Detection of a large Be circumstellar disk during X-ray quiescence of XTE J1946+274

TL;DR

This study reveals a large Be circumstellar disk during X-ray quiescence in XTE J1946+274, suggesting disk presence does not always trigger X-ray activity, and provides spectral and timing analysis of the system during its quiet phase.

Contribution

First detailed multiwavelength analysis of XTE J1946+274 during quiescence showing a large circumstellar disk without X-ray outbursts, proposing a tilted and warped disk as a possible mechanism.

Findings

Large Be circumstellar disk present during quiescence.

X-ray spectrum well fitted by blackbody or power-law models.

Detected pulsations with a period of 15.757 seconds.

Abstract

We present a multiwavelength study of the Be/X-ray binary system XTE J1946+274 with the main goal of better characterizing its behavior during X-ray quiescence. We aim to shed light on the mechanism which triggers the X-ray activity for this source. XTE J1946+274 was observed by Chandra-ACIS during quiescence in 2013 March 12. In addition, this source has been monitored from the ground-based astronomical observatories of El Teide (Tenerife, Spain), Roque de los Muchachos (La Palma, Spain) and Sierra Nevada (Granada, Spain) since 2011 September, and from the TUBITAK National Observatory (Antalya, Turkey) since 2005 April. We have performed spectral and photometric temporal analyses in order to investigate the quiescent state and transient behavior of this binary system. In 2006, a long mass ejection event took place from the Be star, lasting for about seven years. We also found that a large Be circumstellar disk was present during quiescence, although major X-ray activity was not observed. We made an attempt to explain this scenario by assuming the permanently presence of a tilted and warped Be circumstellar disk. The 0.3-10 keV X-ray spectrum during quiescence was well fitted with either an absorbed blackbody or absorbed power-law models. The main parameters obtained for these models were kT=1.43+/-0.17 and Gamma=0.9+/-0.4 (with N_H~2-7E+22 cm-2). The 0.3-10 keV flux of the source was ~0.8E-12 erg-1 cm-2 s-1. Pulsations were found with P_pulse=15.757(1) s (MJD 56363.115) and an rms pulse fraction of 32.1(3)%. The observed X-ray luminosity during quiescent periods may be explained by the neutron star being in supersonic propeller regimen.