Pulsating ULXs: large pulsed fraction excludes strong beaming
  The recent discovery of pulsating ultra-luminous X-ray sources (ULXs) shows
that the apparent luminosity of accreting neutron stars (NSs) can exceed the
Eddington luminosity by a factor of hundreds. The relation between the actual
and apparent luminosity is a key ingredient in theoretical models of ULXs but
it is still under debate. A typical feature of the discovered pulsating ULXs is
a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a
simple geometry of accretion flow and test the possibility of simultaneous
presence of a large luminosity amplification due the geometrical beaming and a
high PF. We argue that these factors largely exclude each other and only a
negligible fraction of strongly beamed ULX pulsars can show PF above 10 per
cent. Discrepancy between this conclusion and current observations indicate
that pulsating ULXs are not strongly beamed and their apparent luminosity is
close to the actual one.
