Propeller states in locally super-critical ULXs

TL;DR

This paper investigates the luminosity and spectral changes in ultraluminous X-ray sources (ULXs) that could indicate neutron star presence and propeller states, especially in super-critical accretion regimes, and identifies candidate sources.

Contribution

It predicts luminosity variations in super-critical ULXs during propeller states and suggests observational signatures, providing constraints on magnetic field strengths.

Findings

Identified three ULXs with potential propeller state signatures.

Found that luminosity changes can be less than an order of magnitude.

Placed constraints on the magnetic field strength of NGC 1313 X-1.

Abstract

An expected signature of the presence of neutron stars in the population of ultraluminous X-ray sources (ULXs) are large scale changes in X-ray luminosity, as systems reach spin equilibrium and a propeller state ensues. We explore the predicted luminosity changes when the disc is locally super-critical, finding that a significant parameter space in dipole field strength and accretion rate (at large radius) can be accompanied by changes of less than an order of magnitude in luminosity. We discuss the spectral signature and locate three ULXs (IC 342 X-1, Cir ULX-5 and NGC 1313 X-1) which appear to show changes consistent with super-Eddington systems entering a propeller state, and place rough constraints on the dipole field strength of NGC 1313 X-1 of $<$ 10$^{10}$ G. This work implies that the most reliable means by which to search for putative propeller states will be to search for changes in hardness ratio and at high energies.