Precessing Warped Be Disk Triggering the Giant Outbursts in 2009 and 2011 in A 0535+262/V725 Tau

TL;DR

This study monitored the Be disk in A 0535+262/V725 Tau from 2009 to 2012, revealing that a precessing warped disk near periastron triggered the 2009 and 2011 giant X-ray outbursts.

Contribution

It provides the first detailed analysis linking a precessing warped Be disk to giant outbursts in a Be/X-ray binary, based on spectroscopic observations.

Findings

Warped Be disk was present during X-ray active phases.

Redshifted emission components indicated disk warping near periastron.

Warped disk likely triggered the giant outbursts in 2009 and 2011.

Abstract

We carried out optical high-dispersion spectroscopic monitoring of the Be disk in a Be/X-ray binary A 0535+262/V725 Tau from 2009 to 2012, covering two giant outbursts and several normal outbursts. This monitoring was performed in order to investigate variabilities of the Be disk due to the interaction with the neutron star in recent X-ray active phase from 2008 to 2011. Such variabilities give a clue to uncleared detailed mechanism for very bright X-ray outbursts, which are unique to some Be/X-ray binaries with relatively wide and eccentric orbit. In the previous letter (Moritani et al. 2011), a brief overview of line profile variabilities around the 2009 giant outburst was given and the possibility of a warped Be disk was discussed. In this paper, a full analysis of the Halpha line profiles as well as other line profiles is carried out. A bright blue component, or blue "shoulder", showing up after periastron indicates the presence of a dense gas stream toward the neutron star, which is associated with observed outbursts. We re-analyze the Halpha line profiles before 2009 (down to 2005) in order to investigate the variability of the the disk structure in the innermost region, which seems to have detached from the Be star surface by 2008. A redshifted enhanced component is remarkable in all emission lines observed around the 2009 giant outburst, occasionally forming a triple peak. These features indicate that the Be disk was warped in X-ray active phase. We estimate the position of the warped region from fitting the radial velocity of the redshifted enhanced component of Halpha, and find that it was very close to the periastron when two giant outbursts in 2009 and 2011 and a bright normal outburst in 2010 March occurred. These facts strongly suggest that the warped Be disk triggered these giant outbursts.