where Y and Z are the auxiliary variables of X used in the variable splitting technique. ι Rě0 pXq " ř ij ι Rě0 pX ij q is the indicator function, with ι Rě0 pxq taking 0 when x ě 0 and 8 otherwise. This ensures the nonnegativity of data entries as assumed in (A1). D β pM|Xq denotes β-divergence between the estimated signal X and the observed data matrix M, and is introduced following assumption (A2). We formulate with D β , in general, to compare the behaviors when using KL divergence (β " 1) and Euclidean distance (β " 2). The nuclear norm of the data matrix }Y} ˚is the sum of the singular values of Y. This term reflects assumption (A3) as a convex surrogate, and µ pě 0q balances its weight. Each column of B 2 Z{BE 2 is the second-order derivative of the EDX spectrum with respect to the energy of X-rays. This term is introduced following assumption (A4) regarding the smoothness of EDX spectra, and λ pě 0q balances its weight.