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Unraveling the origin of high-energy neutrino sources: follow-up searches of IceCube alert events


Karl,  Martina S.
Max Planck Institute for Physics, Max Planck Society and Cooperation Partners;

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Karl, M. S. (2022). Unraveling the origin of high-energy neutrino sources: follow-up searches of IceCube alert events. PhD Thesis, TU München, München.

Cite as: https://hdl.handle.net/21.11116/0000-000C-B5F6-2
I investigate possible high-energy neutrino production sites --- based on IceCube alert events --- for additional neutrino emission. For this, I look at 11 years of muon data from the IceCube Neutrino Observatory. In total, this analysis covers the origin region of 122 alerts that were detected between 2009 and the end of 2021. The signal I aim to find are neutrinos originating from a specific source with an energy distribution following $E^{-\gamma}$, whereas the background is a diffuse neutrino flux. This work investigates two main source classifications: sources with continuous neutrino emission and sources with transient neutrino emission. For the continuous case, the single strongest source out of 122 defined regions has a global p-value of 0.98 and is compatible with the background hypothesis. The upper flux limit for that source (with 90\% confidence level) is $\Phi_{90\%,100\rm{TeV}} = 6.9 \times 10^{-17}$ (TeV cm$^2$ s)$^{-1}$. $\Phi_{90\%, 100\rm{TeV}}$ is for a spectral index of $\gamma =2$ and normalized at 100~TeV. When looking at the combined lower-energy emission IceCube measures from all 122 alert origins (excluding the alert events from analyzed data), I find a p-value of 8\%, which is also compatible with the background hypothesis. In total, the 90\% confidence level upper flux limit is $\Phi_{90\%,100\rm{TeV}} = 4.2 \times 10^{-16}$~(TeV cm$^2$ s)$^{-1}$ for an energy spectral index of $\gamma=2$ for the lower-energy component of all positions combined. This corresponds to 1.6\% of IceCube's astrophysical diffuse flux. When investigating the maximal contribution from all positions including the alert events, the maximal overall flux is $\Phi_{90\%,100\rm{TeV}} = 1.2 \times 10^{-15}$~(TeV cm$^2$ s)$^{-1}$ ($\gamma =2$) coming from all regions combined. This flux is 4.6\% of IceCube's astrophysical diffuse flux. Next, I search for transient neutrino emission and present new methods for identifying them. I find the transient neutrino emission associated with the blazar TXS~0506+056 as the most significant neutrino flare. The local p-value is $p_{local} = 0.14\% = 2.99 \sigma$. The flare parameters of this work agree with previous works. I find a Gaussian flaring time window centered at $\mu_T = 57001 ^{+52}_{-44}$~MJD with a width of $\sigma_T = 64 ^{+58}_{-15}$~days. The likelihood maximization finds $ 12 ^{+9}_{-6}$ neutrinos from the source following a source spectral index of $\gamma = 2.3 \pm 0.4$. The corresponding time-integrated flux --- the fluence --- $J_{100\rm{TeV}} = \int_{t_{start}} ^{t_{end}} \Phi_{100\rm{TeV}} dt$, normalized at 100~TeV, is $J_{100\rm{TeV}} = 1.2 ^{+1.1} _{-0.8} \times 10^{-8} $~(TeV~cm$^2$)$^{-1}$. The average flux during the $2\sigma_T$ time window is $\Phi_{100\rm{TeV}} = 1.1 ^{+1.0}_{-0.7} \times 10^{-15}$~(TeV cm$^2$ s)$^{-1}$. When correcting for testing 122 positions, the global p-value is $p_{global} = 0.156$ and is compatible with background. In general, the lack of a softer neutrino component agrees with expectations from sources with hard neutrino emission. Finding the transient neutrino emission associated with TXS~0506+056 with IceCube data as the only flare with a local p-value at the level of $\sim 3\sigma$ strengthens the hypothesis of TXS~0506+056 as a neutrino source.