Why is IGR J17091-3624 so faint? Constraints on distance, mass, and spin from `phase-resolved' spectroscopy of the `heartbeat' oscillations

TL;DR

This study uses phase-resolved spectroscopy of heartbeat oscillations in IGR J17091-3624 to constrain its system parameters, revealing a high inclination, low or retrograde black hole spin, and a large distance, explaining its faintness.

Contribution

It provides the first detailed phase-resolved spectral analysis of IGR J17091-3624's heartbeat oscillations, constraining key system parameters and explaining its low luminosity.

Findings

The source is a high inclination binary with i > 53°.

Black hole spin is very low or negative, indicating retrograde spin.

The distance to the source is greater than 20 kpc, with a black hole mass less than 5 solar masses.

Abstract

IGR J17091--3624 is a transient X-ray source and is believed to be a Galactic black hole candidate. Recently, it has received a considerable attention due to the detection of peculiar variability patterns known as `heartbeats', which are quasi-periodic mini-outbursts repeated over timescales ranging between 5 and 70 s. So far, such variability patterns have been observed only in GRS 1915+105 and these are classified as $\rho$- and $\nu$-variability classes. Here, we present the results of `phase-resolved' spectroscopy of the `heartbeat' oscillations of IGR J17091-3624 using data from simultaneous observations made by RXTE and XMM-Newton. We find that the 0.7--35 keV spectra can be fitted with a `canonical' model for black hole sources consisting of only two components---a multi-temperature disk black body and a power law (or its equivalent). We attempt to constrain the system parameters of the source by simultaneously fitting spectra during different phases of the burst profile while tying the system parameters across the phases. The results indicate that the source is a high inclination binary ($i$>53$^\circ$). Further, the observed low flux from the source can be explained only if the black hole spin is very low, along with constraints on the black hole mass (<5 M$_\odot$) and the distance (>20 kpc). For higher inclination angles, which is favored by the data, the black hole spin is required to be negative. Thus, low or retrograde spin could be the reason for the low luminosity of the source.