Search for gamma-ray emission from the 12 nearby dwarf spheroidal galaxies with 12 years of Fermi-LAT data

TL;DR

This study updates the search for gamma-ray signals from nearby dwarf spheroidal galaxies using 12 years of Fermi-LAT data, finding weak excesses likely due to astrophysical sources rather than dark matter.

Contribution

It provides an updated analysis with longer data and improved models, identifying potential gamma-ray excesses and clarifying their possible astrophysical origins.

Findings

Weak gamma-ray excesses detected in Reticulum II, Bootes II, and Willman 1.

Excess in Reticulum II likely due to astrophysical contamination.

Possible dark matter particle masses around 14 GeV and 80 GeV for Bootes II and Willman 1.

Abstract

Previously, we have shown in Li et al. (2018) that very weak $\gamma$-ray excesses ($\sim 2\sigma$) appear in some Milky Way dwarf spheroidal galaxies (dSphs, including candidates) and the combination analysis of 12 nearby dSphs yields a local significance of $>4\sigma$. In this work, we adopt a longer data set (i.e., the 12 years of Fermi-LAT data), the latest Fermi-LAT software as well as background models to update the searches of $\gamma$-ray emission from these sources. Very weak $\gamma$-ray excesses ($>$ 2 $\sigma$) are found in the directions of three dSphs, including Reticulum II, Bootes II and Willman 1. In the direction of Reticulum II, the peak TS value of the excess reaches $\sim$11. However, different from the previous analysis with the 9 years of Fermi-LAT data, now the location of the gamma-ray emission is significantly away from the center of Reticulum II because of the enhancement of "offset" $\gamma$-rays above 10 GeV since 2017. The detected weak excess is likely due to the contamination of an astrophysical $\gamma$-ray source with a TS value of $\sim 22$, irrelevant to the dark matter inside Reticulum II. The possible excesses in the directions of Bootes II and Willman 1 are weaker with lower peak TS values ($\sim 7$). If interpreted as annihilation of dark matter particles into $\tau^+\tau^-$, the dark mass mass of $m_\chi \sim 14$ GeV and $\sim 80$ GeV are found for Bootes II and Willman 1, respectively. Much more data are needed to clarify whether these two potential signals are real and then reveal their origins.