Shock fronts, electron-ion equilibration and ICM transport processes in
  the merging cluster Abell 2146
  We present a new 400 ks Chandra X-ray observation of the merging galaxy
cluster Abell 2146. This deep observation reveals detailed structure associated
with the major merger event including the Mach M=2.3+/-0.2 bow shock ahead of
the dense, ram pressure stripped subcluster core and the first known example of
an upstream shock in the ICM (M=1.6+/-0.1). By measuring the electron
temperature profile behind each shock front, we determine the timescale for the
electron population to thermally equilibrate with the shock-heated ions. We
find that the temperature profile behind the bow shock is consistent with the
timescale for Coulomb collisional equilibration and the postshock temperature
is lower than expected for instant shock-heating of the electrons. Although
like the Bullet cluster the electron temperatures behind the upstream shock
front are hotter than expected, favouring the instant heating model, the
uncertainty on the temperature values is greater here and there is significant
substructure complicating the interpretation. We also measured the width of
each shock front and the contact discontinuity on the leading edge of the
subcluster core to investigate the suppression of transport processes in the
ICM. The upstream shock is ~440 kpc in length but appears remarkably narrow
over this distance with a best-fit width of only 6^{+5}_{-3} kpc compared with
the mean free path of 23+/-5 kpc. The leading edge of the subcluster core is
also narrow with an upper limit on the width of only 2 kpc separating the cool,
multiphase gas at 0.5-2 keV from the shock-heated surrounding ICM at ~6 keV.
The strong suppression of diffusion and conduction across this edge suggests a
magnetic draping layer may have formed around the subcluster core.[abridged]
