Spectral and Timing Properties of IGR J17091-3624 in the Rising Hard State During its 2016 Outburst

TL;DR

This study analyzes the spectral and timing behavior of IGR J17091-3624 during its 2016 outburst, revealing disk truncation, evolving electron temperature, and complex QPO features in the hard state.

Contribution

It provides detailed spectral and timing analysis of IGR J17091-3624 during its 2016 outburst, highlighting disk truncation and QPO characteristics with relativistic reflection modeling.

Findings

Disk is truncated with inner radius >10 r_g.

Electron temperature decreases from ~64 keV to ~26 keV.

Type-C QPOs vary between 0.131 Hz and 0.327 Hz, with a secondary peak at ~2.3 times the QPO frequency.

Abstract

We present a spectral and timing study of the NuSTAR and Swift observations of the black hole candidate IGR J17091-3624 in the hard state during its outburst in 2016. Disk reflection is detected in each of the NuSTAR spectra taken in three epochs. Fitting with relativistic reflection models reveals that the accretion disk is truncated during all epochs with $R_{\rm in}>10~r_{\rm g}$, with the data favoring a low disk inclination of $\sim 30^{\circ}-40^{\circ}$. The steepening of the continuum spectra between epochs is accompanied by a decrease in the high energy cut-off: the electron temperature $kT_{\rm e}$ drops from $\sim 64$ keV to $\sim 26$ keV, changing systematically with the source flux. We detect type-C QPOs in the power spectra with frequency varying between 0.131 Hz and 0.327 Hz. In addition, a secondary peak is found in the power spectra centered at about 2.3 times the QPO frequency during all three epochs. The nature of this secondary frequency is uncertain, however a non-harmonic origin is favored. We investigate the evolution of the timing and spectral properties during the rising phase of the outburst and discuss their physical implications.