Black hole-Neutron star distinction based on long-term MAXI and Swift study of 42 low mass X-ray binaries

TL;DR

This study analyzes 13 years of MAXI and Swift data on 42 low-mass X-ray binaries to distinguish black hole and neutron star systems based on long-term X-ray behavior and hardness ratios.

Contribution

It provides a long-term statistical method to differentiate black hole and neutron star X-ray binaries using hardness ratios and outburst durations.

Findings

NSXBs spend longer in the hard state.

BHXBs have longer outburst durations (>100 days).

Distinct hardness ratio distributions separate BHXBs and NSXBs.

Abstract

In this study, we analysed about $\sim$13 years of publicly available data from MAXI and Swift/BAT to examine the long-term source evolution of 42 transient low-mass X-ray binaries. The sample consists of 11 confirmed black hole X-ray binaries (BHXBs), 10 black hole candidates (BHC), and 21 neutron star X-ray binaries (NSXBs). Outbursts and flaring activities studied over 13 years show that 19/21 NSXBs spend significantly longer time in the hard state (observations for which hardness ratio is $\geq$ 0.2) while 15/21 BHXB+XRC spend substantially longer time in the soft state (observations for which hardness ratio is $<$ 0.2). The frequency distribution of the hardness ratio clearly shows two distinct distributions for BHXBs and NSXBs, with their peaks separated: NSXBs prefer harder values, while BHXBs prefer softer values of hardness. Our model-independent analysis for 42 transient sources shows that statistically NSXBs do not prefer to show a canonical high soft state as observed in BHXBs. Additionally, the probability distribution of the duration of the 2-20 keV X-ray outburst is observed to peak at a significantly longer duration ($>$100 days) for BHXBs than for NSXBs (15-60 days). Our analysis shows that among candidate sources, Swift J1728.9-3613, MAXI J1535-571, MAXI J1659-152, EXO 1846-031 show a `q' diagram in the HID and prefer to align with the HID frequency distribution of BHXBs that show `q' diagram, MAXI J1305-704 and MAXI J1836-194 align with frequency distribution of black hole sources without `q' diagram while MAXI J1848-015 shows the HID distribution similar to NSXBs, indicating a neutron star accretor. Therefore, a long-term statistical study of MAXI and Swift/BAT X-ray outbursts from a large sample of transient sources may be used to distinguish BHXB from NSXB.