# Face changing companion of the redback millisecond pulsar PSR J1048+2339

## TL;DR

This study presents detailed optical observations of the redback millisecond pulsar PSR J1048+2339, revealing rapid changes in orbital modulation and brightness due to pulsar wind heating, and models system properties including a high pulsar mass.

## Contribution

The paper provides the first detailed optical light curves showing rapid modulation changes and models the system, revealing a high pulsar mass and variable heating effects in PSR J1048+2339.

## Key findings

- Orbital modulation changed from ellipsoidal to sinusoidal in less than 14 days.
- Brightness increased by one magnitude, indicating dominant pulsar wind heating.
- Derived pulsar mass of 2.1 solar masses and increased irradiation power.

## Abstract

We present optical observations of the redback millisecond pulsar PSR J1048+2339, which is a 4.66 ms radio pulsar in a compact binary with an orbital period of six hours. We obtained high-quality light curves of PSR J1048+2339 with the Lulin 1 m Telescope. The system shows two distinct six-hour orbital modulations, in which an ellipsoidal modulation changes into a sinusoidal-like profile in less than 14 days. In addition to the change, the brightness of the companion increased by one magnitude, suggesting that the latter type of modulation is caused by the pulsar wind heating of the companion and that the heating became dominant in the system. While the changes are not unexpected, such a timescale is the shortest among similar systems. We performed modeling analysis to extract the properties of the system. We obtained a derived pulsar mass of 2.1 M$_{\odot }$ and a companion star mass of 0.4 M$_{\odot }$ for the system. The irradiation power increased by a factor of 6 during which the pulsar wind heating dominates. We also report on the two archival Chandra X-ray observations and discuss several possibilities that might cause the varying heating on the companion.