Unravelling the eclipse mechanism of a binary millisecond pulsar using broad band radio spectra

The frequency dependent eclipses of the radio emission from millisecond pulsars (MSPs) in compact binary systems provide an opportunity to understand the eclipse mechanism and to determine the nature of the eclipsing medium. We combine multi-frequency observations from the upgraded Giant Metrewave Radio Telescope (uGMRT) and model the broadband radio spectrum in the optically thick to thin transition regime to constrain the eclipse mechanism. The best fit model to the eclipse phase spectra favours synchrotron absorption by relativistic electrons. We are able to strongly constrain the frequency of onset of the eclipse to 345$\pm$5 $\mathrm{MHz}$, which is an order of magnitude more precise than previous estimates. The dependence on the magnetic field strength of synchrotron absorption allowed us to estimate the magnetic field strength of the eclipse medium to be $\sim$13 $\mathrm{G}$, which is very similar to the values obtained by considering a pressure balance between the incident pulsar wind and the stellar wind of the companion. Applying this method to other millisecond binary pulsars will enable us to determine if the eclipse mechanisms are all the same and also estimate the wind and magnetic field properties of the companion stars. The method could also be applied to other systems where pulsars interact with companion winds in binary systems and in all cases it will lead to a better understanding of the evolutionary processes.