Propeller effect in the transient X-ray pulsar SMC X-2

TL;DR

This study monitored the transient X-ray pulsar SMC X-2 during an outburst, observing a dramatic luminosity drop indicating a transition to the propeller regime, and estimating the neutron star's magnetic field.

Contribution

First detection of the propeller transition in SMC X-2, providing insights into its magnetic field and accretion physics during a Type II outburst.

Findings

Observed luminosity drop by over 100 times below the limiting value.

Identified the transition to the propeller regime in SMC X-2.

Estimated the neutron star's magnetic field as approximately 3×10^{12} G.

Abstract

We report the results of the monitoring campaign of the transient X-ray pulsar SMC X-2 performed with the Swift/XRT telescope over the period of 2015 September - 2016 January during the Type II outburst. During this event, the bolometric luminosity of the source ranged from $\simeq10^{39}$ down to several$\times10^{34}$ erg/s. Moreover, we discovered its dramatic drop by a factor of more than 100 below the limiting value of $L_{\rm lim}\simeq4\times10^{36}$ erg/s, which can be interpreted as a transition to the propeller regime. These measurements make SMC X-2 the sixth pulsating X-ray source where such a transition is observed and allow us to estimate the magnetic field of the neutron star in the system $B\simeq3\times10^{12}$ G, which is in agreement with independent results of the spectral analysis.