Monitoring Observations of SMC X-1's Excursions (MOOSE) III: X-ray Spectroscopy of a Warped, Precessing Accretion Disc

TL;DR

This study uses NICER X-ray observations to analyze the spectral properties of SMC X-1 during superorbital excursions, revealing that obscuration of the accretion disk causes its high and low states.

Contribution

It provides the first detailed spectral analysis of SMC X-1 during superorbital excursions using a unified model across different states.

Findings

Partial covering fraction varies with superorbital state.

High obscuration causes the low/high states.

Spectral model fits well across different states.

Abstract

The MOOSE (Monitoring Observations of SMC X-1 Excursions) program uses the Neutron Star Interior Composition Explorer Mission (NICER) to monitor the high mass X-ray binary SMC X-1 during its superorbital period excursions. Here we perform X-ray spectral analyses of 26 NICER observations of SMC X-1, taken at the tail-end of the excursion between 2021-04-01 and 2022-01-05. We use a single spectral model to fit spectra observed in high, intermediate and low states, using a combination of a partial covering fraction model, a black-body disc, and a power-law component. We find that the partial covering fraction varies significantly with the superorbital state during superorbital excursion. Our findings suggest that the low/high state in SMC X-1 is caused by a very high obscuration of the accretion disk.