Accretion Flow Properties of EXO 1846-031 During its Multi-Peaked Outburst After Long Quiescence

TL;DR

This study analyzes the 2019 multi-peaked outburst of the black hole candidate EXO 1846-031 after 34 years of quiescence, using multi-instrument data and the TCAF model to understand accretion flow evolution and black hole properties.

Contribution

It applies the TCAF model to multi-instrument data to analyze the accretion flow and determine the black hole's mass and viscous timescale during a complex outburst.

Findings

Multiple peaks in flux linked to variable accretion supply.

Type-C QPOs observed in different spectral states.

Black hole mass estimated at approximately 10.4 solar masses.

Abstract

We study the recent outburst of the black hole candidate EXO 1846-031 which went into an outburst in 2019 after almost 34 years in quiescence. We use archival data from Swift/XRT, MAXI/GSC, NICER/XTI and NuSTAR/FPM satellites/instruments to study the evolution of the spectral and temporal properties of the source during the outburst. Low energy (2-10 keV) X-ray flux of the outburst shows multiple peaks making it a multipeak outburst. Evolving type-C quasi-periodic oscillations (QPOs) are observed in the NICER data in the hard, hard intermediate and soft intermediate states. We use the physical Two Component Advective Flow (TCAF) model to analyze the combined spectra of multiple satellite instruments. According to the TCAF model, the accreting matter is divided into Keplerian and sub-Keplerian parts, and the variation in the observed spectra in different spectral states arises out of the variable contributions of these two types of accreting matter in the total accretion rate. Studying the evolution of the accretion rates and other properties of the accretion flow obtained from the spectral analysis, we show how the multiple peaks in the outburst flux arises out of variable supply of accreting matter from the pile-up radius. We determine the probable mass of the black hole to be $10.4^{+0.1}_{-0.2}~M_\odot$ from the spectral analysis with the TCAF model. We also estimate viscous time scale of the source in this outburst to be $\sim 8$ days from the peak difference of the Keplerian and sub-Keplerian mass accretion rates.