Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and V 0332+53

TL;DR

This study monitored two bright transient X-ray pulsars, detecting the propeller regime transition at specific luminosities, and found spectral softening and brief accretion resumption near periastron, supporting magnetic field estimates from cyclotron lines.

Contribution

First direct detection of propeller regime transition luminosities in two X-ray pulsars, confirming magnetic field estimates and spectral behavior during low states.

Findings

Transition luminosities are approximately 1.4 and 2.0 x 10^{36} erg/s.

Spectra are significantly softer during the low flux state.

Brief accretion resumption occurs near periastron after the transition.

Abstract

We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well known X-ray pulsars 4U 0115+63 and V 0332+53 during their giant outbursts in 2015. Such transitions were detected at the threshold luminosities of $(1.4\pm0.4)\times10^{36}$ erg s$^{-1}$ and $(2.0\pm0.4)\times10^{36}$ erg s$^{-1}$ for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.