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We perform a systematic study of the $^{56}$Ni mass ($M_{\rm Ni}$) of 27stripped envelope supernovae (SESNe) by modeling their light-curve tails,highlighting that use of ``Arnett's rule'' overestimates $M_{\rm Ni}$ for SESNby a factor of $\sim$2. Recently, \citet{Khatami2019} presented a new modelrelating the peak time ($t_{\rm p}$) and luminosity ($L_{\rm p}$) of aradioactive-powered SN to its $M_{\rm Ni}$ that addresses several limitationsof Arnett-like models, but depends on a dimensionless parameter, $\beta$. Usingobserved $t_{\rm p}$, $L_{\rm p}$, and tail-measured $M_{\rm Ni}$ values for 27SESN, we observationally calibrate $\beta$ for the first time. Despite scatter,we demonstrate that the model of \citet{Khatami2019} withempirically-calibrated $\beta$ values provides significantly improvedmeasurements of $M_{\rm Ni}$ when only photospheric data is available. However,these observationally-constrained $\beta$ values are systematically lower thanthose inferred from numerical simulations, primarily because the observedsample has significantly higher (0.2-0.4 dex) $L_{\rm p}$ for a given $M_{\rmNi}$. While effects due to composition, mixing, and asymmetry can increase$L_{\rm p}$ current models cannot explain the systematically low $\beta$values. However, the discrepancy can be alleviated if $\sim$7--50\% of $L_{\rmp}$ for the observed sample originates from sources other than $^{56}$Ni.Either shock cooling or magnetar spin-down could provide the requisiteluminosity. Finally, we find that even with our improved measurements, the$M_{\rm Ni}$ values of SESN are still a factor of $\sim$3 larger than those ofhydrogen-rich Type II SN, indicating that these supernovae are inherentlydifferent in terms of their progenitor initial mass distributions or explosionmechanisms.

The Nickel Mass Distribution of Stripped-Envelope Supernovae: Implications for Additional Power Sources