2. Observations and data analysis
The BeppoSAX satellite (Boella et al. 1997a) carries a complement of several imaging and non-imaging X-ray detectors, covering a broad energy range from 0.1 keV to 300 keV. Only the Medium and Low Energy Concentrator Spectrometers instruments are relevant for the results reported here. Both consist of position-sensitive gas-scintillation proportional counters, placed in the focal plane of the four coaligned grazing incidence X-ray mirrors carried by BeppoSAX. The Medium-Energy Concentrator Spectrometer (MECS, Boella et al. 1997b) consists of three identical units covering the nominal energy range 1.3-10 keV. Its field of view has a diameter of . The Low Energy Concentrator Spectrometer (LECS, Parmar et al. 1997) has a thinner window that allows to extend the low energy response to the range 0.1-10 keV. Both instruments have FWHM energy resolution of %, where E is the energy in keV.
The location of G0.9+0.1 was imaged during a BeppoSAX observation pointed on the SgrB2 molecular cloud. The observation was performed on April 5-6, 1997, and yielded net exposure times of 47 ks for each of the 3 MECS units and 19 ks for the LECS. Though G0.9+0.1 lies 14 arcmin off-axis, it was clearly detected in the MECS, with a total count rate of 0.023 0.002 counts s-1. The centroid of the X-ray emission (, J2000) is consistent with the position of the radio core of G0.9+0.1 (Helfand & Becker 1987).
The on-axis angular resolution of the MECS is (FWHM), but it degrades to at the source off-axis location. Since the low statistics hampers a more detailed investigation of the source radial profile, to check whether the observed X-ray emission comes from an extended source, we performed the following analysis. The ratio of the net counts within a radius to those within a corona from to centered on the source position was compared to that expected from a point source. The reference data were obtained from ground based calibrations and from a ks exposure of Cyg X-1 at the same off-axis angle. The comparison was done for different energy bands and different values of and , always giving values compatible with emission from an unresolved source.
The MECS counts used in the spectral analysis were extracted from circular regions of radius, and the 256 original channels were grouped in order to have at least 20 net counts in each energy bin. The background spectrum was derived from a source free region of the same observation.
The spectrum is well fit by a simple power-law model, giving a reduced (42 d.o.f.) for photon index , cm-2, and flux erg cm-2 s-1 (1-10 keV, corrected for the absorption). Acceptable fits were also obtained with blackbody and thermal bremsstrahlung spectra (see Table 1), while a Raymond-Smith thermal plasma model with abundances fixed at the solar values gave a worse result. There is no evidence for lines in the X-ray spectrum. In particular, we can set an upper limit of 400 eV (90% confidence level) on the equivalent width of Fe K lines. The luminosity corresponding to the best fit power-law spectrum is erg s-1 in the 1-10 keV range, but note that some uncertainty on this value is introduced by the relatively poorly constrained spectral parameters (see Table 1). This is particularly evident for the power-law case ([ ] erg s-1), while the luminosity uncertainty is smaller for the other models.
Table 1. Results of the Spectral Fits (errors are 90% c.l.).
G0.9+0.1 was also detected in the LECS instrument, but only above 2 keV, due to the high interstellar absorption. We verified that the inclusion of the LECS counts in the spectral fits does not significantly change the results described above.
The same counts extracted for the spectral study were used for the timing analysis, after the correction of their times of arrival to the solar system barycenter. No flux variations were seen during the BeppoSAX observation. A search for pulsations for periods in the range 8 ms to 2048 s gave a negative result. With the hypothesis of a sinusoidal modulation, we can set the following upper limits on the pulsed fraction: 53% for 8 P 16 ms, 38% for 16 P 32 ms, and 33% for P 32 ms.
© European Southern Observatory (ESO) 1998
Online publication: March 3, 1998