Документ взят из кэша поисковой машины. Адрес оригинального документа : http://xmm.vilspa.esa.es/external/xmm_sw_cal/background/files/MSSLbckgd_v6.pdf Дата изменения: Wed Apr 19 19:57:33 2006 Дата индексирования: Sat Dec 22 14:50:26 2007 Кодировка: Поисковые слова: zodiacal light

Sky Background Calculations for the Optical Monitor (Version 6)
T. S. Poole August 3, 2005
Abstract Instructions on how to calculate the sky background flux for the Optical Monitor on XMM-Newton, by considering diffuse galactic, zo diacal light, and the average dark count rate of the instrument.

Contents
1 Calculating Sky Background 2 Diffuse Galactic Light 2.1 2.2 2.3 2.4 Background Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Throughput of filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intensity of reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 3 3 4 5 5 5 5 7 7 8

3 Zo diacal Light 3.1 3.2 3.3 3.4 Background Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Throughput of filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Intensity of reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4 Average Dark Count rate

1


1

Calculating Sky Background

The total sky background count rate (Btotal ) is calculating by considering the diffuse galactic background (Bdg ), the background due to zo diacal light (Bz l ), and the average dark count rate of the instrument (Bad ), and is therefore dep endent on sky co ordinates. Btotal is calculated using Btotal = Bdg + Bz l + Bad , (1) where the diffuse galactic and zo diacal light backgrounds are calculated using BX = IX ftp , Iref (2)

where IX is the intensity of background X at a given co ordinate (X = dg - diffuse galactic; X = z l - zo diacal light), ftp is the filter throughput for the background, and Iref is the reference intensity.

2

Diffuse Galactic Light

The diffuse comp onent of the galactic background radiation (Bdg ) is pro duced by scattering of stellar photons by dust grains in interstellar space, and effects wavelengths from the far ultraviolet to the near infrared. This scattering pro cess dominants the general interstellar extinction of starlight, and is therefore more intense at low galactic latitudes where the dust column density and integrated stellar emissivity are b oth high. Typically, diffuse galactic light (DGL) contributes 20% to 30% of total integrated light from the Milky Way [3]. Due to the strongly forward scattering nature of interstellar grains the DGL, intensity generally tracks the LOS* intensity at constant latitude, but large differences of LOS* intensity may o ccur with galactic longitude, therefore ratios must b e used. Table 1 shows the ratio of DGL to LOS* from [3], and shows the variation in the ratios with galactic longitude. Galactic Longitude (o ) 0-5 5 - 10 10 - 15 15 - 20 20 - 30 30 - 40 40 - 60 60 - 90 DGL/LOS* S10 0.21 ± 0.05 0.34 ± 0.07 0.31 ± 0.03 0.19 ± 0.04 0.25 ± 0.04 0.17 ± 0.04 0.17 ± 0.02 0.12 ± 0.02

Table 1: Ratio of DGL to LOS* from [3] for 440nm based on Toller's data ([9]). 2


2.1

Background Intensity

The DGL background Intensity (Idg ) was calculated using background starlight at 440nm (from [3] Table 35), and the DGL/LOS* ratio in units of S101 from Table 1. The final DGL intensity map (after correcting for DGL/LOS* ratio) can b e seen in Figure 1.

dgl_map.fits_0
galactic latitude / [deg]

50

Galactic Latitude ( o)

0

-50

0

100 200 Galactic Longitude ( ) galactic longitude / o [deg]

300

50

(counts)
(counts)

100

Figure 1: Final DGL intensity map.

dgl_map.fits_0 Colorbar

2.2

Throughput of filter

To find the throughput (ftp ) of each OM filter, the sp ectrum for DGL needs to b e considered. This sp ectrum was constructed using data from [7], [2] and [8], and has a reference intensity of 43 S10. Using data from [7], wavelengths from 155 - 435nm were considered. The flux of these values was calculated by multiplying the DGL intensity ([7], Table 2 units of S10) values by · a0 sp ectrum (interp olated at the same wavelengths) · 1 в 10
1

-4

(to scale by 10 magnitudes)

S10 is units of a standard star with a visual magnitude of 10.

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Data from [2] at wavelengths 92.5 - 147.5nm, and [8] at wavelengths 450 - 1000nm, were scaled to match the values of [7]. The resulting sp ectrum can b e seen in Figure 2.

Figure 2: Basic sp ectrum of galactic light constructed from the data of [7], [2] and [8]. This DGL sp ectrum was then run through the IDLSimulator to calculate the throughput for each OM filter. The final throughput results can b e seen in Table 2. Filter V U B White UVW1 UVM2 UVW2 Diffuse Galactic (count.s-1 arcsec-2 ) 0.0127473 0.00819123 0.0214153 0.069187 0.00258478 0.000496318 0.000204369

Table 2: Filter throughput for diffuse galactic sp ectrum of reference intensity 43 S10.

2.3

Intensity of reference

Intensity of reference sp ectrum is 43 S10.

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2.4

Results

The total background intensity of DGL can b e found for a given galactic longitude and latitude in an OM filter, using Equation 3 derived from Equation 2 B
dg

=

Idg ftp , Iref

(3)

where Idg is the intensity of DGL background given by the intensity map in Figure 1, ftp is the filter throughput for the DGL background given in Table 2, and Iref is 43. Results of the minimum and maximum count rates can b e seen in Table 3, where the minimum DGL is found using the UVW2, and the maximum using the White filter. Bdg longitude (count.s-1 arcsec-2 ) (o ) UVW2 7.985 в 10-6 10 white 0.214 290 Table 3: Results of the DGL background. filter latitude (o ) 90 0

minimum maximum

3

Zodiacal Light

Zo diacal light (Bz l ) is due to sunlight scattered by interplanetary dust particles, and is seen at ultraviolet, optical and near infrared wavelengths. Its brightness is a function of wavelength, helio centric distance, and p osition of the observer relative to symmetry plane of interplanetary dust. In general zo diacal light (ZL) is smo othly distributed with small scale structures app earing only at the level of few %; its brightness do es not vary with the solar cycle to within 1% ([1] and [4]).

3.1

Background Intensity

The ZL background Intensity (Iz l ) was pro duced using the smo othed brightness values from [6] (Table 2 - in units of S10) for eliptic longitudes of 50 - 130o . The ZL intensity map can b e seen in Figure 3.

3.2

Throughput of filter

To find the throughput (ftp ) of each OM filter, the sp ectrum for ZL needs to b e considered. The sp ectrum used in this case was a G2 typ e star (as the Sun is a G2V star). The sp ectrum used can b e seen in Figure 4.

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zodiacal_map.fits_0
ecliptic latitude

50

Eliptic Latitude ( o)

0

-50

60 80 100 120 Eliptic Longitude ecliptic longitude ( o)

100

200

300 400 (counts) (counts)

500

zodiacal_map.fits_0 Colorbar
Figure 3: ZL intensity map. NOTE - when using the G2 sp ectrum provided in the IDLSimulator, a scaling of the sp ectrum of 0.01 is required to obtain the intensity of 100 S10 (i.e. 1 в 10-4 * 100), as the visual magnitude of the G2 sp ectrum is 0 . This sp ectrum was then run through the IDLSimulatior to calculate the throughput for each OM filter, with a reference intensity of 100 S10. The final throughput results can b e seen in Table 4. 6


Figure 4: Corrected sp ectrum of G2 typ e star used for ZL. Filter V U B White UVW1 UVM2 UVW2 Zo diacal Light (count.s-1 arcsec-2 ) 0.0115465 0.00737317 0.0198009 0.057095 0.00139419 0.0000425153 0.00000864731

Table 4: Filter throughput for Zo diacal Light sp ectrum of reference intensity 100 S10.

3.3

Intensity of reference

Intensity of reference sp ectrum is 100 S10.

3.4

Results

The total background intensity of ZL can b e found for a given eliptic longitude and latitude in an OM filter, using Equation 4 derived from Equation 2 Bz l = Iz l ftp , Iref (4)

where Iz l is the intensity of ZL background given by the intensity map in Figure 3, ftp is the filter throughput for the DGL background given in Table 4, and Iref is 100. Results 7


of the minimum and maximum count rates can b e seen in Table 3, where the minimum ZL is found using the UVW2, and the maximum using the White filter. Filter min max UVW2 White background longitude (count s-1 arcsec-2 ) (o ) 5.275 в 10-6 105 0.169 70 Table 5: Longitude latitude (o ) 70 0

4

Average Dark Count rate

The mean OM detector dark count rate (Bad ) is 2.56 в 10-4 count.s-1 pixel-1 . Converting this into arcsec2 , pro duces a value of 0.001024 count.s-1 arcsec-2 .

References
[1] Dumont, R., & Levasseur-Regourd, A. C., 1978, Astr. & Astrophys., 64, 9. [2] Gondhalekar, P. M., & Wilson, R., 1975, Astr. & Astrophys., 38, 329. [3] Leinert, C. et al., 1998, Astr. & Astrophys. Suppl., 127, 1. [4] Leinert, C., & Pitts, E., 1989, Astr. & Astrophys., 210, 399. [5] Leinert, C., Richter, I., Pitz, E., & Hanner, M., 1982, Astr. & Astrophys., 110, 355. [6] Levasseur-Regourd, A. C., & Dumont, R., 1980, Astr. & Astrophys., 84, 277. [7] Lillie, C. F., & Witt, A. N.,1976, Astrophys. J., 208, 64. [8] Mattila, K., 1980, Astr. & Astrophys., 82, 373. [9] Toller, G. N., 1981, Ph.D. Thesis, State University of New York at Stony Bro ok.

2

1pixel = 0.5в 0.5 arcsec = 0.25 arcsec

8