Neutrino lighthouse at Sagittarius A*

We investigate whether a subset of high-energy events observed by IceCube may be due to neutrinos from Sagittarius A*. We check both spatial and temporal coincidences of IceCube events with other transient activities of Sagittarius A*. Among the seven IceCube shower events nearest to the Galactic center, we have found that event 25 has a time very close to (around three hours after) the brightest x-ray flare of Sagittarius A* observed by the Chandra X-ray Observatory with a $p$-value of 0.9%. Furthermore, two of the seven events occurred within one day of each other (there is a 1.6% probability that this would occur for a random distribution in time). Thus, the determination that some IceCube events occur at similar times as x-ray flares and others occur in a burst could be the smoking gun that Sagittarius A* is a point source of very-high-energy neutrinos. We point out that if IceCube Galactic center neutrino events originate from charged pion decays, then TeV gamma rays should come from neutral pion decays at a similar rate. We show that the CTA, HAWC, H.E.S.S. and VERITAS experiments should be sensitive enough to test this.

Figure 1

Sky map showing the IceCube neutrino events (labeled by event number; $\text{blue}=\text{shower}$, $\text{green}=\text{track}$, $\text{red}=\mathrm{PeV}$ shower) in galactic coordinates. The size of each neutrino event number label reflects the event’s energy. High-energy neutrinos are absorbed by the Earth; the gray shading and dashed contours denote the regions where the flux attenuation is significant for $E_{\nu }=200\mathrm{TeV}$ and $E_{\nu }=1\mathrm{PeV}$ [21]. The black curve separates the northern and southern sky. The horizontal yellow band denotes the galactic plane region. The dotted circles are $1\sigma$ angular regions for the IceCube events.

Figure 2

Sky map showing the IceCube neutrino events (labeled by event number; $\text{blue}=\text{shower}$, $\text{green}=\text{track}$, $\text{red}=\mathrm{PeV}$ shower) in galactic coordinates. The black curve separates the northern and southern sky. The horizontal yellow band denotes the galactic plane region. The IceCube events that are positionally consistent with the Galactic center are approximated by the dark (light) blue shaded ellipses within 30° (45°) angular distance from the Galactic center.

Figure 3

Left: Swift XRT observations of Sgr A* over six years [49]. The events numbered #1, #2, … #6 in blue are Swift flares. The red dashed line shows the timing of IceCube #2. Right: Swift XRT observations during the IceCube observation period, with the times of potential Galactic center neutrino events marked with red dashed lines. The x-ray data were generated using Swift’s online tools, see [50].

Figure 4

Left: Chandra HETGS observations of Sgr A* in 2012 [52]. The blue regions along the bottom are the quiescent levels and show when Chandra observations were being made. Right: A close-up on the largest flare, observed February 9th [53]. The red dashed line shows the timing of IceCube #25. IceCube #25 occurred 2:38:29 hours after the Chandra giant flare.

Figure 5

Fermi observations of Sgr A* within a 1° cone. The red dashed lines show the timing of IceCube neutrino events. To identify activity above quiescent levels, we simulated the expected data (assuming Sgr A* is a steady source) and subtracted the simulated data (middle, green) from the observed data (top, blue). We show the normalized difference in the bottom panel.

Figure 6

Time sequence of IceCube events that are positionally consistent within 45° of the Galactic center. Events #12 and #33 are more than 30° from the Galactic center. For comparison, the time sequence of all IceCube events is also shown.

Figure 7

The time and RA of all IceCube events within 30° of declination of the Galactic center. The inner [outer] blue band has $\mathrm{RA}\in (236°,296°)=({\mathrm{RA}}_{\mathrm{GC}}-30°,{\mathrm{RA}}_{\mathrm{GC}}+30°)$ [$\mathrm{RA}\in (221°,311°)=({\mathrm{RA}}_{\mathrm{GC}}-45°,{\mathrm{RA}}_{\mathrm{GC}}+45°)$]. Events positionally consistent within 30° of the Galactic center, which fall within the inner blue band, show more time clustering than events away from the Galactic center.

Figure 8

The ratio of the gamma-ray flux to the flavor-summed neutrino flux as a function of energy. See Eq. (6.2) for the definition of $\alpha$ and $E_{\text{cut}}$.

Figure 9

The correlated gamma-ray flux from the IceCube neutrino flux based on $pp$ collisions. The predicted values of the gamma flux based on primary proton spectra with a power law, $E^{-\alpha }$, are shown for $\alpha =2.3$ (green) and $\alpha =2.0$ (red). The breaks in the predicted gamma-ray flux are due to the attenuation effects. Here, the unit $1\mathrm{erg}=624.15\mathrm{GeV}$. Future experimental sensitivities are taken from Ref. [64].