Automating NuSTAR/AIA Coalignment Techniques for NuSTAR Solar Observations

The Nuclear Spectroscopic Telescope ARray (NuSTAR) uses direct-focusing Hard X-Ray (HXR) telescopes to study astrophysical sources, but is capable of performing 12x12 arcmin solar observations between 2.5-13 KeV. Direct-focusing provides higher sensitivity in the HXR range compared to previous indirect imagers, valuable for the study of faint solar flares due to NuSTARs ability to directly observe their accelerated regions. Nanoflares sudden releases of magnetic energy about nine orders of magnitude smaller than the largest X class solar flares, are theorized to provide energy to the solar corona through magnetic reconnection and are subsequently candidates for coronal heating (Grefenstette 2016). However, complications arise when conducting solar observations. The frontmost of NuSTARs four star trackers is blinded when facing the sun, and unable to contribute to the alignment of the telescope. Without the blinded star tracker, thermal fluctuations in optics and tilting in NuSTARs deployed mast cause a 1-2 arcmin uncertainty in absolute pointing. This can be corrected through coalignment with trustworthy data. The Atmospheric Imaging Assembly (AIA) provides full-sun, spatially accurate images in seven wavelengths. Coalignment is often done by manually aligning the NuSTAR and 94 A AIA images, which is time consuming and introduces human bias. Automation of the coalignment saves time, reduces human bias, provides easier access to the use of NuSTAR data for solar observations, and enables the introduction of quantified uncertainties. One automated coalignment method is centroid matching, where the centroids of both the NuSTAR and AIA images are calculated and subsequently overlaid. Three centroid calculation techniques are being assessed: finding the center of mass using intensities of image pixels, fitting 1D gaussians to the marginal x and y distributions of data, and fitting a 2D gaussian to the 2D distribution of data. Sample events with ranging flare geometries will be tested on all three centroid techniques to compare methods. The primary factors in evaluating success of the coalignment code are accuracy in centroid calculations, user-friendliness, and robustness with differing flare geometries. This project presents a preliminary assessment of the centroid matching coalignment technique.