Search for line-like signals in the all-sky Fermi-LAT data

In order to search for the linelike signals in the Fermi-LAT data, we have analyzed totally 49152 regions of interest (ROIs) that cover the whole sky. No ROI displays a line signal with test statistic (TS) value above 25, while for 50 ROIs weak linelike excesses with $\mathrm{TS}>16$ are presented. The intrinsic significances of these potential signals are further reduced by the large trial factor introduced in such kind of analysis. For the largest TS value of 24.3 derived in our analysis, the corresponding global significance is only $0.54\sigma$. We thus do not find any significant linelike signal and set up constraints on the cross section of dark matter annihilating to gamma-ray lines, ${⟨\sigma v⟩}_{\gamma \gamma }$.

Figure 1

The $T{S}_{\max }$ distribution of 49152 searched ROIs. The dashed line is the best fit trial-corrected ${\chi }^2$ distribution. The inserted subfigure is the same distribution plotted in a y-log framework for better showing the tails of the distribution.

Figure 2

ROIs showing linelike excesses with $\mathrm{TS}>16$ overlaid on a Hammer-Aitoff projection of Fermi-LAT counts map ($E>1\mathrm{GeV}$).

Figure 3

Left panel: The line signal detection threshold ${\mathrm{\Phi }}_0$ as a function of the line energy $E_{\gamma }$. Right panel: The expected number of subhalos that can yield line signal significantly detectable by Fermi-LAT as a function of the line signal cross section assuming a DM mass of 100 GeV and a detection threshold of $1.0\times {10}^{-11}\mathrm{ph}/{\mathrm{cm}}^2/\mathrm{s}$. Note that ${\mathrm{\Phi }}_0$, $M_{\mathrm{DM}}$ and $⟨\sigma v⟩$ are actually three degenerate parameters, the curve in the right panel applies to any values of ${\mathrm{\Phi }}_0$ and $M_{\mathrm{DM}}$.

Figure 4

The 95% confidence level upper limits on the cross sections of DM annihilating into double $\gamma$-rays derived in our analysis. As a comparison, we also plot the constraints set by the Fermi-LAT observations of the Galactic central regions [15].

Figure 5

Top: Distribution of TS values from individual fits of a 10 GeV (left panel) or 30 GeV (right panel) line to the 3000 background-only simulations. Bottom left: The $T{S}_{\max }$ distribution of 3000 Monte Carlo spectra generated from background-only simulations. The dashed line is the best fit trial-corrected ${\chi }^2$ distribution. Bottom right: We inject a pseudoline signal with photon number $N_{\mathrm{line}}=30$ at 10 GeV into the MC data and fit them with $\mathrm{background}+\mathrm{line}$ model. This plot shows that our fits can well recover the input parameter (red line).

Figure 6

The observed spectrum of the ROI displaying linelike excess with $TS=24.3$, which is the highest one in our analysis. For the energy bins with photon number less than 2, we plot $1\sigma$ confidence level upper limits for better visualization. Note that the linelike excess shown here is most likely from background fluctuation since a very large trial factor is introduced in the analysis.