Per-Observation Source Detection Aperture Photometry File (phot3.fits)

The Per-Observation Source Detection Aperture Photometry Files consists of a single FITS format file for each source region and stack of source regions and science energy band, containing uniformly sampled arrays of the marginalized posterior probability distribution (PDF) for source intensity, for both source region and 90% encircled counts fraction (ECF) apertures.

The formalism described in Primini & Kashyap (2014, ApJ 796 24) is used to express the source intensities for one or more near-by sources in terms of the counts and exposures in source and background apertures, in one or more observations. Sherpa is then used to determine the maximum likelihood values for the intensities, requiring a single source intensity for all observations, but allowing separate backgrounds in each observation. These are then used as a starting point for an MCMC procedure using the pyBLoCXS get_draws routine to sample the posterior probability distribution, assuming uniform, non-negative priors. Kernel-density estimates of the distribution of draws with respect to each source intensity are then used to describe the PDFs.

FITS HDUs (CXC Data Model blocks):

• PHOTFLUX: photon fluxes and corresponding PDF values for both the source region aperture (intensity, MPDF) and 90% ECF aperture (intensity90, MPDF90), for the group of observations comprising the flux-ordered best block.
• SRC_CNTS: net source counts and corresponding PDF values for both the source region aperture (intensity, MPDF) and 90% ECF aperture (intensity90, MPDF90), for the group of observations comprising the flux-ordered best block.
• SRC_RATE: source rates and corresponding PDF values for both the source region aperture (intensity, MPDF) and 90% ECF aperture (intensity90, MPDF90), for the group of observations comprising the flux-ordered best block.
• FLUX: fluxes and corresponding PDF values for both the source region aperture (intensity, MPDF) and 90% ECF aperture (intensity90, MPDF90), for the group of observations comprising the flux-ordered best block. The conversion from photons s^-1 cm^-2 to ergs s^-1 cm^-2 is performed by summing the photon energies for each incident source photon and scaling by the local value of the ARF at the location of the incident photon.

Each HDU will contain 200 rows, each row containing an intensity value and corresponding MPDF, for both source region and 90% ECF apertures.

The header for each HDU will contain keywords APER_MDE, APER_LO, APER_HI describing the mode and 68% confidence region for the source region MPDF, and AP90_MDE, AP90_LO, AP90_HI, for the 90% ECF aperture values.

The per-observation source region aperture photometry file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈r〉〈b〉_phot3.fits

where 〈i〉 is the instrument designation, 〈s〉 is the data source; 〈obs〉 is the observation identification; 〈obi〉 is the observation interval identification; 〈v〉 is the data product version number; 〈c〉 is the cycle; 〈r〉 is the region ID, formatted as "rnnnn", where nnnn is the 4-digit region number, formatted with leading zeros; and 〈b〉 is the energy band designation. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.