  Swift, Chandra and XMM have found a weak but nearly constant X-ray component
from Swift J1644+57 that appeared at ~500 days and was visible at least until ~
1400 days after the stellar capture, which cannot be explained by standard
tidal disruption theories. We suggest that this X-ray afterglow component may
result from Thomson scattering between the primary X-rays and its surrounding
plasma, i.e. the Compton echo effect. Similar phenomena has also been observed
from molecular clouds in our Galactic Center, which were caused by the past
activity of Srg A*. If this interpretation of Swift J1644+57 afterglow is
correct, this is the first Compton Echo effect observed in the cosmological
distances.
