Constraints on very high energy gamma-ray emission from the Fermi bubbles with future ground-based experiments

The origin of sub-TeV gamma rays detected by the Fermi-Large Area Telescope (LAT) from the Fermi bubbles (FBs) at the Galactic center is still uncertain. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce the observed gamma rays. Recently the High Altitude Water Cherenkov (HAWC) observatory reported an absence of gamma-ray excess from the Northern FB at $b\gtrsim 6°$ Galactic latitude, which resulted in flux upper limits in the energy range of 1.2–126 TeV. These upper limits are consistent with the gamma-ray spectrum measured by Fermi-LAT at $|b|\ge 10°$, where an exponential cutoff at energies $\gtrsim 100\mathrm{GeV}$ is evident. However, the FB gamma-ray spectrum at $|b|\le 10°$, without showing any sign of cutoff up to around 1 TeV in the latest results, remains unconstrained. The upcoming Cherenkov Telescope Array (CTA) will perform a Galactic center survey with unprecedented sensitivity in the energy between 20 GeV and 300 TeV. In this work, we perform both morphological and classic on/off analyses with planned CTA deep central and extended survey and estimate the sensitivity of CTA to the FB hadronic gamma-ray flux models that best fit the spectrum at $|b|\le 10°$ and whose counterpart neutrino flux model best fits the optimistic neutrino spectrum from IceCube Neutrino Observatory. We also perform sensitivity analysis with a future ground-based Cherenkov detector the Large High Altitude Air Shower Observatory (LHAASO). We find that CTA will be able to discover or constrain the FB gamma-ray flux at $|b|\le 10°$ in the $\approx 200\mathrm{GeV}-100\mathrm{TeV}$ range with planned observation strategy, while LHAASO may constrain emission in the $\approx 100\mathrm{GeV}-100\mathrm{TeV}$ range if $\lesssim 10\%$ systematic uncertainties can be achieved.

Figure 1

Proposed CTA Galactic center and extended survey strategy in Galactic coordinate [5]. Upper panel shows the survey region as blue box and the Fermi bubbles in gray contours. Lower panel presents the central 9-point scanning (525 hour) as blue circles, extended 15-point scanning (300 hour) with red circles, and the Fermi bubbles in black dotted contours. For each pointing, the radius is 3°.

Figure 2

Gamma-ray spectra from the FB at the low-latitude and high-latitude regions [24]. The HAWC upper limits are derived in the $b>6°$ region [12]. The neutrino upper limits are calculated from IceCube events spatially correlated with the FB [21]. The hadronic models are shown with primary proton spectral index $k$ and exponential cutoff energy $E_0$ combination for $(k,E_0)=(2.0,1.6\mathrm{TeV})$ blue dashed line; (2.15, 30 TeV) red dotted line; and (2.2, 3 PeV) red solid lines. The magenta dashed line shows the neutrino flux (all three flavors) for the model with $(k,E_0)=(2.2,3\mathrm{PeV})$. Figure adapted from Ref. [22].

Figure 3

CTA sensitivity to hadronic models for the FB gamma-ray spectra (red curves). Top panel is for the high-cutoff spectral model ($k=2.2$, $E_0=3\mathrm{PeV}$), middle panel is for the low-cutoff spectral model ($k=2.15$, $E_0=30\mathrm{TeV}$) and bottom panel is the model at high attitude ($k=2.0$, $E_0=1.6\mathrm{TeV}$) respectively. The CR and GDE backgrounds are shown with purple long-dashed and gray solid lines respectively. The red points in upper and middle (bottom) panels are data from the low- (high-)latitude region of Fermi observation [24]. The CTA sensitivities with statistical uncertainty to the hadronic models in the low-latitude region are shown in blue shaded region in case of no systematic uncertainty and shown as blue dotted curves with 3% systematic uncertainty.

Figure 4

Calculated significance ${\mathrm{\Delta }}_{ij}$ to the FB with CTA in each pixel of four energy bins, 228–342 GeV (top-left), 1.15–1.73 TeV (top-right), 5.85–8.77 TeV (bottom-left), and 29.6–44.4 TeV (bottom-right). The selected ON and OFF regions are encircled with black and blue lines, respectively. High spectral cutoff model ($k=2.2$ $E_0=3\mathrm{PeV}$) has been used here for the low-latitude FB spectrum.

Figure 5

CTA sensitivity for detecting FBs using ON/OFF analysis for two hadronic gamma-ray spectra for the low-latitude region, namely the high-cutoff ($k=2.2$, $E_0=3\mathrm{PeV}$) and low-cutoff ($k=2.15$, $E_0=30\mathrm{TeV}$) models, are shown as red dot-dashed and black dotted curves, where the lower (upper) curve is for the case without (with) 3% systematics respectively.

Figure 6

Field of view of LHAASO in 4 zenith-angle bands [0°, 15°], [15°, 30°], [30°, 45°], and [45°, 60°] as labeled in grey, red, green, and yellow. The bubbles are shown with dot-dashed contours. The black dotted curve represents the Galactic plane.

Figure 7

LHAASO 5 sigma sensitivity to the FB low-latitude high- (low-) cutoff flux model for 30-day observation with and without 10% systematic uncertainty is presented in red dot-dashed (black dotted) curves in the energy between 100 GeV to 100 TeV respectively.