Sharma et al. (2004) analysed XMM-Newton data and detected an asymmetric surface
brightness distribution with respect to the central galaxy. These authors
detected bright knots of soft X-ray emission embedded in a cometary structure
located north of the optical centre of the cD galaxy. According to these
authors, the knots have no obvious association with the radio source.
A1991 was also studied by V05 and V06. Our derived system properties at r_500_
and r_2500_ agree very well with those of V05 and V06. The temperature decline
at large radii in this system was also found from the XMM-Newton data by Snowden
et al. (2008) with consistent values.
A1991.-As with A1795, the long-wavelength images were affected by scattered
light, requiring masking, and the seeing was poor. McNamara & O'Connell (1989)
report spectral evidence for a small SFR, and McNamara & O'Connell (1992) report
a positive U - I color gradient in the core.
A1991. There is evidence of excess absorption in the center
and marginal soft excess in the outskirts of this X-ray-faint cluster.