Accretion properties and estimation of spin of galactic black hole candidate Swift J1728.9-3613with NuSTAR during its 2019 outburst

Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the accretion of matter onto a black hole. Here, we focus on exploring the transient BHXRB Swift J1728.9-3613 discovered by Swift/BAT and MAXI/GSC during its January 2019 outburst. We present measurements on its accretion properties, long time-scale variability, and spin. To probe these properties, we make use of several NICER observations and an unexplored data set from NuSTAR, as well as long-term light curves from MAXI/GSC. In our timing analysis, we provide estimates of the cross-correlation functions between light curves in various energy bands. In our spectral analysis, we employ numerous phenomenological models to constrain the parameters of the system, including flavours of the relativistic reflection model RELXILL to model the Fe Kα line and the >15 keV reflection hump. Our analysis reveals that: (i) Over the course of the outburst, the total energy released was ~5.2 × 10⁴⁴ ergs, corresponding to roughly 90 per cent of the mass of Mars being devoured. (ii) We find a continuum lag of 8.4 ± 1.9 d between light curves in the 2-4 and 10-20 keV bands, which could be related to the viscous inflow time-scale of matter in the standard disc. (iii) Spectral analysis reveals a spin parameter of ~0.6-0.7 with an inclination angle of ~45°-70° and an accretion rate during the NuSTAR observation of ${\sim}17\ \hbox{per cent}\ L_{\rm Edd}$.