Skipping a beat: discovery of persistent quasi-periodic oscillations associated with pulsed fraction drop of the spin signal in M51 ULX-7

Abstract

The discovery of pulsations in (at least) six ultraluminous X-ray sources (ULXs) has shown that neutron stars can accrete at (highly) super-Eddington rates, challenging the standard accretion theories. with a spin signal of $P is the pulsating ULX (PULX) with the shortest known orbital period ($P_ orb and has been observed multiple times by and We report on the timing and spectral analyses of three observations of performed between the end of 2021 and the beginning of 2022, together with a timing re-analysis of and archival observations. We investigated the spin signal by applying accelerated search techniques and studied the power spectrum through the fast Fourier transform, looking for (a)periodic variability in the source flux. We analysed the energy spectra of the 2021--2022 observations and compared them to the older ones. We report the discovery of a recurrent, significant ($>$3sigma ) broad complex at mHz frequencies in the power spectra of We did not detect the spin signal, setting a 3sigma upper limit on the pulsed fraction of $ for the single observation. The complex is significantly detected also in five observations performed in 2012. represents the second PULX for which we have a significant detection of mHz-QPOs at super-Eddington luminosities. These findings suggest that one should avoid using the observed QPO frequency to infer the mass of the accretor in a ULX. The absence of spin pulsations when the broad complex is detected suggests that the mechanism responsible for the aperiodic modulation also dampens the spin signal's pulsed fraction. If true, this represents an additional obstacle in the detection of new PULXs, suggesting an even larger occurrence of PULXs among ULXs.