interpreted as arising from high velocity
outflows intrinsic to the BL Lac object, therefore providing important
information about the inner environment around the central engine. However,
such absorption features have not been observed more recently with
high-resolution X-ray telescopes such as Chandra and XMM-Newton. In this paper,
we report the detection of a transient X-ray absorption feature intrinsic to
the BL Lac object H 2356-309 with the Chandra X-ray Telescope. This BL Lac
object was observed during XMM cycle 7, Chandra cycle 8 and 10, as part of our
campaign to investigate X-ray absorption produced by the warm-hot intergalactic
medium (WHIM) residing in the foreground large scale superstructure. During one
of the 80 ksec, Chandra cycle 10 observations, a transient absorption feature
was detected at 3.3-sigma (or 99.9% confidence level, accounting for the number
of trials), which we identify as the OVIII K-alpha line produced by an absorber
intrinsic to the BL Lac object. None of the other 11 observations showed this
line. We constrain the ionization parameter (25 <~ Xi <~ 40) and temperature
(10^5 < T < 2.5 10^7 K) of the absorber. This absorber is likely produced by an
outflow with a velocity up to 1,500 km/s. There is a suggestion of possible
excess emission on the long-wavelength side of the absorption line; however,
the derived properties of the emission material are very different from those
of the absorption material, implying it is unlikely a typical P Cygni-type
profile.
