Reconstruction of spider system’s observables from orbital-period modulations via the Applegate mechanism

Redback and black-widow pulsars are two classes of peculiar binary systems characterised by very short orbital periods, very low-mass companions, and, in several cases, regular eclipses in their pulsed radio signal. Long-term timing revealed systematic but unpredictable variations in the orbital period, which can most likely be explained by the so-called Applegate mechanism. This relies on the magnetic dynamo activity generated inside the companion star and triggered by the pulsar wind, which induces a modification of the star’s oblateness (or quadrupole variation). This, in turn, couples with the orbit by gravity, causing a consequent change in the orbital period. The Applegate description is limited to providing estimates of physical quantities by highlighting their orders of magnitude. Therefore, we derived the time-evolution differential equations underlying the Applegate model; that is, we tracked such physical quantities in terms of time. Our strategy is to employ the orbital period modulations, measured by fitting the observational data, and implement a highly accurate approximation scheme to finally reconstruct the dynamics of the spider system in question and the relative observables. Among the latter is the magnetic field activity inside the companion star, which is still a matter of debate for its complex theoretical modelling and the ensuing expensive numerical simulations. As an application, we exploited our methodology to examine two spider sources: 47 Tuc W (redback) and 47 Tuc O (black widow). In this paper, the results obtained are analysed and then discussed in relation to the literature.