This page contains a proposal for a new ACIS CC3x3 mode grade table based on the results of an analysis of the grade distribution of background events in detectors S2 and S3 using data from OBSID 536. This was a 119.8 ks, ACIS-I observation of a cluster of galaxies in timed-exposure mode. Since S2 and S3 are significantly off-axis, the contribution from the diffuse X-ray background is reduced.
An image of the ASCA grade 0,2,3,4,6 events in S2 and S3 is presented in Figure 1. The image shows the presence of a few point sources, but they do not appear to be numerous enough nor strong enough to significantly affect the grade histograms of the background events, so no effort was made to remove the source counts.
Figures 2 and 3 show lightcurves of the ASCA grade 0,2,3,4,6 events in each device. S2 shows some background flares at the 20% level, with a few flares as large as a factor of 2. On the other hand, S3 shows numerous extremely strong flares; factors of 2-5 were common and one flare was about 35 times brighter than the baseline level. Therefore, the data was filtered to use only Level 1 events from the time range 5.513x10^7--5.5155x10^7 s (10/01/99, 01:53 -- 10/01/99, 08:50 UT). All subsequent analysis was performed on the data from this time slice.
Figures 3 and 4 show the grade histograms for the S2 and S3 events. ACIS flight grades containing at least 2% of the events are labelled on the plots and listed in Tables 1 and 2. Note that ACIS filtered out flight grades 24, 66, 107, 214, and 255 on-orbit, so the fractions presented here are based on the total number of events telemetered to the ground. In ACIS Memo #162 it was found that over 90% of the background events in S2 and almost 50% of the events in S3 were in these five flight grades. However, that memo analyzed data obtained prior to the sunshade door opening and the subsequent CTI degradation in the front-illuminated CCDs; the current distribution of background events in these grades is not known.
Based on the results presented in Figures 3 and 4, and Tables 1 and 2, a proposal for a new CC3x3 mode grade table has been prepared (see Table 3). The filters in this table combine the dominant background grades in various combinations to allow the user considerable flexibility in choosing which grades to reject based on consideration of telemetry limits, effective area, and background rejection efficiency.
For instance, it has been suggested that ACIS flight grade 66 events should be telemetered in the future to allow reclamation of grade 2 events that were morhped into grade 66 by the effects of CTI in the frontside devices. The grade filter assigned to code 12 in Table 3 would telemeter grade 66 events, while still filtering out the other four most important background event grades (i.e., grades 24,107,214,255).
However, it must be kept in mind that telemetering grade 66 events would substantially increase the amount of bandpass used up by background events. Figure 5 in ACIS Memo #162 shows that about 3% of the background events in S2 and about 15% of the events in S3 were grade 66 events before CTI degradation occured. Thus, the fraction in grade 66 should be higher now by an uncertain amount.
Finally, we note the grade filters proposed in Table 3 would be of use in timed-exposure mode as well.
Figure 1: S2 (left) and S3 (right) image of G02346 events from OBSID 536.
Figure 2: S2 lightcurve of G02346 events from OBSID 536.
Figure 3: S3 lightcurve of G02346 events from OBSID 536.
Figure 4: Grade histogram of background events in ACIS device S2 from OBSID 536.
Figure 5: Grade histogram of background events in ACIS device S3 from OBSID 536.
ACIS Flight Grades with at Least 2% of all
Background Events in Device S2
| ACIS Flight Grade | ASCA Grade | Fraction of Events |
|---|---|---|
| 64 | 2 | 0.0328 |
| 70 | 7 | 0.0209 |
| 86 | 7 | 0.0202 |
| 127 | 7 | 0.0559 |
| 191 | 7 | 0.0206 |
| 194 | 7 | 0.0212 |
| 223 | 7 | 0.0543 |
| 239 | 7 | 0.0283 |
| 247 | 7 | 0.0274 |
| 248 | 7 | 0.1379 |
| 251 | 7 | 0.0511 |
| 254 | 7 | 0.0502 |
| Total | - | 0.5208 |
| Note: ACIS flight grades 24,66,107,214,255 were filtered on-orbit. |
ACIS Flight Grades with at Least 2% of all
Background Events in Device S3
| ACIS Flight Grade | ASCA Grade | Fraction of Events |
|---|---|---|
| 0 | 0 | 0.0436 |
| 2 | 2 | 0.0338 |
| 8 | 3/4 | 0.0311 |
| 11 | 6 | 0.0293 |
| 16 | 3/4 | 0.0286 |
| 22 | 6 | 0.0272 |
| 64 | 2 | 0.0664 |
| 72 | 6 | 0.0411 |
| 75 | 7 | 0.0305 |
| 80 | 6 | 0.0374 |
| 86 | 7 | 0.0288 |
| 104 | 6 | 0.0577 |
| 203 | 7 | 0.0539 |
| Total | - | 0.5094 |
| Note: ACIS flight grades 24,66,107,214,255 were filtered on-orbit. |
"Proposed" ACIS CC3x3 Mode Grade Table
| Code | Description | Grade Filter |
|---|---|---|
| 0 | Reject all grades | 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 |
| 1 | Reject ASCA grades 1,2,3,4,5,6,7 | 0x00000001 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 |
| 2 | Reject ASCA grades 1,5,6,7 | 0x00031105 0x00030004 0x00000033 0x00000000 0x00001104 0x00000004 0x00000000 0x00000000 |
| 3 | Reject ASCA grades 1,5,7 | 0x00471d05 0x00470004 0x00031133 0x00001100 0x00001d04 0x00000004 0x00030000 0x00000000 |
| 4 | UNDEF | 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 |
| 5 | UNDEF | 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 |
| 6 | UNDEF | 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 0x00000000 |
| 7 | Reject ACIS flight grades 24,66,107,127,214,223,248,251,254,255 | 0xfeffffff 0xffffffff 0xfffffffb 0x7ffff7ff 0xffffffff 0xffffffff 0xefbfffff 0x36ffffff |
| 8 | Reject ACIS flight grades 24,107,127,214,223,248,251,254,255 | 0xfeffffff 0xffffffff 0xffffffff 0x7ffff7ff 0xffffffff 0xffffffff 0xefbfffff 0x36ffffff |
| 9 | Reject ACIS flight grades 24,66,107,214,248,255 | 0xfeffffff 0xffffffff 0xfffffffb 0xfffff7ff 0xffffffff 0xffffffff 0xffbfffff 0x7effffff |
| 10 | Reject ACIS flight grades 24,66,107,214,255 | 0xfeffffff 0xffffffff 0xfffffffb 0xfffff7ff 0xffffffff 0xffffffff 0xffbfffff 0x7fffffff |
| 11 | Reject ACIS flight grades 24,107,214,248,255 | 0xfeffffff 0xffffffff 0xffffffff 0xfffff7ff 0xffffffff 0xffffffff 0xffbfffff 0x7effffff |
| 12 | Reject ACIS flight grades 24,107,214,255 | 0xfeffffff 0xffffffff 0xffffffff 0xfffff7ff 0xffffffff 0xffffffff 0xffbfffff 0x7fffffff |
| 13 | Reject ASCA grade 7 | 0x00773f7f 0x0077117f 0x00031133 0x00001133 0x00033f7f 0x0003117f 0x00030033 0x00000000 |
| 14 | Reject ACIS flight grade 255 | 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0x7fffffff |
| 15 | Accept all grades | 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff 0xffffffff |
Dr. Frederick K. Baganoff --- MIT/CSR Last modified: Tue Aug 7 10:44:16 EDT 2007