A groundbreaking discovery has been made by astronomers using ESA's XMM-Newton satellite: a new pulsating ultraluminous X-ray source (ULX) in the galaxy NGC 4631, known as the Whale galaxy. This discovery, published on November 6th on the arXiv preprint server, adds a crucial piece to the puzzle of ULX nature, which has long baffled scientists. ULXs are incredibly bright X-ray sources, each emitting more radiation than a million suns across all wavelengths, yet they are less luminous than active galactic nuclei but more consistently luminous than any known stellar process. While numerous studies have been conducted, the fundamental nature of ULXs remains a mystery.
Some ULXs exhibit pulsations, categorizing them as ultraluminous X-ray pulsars (ULXPs). Discovering and studying these objects is vital for advancing our understanding of accretion physics, particularly mechanisms enabling ULXs to sustain X-ray luminosities exceeding the Eddington limit. NGC 4631, a late-type starburst galaxy located 24.45 million light-years away, is known to host at least seven ULXs. A team led by Lorenzo Ducci from the University of Tübingen in Germany used XMM-Newton's European Photon Imaging Camera (EPIC) to investigate this galaxy in detail, focusing on its ULXs.
Their findings revealed a new pulsating ULX, designated X-8, located in a crowded region of the galactic disk. The observations, conducted in July 2025, identified a significant signal with a period of approximately 9.66 seconds from a new source near the previously known ULX designated X-2. X-8 proved to be brighter than X-2, with an X-ray luminosity of 3.4 duodecillion erg/s. The study also determined that X-8 has a spin period derivative of about -9.6 × 10^-8 seconds/second, indicating one of the most rapid spin-up rates observed in pulsating ULXs, with a spin-up timescale of just 3.2 years.
The astronomers proposed that this high spin-up rate could originate from orbital motion of the pulsar, accretion torque on the neutron star, or a combination of both mechanisms. Additionally, the magnetic field strength of the newfound ULXP was estimated to be within the range of 10–200 trillion Gauss, consistent with other pulsating ultraluminous X-ray sources.
The authors emphasized the significance of their discovery in the context of super-Eddington luminosities studies, stating that this new ULX pulsar adds a key source to the small known population, enabling future studies to better constrain the physical mechanisms responsible for their super-Eddington luminosities.
