Searching for coherent pulsations in ultraluminous X-ray sources

Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large when compared to the Eddington limit for isotropic accretion onto stellar-mass object. Most often either supercritical accretion onto stellar mass black holes or accretion onto intermediate mass black holes is invoked the high luminosities of ULXs. However, the recent discovery of coherent pulsations from ULX M82 X-1 with NuStar has shown that another scenario implying accretion onto a magnetized neutron star is possible for ULXs. Motivated by this discovery, we re-visited the available XMM-Newton archival observations of several bright ULXs with a targeted search for pulsations to check whether accreting neutron stars might power other ULXs as well. We have found no evidence for significant coherent pulsations in any of the sources including the M82 X-2. We provide upper limits for the amplitude of possibly undetected pulsed signals for the sources in the sample.