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Black hole X-ray binaries (BHXRBs) play a crucial role in understanding theaccretion of matter onto a black hole. Here, we focus on exploring thetransient BHXRB \source~discovered by Swift/BAT and MAXI/GSC during its January2019 outburst. We present measurements on its accretion properties, longtime-scale variability, and spin. To probe these properties we make use ofseveral NICER observations and an unexplored data set from NuSTAR, as well aslong term light curves from MAXI/GSC. In our timing analysis we provideestimates of the cross-correlation functions between light curves in variousenergy bands. In our spectral analysis we employ numerous phenomenologicalmodels to constrain the parameters of the system, including flavours of therelativistic reflection model Relxill to model the Fe K$\alpha$ line and the$>15$ keV reflection hump. Our analysis reveals that: (i) Over the course ofthe outburst the total energy released was $\sim 5.2 \times 10^{44}$~ergs,corresponding to roughly 90\% the mass of Mars being devoured. (ii) We find acontinuum lag of $8.4 \pm 1.9$ days between light curves in the $2-4$ keV and$10-20$ keV bands which could be related to the viscous inflow time-scale ofmatter in the standard disc. (iii) Spectral analysis reveals a spin parameterof $\sim 0.6 - 0.7$ with an inclination angle of $\sim 45^{\circ}-70^{\circ}$,and an accretion rate during the NuSTAR observation of $\sim 17\% ~L_{\rmEdd}$.

Accretion Properties and Estimation of Spin of Galactic Black Hole Candidate Swift J1728.9-3613 with NuSTAR during its 2019 outburst