HMN 2026: How Ultraluminous X-ray source in Whale galaxy investigated for spectral and timing variability

Astronomers from Germany and Turkey have analyzed available data from various space telescopes to investigate an ultraluminous X-ray source designated X-4, which is located in the nearby galaxy NGC 4631. Results of the new study, published June 22 on the preprint server arXiv, yield important insights into the spectral and timing variability of this source.

Ultraluminous X-ray sources (ULXs) are point sources in the sky that are so bright in X-rays that each emits more radiation than a million suns emit at all wavelengths. They are less luminous than active galactic nuclei but more consistently luminous than any known stellar process. Although numerous studies of ULXs have been conducted, the basic nature of these sources remains unknown.

A ULX-rich galaxy

Located some 24.45 million light-years away, NGC 4631, dubbed the Whale galaxy, is an edge-on, star-forming spiral galaxy known for its active star formation and prominent extraplanar emission. These features provide a favorable environment for the formation and evolution of luminous X-ray sources.

To date, eight ULXs have been identified in NGC 4631, designated X-1 to X-8. A team of astronomers led by Sinan Allak of the Institute for Astronomy and Astrophysics in Tübingen, Germany, decided to take a closer look at one of these ULXs—X-4, which is known to have a highly asymmetric bubble nebula around it that is likely powered by jet- or outflow-driven shocks.

“In this work, we present the first dedicated spectral and timing study of the transient ULX X-4 based on a comprehensive set of archival Chandra, XMM-Newton, and Swift/XRT observations. By combining multi-epoch data, we probe the source variability over a wide range of timescales, from years down to kilosecond intervals,” the researchers wrote.

A highly variable source

The study conducted by Allak’s team found that X-4 exhibits strong long-term X-ray variability. Its 0.3–10 keV luminosity varies by more than two orders of magnitude over the full observational baseline. This confirms the source’s transient nature suggested by previous observations.

When it comes to short-term variability, the Chandra light curve of X-4 showcases several peak-like structures with characteristic durations of about 1,000–5,000 seconds and aperiodic fluctuations. These findings suggest that the emission from X-4 is shaped by a radiatively driven, optically thick wind.

Furthermore, it was found that the luminosity-temperature relation and the nonmonotonic hardness evolution of X-4 do not follow the behavior expected for a standard thin accretion disk. The astronomers assume that the observed spectral evolution of X-4 can therefore be explained by super-Eddington accretion flows, where radiatively driven winds and viewing-angle effects affect the observed spectral appearance.

According to the paper, no coherent pulsations, quasi-periodic oscillations or statistically significant periodic signals were identified in the Chandra and XMM-Newton data.

Constraining the nature of X-4

Summing up the results, the scientists conclude that the spectral and timing properties of X-4 place it within the population of super-Eddington accreting ULXs, where the observed emission is governed by both the accretion rate and the viewing geometry. They add that when it comes to the nature of X-4, the collected data appear to point to a stellar-mass compact object, consistent with either a neutron star or a stellar-mass black hole accretor.

“Future deeper observations with a higher signal-to-noise ratio will be crucial to constrain the nature of the compact object and to probe the structure of the supercritical accretion flow,” the authors of the study note.

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