A Radio Pulsar Search of the Gamma-ray Binaries LS I +61 303 and LS 5039

TL;DR

This study conducted radio pulsar searches in gamma-ray binaries LS I +61 61 303 and LS 5039, finding null results and discussing how stellar winds affect pulsar detectability, thus informing the nature of these systems.

Contribution

First radio pulsar search in these gamma-ray binaries, providing flux upper limits and analyzing wind absorption effects on pulsar detectability.

Findings

No pulsar signals detected within sensitivity limits.

Stellar wind absorption likely prevents pulsar detection unless specific orbital conditions occur.

Wind models suggest pulsar emission can be absorbed by dense stellar winds.

Abstract

LS I +61 303 and LS 5039 are exceptionally rare examples of HMXBs with MeV-TeV emission, making them two of only five known or proposed "gamma-ray binaries". There has been disagreement within the literature over whether these systems are microquasars, with stellar winds accreting onto a compact object to produce high energy emission and relativistic jets, or whether their emission properties might be better explained by a relativistic pulsar wind colliding with the stellar wind. Here we present an attempt to detect radio pulsars in both systems with the Green Bank Telescope. The upper limits of flux density are between 4.1-14.5 uJy, and we discuss the null results of the search. Our spherically symmetric model of the wind of LS 5039 demonstrates that any pulsar emission will be strongly absorbed by the dense wind unless there is an evacuated region formed by a relativistic colliding wind shock. LS I +61 303 contains a rapidly rotating Be star whose wind is concentrated near the stellar equator. As long as the pulsar is not eclipsed by the circumstellar disk or viewed through the densest wind regions, detecting pulsed emission may be possible during part of the orbit.