  The Solar X-ray Monitor (abbreviated as XSM) on board India's Chandrayaan-2
mission is designed to carry out broadband spectroscopy of the Sun from lunar
orbit. It observes the Sun as a star and measures the spectrum every second in
the soft X-ray band of 1 - 15 keV with an energy resolution better than 180 eV
at 5.9 keV. The primary objective of the XSM is to provide the incident solar
spectrum for the X-ray fluorescence spectroscopy experiment on the
Chandrayaan-2 orbiter, which aims to generate elemental abundance maps of the
lunar surface. However, observations with the XSM can independently be used to
study the Sun as well. The Chandrayaan-2 mission was launched on 22 July 2019,
and the XSM began nominal operations, in lunar orbit, from September 2019. The
in-flight observations, so far, have shown that its spectral performance has
been identical to that on the ground. Measurements of the effective area from
ground calibration were found to require some refinement, which has been
carried out using solar observations at different incident angles. It also has
been shown that the XSM is sensitive enough to detect solar activity well below
A-class. This makes the investigations of microflares and the quiet solar
corona feasible in addition to the study of the evolution of physical
parameters during intense flares. This article presents the in-flight
performance and calibration of the XSM instrument and discusses some specific
science cases that can be addressed using observations with the XSM.
