Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e., the first $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly ${\nu }_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle ${\vartheta }_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

Figure 1

Luminosities as functions of time for ${\nu }_e$ (top), ${\overline{\nu }}_e$ (middle), and heavy-lepton neutrinos (bottom). In the left column results for different progenitor stars between $11.2M_{\odot }$ and $25M_{\odot }$ (left column; the progenitor mass is indicated by the number after the first letter of the model name [21]) are shown, in the middle column for simulations with the new treatment of electron captures by nuclei during stellar core collapse according to LMS (solid and dotted lines corresponding to the right pair of curves) compared to the traditional description (lines corresponding to the left pair of curves) in case of a $15M_{\odot }$ and a $25M_{\odot }$ star. The right column shows results for three different nuclear equations of state applied to the collapse of a $15M_{\odot }$ progenitor (see text for more details). The luminosities are given for an observer at rest, evaluated at a radius of 400 km with a corresponding time retardation of about 1 ms. Time is normalized to the moment of shock formation defined by the instant when the entropy behind the shock first exceeds a value of $3k_B$ per nucleon.

Figure 2

Same as Fig. 1 but for the rms neutrino energies as defined in Eq. (1). Note that during core collapse and, in particular, before shock formation only ${\nu }_e$ were taken into account in the simulations because the production of ${\overline{\nu }}_e$ and heavy-lepton neutrinos is suppressed due to the low entropy and correspondingly high electron and ${\nu }_e$ degeneracy.

Figure 3

Energy and angular distribution of all events with $t<t_{\max }=18\mathrm{m}\mathrm{s}$, assuming $t_{\mathrm{b}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{c}\mathrm{e}}=0\mathrm{m}\mathrm{s}$ at bounce, in a megaton water Cherenkov detector for a SN with a progenitor mass of $15M_{\odot }$ at 5 kpc, and scenario C. The different reaction channels shown are elastic scattering on electrons (concentrated at low energies and low angles), inverse beta decay (isotropically distributed), CC events on oxygen (concentrated at high energies and high angles), and NC events on oxygen (concentrated at low energies and isotropically distributed); also shown is the cut $E(\mathrm{M}\mathrm{e}\mathrm{V})\times \mathrm{a}\mathrm{n}\mathrm{g}\mathrm{l}\mathrm{e}(\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s})=500$.

Figure 4

Number of events per time bin from the elastic scattering on electrons, inverse beta decay assuming 90% tagging efficiency of Gadolinium, and reactions on oxygen in a megaton water Cherenkov detector for a SN with a progenitor mass of $15M_{\odot }$ at 10 kpc, and case C. Solid lines stand for the number of events without cut and dashed lines with the cut $(E/\mathrm{M}\mathrm{e}\mathrm{V})\times (\mathrm{a}\mathrm{n}\mathrm{g}\mathrm{l}\mathrm{e}/\mathrm{d}\mathrm{e}\mathrm{g}\mathrm{r}\mathrm{e}\mathrm{e}\mathrm{s})=500$.

Figure 5

Number of events per time bin from the elastic scattering on electrons: ${\nu }_e{e}^{-}$ and ${\nu }_x{e}^{-}$, and the total $\nu e^{-}$, for the mixing scenarios C (solid lines) and A (dashed lines). A megaton water Cherenkov and a SN with progenitor mass $15M_{\odot }$ at 10 kpc is assumed.

Figure 6

Distribution of fit values $a$ and $b$ for different SN distances, $D=2$, 5, and 10 kpc, and a $15M_{\odot }$ SN progenitor.

Figure 7

Number of events per time bin for different reactions in a megaton water Cherenkov detector for a SN at 10 kpc for cases A (dashed lines) and C (solid lines) and for different progenitor masses: $13M_{\odot }$ (n13), $15M_{\odot }$ (s15s7b2), and $25M_{\odot }$ (s25a28). Statistical errors are also shown for the $15M_{\odot }$ case.

Figure 8

Same as Fig. 7 for mixing scenarios A (dashed lines) and C (solid lines) for a SN at 10 kpc and a progenitor mass of $15M_{\odot }$ (s15a28). We show the SN model with the electron capture rates on heavy nuclei adopted from Ref. 25 (LMS), and with the traditional description. Statistical errors are also shown for the (LMS) models.

Figure 9

Same as Fig. 7 for mixing scenarios A (dashed lines) and C (solid lines) for a SN at 10 kpc and a progenitor mass of $15M_{\odot }$ (s15a28). We show the SN models computed with the equations of state “L&S,” Shen, and Wolff, respectively. Statistical errors are displayed for the Shen model.

Figure 10

Distribution of the probability to misidentify cases A and C for all SN models considered in the previous sections combined (solid lines), and only for those with different progenitor mass (dashed lines).

Figure 11

Same as Fig. 7 for a SN at 10 kpc and a progenitor mass of $15M_{\odot }$ (s15a28), for different neutrino parameters. We show with a dashed line case B, inverted mass hierarchy, and with solid lines a normal mass hierarchy for different values of ${sin}^2{\vartheta }_{13}$. Statistical errors are shown for ${sin}^2{\vartheta }_{13}=2\times {10}^{-5}$.

Figure 12

Distribution $p_{\alpha }(N)$ of the observed total number of events $N$ for 20 000 simulated SNe with progenitors.

Figure 13

Same as Fig. 12 for the SN progenitor s15a28 and three different EoS: L&S, Shen, and Wolff.