The table gives for all SAX NFI instruments the intensity of the background, as measured in orbit during the SVP, in given regions and/or energy ranges. For the MECS and the PDS we give the total background intensity relative to the three and four units respectively. Fot THE HPHSPC we quote the results for the off- collimator position (corresponding to -210' from the nominal pointing), which yields the most stable background estimation.
arcmin-2 s-1 keV-1 arcmin-2 s-1
10-6 10-5
MECS internalcentral 10' 1-11 keV 6.7 6.7
MECS internalouter 18'-25'1-11 keV 5.8 5.8
MECS internalcentral 10' 1.5-5.75 keV 5.1 2.8
MECS internalcentral 10' 5.75-6.25 keV 15.2 1.31
MECS internalcentral 10' 6.25-11keV 7.3 2.77
MECS internalouter 18'-25'1-5.75 keV 3.1 1.7
MECS internalouter 18'-25'5.75-6.25 keV 27.1 2.3
MECS internalouter 18'-25'6.25-11 keV 5.1 1.9
MECS tot whole det. 1-11 keV 16.4 16.4
MECS tot central 10' 1-11 keV 16.4 16.4
LECS tot. whole det. 0.1-10 keV 15.815.8
LECS tot. central 10'0.1-10 keV 9.79.7
LECS tot. central 10'0.1-2 keV 17.23.5
LECS tot. central 10'2-10 keV 8.36.6
Energy rangeRiseTime ch. range counts s-1
PDS tot.4 units12-300 keV3-150 34.4
PDS tot.4 units12-30 keV3-150 7.6
PDS tot.4 units30-100 keV3-150 13.5
PDS tot.4 units12-200 keV3-150 29.2
HPGSPC tot. 4-100 keV85-115 110.7
HPGSPC tot. 7-34 keV85-115 52.1
HPGSPC tot. 34-60 keV85-115 19.6

For a comparison, the internal background in the 1-10 keV band in the ASCA GIS is ~8 × 10-5 counts s-1 arcmin-2 (both units), and the background in the 15-250 keV band in the XTE HEXTE is 50-100 cts/s per cluster of four units.

The MECS internal background is slightly non-uniform on the FOV. This is probably due to re-absorption, far from the primary interaction point, of fluorescence emitted from the 55Fe calibration sources. This results in a background enhancement near the position of the calibration sources.

Dark Earth background events have been accumulated during the SVP observations to quantify this effect. The following figures show the Dark Earth background maps (three detectors added togetherin different energy bands:

MECS background 1.5-5.5 keV
MECS background 5.5-6.5 keV
MECS background 6.5-11 keV

From the analysis of these maps is clear that the contribution of the fluorescence photons of the calibration sources is non-negligible at the center on the detector too. The three MECS spectrum of the Dark Earth background accumulated on the central 10 arcmin shown below is in fact dominated by a line feature at about 6 keV, due to this effect.

LECS internal background is more difficult to characterize because the LECS was operated during occultations for a short time. The following maps show the total (internal+cosmic) LECS background:

LECS background 0.1-2 keV and 2-10 keV

LECS and MECS background event files, obtained by adding together several long observations of empty fields, are available from our anonymous ftp.

The high energy instruments, PDS and HPGSPC, background is slightly variable depending on the orbit position and on the dept of the passage of the satellite in the South Atlantic Anomaly. This modulation is small, up to about 20 % of the total background rate for the PDS.

For the PDS, the level and shape of the background accumulated in the on source and off source positions is very similar. The systematics associated with the background subtraction procedure are as low as 0.02 cts/s in the 20-200 keV, corresponding to few tenths of mCrab for moderately steep sources (i.e. photon spectral index 1.5-2). An SDC technical report (SDC-TR014) with the most up-to-dated results on the PDS sensitivity is available on our anonymous ftp site ("pub/sax/doc/reports/").

On the other hand, a large part of the HPGSPC background is due to photons emitted by a calibration source, located on the top of the collimator in a central position. The figure shows the spectrum of the HPGSPC background.

Background events are discriminated looking at their position (a central region is excluded from the analysis). However, when the collimators are inclined, the calibration source illuminates not only the central region but also part of the source detection region. This means that the background accumulated when the collimators are in a offset position is higher (mostly in the calibration emission lines), than the background accumulated when the collimators are on axis. The figure shows the difference spectrum background between the "on source" and the "offset" collimator positions.

The level of the difference is not constant with time, because the intensity of the radiactive calibration source decreases esponentially with the time.

To account for this effect The HPGSPC team generated a series of difference background spectra, using on source and off source observations of empty fields. Interpolating on these difference spectra at the time of the observation one can obtain the right difference spectra to be used to correct on source minus off source spectrum. However caution must be employed when analyzing HPGSPC observation of few mCrab (i.e. S/N~1%) sources. Such an algorithm is still under refinement and systematics cannot be excluded depending on the nature of the spectrum in the HPGSPC band.

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