Astron. Astrophys. 334, L13-L16 (1998)

4. Discussion

There is a bright variable source in LH with [1.2 .2] and [2.5 .7] for the 0.7-2 keV and 2-7 keV bands respectively (Ogasaka 1997), consisting about 10% of the total ASCA fluxes in both bands. This source was much fainter during the PSPC observation. Thus one should decrease the GIS and SIS normalizations about lower when comparing with the PSPC data. In this case, the agreement between PSPC and GIS is excellent and falls well within statistical errors of each other. For both LH and LX, the SIS data consistently show lower normalizations compared to GIS. This might be caused by incomplete calibration for the radiation damage of the SIS with the 4CCD mode, which can even exist at this level after a few months after the launch, when LH and LX were observed (Dotani et al. 1995).

In the LX observation, a larger discrepancy exists. The GIS and SIS normalizations are lower than the PSPC value by and respectively and slopes are shallower. The ASCA LX normalizations are also significantly lower than those of LH. There is no variable source which can cause this amount of discrepancy in LX. The fact that the 0.1-10 keV fit still show the disagreement of the normalization (see in B2) shows that this is not a modeling problem (e.g. leak of the keV excess with the PSPC energy resolution). One possible explanation is the LTE (e.g Snowden et al. 1994), which is usually apparent in the keV channels of the PSPC data, but sometimes extends above 1 keV when the activity is high. There may also be an over-subtraction of the NXB background from the ASCA data. Furthermore, the instruments are not looking at exactly the same part of the sky. Due to the stray-light and PSF of the ASCA instruments, of the GIS/SIS flux comes from outside of the designated FOV (estimated using our ray-tracing program). These effect may also contribute to this discrepancy.

The best consistency for a certain region of the sky from this work is seen for the PSPC and GIS data on LH. Thus it is instructive to compare the observed spectra of these with previous CXRB measurements. The comparison is shown in Fig. 3. Fig. 3 shows a large excess at keV on the PSPC data, inconsistent with Gendreau et al.'s ASCA SIS data. This may be partially due to the low Galactic column density of LH. The LH GIS data for keV are above the HEAO-1 A2 and Gendreau et al. (1995) SIS values. We note, however, that about 10% of source fluctuation is expected over this small area. Since the ASCA LH data contains a bright source ( in 2-10 keV), this field should be one of the brighter ones. We also note, however, that an integration from the brightest source in the field to the faintest source excluded in the collimator experiments (e.g. in 2-10 keV for HEAO-1 A2 measurement by Marshall et al. 1980) would add of intensity. A thorough treatment of source fluctuations using a larger area and comparing spectra with appropriate source removal will be presented in a future paper.

Fig. 3. The PSPC and GIS spectra (using a two power-law model as an appropriate smooth function for unfolding purpose) of LH are shown and compared with previous measurements: the thick solid bowtie is from Hasinger (1992); the dot-dashed bowtie from Georgantopoulos et al. (1996), both used ROSAT PSPC. The dotted line is from rocket measurements (McCammon & Sanders 1990). The long-dashed horn is from an ASCA SIS measurement by Gendreau et al. 1995and the thin solid bowtie is a joint ROSAT PSPC/ASCA SIS analysis of QSF3 by Chen et al. (1997) for keV. The thick solid line represents the HEAO-1 A2 measurement by Marshall et al. (1980)

In summary, a close look at ROSAT and ASCA spectra for the same regions of the sky have revealed systematic errors caused by response calibration problems and non cosmic background subtraction of up to for one set of observations. These probably caused the reported disagreements between ASCA and ROSAT measurements (Hasinger 1996), while modelings and sky selection can also contribute. We have obtained a fair description of the CXRB spectrum over 0.1-10 keV range cosisting of a extragalactic power-law component (either single or broken below 1 keV), hard and soft thermal components with a satisfactory fit to all instruments.

© European Southern Observatory (ESO) 1998

Online publication: May 12, 1998

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