Effective Area and Energy Resolution vs. Grade Selection; Pre-launch Results

The HRMA/ACIS effective area and energy resolution were calculated for the ASCA grade selections g0, g01, g012, g0123, g01234, g012345, g02346.^6.1 We also considered the special case of including all grades except ACIS grade 255. ACIS 255 (which corresponds to all eight neighbors of the central pixel being above the Split Event Threshold) is considered to contain mostly events that result from interactions of cosmic ray particles with ACIS (and not X-ray photons).  Figure  6.1 and figure 6.2 show the dependence of effective area with grade selection for ACIS CCD's I1 and S3. Tables 6.8 and 6.9 provide numerical forms of the results.
 

Figure 6.1: Effective Area vs. Energy and Grade Selection for ACIS CCD I1 calculated for grade selections of g0, g01, g02, g023, g01234, g012345, g02346, all grades - ACIS255

 
 

Energy keV	Effective Area
	g0	g01	g02	g023	g01234	g012345	g02346	all - ACIS(255)
	cm2	cm2	cm2	cm2	cm2	cm2	cm2	cm2
0.277	2.33	2.33	2.37	2.37	2.38	2.38	2.38	2.38
0.525	96.82	101.15	106.49	109.60	117.11	118.37	118.87	118.96
0.93	283.51	289.22	309.47	317.53	330.60	333.30	334.75	335.31
1.4867	412.15	426.40	480.60	506.20	545.22	558.62	570.06	572.62
3	213.56	217.10	248.15	260.79	276.30	279.77	286.28	287.23
4	204.53	216.40	260.17	280.58	312.61	326.90	345.91	351.00
5	136.17	151.35	193.68	215.76	253.12	277.71	320.80	341.08
6	51.96	60.69	81.09	92.91	112.95	130.95	168.88	207.70
7	21.77	25.13	35.76	40.97	49.48	56.26	79.82	109.90
8	8.93	9.98	14.83	17.05	20.28	22.62	34.23	52.95
9	3.60	4.03	6.00	6.96	8.34	9.00	14.44	23.77

The effective area values listed in this table apply for CCD ID = 1, a HRMA Pitch angle of 5 arcmin, a HRMA Yaw angle of -3.7 arcmin, and a split event threshold of 13 adu. Events within the entire CCD were considered.

Figure 6.2: Effective Area vs. Energy for ACIS CCD S3 calculated for grade selections of g0, g01, g02, g023, g01234, g012345, g02346, all grades - ACIS255

 

Energy keV	Effective Area
	g0	g01	g02	g023	g01234	g012345	g02346	all - ACIS(255)
	cm2	cm2	cm2	cm2	cm2	cm2	cm2	cm2
0.277	109.09	113.72	137.46	149.75	166.9	167.65	168.43	168.49
0.522	141.25	147.66	236.67	279.46	330.76	354.62	394.43	405.34
1.4867	154.10	160.54	305.89	373.49	451.76	480.94	591.09	611.30
3	39.15	40.90	119.42	155.51	196.07	209.37	348.93	380.83
4	32.02	35.22	92.73	115.54	148.12	170.75	307.53	387.58
5	26.06	29.64	67.36	80.18	104.11	125.00	224.27	322.95
6	15.96	18.27	39.48	46.06	59.37	73.77	129.18	204.34
7	12.68	14.15	29.06	33.79	43.54	51.44	87.31	125.03
8	5.98	6.42	13.10	14.90	19.23	22.23	37.13	51.08
9	3.50	3.77	8.14	9.22	12.11	14.01	24.54	32.72

The effective area values listed in this table apply for CCD ID = 7, a HRMA Pitch angle of 0 arcmin, a HRMA Yaw angle of 0 arcmin, and a split event threshold of 13 adu. Events within the entire CCD were considered.

The effective area measurements for the FI and BI devices presented here were made with HRMA (Pitch,Yaw) values of (5',-3.7') and (0,0) respectively. For the purpose of comparing FI with BI performance we present in Table 6.10 a list of HRMA effective area correction factors for converting the off-axis (5',-3.7') FI effective areas to on-axis ones.
 

Table 6.10: HRMA Effective Area (EA) Correction Factors for a HRMA Pitch Angle of 5 arcmin and a HRMA Yaw Angle of -3.7 arcmin.

Energy keV	HRMA EA	HRMA EA	Correction Factor
	off-axis	on-axis
	cm2	cm2
0.277	766.71863	812.18695	0.9440
0.525	737.62781	781.42084	0.9440
0.93	740.83380	784.86621	0.9439
1.4867	732.67267	776.44696	0.9436
3	418.14990	445.54376	0.9385
4	432.86209	465.34161	0.9302
5	379.03247	417.80930	0.9072
6	251.41301	295.91565	0.8496
7	144.60548	190.16235	0.7604

The HRMA effective area off-axis correction factor is defined as the ratio of the off-axis effective area to the on-axis value.

The first significant result is that the HRMA/ACIS effective area varies considerably with grade selection for both front illuminated (FI) and back illuminated (BI) CCD's. A possible use of this effect is in the analysis of grating observations of faint objects where CCD energy resolution only influences the separation of overlapping spectral orders. The exclusion of only ACIS grade 255 will result in a relatively high effective area and a higher signal to noise spectrum of the faint object.

Figure 6.3 and figure 6.4 show the dependence of energy resolution with grade selection for ACIS CCD's I1 and S3. (Tables 6.11 and  6.12 give numeric values of this data.) The energy resolution for FI chips is fairly insensitive to grade selection in the 3-6 keV range. For energies above 6 keV the exclusion of grades begins to improve energy resolution for FI chips. In the energy range of 3-9 keV investigated so far, the energy resolution for back sided CCD's improves significantly with the exclusion of grades.

A possible astrophysical observation in which this effect may be exploited is ACIS imaging of a bright source with the BI chip. For bright sources imaged on a BI chip it may be desirable to sacrifice source counts for spectral resolution by selecting only single events.
 

Figure 6.3: Energy Resolution (FWHM) vs. Energy for ACIS CCD I1 calculated for grade selections of g0, g01, g012, g0123, g01234, g012345, g02346, all grades - ACIS255

 
 
 

Table 6.11:  The energy resolution (FWHM) values listed in this table apply for CCD ID = 1, a HRMA Pitch angle of 5 arcmin, a HRMA Yaw angle of -3.7 arcmin, and a split event threshold of 13 adu. Events within the entire CCD were considered.

Energy keV	Energy Resolution
	g0	g01	g012	g0123	g01234	g012345	g02346	all - ACIS(255)
	eV	eV	eV	eV	eV	eV	eV	eV
3	89.40	89.86	90.10	90.08	89.78	90.03	90.59	90.19
4	100.80	101.57	103.14	103.17	103.00	102.81	103.59	103.67
5	115.21	114.78	115.44	115.92	116.01	116.19	116.50	116.52
6	126.29	126.06	129.75	130.70	132.07	133.08	134.67	134.44
7	137.47	138.59	141.74	142.48	143.12	146.60	151.75	151.84
8	139.43	141.91	146.13	148.93	152.01	154.79	164.02	165.79
9	158.36	154.01	165.58	167.14	168.19	175.50	188.22	196.76

 
 

Figure 6.4: Energy Resolution (FWHM) vs. Energy for ACIS CCD S3 calculated for grade selections of g0, g01, g02, g023, g01234, g012345, g02346, all grades - ACIS255

 
 

Energy keV	Energy Resolution
	g0	g01	g02	g023	g01234	g012345	g02346	all - ACIS(255)
	eV	eV	eV	eV	eV	eV	eV	eV
0.277	77.18	77.22	73.07	71.20	69.84	69.89	70.07	69.96
0.525	90.86	90.86	92.83	93.11	93.12	93.20	92.34	92.70
1.4867	98.11	97.41	99.14	98.0	99.66	99.93	102.10	102.43
3	119.18	120.74	133.01	128.95	133.87	134.82	137.55	139.11
4	130.11	131.13	139.07	140.48	142.43	143.16	149.48	151.17
5	142.36	142.98	161.41	159.14	163.89	165.92	173.58	177.65
6	148.95	149.94	171.94	169.68	172.56	174.57	184.27	190.27
7	157.89	157.48	182.49	180.67	183.12	184.56	193.79	197.96
8	180.15	178.86	203.08	200.06	200.22	200.50	205.97	211.79
9	192.39	191.20	204.37	202.54	200.05	200.75	207.44	216.57