ICM Signatures from Multiple Outbursts in the Galaxy Group NGC 5813: A   Window to AGN Feedback
  We present results from Chandra observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. We focus on three main results. 1) The diffuse gas shows clear signatures from three distinct outbursts of the central AGN, with three pairs of roughly collinear cavities. The inner two cavity pairs are associated with unambiguous elliptical shock fronts, with Mach numbers M~1.7 and M~1.5 for the inner and outer shocks, respectively. 2) The mean power of the two most recent outbursts differs by a factor of six, indicating that the mean jet power varies over long (~10^7 yr) time scales. 3) The heating from the shocks alone is sufficient to balance radiative cooling of the gas within at least the central 10 kpc, allowing feedback to operate isotropically at small radii. 
S. W. Randall, W. R. Forman, S. Giacintucci, P. Nulsen, M. Sun, C.   Jones, E. Churazov, L. David, R. Kraft, M. Donahue, E. ...
astro-ph.CO