Research Byte
Published in the RSAA Lunations
Vol1 Issue27 1–30 April 2022
An astrophysical origin for a putative dark matter signal
For something like 40 years a favourite scenario for the dark matter — the enduringly mysterious stuff that makes up four fifths of the matter content of the cosmos — has been that it is some type of Weakly Interacting Massive Particle or WIMP. As the name suggests, such particles would only interact weakly with baryons (from which we and the rest of the visible universe are made) and between themselves. Despite this, a sufficiently large and concentrated agglomeration of WIMP dark matter may actually produce a significant rate of ‘self-annihilation’, with each such self-annihilation event generically expected to produce an electromagnetic signal that might be detectable. Theory suggests that such signals should emerge (at least partially) at gamma-ray energies.
But where should we look for such signals? Over the entire sky, perhaps the best-motivated place to search is the Galactic centre. Here, at the bottom of the Milky Way’s gravitational potential, we expect the dark matter density in the Galaxy to peak and, consequently, the largest volumetric rate of self-annihilation. Unfortunately, this region is also host to many potential sources of confounding, astrophysical ‘background’. These include the Galaxy’s resident super-massive black hole, an intense region of star formation in the vicinity of the black hole, and, of most relevance to this story, the concentrated and old population of stars that make up the Galactic bulge.
The existence of all these confounds did not, however, dissuade Dan Hooper and co-workers from claiming detection of a dark-matter-compatible gamma-ray signal from the inner Galaxy in data collected by NASA’s orbiting Fermi Space Telescope. Hooper et al. noted that the Fermi data pointed to the existence of a signal, symmetric around the Galactic centre, with a strength and radial intensity profile that seemed an excellent match to self-annihilation of WIMP dark matter in an ‘NFW’ density profile. The spectral peak of the signal, at a gamma-ray energy around 2 GeV (giga electronvolts), was also suggestive of a WIMP origin. Remarkably, despite the fact that this discovery claim was made in 2010 on the basis of only two years’ data-taking by Fermi, the existence of the signal has been sustained and, in fact, reinforced by further data and subsequent analyses, much of it by independent groups, including the scientific collaboration behind the Fermi Telescope itself.
Thus, the existence of the ‘Galactic Centre Excess’ or GCE is not in doubt and spurred over 1000 particle physics papers. But while certainly real, whether, in fact, the GCE is actually a dark matter signal is more contentious. In fact, as I will now explain, our recent work (Gautam, Crocker, Ferrario, Ruiter, Ploeg, Gordon, and Macias, in print, Nature Astronomy) points firmly in the direction of this signal having an astrophysical origin.
The leads we pursued in our work included the fact that the GCE, while centred on the Galactic nucleus, extends well into the bulge. Thus any putative astrophysical origin for the signal must be associated with the sort of (~> 10 Gyr) old stellar population that characterises this structure. Further, whatever the astrophysical source, it must also produce gamma-rays with a spectral distribution that resembles the GCE (i.e., with a clear bump in the spectral energy distribution at ~2 GeV, but also a high energy tail that extends to ~10 GeV or even higher energies). Finally, this source or class of sources, must not produce too much emission at other wavelengths given observational constraints (e.g., in X-rays) or individually-bright gamma-ray objects that would already have been detected as point sources by Fermi.
Is there any source that naturally accommodates all these requirements? Yes, it turns out, though it has taken a significant effort, led by former RSAA Masters student Anuj Gautam, to demonstrate this. Under the supervision of me, Lilia Ferrario (MSI, ANU) and Ashley Ruiter (UNSW Canberra), Anuj constructed a binary population synthesis (BPS) model for the formation of millisecond pulsars (MSPs) amongst the bulge stellar population. The specific MSP-formation channel that Anuj investigated was ‘accretion induced collapse’ wherein an O-Ne white dwarf accretes sufficient material from a binary companion that it collapses directly into a neutron star as it approaches the Chandrasekhar mass limit. Angular momentum conservation in this process sees the formation of a rapidly spinning, magnetised neutron star with a period of a few ms, i.e., a millisecond pulsar.
Remarkably, Anuj’s work demonstrated that, by taking off-the-shelf parameters for quantities like the initial period and mass distributions of the binary stars, we could predict a gamma-ray signal emerging from the bulge that was a match to the spectrum, intensity, and spatial distribution of the GCE. At the same time, the MSP population pointed to by our model neatly stepped around other observational constraints. For instance, Anuj showed that the MSP population hosts few low mass X-ray binaries (as required by observations) and that the mean gamma-ray luminosity of the MSPs is low (so we do not expect any MSP to be individually-detectable; i.e., the population is only detectable in aggregate).
Where does this result lead? Overall, it points to the previously-unrecognised importance of millisecond pulsars as sources of cosmological gamma-ray production; low specific star formation rate stellar populations in general probably have a gamma-ray luminosity dominated by MSPs born of accretion induced collapse. For instance, the measured gamma-ray signal from M31 (Andromeda), the next closest large spiral galaxy, is probably mostly due to MSPs, our ongoing research indicates. Unfortunately, it is amongst such old, quiescent stellar populations (e.g., globular clusters and dwarf spheroidal satellite galaxies) that those motivated to search for gamma-ray evidence of WIMP dark matter have looked the hardest over the last couple of decades; these sorts of systems, devoid of recent star formation, have been thought to be relatively background free. Our recent work demonstrates that the community may have to reconsider where best to look for clean, indirect signals of WIMP dark matter.
Roland Crocker
