Does the gamma-ray binary LS I +61{\deg}303 harbor a magnetar?

TL;DR

This paper investigates whether the gamma-ray binary LS I +61°303 contains a magnetar by analyzing observational data and accretion dynamics, questioning the magnetar interpretation based on magnetic field estimates and neutron star evolution.

Contribution

The study critically examines the magnetar hypothesis for LS I +61°303, considering accretion, spin evolution, and observational constraints to challenge previous magnetic field estimates.

Findings

Neutron star age and spin evolution are hard to reconcile without a strong propeller torque.

Propeller torque could significantly reduce the inferred magnetic field strength.

Magnetar interpretation of LS I +61°303 is less likely given the data analysis.

Abstract

The high-mass X-ray binary LS I +61{\deg}303 is also cataloged as a gamma-ray binary as a result of frequent outbursts at TeV photon energies. The system has released two soft-gamma flares in the past, suggesting a magnetar interpretation for the compact primary. This inference has recently gained significant traction following the discovery of transient radio pulses, detected in some orbital phases from the system, as the measured rotation and tentative spin-down rates imply a polar magnetic field strength of $B_p \gtrsim 10^{14}\,\mbox{G}$ if the star is decelerating via magnetic dipole braking. In this paper, we scrutinize magnetic field estimates for the primary in LS I +61{\deg}303 by analyzing the compatibility of available data with the system's accretion dynamics, spin evolution, age limits, gamma-ray emissions, and radio pulsar activation. We find that the neutron star's age and spin evolution are theoretically difficult to reconcile unless a strong propeller torque is in operation. This torque could be responsible for the bulk of even the maximum allowed spin-down, potentially weakening the inferred magnetic field by more than an order of magnitude.