Study of spider pulsar binary eclipses and discovery of an eclipse mechanism transition

We present a comparative study of the low-frequency eclipses of spider (compact, irradiating binary) PSRs B1957+20 and J1816+4510. Combining these data with those of three other eclipsing systems we study the frequency dependence of the eclipse duration. PSRs B1957+20 and J1816+4510 have similar orbital properties, but the companions to the pulsars have masses that differ by an order of magnitude. A dedicated campaign to simultaneously observe the pulsed and imaged continuum flux densities throughout the eclipses reveals many similarities between the excess material within the two binaries, irrespective of the companion star properties. The observations show that the pulsar fluxes are removed from the line of sight throughout the main body of the eclipses. For PSR J1816+4510 we present the first direct evidence of an eclipse mechanism that transitions from one that removes the pulsar flux from the line of sight to one that merely smears out pulsations, and claim that this is a consequence of scattering in a tail of material flowing behind the companion. Inferred mass loss rates from the companion stars are found to be $\dot{M}_{\text{C}} \sim 10^{-12}~M_\odot$~yr$^{-1}$ and $\dot{M}_{\text{C}} \sim 2 \times 10^{-13}~M_\odot$~yr$^{-1}$ for PSR B1957+20 and PSR J1816+4510, respectively; seemingly too low to evaporate the stars within Hubble time. Measurements of eclipse durations over a wide range of radio-frequencies show a significant dependence of eclipse duration on frequency for all pulsars, with wider eclipses at lower-frequencies. These results provide a marked improvement in the observational constraints available for theoretical studies of the eclipse mechanisms.