Long-term spin-down and low luminosity regime in the Be/X-ray binary pulsar GX 304-1

• Published in: arXiv.org 2024-11
• Main Author: Amar Deo Chandra
• Format: Article
• Language: English
• Subjects: Companion stars; Deposition; Outbursts; Pulsars; Torque; X ray binaries; X ray stars; X-ray astronomy
• ISSN: 2331-8422

We carry out timing and spectral studies of the Be/X-ray binary pulsar GX 304-1 using NuStar and XMM-Newton observations. We construct the long-term spin period evolution of the pulsar which changes from a long-term spin-up (\(\sim 1.3 \times 10^{-13} \)Hz \~s\(^{-1}\)) to a long-term spin-down (\(\sim -3.4 \times 10^{-14} \)Hz \~s\(^{-1}\)) trend during a low luminosity state (\(\sim 10^{34-35} \)erg \~s\(^{-1}\)). A prolonged low luminosity regime (\(L_X \sim 10^{34-35} \)erg \~s\(^{-1}\)) was detected during 2005-2010 and spanning nearly five years since 2018 December. The XMM-Newton and NuStar spectra can be described with a power law plus blackbody model having an estimated luminosity of \(\sim 2.5 \times 10^{33} \)erg \~s\(^{-1}\) and \(\sim 3.6 \times 10^{33} \)erg \~s\(^{-1}\) respectively. The inferred radius of the blackbody emission is about 100-110 m which suggests a polar-cap origin of this component. From long-term ultraviolet observations of the companion star, an increase in the ultraviolet signatures is detected preceding the X-ray outbursts. The spectral energy distribution of the companion star is constructed which provides a clue of possible UV excess when X-ray outbursts were detected from the neutron star compared to the quiescent phase. We explore plausible mechanisms to explain the long-term spin-down and extended low luminosity manifestation in this pulsar. We find that sustained accretion from a cold disc may explain the prolonged low luminosity state of the pulsar since December 2018 but the pulsar was undergoing normal accretion during the low luminosity period spanning 2005-2010.