The log-likelihood functiono of the TPXG distribution is given by:
$$\ln L(\alpha, \theta|x) = 2n \ln \theta - n \ln (\alpha + \theta) - \theta \left(\sum_{i=1}^{n} x_i\right)
+ \sum_{i=1}^{n} \ln \left(1 + \frac{\alpha\theta}{2}x_i^2\right)$$
References
"Sen, S., Chandra, N. and Maiti, S. S. (2018). On properties and applications of a two-parameter XGamma distribution. Journal of Statistical Theory and Applications, 17(4): 674--685."