Characteristics of the Galactic Center excess measured with 11 years of $Fermi$-LAT data

The excess of $\gamma$ rays in the data measured by the Fermi Large Area Telescope from the galactic center region is one of the most intriguing mysteries in astroparticle physics. This “galactic center excess” (GCE) has been measured with respect to different interstellar emission models (IEMs), source catalogs, data selections, and techniques. Although several proposed interpretations have appeared in the literature, there are no firm conclusions as to its origin. The main difficulty in solving this puzzle lies in modeling a region of such complexity and thus, precisely measuring the characteristics of the GCE. In this paper, we use 11 years of Fermi-LAT data, state of the art IEMs, and the newest 4FGL source catalog to provide precise measurements of the energy spectrum, spatial morphology, position, and sphericity of the GCE. We find that the GCE has a spectrum that is peaked at a few GeV and is well fit with a log parabola. The normalization of the spectrum changes by roughly 60% when using different IEMs, data selections, and analysis techniques. The spatial distribution of the GCE is compatible with a dark matter (DM) template produced with a generalized Navarro-Frenk-White density profile with slope $\gamma =1.2–1.3$. No energy evolution is measured for the GCE morphology between 0.6–30 GeV at a level larger than 10% of the $\gamma$ average value, which is 1.25. The analysis of the GCE modeled with a DM template divided into quadrants shows that the spectrum and spatial morphology of the GCE is similar in different regions around the galactic center. Finally, the GCE centroid is compatible with the galactic center, with a best-fit position between $l=[-0.3°,0.0°],b=[-0.1°,0.0°]$, and it is compatible with a spherical symmetric morphology. In particular, fitting the DM spatial profile with an ellipsoid gives a major-to-minor axis ratio (aligned along the galactic plane) between 0.8–1.2.

Figure 1

Map of the flux calculated at 1 GeV for the $\gamma$-ray emission produced for bremsstrahlung and ${\pi }^0$ from cosmic-ray protons (left panel) and for inverse Compton scattering from electrons and positrons injected from the galactic center (right panel). The color bar represents the intensity of the emission in the different directions of the ROI in units of $1/\mathrm{MeV}/{\mathrm{cm}}^2/\mathrm{s}/\mathrm{sr}$. We consider the case with a vertical size of the galactic halo of $z=8\mathrm{kpc}$. The maps reported here have been produced in Ref. [11].

Figure 2

Weight maps generated for $E=0.27\mathrm{GeV}$ (left panel) and 1.1 GeV (right panel). The color bar represents the ${\mathrm{Log}}_{10}$ of the weight values. See the main text for further details on the weighted likelihood technique.

Figure 3

Left panel: value of the $\mathrm{Log}(\mathcal{L})$ and sum of the absolute value of the residuals obtained when we run the analysis pipeline to Model i with $i=[1,11]$ (see Table 2 for further details.). Right Panel: SED of the GCE obtained with the different Models. In parenthesis of the legend, we report the number of components present for each Model.

Figure 4

Energy spectrum between 1 and 10 GeV of the different components included in our analysis with Model 2. Here, we use the components generated with the Baseline IEM (see Sec. 2b for further details).

Figure 5

We show in the left panel the map of the residuals obtained using the Model 2 and the Baseline IEM to fit the ROI in the energy range 1–10 GeV. Instead, in the right panel, we have added to the residuals the counts of the GCE. The different colors represent the fractional residual, i.e., the residual counts (data minus model) divided by the total counts.

Figure 6

Flux, integrated between 1–10 GeV, absorbed by the DM template when it is placed at different positions along the galactic plane. We show the results obtained with three different IEMs.

Figure 7

GCE SED measured when using the Baseline, Yusifov, SNR, OBstars, Pulsars, SLext, ICS combined, and PlanckGASS IEMs. We also show the results obtained with different selections of the data (SOURCE, ULTRACLEANVETO, and PSF23), with a smaller ROI ($30\times 30$ ROI) and without using the weight map in the maximum likelihood analysis (no weights).

Figure 8

Comparison between the results for the GCE SED obtained in our analysis and in [9, 11]. The bands represent the variation of the GCE SED obtained by using all the IEMs and analysis techniques shown in Fig. 7 and the results found in [9, 11] when using different IEMs. See the text for further details on the conversion of the GCE SED found in our analysis and in [9, 11] into flux per solid angle (i.e., in units of $\mathrm{MeV}/{\mathrm{cm}}^2/\mathrm{s}/\mathrm{sr}$). We also display the best fit to the GCE SED, obtained with the Baseline IEM, by using a log-parabola function.

Figure 9

GCE SED obtained in case we use the Baseline, CMZ 4 kpc, CMZ 8 kpc, IC bulge, and No low-lat Bubbles IEMs.

Figure 10

Surface brightness data ($dN/d\mathrm{\Omega }$) obtained with the analysis in annuli of size 1° and with the Baseline IEM in the energy range 1–10 GeV. We also show the best fit obtained with a NFW density profile for $\gamma =1.27$ (dashed blue line) and for the IC bulge model at $E=0.7$ (dotted green line) and 20 GeV (dot-dashed green line), for which we fit the GCE data with the $\gamma$ rays emitted for inverse Compton scattering by cosmic-ray electrons injected from the galactic bulge (see Sec. 2b for a complete description of this model). Red data points show the results obtained in [8] at 2.67 GeV.

Figure 11

Best fit for $\gamma$ obtained by fitting the surface brightness data obtained in the following energy bins: 0.6–1.0, 1.0–1.8, 1.8–3.2, 3.2–5.6, 5.6–10, 10–32 GeV. We show the results found using following IEMs: Baseline, Yusifov, SNR, OBstars, Pulsars, SLext, ICS combined, and PlanckGASS IEM. We also display the values of $\gamma$ we obtained with different selections of the data (SOURCE, ULTRACLEANVETO, and PSF23), with a smaller ROI ($30\times 30$ ROI) and without using the weight map in the maximum likelihood analysis (o weights).

Figure 12

SED of the quadrants for the quad+ configuration in the top panel and for quadx in the bottom panel. We also display the GCE SED, obtained with the DM template considered as a whole, divided by 4. The data have been obtained with the Baseline IEM.

Figure 13

Best fit for $\gamma$ found by fitting the surface brightness of each quadrant and using different IEMs. We show here the results obtained with the quadrant configuration quad+.

Figure 14

Value of the likelihood as a function of the galactic longitude and latitude $\mathrm{Log}(\mathcal{L})(l,b)$ found with the Baseline IEM for the energy range between 1–10 GeV. We show with a cyan star the position, for which we maximize the $\mathrm{Log}(\mathcal{L})(l,b)$.

Figure 15

Value of the likelihood as a function of the parameters ratio and $\gamma \mathrm{Log}(\mathcal{L})(ratio,\gamma )$ found with the Baseline (top left), SNR (top right), Yusifov (bottom left), and SL ext (bottom right) IEM for the energy range between 1–10 GeV.