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.