Beyond Accretion Limits: The Rise of Pulsating Gems

Abstract

The discovery of several ultraluminous X-ray sources exhibiting fast and rapidly evolving X-ray pulsations unequivocally associates these sources with accreting neutron stars orbiting relatively massive companion stars (> 8M${\hspace{0.17em}}_{\odot }$). Among these ULXs, the brightest pulsating ULX (PULX), NGC 5907 ULX-1, displays a peak luminosity (~2 × 10⁴¹ erg s⁻¹) that exceeds its Eddington limit by ~1000 times. These discoveries have raised several key questions, the most urgent of which include: what physical process (or processes) is driving the observed luminosities? What is the nature of compact objects in the still non-pulsating ULXs, and how can we unambiguously ascertain it? Why are PULXs so rare and elusive, and how can we identify more members of this class? In this contribution, a brief overview of the ULX class is provided focusing on PULXs, presenting the most recent results obtained for NGC 5907 ULX-1, NGC 7793 P13, M82 X-2 and M51 ULX-7. How current-generation X-ray missions are already providing (and can continue to do so in the next years) a wealth of information to address the aforementioned questions is also outlined.