  In this work, we study the parameter space of neutrino-emitting BL Lacs under
the framework of the one-zone lepto-hadronic model. We show that constraints on
the model come from various aspects of observations such as the variability
timescale of blazar flares, gamma-ray opacity and the spectral energy
distribution of electromagnetic emission, as well as the inferred neutrino
emissivity of the blazar. We apply our method to two potential neutrino
sources, i.e., TXS 0506+056 and PKS 0735+178, which are BL Lacs. Then, we
explore and summarize the allowed range of parameters such as the bulk Lorentz
factor and the blob radius under different distributions of injected protons.
We find that the parameter space that is available to explain the BL
Lac--neutrino association is sensitive to the proton distribution, and usually,
an injected proton luminosity significantly exceeding the Eddington luminosity
is required for both sources. Our results suggest that the simple
lepto-hadronic one-zone model may not be a reasonable interpretation for BL
Lac--neutrino associations.
