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

¹ Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
² INAF – Osservatorio Astronomico di Roma, Via Frascati 33, I-00078 Monte Porzio Catone, (RM), Italy
³ Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza”, piazzale Aldo Moro 5, I-00185 Roma, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate, (LC), Italy
⁵ Astrophysics Sub-department, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁶ INAF/IASF Palermo, Via Ugo La Malfa 153, I-90146 Palermo, Italy
⁷ Cahill Center for Astrophysics, California Institute of Technology, 1216 East California Boulevard, Pasadena, CA I-91125, USA
⁸ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius, (CA), Italy
⁹ European Space Astronomy Centre (ESAC), ESA, Camino Bajo del Castillo s/n, Villanueva de la Cañada, I-28692 Madrid, Spain
¹⁰ INFN – Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
¹¹ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK
¹² Scuola Universitaria Superiore IUSS Pavia, Palazzo del Broletto, piazza della Vittoria 15, I-27100 Pavia, Italy
¹³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, Via Alfonso Corti 12, I-20133 Milano, Italy
¹⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, I-20121 Milano, Italy
¹⁵ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁶ Centre for Extragalactic Astronomy & Dept of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁷ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy

Received: 18 April 2024
Accepted: 12 July 2024

Abstract

Context. 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. M51 ULX-7, with a spin signal of P ≃ 2.8 s, is the pulsating ULX (PULX) with the shortest known orbital period (P_orb ≃ 2 d) and has been observed multiple times by XMM-Newton, Chandra, and NuSTAR.

Aims. We report on the timing and spectral analyses of three XMM-Newton observations of M51 ULX-7 performed between the end of 2021 and the beginning of 2022, together with a timing re-analysis of XMM-Newton, Chandra, and NuSTAR archival observations.

Methods. 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.

Results. We report the discovery of a recurrent, significant (> 3σ) broad complex at mHz frequencies in the power spectra of M51 ULX-7. We did not detect the spin signal, setting a 3σ upper limit on the pulsed fraction of ≲10% for the single observation. The complex is significantly detected also in five Chandra observations performed in 2012.

Conclusions. M51 ULX-7 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.