Revealing the state transition of Cen X-3 at high spectral resolution with Chandra

¹ Instituto Universitario de Física Aplicada a las Ciencias y las Tecnologías, Universidad de Alicante, 03690 Alicante, Spain
² Institute for Physics and Astronomy, Universität Potsdam, 14476 Potsdam, Germany
³ Sternberg Astronomical Institute, Moscow M. V. Lomonosov State University, Universitetskij Pr, 13, Moscow 119234, Russia
⁴ MIT Kavli Institute for Astrophysics and Space Research, Cambridge, Massachussetts, USA
⁵ Department of Physics, Washington University in St. Louis, Missouri, USA

Received: 25 February 2024
Accepted: 8 August 2024

Abstract

Cen X-3 is a compact, high-mass X-ray binary (HMXRB), likely powered by Roche lobe overflow. We present a phase-resolved X-ray spectral and timing analysis of a target of opportunity Chandra observation made during a low-flux to high-flux transition. The high-resolution spectra allow us to delve into the events that occurred during this episode. The spectrum is described by a single black body absorbed by a local column density of the order of 10^{23 − 24} cm⁻², which is one to two orders of magnitude higher than found for previous analyses of data taken at similar orbital phases. Such a large column produces a Compton shoulder in the Fe Kα line. The transition appears to be caused by the onset of efficient cooling, which cools the plasma by 10 million degrees in just 10 ks, allowing matter to enter the magnetosphere. This happens after a major disturbance, probably the arrival of a train of wind clumps with individual masses in the range 10^{19 − 20} g. This train moves ballistically in an eccentric orbit around the NS, producing a distinctive Doppler modulation in the Fe XXV line.