Chandra’s Hypersoft Hunt Reveals 84 Mysterious Cosmic X-Ray Beacons That Surveys Kept Missing

Mustafa Muhibullah did not set out to name a new class of object. He was a University of Alabama graduate student working through public Chandra X-ray Observatory files that other groups had already used. In the softest sliver of those files, sources lit up and then disappeared as soon as the energy cut moved a little higher. Artifacts were the first suspicion. Stacked exposures and careful cleaning left something real.

We’re talking about Hypersoft X-ray sources, or HSS. They mostly emit energy between 0.15 and 0.3 keV, which is below the Chandra telescope’s detection limit, and emit significantly less energy above 0.3 keV. A useful cut is the photon ratio in these two bands, which is around 8 to 1, as used in a Nature Astronomy publication. Unfortunately, most of these sources are left out in the cold by typical catalogs, as the lowest band is usually only recorded as background noise.

Eighty-four of these sources have survived the cut in six nearby galaxies, including Andromeda (18) and M101, the Pinwheel Galaxy (7). You can see them circled in Chandra’s new composite with Hubble. The collection is then completed by four ellipticals: NGC 3115 (18), NGC 3379 (12), NGC 4697 (8), and the gigantic ellipse NGC 4472 (21). They can be found in spiral arms where new stars are being formed, or in older populations where this has already been completed. M101’s seven are indistinguishable from other tiny specks of paired stars.

Narrow band luminosities range from 10^35 erg/s in nearby M31 to 10^38 erg/s in distant ellipticals. Meanwhile, models of the entire image imply that the ‘proper’ output is substantially larger; much of that energy will be far beyond Chandra’s range, in the extreme ultraviolet range, but the majority of it will be absorbed by interstellar hydrogen and helium. What little bit makes it through is barely discernible in Chandra’s softest band. Temperatures range from 200,000 to 280,000 degrees Celsius, which is quite cold compared to many other X-ray sources but still rather high, the kind of temperatures at which ultraviolet begins to show.

One of the advisors, Jimmy Irwin, stated that he believes white dwarfs would account for at least a portion of this group. White dwarfs can readily handle temperatures like this. Rosanne Di Stefano of the Center for Astrophysics is less sure, envisioning a combination of accreting white dwarfs, post-nova systems, black holes, and neutron stars. As it happens, a few of the dim M31 sources appear to line up with known novae that have cooled down to this state several years after the massive optical explosion that gave birth to them. Di Stefano doesn’t mince her words, because these hypersoft sources don’t just come from one type of engine or radiation source.