Investigation of X-ray cavities in the cooling flow system Abell 1991
  We present results based on the systematic analysis of \textit{Chandra}
archive data on the X-ray bright Abell Richness class-I type cluster Abell 1991
with an objective to investigate properties of the X-ray cavities hosted by
this system. The unsharp masked image as well as 2-d $\beta$ model subtracted
residual image of Abell 1991 reveals a pair of X-ray cavities and a region of
excess emission in the central $\sim$12 kpc region. Both the cavities are of
ellipsoidal shape and exhibit an order of magnitude deficiency in the X-ray
surface brightness compared to that in the undisturbed regions. Spectral
analysis of X-ray photons extracted from the cavities lead to the temperature
values equal to $1.77_{-0.12}^{+0.19}$ keV for N-cavity and
$1.53_{-0.06}^{+0.05}$ keV for S-cavity, while that for the excess X-ray
emission region is found to be equal to $2.06_{-0.07}^{+0.12}$ keV. Radial
temperature profile derived for Abell 1991 reveals a positive temperature
gradient, reaching to a maximum of 2.63 keV at $\sim$ 76 kpc and then declines
in outward direction. 0.5$-$2.0 keV soft band image of the central 15\arcsec
region of Abell 1991 reveals relatively cooler three different knot like
features that are about 10\arcsec off the X-ray peak of the cluster. Total
power of the cavities is found to be equal to $\sim 8.64\times 10^{43}$ \lum,
while the X-ray luminosity within the cooling radius is found to be 6.04
$\times 10^{43}$ \lum, comparison of which imply that the mechanical energy
released by the central AGN outburst is sufficient to balance the radiative
loss.
