Monitoring Observations of SMC X-1's Excursions (MOOSE) I: Program Description and Initial High-State Spectral Results

TL;DR

This paper presents initial spectral analysis of SMC X-1 during super-orbital excursions using NICER data, finding stable spectral shapes but variable absorption, and introduces the MOOSE program for ongoing monitoring.

Contribution

It introduces the MOOSE observational program and provides the first high-state spectral results during recent super-orbital excursions of SMC X-1.

Findings

Spectral shapes are consistent across different super-orbital phases.

Evidence for >3 sigma change in absorption during excursions.

Little intrinsic spectral variability observed in high-states.

Abstract

SMC X-1 has exhibited three super-orbital period excursions since the onset of X-ray monitoring beginning with RXTE's launch in 1995. NICER has recently probed a fourth observed excursion beginning in 2021 with our program Monitoring Observations of SMC X-1's Excursions (MOOSE). These sensitive new MOOSE data probe different super-orbital periods and phases within them. Spectral fits to the high-state continuum during April 2021 to January 2022 show that the intrinsic spectral shapes are characterised by a soft (kT~0.19 keV) disc component and a hard (Gamma~0.7) power-law tail. When the 2021-2022 NICER observations, taken during an excursion, are compared to 2016 XMM-Newton observations (outside of an excursion), we find little evidence for intrinsic spectral variability across the high-states, but find evidence for a >3 sigma change in the absorption, although we caution that there may be calibration differences between the two instruments. Thus, over different lengths of super-orbital periods, we see little evidence for intrinsic spectral changes in the high-state. Upcoming studies of the pulse profiles may shed light on the mechanism behind the excursions.