Mid-UV Studies of the Transitional Millisecond Pulsars XSS J12270-4859 and PSR J1023+0038 During Their Radio Pulsar States

TL;DR

This study uses mid-UV observations from multiple space telescopes to analyze the emission and variability of two transitional millisecond pulsars during their radio pulsar states, revealing the need for additional radiation sources beyond direct heating.

Contribution

It provides new mid-UV observational data and modeling insights, highlighting the limitations of simple heating models and suggesting the role of intrabinary shocks in these pulsars.

Findings

Mid-UV luminosities are similar at comparable orbital phases.

Models with fixed parameters do not fit the observed MUV light curves.

An intrabinary shock likely contributes to the MUV and optical emissions.

Abstract

We report mid-UV (MUV) observations taken with HST/WFC3, Swift/UVOT and GALEX/NUV of the transitional millisecond pulsars XSS J12270-4859 and PSR J1023+0038 during their radio pulsar states. Both systems were detected in our images and showed MUV variability. At similar orbital phases, the MUV luminosities of both pulsars are comparable. This suggests that the emission processes involved in both objects are similar. We estimated limits on the mass ratio, companion's temperature, inclination, and distance to XSS J12270-4859 by using a Markov Chain Monte Carlo algorithm to fit published folded optical light curves. Using the resulting parameters, we modeled MUV light curves in our HST filters. The resulting models failed to fit our MUV observations. Fixing the mass ratio of XSS J12270-4859 to the value reported in other studies, we obtained a distance of ~3.2 kpc. This is larger than the one derived from radio measurements (~1.4 kpc). Assuming a uniform prior for the mass ratio, the distance is similar to that from radio measurements. However, it requires an undermassive companion (~0.01 M_sun). We conclude that a direct heating model alone can not fully explain the observations in optical and MUV. Therefore, an additional radiation source is needed. The source could be an intrabinary shock which contributes to the MUV flux and likely to the optical one as well. During the radio pulsar state, the MUV orbital variations of PSR J1023+0038 detected with GALEX, suggests the presence of an asymmetric intrabinary shock.