Studying the accretion geometry of EXO 2030+375 at luminosities close to the propeller regime

TL;DR

This study analyzes the accretion geometry of EXO 2030+375 during a low luminosity state, providing evidence that accretion persists at very low rates and exploring the potential onset of the propeller effect.

Contribution

It offers a detailed spectral and timing analysis of low-luminosity accretion in a Be X-ray binary, highlighting continued pulsations and spectral changes near the propeller regime.

Findings

Pulsations detected at luminosities ~6.8e35 erg/s.

Spectral variation suggests accretion column passing through line of sight.

Softer spectrum at lower luminosity possibly indicates propeller effect onset.

Abstract

The Be X-ray binary EXO 2030+375 was in an extended low luminosity state during most of 2016. We observed this state with NuSTAR and Swift, supported by INTEGRAL observations as well as optical spectroscopy with the NOT. We present a comprehensive spectral and timing analysis of these data here to study the accretion geometry and investigate a possible onset of the propeller effect. The H-alpha data show that the circumstellar disk of the Be-star is still present. We measure equivalent widths similar to values found during more active phases in the past, indicating that the low-luminosity state is not simply triggered by a smaller Be disk. The NuSTAR data, taken at a 3-78 keV luminosity of ~6.8e35 erg/s (for a distance of 7.1 kpc), are well described by standard accreting pulsar models, such as an absorbed power-law with a high-energy cutoff. We find that pulsations are still clearly visible at these luminosities, indicating that accretion is continuing despite the very low mass transfer rate. In phase-resolved spectroscopy we find a peculiar variation of the photon index from ~1.5 to ~2.5 over only about 3% of the rotational period. This variation is similar to that observed with XMM-Newton at much higher luminosities. It may be connected to the accretion column passing through our line of sight. With Swift/XRT we observe luminosities as low as 1e34 erg/s during which the data quality did not allow us to search for pulsations, but the spectrum is much softer and well described by either a blackbody or soft power-law continuum. This softer spectrum might be due to the fact that accretion has been stopped by the propeller effect and we only observe the neutron star surface cooling.