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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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