Caveat: ACIS Event Thresholds
A number of Chandra ACIS observations which have been processed with the Level 3 pipeline and are contained in the Chandra Source Catalog have zero sensitivity in parts of the event energy range encompassed by the CSC science energy bands (0.2 keV - 7 keV). This is on account of the observer's ability to alter the default on-board energy filters for ACIS, which is appropriate when sources are expected to have spectra in a restricted energy range or count rate saturation is a concern (see the CIAO caveats page for more details). In other words, the low energy threshold of events in some ObsIDs lies above the lowest energy considered by the CSC (0.2 keV), and/or the high event energy threshold lies below the highest energy considered (7 keV). In the first release there are no sources in the catalog that have a lower high event energy threshold. Catalog source properties which are affected by these energy gaps are assigned a value of "INDEF" in processing, and are reported with a null value in the catalog, where appropriate. For example, a source with a low event energy threshold of 1.0 keV would have a value of 0.0 for certain ultrasoft (u), soft (s), and broad (b) band flux properties in the catalog (the source has zero flux in the u band and only partial coverage in the s and b bands).
The effective energy thresholds vary by chip, node, time, and grade, and the ARFs and RMFs created for the affected observations may not correctly represent the instrument settings. For the CSC, this means that certain aperture model energy fluxes, spectral model fit parameters, and hardness ratios may be affected, since the spectral instrumental signature is removed from these values (but not from the counts values). Specifically, the following per-observation source properties for the u-, b-, and s-bands must be considered:
Aperture Model Energy Fluxes
o.flux_powlaw_aper + limits
o.flux_powlaw_aper90 + limits
o.flux_bb_aper + limits
o.flux_bb_aper90 + limits
Spectral Model Fits
o.alpha + limits
o.flux_powlaw + limits
o.nh_powlaw + limits
o.powlaw_stat
o.kT + limits
o.flux_bb + limits
o.nh_bb + limits
o.bb_stat
Hardness Ratios
o.hard_hm + limits
o.hard_hs + limits
o.hard_ms + limits
The ObsIds with partial coverage of the CSC science energy bands are listed below. The ones with high lower thresholds (0.3, 0.5, and 1.0 keV) will have null values for the u-, s-, and b-band per-observation source properties listed above.
ObsIDs with Lower Energy Threshold: |
U Band Source Fluxes(flux_powlaw_*_u, flux_bb_*_u) |
S Band Source Fluxes(flux_powlaw_*_s, flux_bb_*_s) |
B Band Source Fluxes(flux_powlaw_*_b, flux_bb_*_b) |
Hardness Ratios |
Spectral Model Fit Parameters and Fluxes |
|---|---|---|---|---|---|
| 0.3 | null | ||||
| 0.5 | null | null | null | ||
| 1.0 | null | null | null | null | null |
The corresponding master source properties will have null values as well if all observations contributing to a master source entry have null values; otherwise, the master source properties will be computed from the non-null observations. Be aware that the inter-observation variability of these master sources may not be reliable. Finally, bear in mind that certain aperture source energy fluxes (photflux_* values) are not set to null values because the flux_significance is derived from them.
The 0.3 keV threshold (rejected u-band source fluxes) applies to ObsIds:
ObsID status grating exp_mode eventfilter_lower eventfilter_higher
------ -------- ---------------- ------- -------- -------------------- --------------------
5547 archived G21.6-0.0 NONE TE 0.300000 12.000000
5563 archived G20.0-0.2 NONE TE 0.300000 12.000000
3507 archived XTE J1709-267 NONE TE 0.300000 13.000000
3499 archived RCRADARKCLOUD NONE TE 0.300000 12.000000
3510 archived 4U1755-33 NONE TE 0.300000 13.000000
3877 archived SGR 1627-41 NONE TE 0.300000 12.000000
4475 archived RCrADarkCloud NONE TE 0.300000 12.000000
7576 archived PSR J0108-1431 NONE TE 0.300000 12.700000
7577 archived PSR B1257+12 NONE TE 0.300000 12.700000
3508 archived 2S1711-339 NONE TE 0.300000 13.000000
3509 archived A1744-361 NONE TE 0.300000 13.000000
The 0.5 keV threshold (rejected u- and s-band fluxes and hardness ratios) applies to ObsIds:
5530 archived G15.9+0.2 NONE TE 0.500000 12.000000
2785 archived G349.7+0.2 NONE TE 0.500000 12.000000
6289 archived G15.9+0.2 NONE TE 0.500000 12.000000
6288 archived G15.9+0.2 NONE TE 0.500000 12.000000
The 1.0 keV threshold (rejected u-, s-, and b-band fluxes, hardness ratios, and all spectral model fit parameters) applies to ObsIds:
2267 archived GCS 20 NONE TE 1.000000 10.000000
2268 archived GCS 21 NONE TE 1.000000 10.000000
2269 archived GCS 1 NONE TE 1.000000 10.000000
2270 archived GCS 22 NONE TE 1.000000 10.000000
2271 archived GCS 2 NONE TE 1.000000 10.000000
2272 archived GCS 23 NONE TE 1.000000 10.000000
2273 archived GCS 10 NONE TE 1.000000 10.000000
2274 archived GCS 3 NONE TE 1.000000 10.000000
2275 archived GCS 24 NONE TE 1.000000 10.000000
2277 archived GCS 4 NONE TE 1.000000 10.000000
2279 archived GCS 12 NONE TE 1.000000 10.000000
2280 archived GCS 5 NONE TE 1.000000 10.000000
2281 archived GCS 26 NONE TE 1.000000 10.000000
2283 archived GCS 27 NONE TE 1.000000 10.000000
2285 archived GCS 6 NONE TE 1.000000 10.000000
2286 archived GCS 28 NONE TE 1.000000 10.000000
2287 archived GCS 15 NONE TE 1.000000 10.000000
2288 archived GCS 7 NONE TE 1.000000 10.000000
2290 archived GCS 30 NONE TE 1.000000 10.000000
2291 archived GCS 16 NONE TE 1.000000 10.000000
2292 archived GCS 8 NONE TE 1.000000 10.000000
2293 archived GCS 17 NONE TE 1.000000 10.000000
2294 archived GCS 9 NONE TE 1.000000 10.000000
2295 archived GCS 18 NONE TE 1.000000 10.000000
2296 archived GCS 19 NONE TE 1.000000 10.000000