Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy due to millisecond pulsars

• Published in: Nature astronomy Vol. 6; no. 11; pp. 1317 - 1324
• Main Authors: Crocker, Roland M., Macias, Oscar, Mackey, Dougal, Krumholz, Mark R., Ando, Shin’ichiro, Horiuchi, Shunsaku, Baring, Matthew G., Gordon, Chris, Venville, Thomas, Duffy, Alan R., Yang, Rui-Zhi, Aharonian, Felix, Hinton, J. A., Song, Deheng, Ruiter, Ashley J., Filipović, Miroslav D.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2022; Nature Publishing Group; Springer Nature
• Subjects: 639/33/34/864; 639/33/34/866; Astronomy; Astronomy & Astrophysics; Astrophysics and Cosmology; Bubbles; Emissions; Gamma rays; Physics; Physics and Astronomy; Pulsars; Space telescopes; Spatial analysis; Star & galaxy formation
• ISSN: 2397-3366; 2397-3366
• DOI: 10.1038/s41550-022-01777-x

The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s supermassive black hole. Here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the Sagittarius dwarf spheroidal galaxy (dSph). This large Milky Way satellite is viewed through the Fermi bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sagittarius dSph has no ongoing star formation, but we nevertheless demonstrate that the dwarf’s millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron–positron pairs injected by millisecond pulsars belonging to the Sagittarius dSph, combined with these objects’ magnetospheric emission. This finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the Galactic Centre, the Andromeda galaxy and other massive Milky Way dSphs. A bright patch in the Fermi bubbles, previously attributed to a jet launched by the Galaxy’s central black hole, is actually due to gamma-ray emission by millisecond pulsars in a background, satellite galaxy of the Milky Way.