Searching for coherent pulsations in ultraluminous X-ray sources

TL;DR

This study searched for pulsations in multiple ultraluminous X-ray sources using archival data, finding no evidence of neutron star pulsations, thus constraining the nature of these luminous objects.

Contribution

The paper conducts a systematic search for pulsations in ULXs, providing upper limits and exploring the neutron star accretion scenario beyond the known case of M82 ULX.

Findings

No significant pulsations detected in the sample

Upper limits set on pulsed signal amplitudes

Constraints on neutron star presence in ULXs

Abstract

Luminosities of ultraluminous X-ray sources (ULXs) are uncomfortably large if compared to the Eddington limit for isotropic accretion onto stellar-mass object. Most often either supercritical accretion onto stellar mass black hole or accretion onto intermediate mass black holes is invoked the high luminosities of ULXs. However, the recent discovery of coherent pulsations from M82 ULX with NuSTAR showed 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 ULX. We provide upper limits for the amplitude of possibly undetected pulsed signal for the sources in the sample.