Space and Physics
PUBLISHED
What you will discover in this article
- Astronomers report the discovery of a new class of X-ray sources that emit low-energy X-rays at the very edge of space telescopes like NASA's Chandra's ability.
- Eighty-four of these sources have been found in six different galaxies.
- “We're not really sure yet what they are,” the researchers told IFLScience.
Astronomers have reported an exciting discovery in X-rays that might have important consequences well beyond the confines of the field. These objects belong to a new class of hypersoft X-ray sources and likely involve a compact object stealing material from a companion.
The hypersoft in the class name means that these objects only emit at the lowest energy end of the X-ray spectrum. The expectation is that whatever is powering these sources is bright and mighty and peaking in the extreme ultraviolet.
You might be wondering then why they have not been detected in that portion of the light spectrum. We do not have a dedicated mission studying the universe in extreme UV because the helium and hydrogen spread in the galaxy are extremely good at absorbing this light, shrouding these sources.
Using NASA’s Chandra telescope, the researchers were able to spot the tail of these objects' emission. They used archival data across many years to find them – and it was not guaranteed that they’d find anything. Chandra is less sensitive in this region, and people have been thinking that the data might not be useful.
They looked at six nearby galaxies: Andromeda (M31), the Pinwheel Galaxy (M101), and four ellipticals, finding 84 hypersoft X-ray sources.
“This study actually focuses on kind of like a missing population in X-rays,” first author Mustafa Muhibullah of the University of Alabama told IFLScience. “We find sources that are only visible or detectable in the low-energy X-rays, but not in the high-energy X-rays.”
An intense source of uncertain origin
The true nature of these sources is currently unknown. The researchers put forward the explanation that they ought to be compact objects, which is astro-slang for either a black hole, a neutron star, or a white dwarf. These are ways for stars to end, depending on their mass.
White dwarfs are the fate of stars like our Sun. Once they finish burning the fuel at their core, that very core collapses into a very hot object and blows away the other layers.
Neutron stars and black holes are the product of stars more massive than the Sun going supernova. No matter the compact object, the source of energetic emission comes from what’s happening in the system. The compact object has a companion, and it is stealing material. The process is very hot, reaching temperatures between 200,000 and 250,000°C (360,000 to 450,000°F).
“The bottom line is we're not really sure yet what they are,” co-author Professor Jimmy Irwin, also of the University of Alabama, told IFLScience.
“At least in my own personal opinion, they are probably white dwarfs just because they can more easily obtain this kind of temperature.”
If part or a majority of these objects are white dwarfs, that has huge potential. White dwarfs would never normally go supernova, unless they have a companion from which they are stealing material.
In that case, once enough material is stolen, they could go boom. The explosions tend to be of a similar luminosity every time, making them ideal “standard candles," a way to measure distance in the universe. They are fundamental to studying the currently controversial expansion rate of the cosmos.
If hypersoft X-ray sources are white dwarfs in the process of becoming Type Ia supernovae, it would be great to study how they behave and maybe have a way to spot them before they explode.
There should be quite a fair bit of these objects in galaxies, so why were they not more obvious? It could be that physical processes are at play that make them disappear from us for a while.
“Perhaps because of a change in accretion rate, the photosphere (the surface that we can detect) expands, and when it expands, then it's no longer an extreme EUV source with an X-ray tail,” co-author Professor Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian told IFLScience.
While the focus and the excitement is on the white dwarf model, it is likely that this class contains different types of objects.
“It could very well be the case that some hypersoft sources are accreting neutron stars, some are accreting black holes, and some are nuclear burning white dwarfs,” Professor Di Stefano told IFLScience.
The team plans to look at more galaxies to find more of these sources and also look at these first 84 objects using the Hubble Space Telescope. Hubble doesn’t see in the extreme UV, but it could see them in lower-energy UV. Finding these objects there too could confirm the estimates from the Chandra data.
The study is published in the journal Nature Astronomy.
