Exploring the spin dependence on mass inclination and distance for the newly discovered black hole X-ray binary Swift J151857.0-572147

Abstract

The black hole X-ray binary (BHXRB) Swift J151857.0-572147 was discovered during its first outburst in March 2024. We review the archive of NICER observations from this outburst, focusing on the soft states. We select spectra for which the disk to total flux ratio exceeds 0.8 and the coronal scattering fraction fsc is less than 25%, conditions under which the accretion disk is expected to extend to the innermost stable circular orbit (ISCO) and remains geometrically thin. Through a continuum fitting analysis, we explore the dependence of spin on mass, inclination, and distance. We constrain the spin within the parameter space typical of stellar-mass black holes (sBHs), assuming a mass around 10 solar masses, inclination angles between 20 and 60 degree, and distances between 4 and 16 kpc. For fiducial parameters: a mass of 10 solar masses, a distance of 10 kpc, and an inclination angle of 40 degree, a moderate spin of approximately 0.7 is obtained. However, precise determination of the spin will require accurate measurements of these parameters. Our analysis provides a framework to infer the spin and estimate its uncertainties once more precise measurements of mass, distance, and inclination become available. As we demonstrate, lower inclination angles, greater distances, or larger black hole masses result in higher spin values.

Figures (14)

• Figure 1: a) NICER/XTI light curves for 2024 outburst of J151857. The top and middle panels display the count rates in the 2–-4 keV and 4–-10 keV energy bands, respectively, with the background indicated below. The bottom panel presents the temporal evolution of the hardness ratio (defined as the 4--10 keV count rate divided by the 2--4 keV count rate). Red arrows indicate the six observations selected for the joint fit. b) HID corresponding to NICER observations shown in panel (a). Colors transitioning from purple (earlier times) to red (later times) depict the temporal sequence. The vertical line at a hardness ratio of 0.5 is included in the plot as a visual guide. The solid star and hollow circle symbols denote the orbit-night and orbit-day data, respectively. For clarity, data points with hardness uncertainties greater than 0.05 were smoothed by averaging the temporally adjacent 50 data points (i.e., generally spanning 1000 s).
• Figure 2: Illustration of spectral characteristics and fitting results for an orbit night spectrum (ObsID: 7661010106, N39) of J151857. Blue represents the signal, while red indicates the background. Left: Spectrum in the 0.4-10 keV range. A significant flat signal below 1 keV is observed. Right: Spectral fitting results. Upper panel: Best-fit with tbfeo(ezdiskbb+nthcomp) shows residuals around 1.3 keV and 1.84 keV. Lower panel: Improved fit with tbvarabs(ezdiskbb+nthcomp) significantly reduces low-energy residuals.
• Figure 3: The joint fit results of Model 1 tbvarabs*(ezdiskbb+nthcomp) for the six spectra from different periods are presented. These include the background data, residuals, and ratio plots, respectively.
• Figure 4: Fitting results of model: tbfeo*(ezdiskbb+nthcomp). The disk temperature and the Eddington ratio throughout the outburst. The grey shaded regions represent data with an Eddington ratio between 3--30% and a disk fraction greater than 0.8.
• Figure 5: The percentage of flux contributions from different components, calculated using cflux. The star shapes represent the orbital night data, while the hollow circular shapes represent the orbital day data. The time sequence follows the same color gradient as in Fig. \ref{['fig:HID']}. The shaded region represents the region where the disk fraction is greater than 0.8, which is consistent with the shaded data in Table \ref{['tab:obs']}.