3FGL J1924.8-1034: A spatially extended stable unidentified GeV source?

Milky Way–like galaxies are predicted to host a very large number of dark matter subhalos. Some massive and nearby subhalos could generate detectable gamma rays, appearing as unidentified, spatially extended and stable gamma-ray sources. We search for such sources in the third Fermi Large Area Telescope source list (3FGL) and report the identification of a new candidate, 3FGL J1924.8-1034. With the Fermi-LAT Pass 8 data, we find that 3FGL J1924.8-1034 is spatially extended at a high confidence level of $5.4\sigma$, with a best-fit extension radius of $\sim 0.15°$. No significant variability has been found and its gamma-ray spectrum is well fitted by the dark matter annihilation into $b\overline{b}$ with a mass of $\sim 43\mathrm{GeV}$. All these facts make 3FGL J1924.8-1034 a possible dark matter subhalo candidate. However, due to the limited angular resolution, the possibility of that the spatial extension of 3FGL J1924.8-1034 is caused by the contamination from the other unresolved point source cannot be ruled out.

Figure 1

$5°\times 5°$ TS map centered at the position of 3FGL J1924.8-1034. In the left (right) panel we show the TS map with (without) a point target source included in the modeling. The green crosses represent the positions of the sources within $5°\times 5°$ listed in Table 1

Figure 2

The $\mathrm{\Delta }{\mathrm{TS}}_{\mathrm{ext}}$ of 3FGL J1924.8-1034 when the point-like template is replaced with a spatial template in the cases of three extension profiles (GAUSS, NFW, PLE). The (GAUSS, NFW, PLE) template fit peaks at ${\theta }_{68}=(0.1°,0.1°,0.15°)$ with a $\mathrm{\Delta }{\mathrm{TS}}_{\mathrm{ext}}=(28.7,26.8,29.1)$, respectively.

Figure 3

The light curve of 3FGL J1924.8-1034. The blue point is the photon flux integrated from 300 MeV to 300 GeV in each time bin, and the dashed line is the mean value of all blue points.

Figure 4

The spectral energy distribution (SED) of 3FGL J1924.8-1034 (blue). Three types of spectra are shown in this figure, including the PLSuperExpCutoff spectrum (red dashed line), the LogParabola spectrum (green dashed line) and the DMFitFunction spectrum of dark matter annihilation into $b\overline{b}$ (for a rest mass of 43.5 GeV) (black dashed line).

Figure 5

The relationship between the subhalo mass ($M_{200}$) and the distance ($D$) of the subhalo. The width of the grey band includes the effect of scatter in the subhalo’s concentration and mass loss for tidal stripping.