Millisecond pulsars from accretion-induced collapse as the origin of the Galactic Centre gamma-ray excess signal

Gamma-ray data from the Fermi Large Area Telescope reveal an unexplained, apparently diffuse, signal from the Galactic bulge^1–3 that peaks near ~2 GeV with an approximately spherical⁴ intensity profile ∝ r^−2.4 (refs. ^3,5), where r is the radial distance to the Galactic centre, that extends to angular radial scales of at least ~10° and possibly to ~20° (refs. ^6,7). The origin of this ‘Galactic Centre excess’ (GCE) has been debated, with proposed sources prominently including self-annihilating dark matter^1,4 and a hitherto undetected population of millisecond pulsars (MSPs)⁸. However, the conventional channel for the generation of MSPs has been found to predict too many low-mass X-ray binary (LMXB) systems⁹ and, because of the expected large natal kicks, may not accommodate¹⁰ the close spatial correspondence^11–13 between the GCE signal and stars in the bulge. Here we report a binary population synthesis (BPS) forward model that demonstrates that an MSP population arising from the accretion-induced collapse (AIC) of O–Ne white dwarfs in Galactic bulge binaries can naturally reproduce the morphology, spectral shape and intensity of the GCE signal while also obeying LMXB constraints. Synchrotron emission from MSP-launched cosmic ray electrons and positrons may simultaneously explain the mysterious microwave ‘haze’¹⁴ from the inner Galaxy.