Long-term optical/IR variability of the Be/X-ray binary LS V +44 17/RX J0440.9+4431

¹ IESL (FORTH) & University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece e-mail: pau@physics.uoc.gr
² Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, 03080 Alicante, Spain
³ Observatorio Astronómico, Universitat de Valencia, 46071 Paterna-Valencia, Spain
⁴ School of Physics and Astronomy, The University, Southampton, SO17 1BJ, UK

Received: 24 March 2005
Accepted: 6 June 2005

Abstract

We present the first long-term study of the optical counterpart to the X-ray pulsar RX J0440.9+4431/LS V +44 17. The data consist of optical spectroscopic and infrared photometric observations taken during the period 1995-2005. The infrared observations are the first published for this source. The results of our photometric and spectroscopic analysis show that RX J0440.9+4431/LS V +44 17 contains a moderately reddened, 5, B0.2V star located at about 3.3 kpc. The Hα line consistently shows a double-peak profile varying from symmetric shape to completely distorted on one side (V/R phases). A correlation between the equivalent width of the Hα line and the infrared magnitudes is seen: as the EW(Hα) decreases the IR magnitudes become fainter. This long-term optical/IR variability is attributed to structural changes in the Be star's circumstellar disc. The observations include a recent decline in the circumstellar disc and subsequent recovery. We have witnessed the cessation of a global oscillation due to the decline of the circumstellar disc. If the present disc growth rate continues we predict the onset of another episode of V/R variability by the end of 2006. We have investigated the typical time scales for disc variability of various Be/X-ray binaries and found a correlation with the orbital period. This correlation is hard to establish due to the difficulty in defining the exact duration of the various activity states, but it is seen both in the duration of the disc growth/dissipation phase and the value of the Hα equivalent width prior to the appearance of asymmetric profiles. These relationships provide further evidence for the interaction of the neutron star with the circumstellar disc of the Be star's companion and confirms the need of a fully developed disc for the V/R variability to be observed.