No Shock Across Part of a Radio Relic in the Merging Galaxy Cluster ZwCl
  2341.1+0000?
  The galaxy cluster ZwCl 2341.1+0000 is a merging system at z=0.27, which
hosts two radio relics and a central, faint, filamentary radio structure. The
two radio relics have unusually flat integrated spectral indices of -0.49 +/-
0.18 and -0.76 +/- 0.17, values that cannot be easily reconciled with the
theory of standard diffusive shock acceleration of thermal particles at weak
merger shocks. We present imaging results from XMM-Newton and Chandra
observations of the cluster, aimed to detect and characterise density
discontinuities in the ICM. As expected, we detect a density discontinuity near
each of the radio relics. However, if these discontinuities are the shock
fronts that fuelled the radio emission, then their Mach numbers are
surprisingly low, both <=2. We studied the aperture of the density
discontinuities, and found that while the NW discontinuity spans the whole
length of the NW radio relic, the arc spanned by the SE discontinuity is
shorter than the arc spanned by the SE relic. This startling result is in
apparent contradiction with our current understanding of the origin of radio
relics. Deeper X-ray data are required to confirm our results and to determine
the nature of the density discontinuities.
