Guaranteed and prospective Galactic TeV neutrino sources

Recent observations, particularly from the HESS Collaboration, have revealed rich Galactic populations of TeV gamma-ray sources, including a collection unseen in other wavelengths. Many of these gamma-ray spectra are well measured up to $\sim 10\mathrm{TeV}$, where low statistics make observations by air Čerenkov telescopes difficult. To understand these mysterious sources, especially at much higher energies—where a cutoff should eventually appear—new techniques are needed. We point out the following: (1) For a number of sources, it is very likely that pions, and hence TeV neutrinos, are produced; (2) As a general point, neutrinos should be a better probe of the highest energies than gamma rays, due to increasing detector efficiency; and (3) For several specific sources, the detection prospects for ${\mathrm{km}}^3$ neutrino telescopes are very good, $\sim 1–10$ events/year, with low atmospheric neutrino background rates above reasonable energy thresholds. Such signal rates, as small as they may seem, will allow neutrino telescopes to powerfully discriminate between models for the Galactic TeV sources, with important consequences for our understanding of cosmic-ray production.

Figure 1

Ranges over which gamma-ray and neutrino observations can measure a source spectrum ($E_{\gamma }\simeq E_{\pi }/2$, $E_{\nu }\simeq E_{\pi }/4$). Squares are HESS measurements of Vela Jr., fit by a power law (dashed line). Neutrino telescopes can probe higher energies to distinguish between pionic spectra with cutoffs (dot-dashed lines) by measuring the ${\nu }_{\mu }$-induced muon and shower spectra. Muon rates of $N(E_{\mu }>1\mathrm{TeV})\sim 3–6{\mathrm{yr}}^{-1}$ correspond to the dot-dashed lines (see Sec. 4), illustrating the sensitivity of neutrino observations.

Figure 2

A demonstration of the sensitivity of neutrino telescopes to varying spectral characteristics. Shown is the (base-10 log) differential (${\nu }_{\mu }+{\overline{\nu }}_{\mu }$)-induced muon spectra for a pionic source spectrum with ${\varphi }_{\gamma }=20\times {10}^{-12}{\mathrm{TeV}}^{-1}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$. In the upper panel, the lines of decreasing height correspond to spectral indices of $\Gamma =1.8$, 2.0, 2.2, and 2.4, with an exponential cutoff $E_{\nu }^{\mathrm{cut}}=50\mathrm{TeV}$. In the lower panel, the lines of decreasing height correspond to neutrino spectrum cutoffs of 100, 50, 25, and 10 TeV, with a fixed spectral index of $\Gamma =2.1$. Each line corresponds to 1 yr of observation.

Figure 3

Vela Jr. (RX $\mathrm{J}0852.0-4622$)—Muons: Integrated (${\nu }_{\mu }+{\overline{\nu }}_{\mu }$)-induced muon rates above a given measured muon energy. The long dashed, dot-dashed, and short dashed lines correspond to neutrino spectrum exponential cutoffs of 50, 25, and 10 TeV, respectively. The dotted line shows the expected atmospheric background in a $7{\mathrm{deg}}^2$ bin, as discussed in the text. Rates are for 1 yr of operation in a ${\mathrm{km}}^3$ Mediterranean detector.

Figure 4

Vela Jr. ($\mathrm{J}0852.0-4622$)—Showers: Integrated neutrino-induced shower rates above a given measured shower energy, with signal lines as in Fig. 3. Note that a different energy scale is used for showers, as measurable shower events are assumed to have a 1 TeV energy threshold. Background rates are for a circle of 10° radius, corresponding to the shower angular resolution in a ${\mathrm{km}}^3$ Mediterranean detector. A significant reduction in background results from improved angular resolution. Rates are for 1 yr of operation.

Figure 5

Galactic Center region: Integrated (${\nu }_{\mu }+{\overline{\nu }}_{\mu }$)-induced muon rates above a given measured muon energy. Shown are the rates from the Galactic Center diffuse source alone (dot-dashed line) and the result of including the Sgr ${\mathrm{A}}^{*}$ source (dashed line). Two cases are considered: a neutrino spectrum exponential cutoff $E_{\nu }^{\mathrm{cut}}=50\mathrm{TeV}$ and a more conservative cutoff of 20 TeV. The dotted line shows the estimated atmospheric background in a $3{\mathrm{deg}}^2$ bin. Rates are for 1 yr of observation in a ${\mathrm{km}}^3$ Mediterranean detector.