A new pulsating neutron star in the Ultraluminous X-ray source NGC 4559 X7?

• DOI: 10.48550/arxiv.2502.10246
• Publication Date: 2025-02-14
• Authors: Fabio Pintore; C. Pinto; G. A. Rodríguez C...; George L. Israel; Nicolò Oreste Pin...; S. Motta; F. Barra; D. J. Walton; F. Fuerst; Peter Kosec; Chiara Salvaggio; M. Del Santo; A. Wolter; Matthew Middleton; A. D’Aì; E. Ambrosi; L. Burderi; M. Imbrogno; R. Salvaterra; A Robba
• Keywords: Star (game theory)

Ultraluminous X-ray sources (ULX) are extragalactic objects with luminosities above the Eddington limit for a 10 Msun black hole (BH). ULXs may host super-Eddington accreting neutron stars or stellar mass BH, although exact proportion of two populations is not yet known. We investigate properties ULX NGC 4559 X7, which shows flux variability up to factor 5 on months-to-years and hours-to-days timescales. A flaring activity was also observed during source highest epochs. Flares unpredictable, different durations all flat-topped in flux. The latter suggests that, at flare peaks, there likely common switch-off mechanism accretion onto compact object. analysed available XMM-Newton Swift/XRT observations spectral temporal evolution looking short long-term variability. look periodicities coherent signals through accelerated searches that included orbital corrections. described X7 spectra thermal components plus cut-off powerlaw model. found three well defined states, where mainly driven by harder components. In addition, pulsed signal 2.6-2.7s detected observations. significance these relatively weak but they same parameter space properties. If confirmed, it would imply high spin-down 1e-9 s/s, could be extreme amongst known pulsating ULXs. become new pulsar. discuss results context stellar-mass object, particular suggesting might star.