Testing the dark matter subhalo hypothesis of the gamma-ray source 3FGL $\mathrm{J}2212.5+0703$

N-body simulations predict that galaxies at the Milky Way scale host a large number of dark matter (DM) subhalos. Some of these subhalos, if they are massive enough or close enough to the Earth, might be detectable in $\gamma$ rays due to the DM annihilation. 3FGL $\mathrm{J}2212.5+0703$, an unidentified gamma-ray source, has been suggested to be the counterpart candidate of a DM subhalo by Bertoni et al. In this work we analyze the Fermi-LAT Pass 8 data of 3FGL $\mathrm{J}2212.5+0703$ to independently test the DM subhalo hypothesis of this source. In order to suppress the possible contamination from two nearby very bright blazars, we just take into account the front-converting gamma rays which have better angular resolutions than that of the back-converting photons. In addition to the spatial distribution analysis, we have extended the spectrum analysis down to the energies of $\sim 100\mathrm{MeV}$, and thoroughly examined the variability of the emission during the past 8 years. We confirm that 3FGL $\mathrm{J}2212.5+0703$ is a steady and spatially extended gamma-ray emitter at a high confidence level. The spectrum is well consistent with that expected from DM annihilation into $b\overline{b}$. The introduction of a phenomenological LogParabola spectrum just improves the fit slightly. All these results suggest that 3FGL $\mathrm{J}2212.5+0703$ could be indicative of a DM subhalo.

Figure 1

$5°\times 5°$ TS maps of the 3FGL $\mathrm{J}2212.5+0703$ region using the front-converting Pass 8 data in the energy range of 1–500 GeV. We show the TS map with (without) target source included in the model in the left (right) panel. The green cross in the center of the figure is the position of target source listed in the catalog, while the magenta circle shows the position fitted with gtfindsrc.

Figure 2

The increase in the TS value of 3FGL $\mathrm{J}2212.5+0703$ when a Gaussian template instead of a point-like one is incorporated in the data fit. The best-fit template is the one with a width of $0°.15$ and the corresponding ${\mathrm{TS}}_{\mathrm{ext}}$ is 22.4.

Figure 3

The light curve of 3FGL $\mathrm{J}2212.5+0703$. Red points are the fluxes integrated from 100 MeV to 500 GeV, while the TS value of each time interval is shown in gray histogram. 95% upper limits are drawn when the TS values are smaller than 5. The solid line and two dashed horizontal lines represent the average flux and its $1\sigma$ range, respectively. The dotted line is the flux derived from the fit of the flux-likelihood relation in different time bins, which is almost the same as the average flux. In this analysis, we find the variability index to be 33.2 with 15 degrees of freedom, indicating no significant ($\sim 2.6\sigma$) deviation from being a steady source.

Figure 4

The spectral energy distribution (SED) of 3FGL $\mathrm{J}2212.5+0703$ and two types of spectral modeling. Gray histograms represent the TS values in different energy bins. We use two types of spectra to fit the data, i.e., the generic LogParabola spectrum and the DMFitFunction spectrum which predicts the annihilation of 18.5 GeV DM particles to $b\overline{b}$, and the corresponding spectra are shown in black (dashed) line and blue (dotted dashed) line, respectively. The TS value difference between these two models is 7.0, implying that both models works almost equally well.