Observations of the Fermi bubbles and the Galactic center excess with the DArk Matter Particle Explorer

TL;DR

This study uses 102 months of DAMPE data to independently detect two diffuse gamma-ray features—the Fermi bubbles and the Galactic Center GeV excess—that were previously seen by Fermi-LAT. By constructing DAMPE-specific templates for the Galactic diffuse emission and other backgrounds and performing a binned likelihood analysis, the authors extract robust spectral and morphological information for both sources. The Fermi bubbles exhibit a hard spectrum with a cutoff around a few hundred GeV, with a combined significance of about $26\sigma$, while the GC excess shows a notable GeV peak that can be described by dark matter annihilation with $m_\chi\sim50$ GeV and $\langle\sigma v\rangle\sim10^{-26}~\mathrm{cm^3\,s^{-1}}$ for $\chi\chi\to b\bar{b}$, consistent with Fermi-LAT results. These results demonstrate DAMPE’s capability for high-precision gamma-ray studies and provide complementary constraints on DM scenarios in the Galactic center.

Abstract

The DArk Matter Particle Explorer (DAMPE) is a space-borne high-energy particle detector that surveys the $γ$-ray sky above$\sim 2~\rm GeV$ with a peak acceptance of $\sim 0.2~\rm m^2\,sr$. With the 102 months of data collected by DAMPE, we show that the Fermi bubbles are detected at a significance of $\sim 26σ$ and identify a GeV excess in the direction of Galactic center at $\sim 7 σ$ confidence. Both spectra and morphology are consistent with those observed by Fermi-LAT and the GeV excess component can be interpreted by the dark matter annihilation with a mass of $\sim 50$ GeV and a velocity-averaged cross section of $\sim 10^{-26}~{\rm cm^{3}~s^{-1}}$ for the $χχ\rightarrow b\bar{b}$ channel. Our results thus provide the first independent detection of these two intriguing diffuse gamma-ray sources besides Fermi-LAT.

Figures (18)

• Figure 1: Upper panels: The intensity map from 2 GeV to 500 GeV in CAR projection, smoothed with a Gaussian kernel with $\sigma=0\fdg75$ for (a) the observed data and (b) best-fit model without the bubbles. The Galactic plane region ($|b|\leq 5^{\circ}\xspace$) is masked. The green crosses mark the point sources in the DAMPE catalog, whereas the gray contour encloses the Fermi bubbles. Lower panels: The significance of the residual maps smoothed with a $1^{\circ}\xspace$ Gaussian kernel for the models (c) without and (d) with the bubbles. The black curve shows the boundary derived in this work, while the other lines represent the ones from Fermi-LAT.
• Figure 2: Left panel: Distribution of the values in the significance map. The map, in HEALPix projection, is smoothed with a $1^{\circ}\xspace$ Gaussian kernel. The black histogram shows the distribution of pixel numbers within the ROI, whereas the red and green ones show those within and outside the bubbles. The blue dashed line is the Gaussian profile with a mean value of zero fitted to the background histogram. The orange solid line represents the $2\sigma_{\rm bg}$ cut adopted to define the bubbles' boundary. Right panel: The map with the significance larger than $2\sigma_{\rm bg}$ cut. The bubble template in this work consists of two polygons whose boundaries are illustrated with the solid orange lines.
• Figure 3: The average residual flux as a function of the angular distance to the edge in the high-latitude region with $|b|>20^{\circ}\xspace$ (blue points). The residual is obtained by extracting the best-fit model except for the bubbles from the data. The negative (positive) $x$-axis values represent the region inside (outside) the bubbles. The green dashed line is the sharp edge convolved with the PSF. The orange band is the average flux of the bubble template as a whole.
• Figure 4: The average flux of the observed data (black hollow points) and various fitted components in the baseline model (solid points connected with lines) within the ROI. The red points show the flux of the bubbles. The masked Galactic plane and point sources are excluded from the calculation.
• Figure 5: The spectral energy distribution (SED) of the Fermi bubbles. The red points show the best-fit SED from DAMPE given the baseline model, whereas the blue points show those from Fermi-LAT Ackermann2014. 95% upper limit is given when the TS value is below 9. The red band exhibits the total errors from the statistical and systematic uncertainties. The optimal single power law model and power law cutoff model are also presented with black dotted and green dashed lines, respectively.