Gamma-ray emission from the Sagittarius Dwarf Spheroidal galaxy due to millisecond pulsars

TL;DR

This paper suggests that millisecond pulsars in the Sagittarius dwarf spheroidal galaxy can account for gamma-ray emissions observed, highlighting their role in old stellar populations and implications for dark matter searches.

Contribution

It demonstrates that MSPs in Sgr dSph can plausibly explain gamma-ray signals, linking pulsar populations to gamma-ray emission in dwarf spheroidals.

Findings

MSPs can produce gamma-ray emission via inverse Compton scattering.

The gamma-ray spectrum matches MSP emission models.

Sgr dSph's gamma-ray signal is likely due to MSPs, not dark matter.

Abstract

The Fermi Bubbles are giant, gamma-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 super-massive black hole. Here we show via a spatial template analysis that much of the gamma-ray emission associated to the brightest region of substructure -- the so-called cocoon -- is likely due to the Sagittarius dwarf spheroidal (Sgr dSph) galaxy. 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 Sgr dSph has no on-going star formation, but we nevertheless demonstrate that the dwarf's millisecond pulsar (MSP) population can plausibly supply the gamma-ray signal that our analysis associates to 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 MSPs belonging to the Sgr dSph, combined with these objects' magnetospheric emission. This finding plausibly suggests that MSPs produce significant gamma-ray emission amongst old stellar populations, potentially confounding indirect dark matter searches in regions such as the Galactic Centre, the Andromeda galaxy, and other massive Milky Way dwarf spheroidals.