Dependence of Background Rejection Efficiency on Event Grade Selection

An important consideration in choosing ACIS event selection criteria is the efficiency of background rejection. To illustrate this point, we present in Tables 4.72, 4.73 and  4.74, the fraction of background events in various grades. For conciseness, we list only the standard ASCA grades, and ACIS grade 255. A similar analysis should be performed (presumably by the AXAF Science Center) for all 255 ACIS grades before a final decision is made on the definition of standard ACIS grades. Of course, the raw XRCF background event lists we have compiled are available to ASC for this purpose.

A cursory analysis of these tables shows, for example, that in device I3 91.21% of background events were ACIS bit map grade 255 events (i.e. all eight neighboring pixels were above the split threshold). However, only 4.14% of the background events in S3 were ACIS grade 255 events. Thus, a large majority of the background events in a frontside device could be rejected simply by filtering ACIS grade 255 events, but only a very small percentage of the background events in a backside device would be rejected. If one were to reject only ACIS grade 255 events, the total on-orbit background rate for six devices should be $\rm\sim40\ ct\ s^{-1}$, which is about 22% of the telemetry bandwidth.
 
 

Table 4.72: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors I0, I1, I2, and I3

 

Flight	Event$^{\dagger}$	Fraction of Events per Event Grade
Device	Grades	Quad A	Quad B	Quad C	Quad D	Mean
		(%)
I0	0	0.387	0.395	0.289	0.325	0.349
	1	0.000	0.000	0.000	0.000	0.000
	2	0.139	0.146	0.133	0.121	0.134
	3	0.050	0.052	0.066	0.047	0.054
	4	0.064	0.057	0.076	0.063	0.065
	5	0.144	0.130	0.152	0.173	0.150
	6	0.159	0.120	0.166	0.236	0.170
	7	99.059	99.101	99.118	99.036	99.078
	157	99.202	99.231	99.270	99.209	99.228
	B255	93.677	88.740	89.959	91.731	91.027
I1	0	0.354	0.423	0.355	0.354	0.371
	1	0.000	0.000	0.005	0.000	0.001
	2	0.204	0.146	0.143	0.106	0.150
	3	0.094	0.146	0.069	0.087	0.099
	4	0.077	0.065	0.103	0.087	0.083
	5	0.160	0.173	0.197	0.143	0.168
	6	0.166	0.179	0.172	0.133	0.163
	7	98.945	98.868	98.956	99.090	98.965
	157	99.105	99.041	99.158	99.232	99.134
	B255	91.637	89.269	90.800	91.361	90.767
I2	0	0.343	0.280	0.273	0.230	0.282
	1	0.000	0.004	0.000	0.004	0.002
	2	0.124	0.068	0.104	0.121	0.104
	3	0.057	0.055	0.056	0.042	0.053
	4	0.033	0.093	0.048	0.079	0.063
	5	0.129	0.140	0.164	0.155	0.147
	6	0.114	0.157	0.130	0.113	0.129
	7	99.199	99.202	99.226	99.256	99.221
	157	99.327	99.346	99.390	99.415	99.370
	B255	94.400	90.730	90.750	93.887	92.441
I3	0	0.247	0.197	0.346	0.327	0.279
	1	0.009	0.000	0.000	0.000	0.002
	2	0.085	0.081	0.059	0.163	0.097
	3	0.052	0.031	0.054	0.058	0.049
	4	0.019	0.063	0.069	0.062	0.053
	5	0.190	0.175	0.139	0.125	0.157
	6	0.166	0.103	0.129	0.192	0.148
	7	99.231	99.350	99.204	99.072	99.214
	157	99.430	99.524	99.342	99.197	99.373
	B255	90.273	90.925	90.304	91.679	90.795
$^{\dagger}$ All event grades except B255 are ASCA grades. Here B255 stands
for ACIS bit grade code 255 (i.e., all pixels above threshold).

 

Table 4.73: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors S0, S2, S4, and S5

 

Flight	Event$^{\dagger}$	Fraction of Events per Event Grade
Device	Grades	Quad A	Quad B	Quad C	Quad D	Mean
		(%)
S0	0	0.195	0.230	0.307	0.263	0.249
	1	0.000	0.000	0.000	0.006	0.001
	2	0.141	0.075	0.080	0.140	0.109
	3	0.060	0.027	0.043	0.053	0.045
	4	0.054	0.016	0.068	0.047	0.046
	5	0.124	0.166	0.141	0.169	0.150
	6	0.135	0.107	0.123	0.105	0.118
	7	99.291	99.379	99.239	99.218	99.282
	157	99.416	99.545	99.380	99.393	99.434
	B255	93.414	90.934	90.052	94.130	92.133
S2	0	0.393	0.298	0.338	0.354	0.346
	1	0.000	0.000	0.000	0.000	0.000
	2	0.179	0.158	0.143	0.156	0.159
	3	0.089	0.084	0.065	0.049	0.072
	4	0.066	0.056	0.068	0.052	0.061
	5	0.241	0.152	0.182	0.192	0.192
	6	0.241	0.155	0.169	0.182	0.187
	7	98.790	99.096	99.034	99.015	98.984
	157	99.031	99.248	99.216	99.207	99.176
	B255	90.966	88.295	88.005	90.258	89.381
S4	0	0.345	0.323	0.411	0.393	0.368
	1	0.000	0.000	0.007	0.008	0.004
	2	0.161	0.232	0.166	0.283	0.210
	3	0.112	0.074	0.151	0.039	0.094
	4	0.032	0.066	0.094	0.063	0.064
	5	0.209	0.182	0.216	0.157	0.191
	6	0.145	0.190	0.144	0.283	0.190
	7	98.996	98.933	98.810	98.774	98.878
	157	99.205	99.115	99.034	98.939	99.073
	B255	92.890	89.010	85.648	89.456	89.251
S5	0	0.222	0.356	0.233	0.415	0.307
	1	0.000	0.000	0.000	0.009	0.002
	2	0.189	0.144	0.141	0.189	0.166
	3	0.072	0.045	0.064	0.028	0.052
	4	0.026	0.083	0.064	0.113	0.072
	5	0.157	0.235	0.162	0.283	0.209
	6	0.091	0.144	0.141	0.217	0.148
	7	99.243	98.991	99.196	98.745	99.044
	157	99.400	99.227	99.358	99.038	99.255
	B255	92.820	88.693	87.609	92.604	90.432
$^{\dagger}$ All event grades except B255 are ASCA grades. Here B255 stands for ACIS bit grade code 255 (i.e., all pixels above threshold).

 

Table 4.74: Fraction of Background Events per Event Grade Measured at XRCF in the Passband 0.1-10 keV for Detectors S1 and S3

 

Flight	Event$^{\dagger}$	Fraction of Events per Event Grade
Device	Grades	Quad A	Quad B	Quad C	Quad D	Mean
		(%)
S1$^{\ast}$	$\cdots\hfil$	$\cdots\hfil$	$\cdots\hfil$	$\cdots\hfil$	$\cdots\hfil$	$\cdots\hfil$
S3	0	5.667	5.579	5.148	5.678	5.518
	1	0.000	0.070	0.071	0.000	0.035
	2	8.000	9.275	9.944	8.754	8.993
	3	2.917	3.138	2.186	2.839	2.770
	4	2.250	2.650	2.680	2.050	2.408
	5	5.250	4.393	4.443	5.836	4.981
	6	14.250	11.018	13.329	12.145	12.685
	7	61.667	63.877	62.200	62.697	62.610
	157	66.917	68.340	66.714	68.533	67.626
	B255	4.333	3.835	4.020	4.180	4.092
$^{\dagger}$ All event grades except B255 are ASCA grades. Here B255 stands for ACIS bit grade code 255 (i.e., all pixels above threshold).
$^{\ast}$ See note for detector S1 in Table 4.70.