We present results from a 38 ks Chandra X–ray observation of the z = 0.059 galaxy cluster A1991 .
The cluster has a bright X-ray core and a central temperature gradient that declines inward from 2.7 keV at 130 kpc to \approx 1.6 keV at the cluster center .
The radiative cooling time of the gas in the inner 10 kpc is about 0.5 Gyr , and rises to 1 Gyr at a radius of 20 kpc .
The cooling rate of the gas within the latter radius is \mathrel { < \kern - 10.0 pt \lower 3.87 pt \hbox { $ \sim$ } } 25 M _ { \odot } \thinspace { yr } ^ { -1 } \thinspace .
The Chandra ACIS-S3 image shows that the intracluster medium has an asymmetric surface brightness distribution with respect to the central galaxy .
Bright knots of soft X-ray emission embedded in a cometary structure are located approximately 10 arcsec north of the optical center of the cD galaxy .
Unlike the structures seen in other cooling flow clusters , the knots have no obvious association with the radio source .
The structure ’ s temperature of 0.83 keV makes it nearly 1 keV cooler than its surroundings , and its mass is 3.4 \times 10 ^ { 9 } M _ { \sun } .
Based on its bow-shaped appearance and modest overpressure with respect to its surroundings , we interpret the structure as a cool mass concentration that is breaking apart as it travels northward through the center of the cluster .