Witnessing the magnetospheric boundary at work in Vela X-1

TL;DR

This study analyzes Vela X-1's off-states using Suzaku data, revealing the source remains active with pulsations, suggesting residual accretion via Kelvin-Helmholtz instabilities rather than complete shutdown.

Contribution

First observation showing Vela X-1 does not turn off during off-states, challenging the propeller regime interpretation.

Findings

Vela X-1's off-states still show pulsations.

Residual flux likely due to matter leaking through magnetosphere.

Supports the gated accretion model involving Kelvin-Helmholtz instabilities.

Abstract

We present an analysis of the Vela X-1's "off-states" based on Suzaku observations taken in June 2008. Defined as states in which the flux sudden decreases below the instrumental sensitivity, these "off-states" have been interpreted by several authors as the onset of the "propeller regime". For the first time ever, however, we find that the source does not turn off and, although the flux drops by a factor of 20 during the three recorded "off-states", pulsations are still observed. The spectrum and the pulse-profiles of the "off-states" are also presented. Eventually, we discuss our findings in framework of the "gated accretion" scenario and conclude that most likely the residual flux is due to the accretion of matter leaking through the magnetosphere by means of Kelvin-Helmholz instabilities (KHI).