  Self-interactions of dark matter particles can potentially lead to an
observable separation between the dark matter halo and the stars of a galaxy
moving through a region of large dark matter density. Such a separation has
recently been observed in a galaxy falling into the core of the galaxy cluster
Abell 3827. We estimated the DM self-interaction cross section needed to
reproduce the observed effects and find that the sensitivity of Abell 3827 has
been significantly overestimated in a previous study. Our corrected estimate is
$\tilde{\sigma}/m_\text{DM} \sim 3\:\text{cm}^2\:\text{g}^{-1}$ when
self-interactions result in an effective drag force and $\sigma/m_\text{DM}
\sim 1.5\:\text{cm}^2\:\text{g}^{-1}$ for the case of contact interactions, in
some tension with previous upper bounds.
