Study of the boxlike dark matter signals from dwarf spheroidal galaxies with Fermi-LAT data

The observation of a special spectral feature in the gamma-ray data would be one of the best ways to identify dark matter (DM). The box-shaped gamma-ray spectra could be generated by the decay of intermediate particles produced by DM annihilation or decay. It provides another kind of signal that can be relatively easily distinguished from astrophysical backgrounds besides the linelike signals. Dwarf spheroidal galaxies are expected to be dominated by DM and may be one of the most promising targets for indirect DM searches. In this paper, we study the box-shaped DM signals with Fermi-LAT observations of dwarf spheroidal galaxies. We analyze 106 months of Fermi-LAT data to derive the upper limits on the annihilation cross section or the decay timescale of DM. In addition, we compare the results for different sample selections and DM density distributions. We expect that more dwarf spheroidal galaxies will be found and the sensitivity of box-shaped gamma-ray signal searches will be significantly improved in the future.

Figure 1

The 95% confidence level upper limits on the cross section of cascade annihilation of DM for 24 dSphs with the $J$-factors of Ref. [48] (H16 sample), for several values of the degeneracy parameter $\mathrm{\Delta }m/m_{\mathrm{DM}}$.

Figure 2

Constraints on the cascade annihilation of DM for different samples of dSphs with $\mathrm{\Delta }m/m_{\mathrm{DM}}=0.05$. The dash-dotted line is the result for all 52 dSphs and dSph candidates collected from the literature, with their $J$-factors estimated from an empirical relation given in Ref. [43]. The numbers in brackets represent the dSph numbers contained in each sample. For H16 result, we also plot its 68% containment (grey band) obtained by considering the $J$-factor uncertainties.

Figure 3

Left: Constraints on the cascade decay lifetime of DM for the H16 sample, for a few degeneracy parameters $\mathrm{\Delta }m/m_{\mathrm{DM}}$. Right: Comparison of constraints on the cascade decay lifetime for different dSph samples, for $\mathrm{\Delta }m/m_{\mathrm{DM}}=0.55$. The grey band is the 68% containment due to the $D$-factor uncertainties for the H16 sample. Also shown are the constraints based on the observation of the Galactic center [18] (dash-dotted line).

Figure 4

Comparison between the constraints on the cascade annihilation of DM derived in this work and those from previous literature. Our result based on 24 dSphs with $J$-factors from Ref. [48] (H16 sample) is shown by the solid line. The dash-dotted, dashed and dotted lines from Refs. [18] and [19] are set by the Fermi-LAT observation towards the center of the Galaxy. All the results plotted here assume $\mathrm{\Delta }m/m_{\mathrm{DM}}=0.001$.