The archives of a space telescope were hiding 84 objects so unusual that they could belong to a previously unnoticed cosmic family.
Researchers found them in old observations from the Chandra space telescope, which specializes in X-rays. They examined six galaxies and spotted these objects in each of them. Two are spiral galaxies, including Andromeda and M101, and four are elliptical galaxies. The sources appear both near young stars and in regions populated by old stars.
Artist’s impression showing a system of two stars, a white dwarf in the foreground and its companion in the background. The white dwarf draws gas from its companion, which is channeled toward its poles. When this material falls onto the hot surface of the white dwarf.
Mark Garlick · CC BY 4.0 · Wikimedia
Their distinctive feature lies in the light they emit. In Chandra images, they appear in the least energetic X-rays, then disappear when more energetic X-rays are examined. Astronomers have therefore named them “hypersoft X-ray sources.” Much of their energy would actually be emitted as highly energetic ultraviolet light, right next to X-rays in the light spectrum.
To understand why they remained hidden, we need to look at how this light travels. Low-energy X-rays are difficult to detect. The ultraviolet light produced by these objects is also easily absorbed by hydrogen and helium present between the stars. Telescopes have therefore long been looking in a region of the spectrum where part of the signal escaped them.
The exact nature of these 84 sources remains unknown. However, researchers think that several types of objects could be behind this same behavior. They could be systems made up of two stars, one of which has become a white dwarf, a neutron star, or a black hole. This extremely dense object then draws matter from its companion.
As it falls toward the compact object, this matter heats up and releases enormous amounts of energy. Such pairs of celestial bodies are already known. Here, the main difference comes from how this energy is distributed: large amounts of highly energetic ultraviolet light and particularly low-energy X-rays. The brightest examples reach approximately 1031 watts in the small X-ray band studied.
This discovery could be of particular interest to researchers studying certain supernovas. A white dwarf that gathers matter from a neighboring star can, in some scenarios, eventually explode as a Type Ia supernova. Astronomers use these explosions to measure distances in the Universe, but the precise identity of the systems that produce them remains debated. Some of the new sources could belong to this sought-after population.
These objects could also affect the gas in their galaxy. Their powerful ultraviolet radiation can strip electrons from atoms between the stars, a phenomenon called ionization. Astronomers observe more ionized gas in some galaxies than the known hot stars appear able to produce on their own. A hidden population of these sources could provide a possible explanation for part of this radiation.
Researchers must now determine what actually lies at the heart of each source. The 84 objects are not necessarily all of the same nature. Follow-up observations could search for their companion stars, track their changes in brightness, and distinguish white dwarfs from systems containing a neutron star or a black hole.