A transient ultraluminous X-ray source in the Whale galaxy defies standard accretion models

The Whale galaxy keeps producing surprises. A new study published June 22 on the preprint server arXiv reveals that a transient ultraluminous X-ray source in NGC 4631, designated X-4, exhibits behavior that cannot be explained by conventional accretion disk physics, pointing instead toward a regime where matter falls onto a compact object at rates far exceeding what standard theory permits.

The study was conducted by astronomers from Germany and Turkey, who combined archival data from multiple space telescopes to examine the spectral and timing properties of X-4 in detail.

What makes a source ultraluminous

Ultraluminous X-ray sources, or ULXs, are point sources that outshine a million suns across all wavelengths combined. They are less luminous than active galactic nuclei but consistently more luminous than any known stellar process, which places them in a category that has challenged astrophysicists for decades. Whether they harbor neutron stars, stellar-mass black holes, or something more exotic remains an open question for most known examples.

NGC 4631, located approximately 24.45 million light-years away and nicknamed the Whale galaxy for its elongated edge-on appearance, is a star-forming spiral galaxy known to host at least eight ULXs. X-4 is one of the most enigmatic among them: it is a transient source, meaning it does not shine constantly but switches on and off in ways that encode critical information about its physical nature.

Extreme variability and super-Eddington accretion

The analysis found that X-4 varies by more than two orders of magnitude in X-ray luminosity across the 0.3–10 keV energy band, a remarkable range that speaks to the instability of the accretion process powering it. On top of that large-scale variability, the source shows strong short-term aperiodic flaring on timescales of roughly one thousand to ten thousand seconds. No coherent pulsations or quasi-periodic oscillations were detected in the data.

Crucially, the relationship between luminosity and temperature, along with how the spectral hardness evolves, is inconsistent with a standard geometrically thin accretion disk. Instead, the data favor a super-Eddington accretion scenario: one where the infalling matter arrives faster than radiation pressure can push it away, inflating the disk into a thick, geometrically puffed structure. Radiatively driven winds and viewing-angle effects, where the observer’s line of sight through the funnel-shaped geometry of the outflow changes what is detected, are both necessary to explain what is seen.

Neutron star or black hole?

The observations are compatible with a stellar-mass compact object at the center of X-4, meaning the accretor is either a neutron star or a stellar-mass black hole rather than an intermediate-mass object. Distinguishing between these two possibilities will require detecting or ruling out pulsations in future observations with greater sensitivity. If pulsations are eventually confirmed, X-4 would join the growing class of ultraluminous X-ray pulsars, systems where the most extreme X-ray luminosities are powered not by massive black holes but by neutron stars accreting at rates that dwarf anything classical physics predicted possible.

Reference: Sinan Allak et al., Spectral and timing variability of the transient ultraluminous X-ray source NGC 4631 X-4, arXiv (2026). DOI: 10.48550/arxiv.2606.23498

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Reproduction in whole or in part is prohibited without express authorization. More information arXiv DOI: 10.48550/arxiv.2606.23498