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

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.

Figures (46)

• Figure 1: Source name vs mass of compact objects. Top panel: Black holes. Middle panel: Candidate sources. Bottom panel: Neutron stars. References for masses of individual objects are provided in the respective sections.
• Figure 2: Lightcurve and HID of MAXI J1820+070 Left panel: The only outburst of MAXI J1820+070 in 2018 observed with MAXI are shown in the 2-4 keV soft band (blue open circles), 10-20 keV hard bands (red solid circles) respectively. The same outburst as observed with Swift/BAT in 15-50 keV is shown using green stars. A multiplication factor of 10 is used for plotting Swift lightcurve for easy comparison. Right panel: Hardness Intensity Diagram (HID) of the same outburst is shown where hardness ratio is defined as the ratio of X-ray intensity in 10-20 keV and 2-4 keV while the intensity is defined as 2-20 keV count rate. Grey triangles denote hard state (when Hardness ratio $>$ 0.2) while black inverted triangles denote soft state (when Hardness ratio $<$ 0.2).
• Figure 3: Lightcurve and HID of GX 339-4: Top left panel: Lightcurves throughout the entire observation duration with MAXI are shown in the 2-4 keV soft band (blue open circles), 10-20 keV hard bands (red solid circles) respectively while the same as observed with Swift/BAT in 15-50 keV is shown using green stars. A multiplication factor of 15 is used for plotting Swift lightcurve for easy comparison. Top right panel: Zoomed-in view of the first two outbursts and a failed outburst as observed with MAXI and Swift are shown. Bottom left panel: for the clarity of outburst features, further zoomed-in view of the third outburst is shown. Bottom right panel: Hardness Intensity Diagram (HID) of all outbursts are shown where hardness ratio is defined as the ratio of X-ray intensity in 10-20 keV and 2-4 keV while the intensity is defined as 2-20 keV count rate. Grey triangles denote hard state (when Hardness ratio $>$ 0.2) while black inverted triangles denote soft state (when Hardness ratio $<$ 0.2). Inset demonstrates the HID during the outburst shown in the bottom left panel.
• Figure 4: Lightcurve and HID of XTE J1752–223: Left panel: Zoomed view of the outburst observed with MAXI in the 2–4 keV (blue open circles) and 10–20 keV (red solid circles) bands, along with Swift/BAT 15–50 keV data (green stars). The Swift lightcurve is scaled by a factor of 10 for clarity. The hard-band flux rises prior to the soft band. Right panel: Corresponding Hardness–Intensity Diagram (HID), where hardness is defined as the ratio of 10–20 keV to 2–4 keV intensity and intensity as the 2–20 keV count rate. Grey triangles denote the hard state and black inverted triangles denote the soft state.
• Figure 5: Lightcurves and HID of 4U 1630–472: Top left panel: Lightcurves during the full observation period obtained with MAXI in the 2–4 keV (blue open circles) and 10–20 keV (red solid circles) bands. The Swift/BAT 15–50 keV data (green stars), available are also shown. The Swift lightcurve is scaled by a factor of 10 for easy comparison. Top right panel: Zoomed view of the hard-band dominated (failed) outburst. Bottom left panel: Zoomed view of a full outburst around 59500 MJD. Bottom right panel: Corresponding Hardness–Intensity Diagram (HID), where hardness is defined as the ratio of 10–20 keV to 2–4 keV intensity and intensity as the 2–20 keV count rate. Grey triangles represent the hard state (when Hardness ratio $>$ 0.2) and black inverted triangles the soft state (when Hardness ratio $<$ 0.2).