Cyclical accretion regime change in the slow X-ray pulsar 4U 0114+65 observed with Chandra

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

^⋆ Corresponding author; graciela.sanjurjo@ua.es

Received: 28 October 2024
Accepted: 13 January 2025

Abstract

4U 0114+65 is a high-mass X-ray binary system formed by the luminous supergiant B1Ia, known as V* V662 Cas, and one of the slowest rotating neutron stars (NSs) with a spin period of about 2.6 hours. This provides a rare opportunity to study interesting details of the accretion within each individual pulse of the compact object. For this paper we analyzed 200 ks of Chandra grating data, divided into nine uninterrupted observations around the orbit. The changes in the circumstellar absorption column through the orbit suggest an orbital inclination of ∼40° with respect to the observer and a companion mass-loss rate of ∼8.6 ⋅ 10⁻⁷M⊙ yr⁻¹. The peaks of the NS pulse exhibit a large pulse-to-pulse variability. Three of them show an evolution from a brighter regime to a weaker one. We propose that the efficiency of Compton cooling in this source fluctuates throughout an accumulation cycle. After significant depletion of matter within the magnetosphere, since the settling velocity is ∼2× lower than the free-fall velocity, the source gradually accumulates matter until the density exceeds a critical threshold. This increase in density triggers a transition to a more efficient Compton cooling regime, leading to a higher mass accretion rate and consequently to an increased brightness.