  X-ray observations of neutron star (NS) low mass X-ray binaries (LMXBs) are
useful to probe physical processes close to the NS and to constrain source
parameters. Aql X-1 is a transient NS LMXB which frequently undergoes outbursts
provides an excellent opportunity to study source properties and accretion
mechanism in strong gravity regime over a wide range of accretion rates. In
this work, we systematically investigate the spectral evolution of Aql X-1
using NICER observations during the source outbursts in 2019 and 2020. The
NICER observations cover the complete transition of the source from its
canonical hard state to soft state and back. The spectra extracted from most
observations can be explained by a partially Comptonised accretion disc. We
find that the system can be described by an accretion disk with an inner
temperature of $\approx0.7$ keV and a Comptonising medium of thermal electrons
at $\approx2$ keV, while the photon index is strongly degenerate with the
covering fraction of the medium. We also find evidence of Fe K$\alpha$
fluorescence emission in the spectra indicating reprocessing of the Comptonised
photons. We observe an absorption column density higher than the Galactic
column density for most of the observations indicating a significant local
absorption. But for some of the observations in 2020 outburst, the local
absorption is negligible.
