Internal working of jaxspec

Data structure for observations

Observation is the main data structure to handle X-ray data. Observations in X-ray carry a lot of data and metadata. The main data are the following :

• counts : the counts in each instrument channel
• folded_counts : the counts after grouping
• grouping : the sparse channel → bin grouping matrix
• quality : the per-channel quality flag

An Observation carries no energy axis of its own: a PHA file records counts per channel, and the mapping from channel to energy lives in the response. That axis is on Instrument (e_min_channel / e_max_channel for the folded side, e_min_unfolded / e_max_unfolded for the model side), which is exactly why the two are joined into an ObsConfiguration before fitting.

Our entrypoint to the X-ray data is the OGIP standard.

flowchart LR
    T("fa:fa-chart-line True spectrum");
    II("fa:fa-satellite Instrument <br> [input channels]");
    IO("fa:fa-eye  Observation <br> [folded channels]");
    O("fa:fa-chart-column Binned Obs <br> [bins]");
    T --> |" Discretisation "| II;
    II --> |" Folding with <br> instrument response "| IO;
    IO --> |" Rebinning "| O;
    O -.Fitting the parameters.-> T;