Soft X-ray Spectroscopy of the Sun with Chandrayaan-2 Solar X-ray Monitor

Abstract:

Solar X-ray Monitor (XSM) onboard the orbiter of the Chandrayaan-2 lunar mission is a part of its remote X-ray fluorescence spectroscopy experiment. XSM observes the Sun as a star in the soft X-ray band of 1 – 15 keV to provide spectral measurements at a cadence of one second. The design features of the XSM such as the use of Silicon Drift Detector, automated filter wheel mechanism, and onboard data processing and intelligence enable XSM to have two unique characteristics: (i) obtain spectra with an energy resolution of better than 180 eV at 5.9 keV; and (ii) maintain this spectral performance over the wide range of incident fluxes expected during different classes of solar flares ranging from sub-A class to X5 flares. Thus, XSM provides the highest energy resolution and time cadence and covers the largest scale in solar X-ray intensity for any broad-band soft X-ray spectrometer flown so far. XSM has been operational in lunar orbit from September 2019, and the in-flight performance of the instrument so far has been excellent and identical to that on the ground. The observations show that the XSM has a very low background as expected and can measure the solar spectrum even during periods of extremely low activity with the flux levels an order of magnitude below the GOES A1 class. Over the two primary observing seasons, XSM has detected many A and B class flares along with several sub-A class flares. With the high sensitivity, energy resolution, and time cadence of XSM, it has become possible to characterize the evolution of the X-ray spectral properties even during very small events like B-class flares. XSM is likely to continue to be operational over the next few years, and with increasing solar activity, it provides a unique opportunity to study the evolution of the soft X-ray spectrum of the Sun with unprecedented details. Here we present an overview of the XSM and initial results from the instrument.