Detection of X-ray pulsations at the lowest observed luminosity of Be/X-ray binary pulsar EXO 2030+375 with AstroSat

TL;DR

This study reports the first detection of X-ray pulsations from the Be/X-ray binary pulsar EXO 2030+375 at an extremely low luminosity, expanding understanding of pulsar behavior at minimal accretion rates.

Contribution

It provides the first firm detection of pulsations at the lowest observed luminosity for EXO 2030+375, demonstrating pulsation detectability at very low accretion rates.

Findings

Pulsations detected at luminosity of 2.5×10^35 erg s^-1

Pulsations observed up to 80 keV at higher luminosity, limited to 25 keV at lowest luminosity

Spectral analysis shows a steep power-law spectrum with index ~2

Abstract

We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of 2.5$\times$10$^{35}$ erg s$^{-1}$ in 0.5--30 keV energy range, $\approx$41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030+375 at an extremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to $\sim$80 keV when the source was brighter, pulsations were limited up to $\sim$25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in 2$\times$10$^{-4}$ Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of $\sim$2. The values of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar.