A method to search for bulk motions in the ICM with {\sl Chandra} CCD
  spectra: application to the Bullet cluster
  We propose a strategy to search for bulk motions in the intracluster medium
(ICM) of merging clusters based on {\sl Chandra} CCD data. Our goal is to
derive robust measurements of the average redshift of projected ICM regions
obtained from the centroid of the $K_\alpha$ line emission. We thoroughly
explore the effect of the unknown temperature structure along the line of sight
to accurately evaluate the systematic uncertainties on the ICM redshift. We
apply our method to the "Bullet cluster" (1E~0657-56). We directly identify 23
independent regions on the basis of the surface brightness contours, and
measure the redshift of the ICM averaged along the line of sight in each. We
find that the redshift distribution across these regions is marginally
inconsistent with the null hypothesis of a constant redshift or no bulk motion
in the ICM, at a confidence level of about $2\, \sigma$. We tentatively
identify the regions most likely affected by bulk motions and find a maximum
velocity gradient of about $(46\pm 13)$ $\rm km~s^{-1}~kpc^{-1}$ along the line
of sight on a scale of $\sim 260 $ kpc along the path of the "bullet." We
interpret this as the possible signature of a significant mass of ICM pushed
away along a direction perpendicular to the merging. This preliminary result is
promising for a systematic search for bulk motions in bright, moderate-redshift
clusters based on spatially resolved spectral analysis of {\sl Chandra} CCD
data. This preliminary result is promising for a systematic search for bulk
motions in bright, moderate-redshift clusters based on spatially resolved
spectral analysis of {\sl Chandra} CCD data.
