Per-Observation Detections Light Curve Files (lc3.fits)
The light curve files consist of a set of FITS format files, one for each source region of each observation, for each science energy band. Each light curve file has two extensions: the first one (LIGHTCURVE) records the multi-resolution light curve output by the Gregory-Loredo analysis of the arrival times of the source events within the source region. The second extension (BKGLIGHTCURVE) records the background light curve, which has an identical time-binning as the source light curve, but is derived from the events within the background region. Each extensions has the following columns:
- Time — arrival time
- COUNT_RATE — count rate
- COUNT_RATE_ERR — count rate error
- Minus_3Sig — \(3\sigma\) count rate lower-limit
- Plus_3Sig — \(3\sigma\) count rate upper-limit
The background light curve is recorded in the FITS HDU (CXC Data Model block) immediately following the source light curve HDUs.
Source Region Light Curve
The per-observation light curve FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈r〉〈b〉_lc3.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.
In CSC 2.0, light curves are computed for each detection at the individual observation level. However, they are not computed if the MLE analysis determines that the observation level detection likelihood is below a threshold that depends on the local background and count rate for the given band. In that case, likelihoods and light curves are not recorded in the database. Consequently, for each source, light curves are only provided for detections that were detected with enough significance.