TeV Gamma-Ray Observations of the Galactic Center

TL;DR

This paper reports a probable TeV gamma-ray detection from the Galactic Center using the Whipple 10 m telescope, based on 26 hours of on-source data collected at large zenith angles. A novel large-zenith-angle, two-dimensional analysis with Crab Nebula calibrations yields a GC excess centered near Sgr A* with a peak energy around 2.8 TeV and an integral flux of about 1.6×10^-8 photons m^-2 s^-1 (≈0.4 Crab) above that energy, with a significance of roughly 3.7σ after accounting for analysis variations. The emission appears consistent with a point source within ~15 arcmin of the GC and shows no strong variability across the observation period, prompting discussion of possible origins near Sgr A* or surrounding structures, or exotic scenarios like dark matter. The study also introduces a methodology for TeV GC measurements at large zenith angles, validated against Crab data, and emphasizes the need for further multiwavelength and higher-sensitivity TeV observations to identify the emission mechanism.

Abstract

We report a possible detection of TeV gamma-rays from the Galactic Center by the Whipple 10m gamma-ray telescope. Twenty-six hours of data were taken over an extended period from 1995 through 2003 resulting in a total significance of 3.7 standard deviations. The measured excess corresponds to an integral flux of \Flux above an energy of $2.8 \mathrm{TeV}$, roughly 40% of the flux from the Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15 arcmin and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence for variability.

Figures (5)

• Figure 1: Plots of our selection criteria for gamma-ray-like events for two of the parameters (LENGTH and WIDTH) for two different zenith angles ($0^\circ$ and $60^\circ$). The dotted lines show the upper and lower cuts on the respective parameters as a function of the SIZE (total signal) parameter. The dots are from simulations of gamma rays with a range of energies.
• Figure 2: Optical image of a star ($\mathrm{Sgr}\ \gamma_2$) used to check the telescope's pointing at low elevation. This image shows that the telescope has an offset of 0.14 degrees down and to the right of camera center position (0,0).
• Figure 3: A gamma-ray image of the region around Sgr A*. The image is of excess counts with overlaid significance contours (1 standard deviation per contour). The axes are labeled in degrees from the assumed camera center. The true center position of the camera, which is not exactly at (0,0) due to flexing of the telescope at low elevation, is marked with a cross. The dashed lines are the RA and Dec contours at this position. Also shown (as a light contour) is the 99% confidence region for the EGRET observations dingus2002.
• Figure 4: A gamma-ray image of the Crab Nebula taken at large zenith angle ($\approx62^\circ$) using the same analysis procedure used for Sgr A*. The offset and pointing variations can be seen in the resulting image.
• Figure 5: Flux of Sgr A* as a function of time. Each data point represents a single 28 minute run. Time gaps in the data have been removed where indicated. The dashed line is a least squares fit of a constant function to the data.