We report on searches for neutrinos and antineutrinos from astrophysical sources performed with the Borexino detector at the Laboratori Nazionali del Gran Sasso in Italy. Electron antineutrinos ($\bar{\nu}_e$) are detected in an organic liquid scintillator through the inverse $\beta$-decay reaction. In the present work we set model-independent upper limits in the energy range 1.8-16.8 MeV on neutrino fluxes from unknown sources that improve our previous results, on average, by a factor 2.5. Using the same data set, we first obtain experimental constraints on the diffuse supernova $\bar{\nu}_e$ fluxes in the previously unexplored region below 8 MeV. A search for $\bar{\nu}_e$ in the solar neutrino flux is also presented: the presence of $\bar{\nu}_e$ would be a manifestation of a non-zero anomalous magnetic moment of the neutrino, making possible its conversion to antineutrinos in the strong magnetic field of the Sun. We obtain a limit for a solar $\bar{\nu}_e$ flux of 384 cm$^{-2}$s$^{-1}$ (90% C.L.), assuming an undistorted solar $^{8}$B neutrinos energy spectrum, that corresponds to a transition probability $p_{ \nu_e \rightarrow \bar\nu_{e}} $ 1.8 MeV. At lower energies, by investigating the spectral shape of elastic scattering events, we obtain a new limit on solar $^{7}$Be-$\nu_e$ conversion into $\bar{\nu}_e$ of $p_{ \nu_e \rightarrow \bar \nu_{e}}<$ 0.14 (90% C.L.) at 0.862 keV. Last, we investigate solar flares as possible neutrino sources and obtain the strongest up-to-date limits on the fluence of neutrinos of all flavor neutrino below 3-7 ,MeV. Assuming the neutrino flux to be proportional to the flare's intensity, we exclude an intense solar flare as the cause of the observed excess of events in run 117 of the Cl-Ar Homestake experiment.

中文翻译：

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使用 Borexino 从天体物理来源寻找低能中微子

我们报告了在意大利国家大萨索实验室使用 Borexino 探测器从天体物理来源搜索中微子和反中微子的情况。电子反中微子 ($\bar{\nu}_e$) 通过逆 $\beta$-衰变反应在有机液体闪烁体中被检测到。在目前的工作中，我们对来自未知来源的中微子通量设置了 1.8-16.8 MeV 能量范围内与模型无关的上限，这将我们之前的结果平均提高了 2.5 倍。使用相同的数据集，我们首先在低于 8 MeV 的先前未探索区域中获得对漫射超新星 $\bar{\nu}_e$ 通量的实验约束。还提出了在太阳中微子通量中寻找 $\bar{\nu}_e$：$\bar{\nu}_e$ 的存在将是中微子非零异常磁矩的表现，使其在太阳强磁场中转化为反中微子成为可能。我们获得了 384 cm$^{-2}$s$^{-1}$ (90% CL) 的太阳能 $\bar{\nu}_e$ 通量的限制，假设太阳能 $^{8} $B 中微子能谱，对应于跃迁概率 $p_{ \nu_e \rightarrow \bar\nu_{e}} $ 1.8 MeV。在较低能量下，通过研究弹性散射事件的光谱形状，我们获得了对太阳 $^{7}$Be-$\nu_e$ 转换为 $p_{\nu_e 的 $\bar{\nu}_e$ 的新限制\rightarrow \bar \nu_{e}}<$ 0.14 (90% CL) 在 0.862 keV。最后，我们研究了太阳耀斑作为可能的中微子来源，并获得了对低于 3-7 ,MeV 的所有风味中微子的中微子通量的最强最新限制。假设中微子通量与耀斑的强度成正比，