Master Source Region Aperture Photometry Draws File (photdraws3.fits)

Aperture Photometry applies a Bayesian algorithm to sample the background-marginalized posterior probability distributions for net source counts, source rate, photon and energy flux. Raw draws histograms are analyzed with an Epanechnikov Kernel Density Estimator, and the smoothed outputs are used as the posterior probability distributions provided in prop3 files and from which credible regions for intensity parameters and hardness ratios are derived.

Aperture Photometry applies the Sherpa implementation of pyBLoCXS (van Dyk et al. 2001), a Markov chain Monte Carlo (MCMC)- based algorithm that samples the posterior probability distribution of the parameters starting from the maximum likelihood step in the optimization. The Sherpa function get_draws is used to run MCMC chains using the model information for a specific dataset, the selected sampler, the defined priors, and the specified number of iterations. This function returns an array of statistic values, an array of acceptance Booleans, and an array of sampled parameter values, i.e., draws.

For dim sources that fail to converge with the CSC 2.0 photometry algorithms, the marginalized posterior distributions are sampled using a more robust MCMC algorithm (NUTS). This improves our ability to provide well-constrained fluxes in the low-count regime. Details can be found in the Implementing pymc3 in CSC 2.1 Aperture Photometry memo.

The Master Source Aperture Photometry file is named: 〈m〉N〈v〉_〈b〉_phot3.fits

where 〈m〉 is the Master Source name formatted as "2CXOJhhmmsss{p|m}ddmmss[{<null>|A|B|C|...|X}]", 〈v〉 is the data product version number, and 〈b〉 is the energy band identifier.

The file contains draws for PHOTFLUX, SRC_CNTS, SRC_RATE, and energy FLUX for the best block and average of all observations, in separate FITS blocks as shown here:

  
unix% dmlist 2CXOJ0111570m460654N027_b_photdraws3.fits blocks

--------------------------------------------------------------------------------
Dataset: 2CXOJ0111570m460654N027_b_photdraws3.fits
--------------------------------------------------------------------------------
 
     Block Name                          		Type         Dimensions
--------------------------------------------------------------------------------
Block    1: PRIMARY                        	Null        
Block    2: PHOTFLUXDRAWS                  	Table         8 cols x 5001     rows
Block    3: SRC_CNTSDRAWS                 	Table         8 cols x 5001     rows
Block    4: SRC_RATEDRAWS                   	Table         8 cols x 5001     rows
Block    5: FLUXDRAWS                           Table         8 cols x 5001     rows
Block    6: PHOTFLUXDRAWS_AVG        	        Table         8 cols x 5001     rows
Block    7: SRC_CNTSDRAWS_AVG        	        Table         8 cols x 5001     rows
Block    8: SRC_RATEDRAWS_AVG         	        Table         8 cols x 5001     rows
Block    9: FLUXDRAWS_AVG                  	Table         8 cols x 5001     rows

Draws are contained in the columns ampl_source and ampl_source90 (for aper90 quantities).