In Fig. 3 (click here) we show the effective area of the BeppoSAX instruments. Note that the effective area of the NFI is increasing with energy following a power law with positive slope. This will partially compensate for the spectral shape of the celestial objects, though not quite completely, because the HPGSPC and PDS are generally background dominated whereas MECS and LECS are source dominated (Fig. 4 (click here)). A relevant property of the BeppoSAX scientific payload is the broad overlapping energy response of the various instruments, that will allow cross-calibration of the instruments.
Figure 3: Effective area of BeppoSAX instruments
Figure 4: Five sigma BeppoSAX NFI instrument sensitivity for a 100 ksec exposure. For the HPGSPC and PDS it was assumed to point for half of the time off-source to measure the background. Spectra of representative galactic and extragalactic sources are shown
The four X-ray optics with GSPC's detector in the focal plane have been designed to deliver:
- an effective area around 7 keV sufficient to detect in an iron line with an equivalent width of 150/ eV at from a source with erg , down to , when the continuum level at 6.4 keV becomes 2 times the background level.
- an energy resolution of 8% @ 6 keV and 25% @ 0.6 keV, that below 0.6 keV becomes comparable with that of a CCD (e.g. Tanaka et al. 1994);
- spectral coverage below the carbon edge (0.3 keV), where the LECS detector will provide 2-3 independent energy bins;
- reasonable imaging capabilities (1.5 arcmin @ 6 keV). Combined with their energy resolution, they will perform spatially resolved spectroscopy of extended sources such as clusters of galaxies and supernova remnants. Furthermore they will do spectral measurements on weak point-like sources ( ), thanks to the low background included in the small size of the source spot.
In Fig. 5 (click here) we show the capability of the combined LECS and MECS to perform spectral measurements of thin plasma spectra. Taking as reference a 30% error on the relevant parameters we see that the temperature can be determined down to , the iron abundance down to and the abundance of elements other than Hydrogen and Iron down to .
In case of sources with power law spectra, such as AGN, spectral index measurements can be carried out down to fluxes , as shown in Fig. 6 (click here). The typical spectrum that could be obtained for an AGN with in is shown in Fig. 7 (click here).
An example of the capability of studying narrow features is shown in Fig. 8 (click here), where an OVII edge at 0.8 keV and a broad iron line at 6.4 keV (FWHM = 0.7 keV) in a typical Seyfert 1 galaxy are well resolved by the LECS and MECS respectively.
Figure 5: Capability of LECS and MECS in measuring spectral parameters of thin plasma spectra. The relative error on the temperature (continuous lines), iron abundance (dotted lines) and lighter elements abundance (dashed lines) are shown as a function of the source flux for exposure times of (thin lines) and (thick lines)
Figure 6: Spectral index determination by the combined NFI. The relative error on the spectral index is given as a function of the flux for three values of the index ( (continuous line), (dotted line), (dashed line)) with and two values of integration time (: thin lines, : thick lines). Below the performances are primarily due to the LECS and MECS
Figure 7: Simulation of a spectrum of an AGN with observed by the LECS and MECS in
Figure 8: Simulation of an observation by BeppoSAX-NFI (40000 s) of a Seyfert 1 galaxy (MCG-6-30-15) with several of the features observed in the last 10 years by different satellites: a soft excess below 1 keV (EXOSAT), an edge of ionized oxygen at 0.8 keV (ROSAT, ASCA), a broad iron line ( 0.7 keV) at 6.4 keV and a high energy bump bewtween 10 and 200 keV (GINGA). The spectrum is fitted with a simple power law and the residuals clearly show all these components. In the blow-up the residuals of the MECS around the broad iron line are plotted, showing that the line is well resolved by the detector
The high energy instruments aboard BeppoSAX are respectively a high pressure (5 atm. Xe) GSPC and a scintillator (NaI/CsI) phoswich detector system. Several solutions have been implemented to minimize systematic effects and the background in these collimated detectors:
- the background is monitored continuosly rocking the collimators on and off source with a typical period of one minute;
- the environmental background due to high energy particles is much lower than that of other current and near future missions, thanks to the low inclination orbit;
- the HPGSPC will be the first detector ever flown on a satellite to implement the technique of the fluorescence gate (Manzo et al. 1980) that will allow to decrease the background substantially above the Xenon edge (34 keV);
- both detectors have an active equalization system that will keep the gain within 0.5-0.25%.
The PDS was specifically designed in order to minimize background so as to increase its sensitivity to background dominated sources (basically below 200 mCrab). From this point of view the expected performance should be comparable or better than that of the HEXTE detector aboard XTE (Bradt et al. 1993). XTE, on the other hand, remains superior for bright sources, given its larger area aimed to priviledge the timing information. The PDS sensitivity allows spectral measurements up to 200 keV for sources of about 1 mCrab ( , Fig. 4 (click here)).
The main scientific motivation that lead to the design of the HPGSPC and its inclusion in the payload is its unprecedented energy resolution, 4% @60 keV, that will allow detailed line spectroscopy in hard X-rays. The spectroscopic capabilities of HPGSPC and PDS are illustrated in Fig. 9 (click here), that represents a simulation of a possible cyclotron absorption spectrum of a bright transient, , a X-ray pulsar. GINGA observed only the first harmonic and tentatively the second one (Makishima et al. 1990). The BeppoSAX high energy instruments can measure very well not only the first harmonic but also determine the possible presence of higher harmonics in a spectral range where the information is - up to now - missing. Pulse phase dependent spectroscopy will be easily achieved for such sources as well.
Figure 9: Cyclotron lines in the transient . The simulation shows a possible spectrum (four harmonics) of the object observed by the HPGSPC (crosses) and the PDS (circles) in 10 000 s. It clearly illustrates the capability of BeppoSAX of measuring such features also in the unexplored region above 40 keV
The sensitivity curves of BeppoSAX NFI are given in Fig. 4 (click here) along with spectra of some typical galactic and extragalactic sources. Basically for AGN-like spectra it is possible to determine the spectrum up to 200 keV for sources down to about 1 mCrab. As an example in Fig. 8 (click here) we show the simulation of the combined BeppoSAX-NFI spectrum of a Seyfert 1 galaxy, namely MCG-6-30-15, with all the spectral component and features detected by several satellites in the past. Starting from the low energy part there is: a soft excess observed by EXOSAT (Pounds et al. 1986), an OVII edge around 0.8 keV observed by ROSAT (Nandra & Pounds 1992) and ASCA (Fabian et al. 1994), an iron line at 6.4 keV and a high energy bump above 10 keV detected by GINGA (Matsuoka et al. 1990). All these components can be measured with good accuracy with BeppoSAX in a single shot for the first time.
The sensitivity of the WFC depends on the pointing direction in the sky, because each source in the field of view contributes to the overall background. Towards high-galactic latitude the Cosmic Diffuse X-Ray Background is the main contributor to the background. In this case the sensitivity is at the order of a few mCrab in (Fig. 10 (click here)). This will allow the monitoring of faint sources like AGN (Fig. 11 (click here)), as well as their prime objective, which is the survey of the galactic plane and the search of X-ray transients for follow-up studies with the narrow field instruments.
On the basis of the distribution of gamma-ray burst and assuming about 1/100 of we expect to detect a few X-ray counterparts to gamma-ray bursts per year, thus positioning the events within about 3' and gathering broad band information with the simultaneous observation of the gamma-ray burst monitor.
Figure 10: Five sigma sensitivity of one WFC
Figure 11: The spectrum of a AGN (2.5 mCrab) observed by one WFC in 40000 s