Title: The effect of an offset polar cap dipolar magnetic field on the modeling of the Vela pulsar's $γ$-ray light curves,
Abstract: We performed geometric pulsar light curve modeling using static, retarded
vacuum, and offset polar cap (PC) dipole $B$-fields (the latter is
characterized by a parameter $\epsilon$), in conjunction with standard two-pole
caustic (TPC) and outer gap (OG) emission geometries. The offset-PC dipole
$B$-field mimics deviations from the static dipole (which corresponds to
$\epsilon=0$). In addition to constant-emissivity geometric models, we also
considered a slot gap (SG) $E$-field associated with the offset-PC dipole
$B$-field and found that its inclusion leads to qualitatively different light
curves. Solving the particle transport equation shows that the particle energy
only becomes large enough to yield significant curvature radiation at large
altitudes above the stellar surface, given this relatively low $E$-field.
Therefore, particles do not always attain the radiation-reaction limit. Our
overall optimal light curve fit is for the retarded vacuum dipole field and OG
model, at an inclination angle $\alpha=78{_{-1}^{+1}}^{\circ}$ and observer
angle $\zeta=69{_{-1}^{+2}}^{\circ}$. For this $B$-field, the TPC model is
statistically disfavored compared to the OG model. For the static dipole field,
neither model is significantly preferred. We found that smaller values of
$\epsilon$ are favored for the offset-PC dipole field when assuming constant
emissivity, and larger $\epsilon$ values favored for variable emissivity, but
not significantly so. When multiplying the SG $E$-field by a factor of 100, we
found improved light curve fits, with $\alpha$ and $\zeta$ being closer to best
fits from independent studies, as well as curvature radiation reaction at lower
altitudes.