Constraining the Frequency of Transient Energy Emission through NuSTAR Spectroscopic Analysis

The Nuclear Spectroscopic Telescope ARray (NuSTAR) is a high-energy X-ray telescope orbiting the Earth. NuSTAR's focusing optics and pixel detectors are capable of measuring the time, energy, and position of each measured photon. Each photon is recorded as an individual event, providing high flexibility in the analysis process. While designed as an astrophysical instrument, NuSTAR's increased sensitivity relative to other solar observers offers a unique view of the solar disk, particularly during sub-B-class GOES levels, through direct imaging of hard X-rays greater than 3 keV.

This presentation features work of an automated detection algorithm to identify faint, sometimes unidentifiable by eye, brightenings measured by NuSTAR. Times of heightened count rates are identified from the pixel light curves and grouped based on the temporal and spatial properties of the pixels in order to reconstruct the brightening events. The found events are then put through a first-pass spectroscopic analysis using the XSPEC software to obtain energy estimates and the relative abundance of thermal and non-thermal plasma. We have estimated that NuSTAR has observed hundreds of microflare-scale events, offering a substantial sample size to constrain the flare frequency-versus-energy distribution with energy estimates obtained from the spectral fits. This distribution reveals the significance of small-scale flaring events in providing energy to the solar corona. As instrumentation approaches the observational regime containing the largest nanoflares, refining the flare-frequency distribution becomes increasingly relevant in explaining coronal heating. Lastly, the spectral fits estimate the thermal and non-thermal emission of the event, which is used to characterize the behavior of the microflares relative to their larger counterparts.