REFINEMENTS TO THE GSPC CALIBRATIONS
An observation of the Crab in October 1985 and a ground calibration have allowed
a further improvement in our understanding of the parameters of the EXOSAT GSPC.
This new information has been incorporated into the procedure used to calibrate
the GSPC and results in a small change in the effective areas and burst length
acceptance efficiencies as a function of energy. Tests on GSPC data from a
selection of sources indicate that these changes produce little noticeable
difference in comparison with results obtained earlier. However, the new
calibrations will be incorporated into the CCF for completeness. In addition,
this opportunity was used to correct a small error made during the earlier
calibrations which gave an equivalent hydrogen column density for the Crab of
5x1021 H cm-2 instead of 3.5x1021 H cm-2. The details of the two improvements are
as follows:
1. The gain jump at the L edge, the cause of the "bump" at 4.7 keV, has now been
measured by Paul Lamb at MSSL using an almost identical instrument flown on
Spacelab 1 (Lamb et al. to be published). Their measurements give a jump at the
LIII edge (4.78 keV) of 60 eV and another jump at the
LIII edge (5.10 keV) of 20 eV. Using these values the
detector offset was recalculated and found to be +140
eV (cf. Express No.11 p.51). The effective
areas were redetermined using these new values and are listed in Table 1.
2. The burst length discrimination efficiency as a function of energy was
originally obtained by making three sequential observations on the Crab in the
two settings used (84-107 and 89-104) and in the wide open setting (39-126) and
dividing the resulting background subtracted spectra. The problem with this
technique is that any small gain drifts during the observation will cause
inaccuracies in the resulting efficiencies which might be noticeable on bright
sources. To improve this, a Crab observation was made in October 1985 using the
GDIR OBC program, which gives both the burst length and energy for each event.
Thus the appropriate burst length setting could be selected from the same data
without any uncertainty concerning gain variations. A new subroutine, listed in
Appendix 1, calculates the burst length efficiency for a given input energy. It
replaces the subroutine given in Express No. 11 p.51.
N.E. White
FUNCTION GSAXE(EIN,IBL)
C
C
C THIS FUNCTION RETURNS THE BURST LENGTH EFFICIENCY AS A FUNCTION
C OF INPUT ENERGY, EIN. NEW 21/2/86
C
C THIS USES CRAB FALL 85 OBSERVATION
C
C ACCEPTANCES IBL=
C O WIDE OPEN
C 1 89-107
C 2 89-104
C
C
DIMENSION E(5),POL1(13),POL2(13)
DATA E/0.0,25.0,55.0,135.0,195.0/
C
C
C
C 89-107/WO: SPLINE FIT TO 85 CRAB DIRECT DATA
C
DATA POLl/0.457,0.124E-01,-0.268E-03,0.375E-05,0.868E-02,
*-0.366E-03,0.662E-05,0.257E-02,-0.129E-04,0.349E-07,-0.72-8E-03,
*0.387E-04,-0.379E-06/
C
C
C 89 - 104/WO
C
DATA POL2/0.38,0.992E-02,-0.225E-03,0.337E-05,0.867E-02,
*-0.435E-03,0.825E-05,0.245E-02,-0.325E-05,-0.262E-07,0.428E-03,
*-0.724E-05,0.194E-06/
C
C
C
DATA NMIN/25/,NMAX/200/
C
C
C
IF(IBL.NE.O)GO TO 1
GSAXE=1.0
RETURN
1 CONTINUE
CHAN=(EIN-0.140)/0.076524+0.5
IF(CHAN.LT.NMIN)CHAN-NMIN
IF(CHAN.GT.NMAX)CHAN-NMAX
CHAN=CHAN-NMIN+1
IF(IBL.EQ.1)CALL SPLIN(POL1,E,4,CHAN,GSAXE)
IF(IBL.EQ.2)CALL SPLIN(POL2,E,4,CHAN,GSAXE)
IF(GSAXE.GT.1.0)GSAXE=1.0
RETURN
END
APPENDIX 1
Table 1 - GSPC Effective Areas (March 1986)
1 1.0000 .0000
2 1.1000 .0000
3 1.2000 .0001
4 1.3000 .0014
5 1.4000 .0131
6 1.5000 .0704
7 1.6000 .2612
8 1.7000 .7349
9 1.8000 1.6708
10 1.9000 3.2393
11 2.0000 5.5522
12 2.1000 8.6421
13 2.2000 12.4647
14 2.3000 16.9171
15 2.4000 21.8623
16 2.5000 27.1513
17 2.6000 32.6401
18 2.7000 38.2001
19 2.8000 43.7231
20 2.9000 49.1231
21 3.0000 54.3351
22 3.1000 59.3124
23 3.2000 64.0235
24 3.3000 68.4498
25 3.4000 72.5825
26 3.5000 76.4198
27 3.6000 79.9655
28 3.7000 83.2270
29 3.8000 86.2141
30 3.9000 88.9382
31 4.0000 91.4116
32 4.1000 93.6467
33 4.2000 95.6557
34 4.3000 97.4505
35 4.4000 99.0425
36 4.5000 100.4420
37 4.6000 107.6631
38 4.7000 105.8454
39 4.8000 105.9191
40 4.9000 104.2960
41 5.0000 112.7703
42 5.1000 115.2408
43 5.2000 116.6736
44 5.3000 117.9951
45 5.4000 119.2110
46 5.5000 120.7518
47 5.6000 121.7985
48 5.7000 122.7561
49 5.8000 123.6291
50 6.3000 126.8723
51 6.8000 128.5724
52 7.3000 129.0935
53 7.8000 128.7253
54 8.3000 127.7065
55 8.8000 126.2384
56 9.3000 124.4961
57 9.8000 122.6334
58 10.3000 120.9184
59 10.8000 119.1298
60 11.3000 117.1375
61 11.8000 114.9598
62 12.3000 112.6135
63 12.8000 110.1148
64 13.3000 107.4805
65 13.8000 104.7276
66 14.3000 101.8735
67 14.8000 98.9366
68 15.3000 95.9356
69 16.3000 89.8161
70 17.3000 83.6596
71 18.3000 77.5942
72 19.3000 71.7242
73 20.3000 66.1269
74 25.3000 43.3278
75 30.3000 28.6124
76 35.3000 74.9352
77 40.3000 61.7569
78 45.3000 50.4695
79 50.3000 41.1802
80 55.3000 33.6961
81 60.3000 27.7215
82 65.3000 22.9619
83 70.3000 19.1610
84 75.30O0 16.1110
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