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

Abstract: 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.