Spectral and timing evolution of MAXI J1631-479 during the 2018-19 outburst with NICER

TL;DR

This study analyzes the spectral and timing evolution of MAXI J1631-479 during its 2018-19 outburst using NICER data, revealing state transitions, QPO characteristics, and spectral properties during decay.

Contribution

It provides the first detailed spectral and timing analysis of MAXI J1631-479 during its outburst decay with NICER, highlighting QPO behavior and spectral state changes.

Findings

Type-C QPOs observed with increasing frequency during decay

Spectral states dominated by thermal disk and Comptonization

RMS spectra show hard shape above 1 keV

Abstract

The X-ray transient MAXI J1631-479 went into outburst on 2018 December 21 and remained active for about seven months. Owing to various constraints it was monitored by NICER only during the decay phase of the outburst for about four months. The NICER observations were primarily in the soft state with a brief excursion to the hard intermediate state. While the soft state spectrum was dominated by thermal disk emission, the hard intermediate state spectrum had maximum contribution from the thermal Comptonization. Almost all intermediate-state power spectra had a Type-C low frequency quasi-periodic oscillation (within 4 - 10 Hz), often accompanied by a harmonic component. The frequency of these oscillations increased and the fractional rms decreased with inner-disk temperature suggesting a geometric origin. One observation in the middle of the outburst during the hard intermediate state had two non-harmonically related peaks. While one of them was definitely a Type-C QPO, the identification of the other one is uncertain. The rms spectra during the intermediate state had a hard shape from above 1 keV. Below 1 keV the shape could not be constrained in most cases, while only a few observations showed a rise in amplitude.