Quiet, but not silent: Uncovering quiescent state properties of two transient High Mass X-ray binaries

TL;DR

This study investigates the spectral and timing properties of two transient high-mass X-ray binaries during quiescence, revealing ongoing low-level accretion, thermal emission consistent with crustal heating, and pulsations in one source.

Contribution

First broadband spectral and timing analysis of two transient X-ray pulsars during quiescence, uncovering thermal emission and pulsations not previously observed in such states.

Findings

Detection of a strong soft power law component in both sources.

Thermal emission consistent with deep crustal heating model.

Pulsations detected in MAXI J1409-619 at 502 s, not in MXB 0656-072.

Abstract

We present the first set of broadband spectral and timing studies of two transient X-ray pulsars, MXB 0656-072 and MAXI J1409-619 using NuSTAR observations conducted during quiescence. Despite being captured at one of their lowest luminosity states, both these targets show signs of ongoing low-level accretion. Results from the time-averaged spectral analysis indicate for the first time, the presence of a strong soft power law component along with thermal emission from the neutron star hot spots. For both targets, the quiescent thermal X-ray emission is consistent with the deep crustal heating model. In MXB 0656-072, we do not detect any pulsations or indications of a cyclotron line during quiescence. However, in MAXI J1409-619 we detect strong pulsations at 502 s with a pulsed fraction of $\sim$66%, which adds this pulsar to the list of a handful of quiescent-state pulsating systems.