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Last modified: 13 December 2012

URL: http://cxc.harvard.edu/csc/event_thresh_caveat.html

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

Last modified: 13 December 2012
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