Last modified: 21 October 2019

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 CSC 2.0 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 Spectral Model Fits Hardness Ratios

The ObsIds with partial coverage of the CSC science energy bands are listed below. The ones with high lower thresholds (0.24-0.4, 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
S Band Source Fluxes
B Band Source Fluxes
Hardness Ratios
Spectral Model Fit Parameters and Fluxes
(flux_powlaw_*_u, flux_bb_*_u)
(flux_powlaw_*_s, flux_bb_*_s)
(flux_powlaw_*_b, flux_bb_*_b)
0.24-0.4 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 0.24-0.4 keV threshold (rejected u-band source fluxes)

ObsID status grating exp_mode eventfilter_lower eventfilter_higher
13811 archived IC 3418 NONE TE 0.240000 12.000000
13907 archived IRAS F15307+3252 NONE TE 0.240000 12.000000
14009 archived RXJ1532.9+3021 NONE TE 0.240000 12.000000
13293 archived Puppis A -micro-X field NONE TE 0.250000 8.250000
13612 archived KIC11560431 NONE TE 0.250000 11.750000
1994 archived Crab Nebula NONE TE 0.300000 11.000000
1995 archived Crab Nebula NONE TE 0.300000 11.000000
1997 archived Crab Nebula NONE TE 0.300000 11.000000
1998 archived Crab Nebula NONE TE 0.300000 11.000000
1999 archived Crab Nebula NONE TE 0.300000 11.000000
2001 archived Crab Nebula NONE TE 0.300000 12.000000
3499 archived RCrADarkCloud NONE TE 0.300000 12.000000
3507 archived XTE J1709-267 NONE TE 0.300000 13.000000
3508 archived 2S1711-339 NONE TE 0.300000 13.000000
3509 archived A1744-361 NONE TE 0.300000 13.000000
3510 archived 4U1755-33 NONE TE 0.300000 13.000000
3838 archived SN1006 NONE TE 0.300000 12.000000
3877 archived SGR 1627-41 NONE TE 0.300000 12.000000
4385 archived SN1006 NONE TE 0.300000 12.000000
4386 archived SN1006 NONE TE 0.300000 12.000000
4387 archived SN1006 NONE TE 0.300000 12.000000
4388 archived SN1006 NONE TE 0.300000 12.000000
4389 archived SN1006 NONE TE 0.300000 12.000000
4390 archived SN1006 NONE TE 0.300000 12.000000
4391 archived SN1006 NONE TE 0.300000 12.000000
4392 archived SN1006 NONE TE 0.300000 12.000000
4393 archived SN1006 NONE TE 0.300000 12.000000
4394 archived SN1006 NONE TE 0.300000 12.000000
4475 archived RCrADarkCloud NONE TE 0.300000 12.000000
4626 archived CTB-109 Lobe NONE TE 0.300000 12.000000
4674 archived Geminga NONE TE 0.300000 10.000000
5547 archived G21.6-0.0 NONE TE 0.300000 12.000000
5563 archived G20.0-0.2 NONE TE 0.300000 12.000000
7031 archived GX9+1 NONE TE 0.300000 8.000000
7576 archived PSR J0108-1431 NONE TE 0.300000 12.700000
7577 archived PSR B1257+12 NONE TE 0.300000 12.700000
8224 archived Cyg X-2 NONE TE 0.300000 9.700000
8444 archived Cyg X-2 NONE TE 0.300000 9.700000
8445 archived Cyg X-2 NONE TE 0.300000 9.700000
8446 archived Cyg X-2 NONE TE 0.300000 9.700000
10711 archived G289.7-0.3 NONE TE 0.300000 12.000000
10712 archived G46.8-0.3 NONE TE 0.300000 12.000000
10713 archived G9.95-0.81 NONE TE 0.300000 12.000000
10880 archived PSR J1357-6429 NONE TE 0.300000 10.000000
11250 archived PSR J1740+1000 NONE TE 0.300000 12.000000
11831 archived 0FGL J1018.2-5858 NONE TE 0.300000 12.000000
11832 archived 0FGL 1844.1-0335 NONE TE 0.300000 12.000000
11833 archived 0FGL J1430.5-5918 NONE TE 0.300000 12.000000
11834 archived 0FGL J1413.1-6203 NONE TE 0.300000 12.000000
11835 archived 0FGL J1821.4-1444 NONE TE 0.300000 12.000000
11836 archived 0FGL J1648.1-4606 NONE TE 0.300000 12.000000
11837 archived 0FGL J1741.4-3046 NONE TE 0.300000 12.000000
12148 archived 1FGL J1954.3+2836 NONE TE 0.300000 12.000000
12149 archived PSR J1958+2846 NONE TE 0.300000 12.000000
12150 archived PSR J1028-5819 NONE TE 0.300000 12.000000
12151 archived PSR J1022-5746 NONE TE 0.300000 12.000000
12152 archived HESS J1848-018 NONE TE 0.300000 12.000000
13139 archived Crab Nebula NONE TE 0.300000 11.000000
13146 archived Crab NONE TE 0.300000 11.000000
13147 archived Crab NONE TE 0.300000 11.000000
13150 archived Crab Nebula NONE TE 0.300000 11.000000
13151 archived Crab Nebula NONE TE 0.300000 11.000000
13152 archived Crab Nebula NONE TE 0.300000 11.000000
13153 archived Crab Nebula NONE TE 0.300000 11.000000
13154 archived Crab Nebula NONE TE 0.300000 11.000000
13204 archived Crab NONE TE 0.300000 11.000000
13205 archived Crab NONE TE 0.300000 11.000000
13206 archived Crab NONE TE 0.300000 11.000000
13207 archived Crab NONE TE 0.300000 11.000000
13208 archived Crab NONE TE 0.300000 11.000000
13209 archived Crab NONE TE 0.300000 11.000000
13210 archived Crab NONE TE 0.300000 11.000000
13285 archived 1FGL J1311.7-3429 NONE TE 0.300000 12.000000
13286 archived PSR J2238+59 NONE TE 0.300000 12.000000
13287 archived HESS J1626-490 NONE TE 0.300000 12.000000
13288 archived PSR J1459-60 NONE TE 0.300000 12.000000
13289 archived PSR J0248+6021 NONE TE 0.300000 12.000000
13290 archived PSR J1731-1847 NONE TE 0.300000 12.000000
13750 archived Crab NONE TE 0.300000 11.000000
13751 archived Crab NONE TE 0.300000 11.000000
13752 archived Crab NONE TE 0.300000 11.000000
13754 archived Crab NONE TE 0.300000 11.000000
13755 archived Crab NONE TE 0.300000 11.000000
13756 archived Crab NONE TE 0.300000 11.000000
13757 archived Crab NONE TE 0.300000 11.000000
13790 archived HESS J1741-302A NONE TE 0.300000 12.000000
14028 archived RX J0334.2-0111 NONE TE 0.300000 12.000000
14207 archived PSR J0357+3205 NONE TE 0.300000 10.000000
14208 archived PSR J0357+3205 NONE TE 0.300000 10.000000
14379 archived RX J0334.2-0111 NONE TE 0.300000 12.000000
14416 archived Crab NONE TE 0.300000 11.000000
14458 archived Crab pulsar NONE TE 0.300000 11.000000
14519 archived The Mouse NONE TE 0.300000 10.000000
14520 archived The Mouse NONE TE 0.300000 10.000000
14523 archived PSR J1509-5850 nebula NONE TE 0.300000 10.000000
14524 archived PSR J1509-5850 nebular NONE TE 0.300000 10.000000
14657 archived 1FLG J1018.6-5856 NONE TE 0.300000 10.000000
14678 archived Crab NONE TE 0.300000 11.000000
14679 archived Crab NONE TE 0.300000 11.000000
14680 archived Crab NONE TE 0.300000 11.000000
14681 archived Crab NONE TE 0.300000 11.000000
14682 archived Crab NONE TE 0.300000 11.000000
14685 archived Crab NONE TE 0.300000 11.000000
14821 archived J1906+0746 NONE TE 0.300000 12.000000
15360 archived HESS J1843-033A NONE TE 0.300000 12.000000
15361 archived PSR J1044-5737 NONE TE 0.300000 12.000000
15362 archived PSR J1231-1411 NONE TE 0.300000 12.000000
15363 archived PSR J0101-6422 NONE TE 0.300000 12.000000
15364 archived PSR B1919+21 NONE TE 0.300000 12.000000
15365 archived HESS J1843-033B NONE TE 0.300000 12.000000
15366 archived PSR J2043+1711 NONE TE 0.300000 12.000000
16245 archived Crab NONE TE 0.300000 11.000000
16257 archived Crab NONE TE 0.300000 11.000000
16560 archived 1FLG J1018.6-5856 NONE TE 0.300000 10.000000
6690 archived Cas A CCO NONE TE 0.350000 11.000000
13783 archived Cas A CCO NONE TE 0.350000 11.000000
2754 archived MSH 11-61A NONE TE 0.400000 13.000000
3720 archived MSH 11-61A NONE TE 0.400000 13.000000
4596 archived RCW103 NONE TE 0.400000 10.000000
4597 archived RCW103 NONE TE 0.400000 10.000000
4598 archived RCW103 NONE TE 0.400000 10.000000
4599 archived RCW103 NONE TE 0.400000 10.000000
5592 archived RCW103 NONE TE 0.400000 10.000000
5593 archived RCW103 NONE TE 0.400000 10.000000
5594 archived RCW103 NONE TE 0.400000 10.000000
5595 archived RCW103 NONE TE 0.400000 10.000000
7639 archived Tycho's SNR NONE TE 0.400000 12.000000
8551 archived Tycho's SNR NONE TE 0.400000 12.000000
10093 archived Tycho's SNR NONE TE 0.400000 12.000000
10094 archived Tycho's SNR NONE TE 0.400000 12.000000
10095 archived Tycho's SNR NONE TE 0.400000 12.000000
10096 archived Tycho's SNR NONE TE 0.400000 12.000000
10097 archived Tycho's SNR NONE TE 0.400000 12.000000
10902 archived Tycho's SNR NONE TE 0.400000 12.000000
10903 archived Tycho's SNR NONE TE 0.400000 12.000000
10904 archived Tycho's SNR NONE TE 0.400000 12.000000
10906 archived Tycho's SNR NONE TE 0.400000 12.000000

The 0.5 keV threshold (rejected u- and s-band fluxes and hardness ratios)

ObsID status grating exp_mode eventfilter_lower eventfilter_higher
2785 archived G349.7+0.2 NONE TE 0.500000 12.000000
5530 archived G15.9+0.2 NONE TE 0.500000 12.000000
6288 archived G15.9+0.2 NONE TE 0.500000 12.000000
6289 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)

ObsID status grating exp_mode eventfilter_lower eventfilter_higher
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
2276 archived GCS 11 NONE TE 1.000000 10.000000
2277 archived GCS 4 NONE TE 1.000000 10.000000
2278 archived GCS 25 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
2282 archived GCS 13 NONE TE 1.000000 10.000000
2283 archived GCS 27 NONE TE 1.000000 10.000000
2284 archived GCS 14 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
2289 archived GCS 29 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