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

¹ Department of Physics, Middle East Technical University, 06531 Ankara, Turkey
e-mail: mehtap@astroa.physics.metu.edu.tr
² Institut de Ciències de l’Espai, (IEEC-CSIC), Campus UAB, Fac. de Ciències, Torre C5 pa., 08193 Barcelona, Spain
³ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna, Dept. Astrofísica, 38206 La laguna, Tenerife, Spain
⁵ Universitad de Valencia, Dept. Didáctica de las Matemática, Avda. Tarongers, 4, 46022 Valencia, Spain
⁶ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁷ Observatorio Astronómico de la Univ. de Valencia, C/Catedrático Jose Beltran, 2, 46980 Paterna (Valencia), Spain
⁸ Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
⁹ Department of Astronomy, Oscar Klein Center, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 8 December 2014
Accepted: 9 August 2015

Abstract

Aims. 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 also aim to shed light on the possible mechanisms which trigger the X-ray activity for this source.

Methods. 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 TÜBİTAK 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.

Results. Our optical study revealed that a long mass ejection event from the Be star took place in 2006, lasting for about seven years, and another one is currently ongoing. We also found that a large Be circumstellar disk is present during quiescence, although major X-ray activity is not observed. We made an attempt to explain this by assuming the permanently presence of a tilted (sometimes warped) Be decretion disk. The 0.3−10 keV X-ray spectrum of the neutron star during quiescence was well fitted with either an absorbed black-body or an absorbed power-law models. The main parameters obtained for these models were kT = 1.43 ± 0.17 and Γ = 0.9 ± 0.4 (with N_H ~ 2−7 × 10²² cm^-2). The 0.3−10 keV flux of the source was ~0.8−1 × 10^-12  erg^-1 cm^-2 s^-1. Pulsations were found with P_pulse = 15.757(1) s (epoch MJD 56 363.115) and an rms pulse fraction of 32.1(3)%. The observed X-ray luminosity during quiescent periods was close to that of expected in supersonic propeller regimen.