Radio pulsations from a neutron star within the gamma-ray binary LS I +61$^{\circ}$ 303

TL;DR

This paper reports the first detection of radio pulsations from LS I +61° 303, providing strong evidence that the compact object in this gamma-ray binary is a rotating neutron star, based on FAST telescope observations.

Contribution

The discovery of transient radio pulsations from LS I +61° 303 confirms the presence of a neutron star in the system, resolving a long-standing uncertainty.

Findings

Detected radio pulsations with a period of ~269 ms

First evidence of pulsations from LS I +61° 303 at any frequency

Supports the neutron star hypothesis for the compact object

Abstract

LS I +61$^{\circ}$ 303 is one of the rare gamma-ray binaries, emitting most of their luminosity in photons with energies beyond 100 MeV. The $\sim$26.5 d orbital period is clearly detected at many wavelengths. Additional aspects of its multi-frequency behavior make it the most interesting example of the class. The morphology of high-resolution radio images changes with orbital phase displaying a cometary tail pointing away from the high-mass star. LS I +61$^{\circ}$ 303 also shows superorbital variability. A couple of energetic ($\sim 10^{37}$ erg s$^{-1}$), short, magnetar-like bursts have been plausibly ascribed to it. LS I +61$^{\circ}$ 303's phenomenology has been put under theoretical scrutiny for decades, but the lack of certainty regarding the nature of the compact object in the binary has prevented advancing our understanding of the source. Here, using observations done with the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we report on the existence of transient radio pulsations from the direction of LS I +61$^{\circ}$ 303. We find a period $P=269.15508 \pm 0.00016$ ms at a significance of $> 20\sigma$. This is the first evidence for pulsations from this source at any frequency, and strongly argues for the existence of a rotating neutron star in LS I +61$^{\circ}$ 303.