Astron. Astrophys. 360, 447-456 (2000)

2. Observations and data reduction

In this paper we present data from the High Resolution Imager (HRI) and the Position Sensitive Proportional Counter (PSPC) on board of the X-ray satellite ROSAT. This X-ray telescope was operating in the energy range of 0.1-2.4 keV. For details concerning ROSAT and its detectors see the ROSAT User's Handbook (Briel et al. 1996).

2.1. HRI

We proposed HRI pointed observations of NGC 4303 (ROSAT sequence number 600854, PI: N. Junkes), which were carried out in the time periods July 2-8, 1996, June 9-23, 1997, and January 6-7, 1998 with integration times of 16055 sec, 27538 sec, and 5600 sec, respectively (see Table 2). All three observations are centered on RA(2000) = ; Dec(2000) = .

The three photon lists were combined to create one single image. The data were analyzed using the commercial interactive data language software package IDL (Interactive Data Language). All presented images contain pixel values with units of counts per pixel and second. The images were corrected for vignetting but not for background. To quantify any source counts three circular source-free fields with radii of 49", 34", and 39", respectively, were selected to determine the background level (Table 2). The averaged background flux amounts to 3.5210 cts s^-1 arcsec^-2.

Table 1. Some basic parameters of NGC 4303.
Notes:
a) References: 1) de Vaucouleurs et al. (1991)
2) NASA/IPAC Extragalactic Database
3) Ferrarese et al. (1996)
4) Tully (1988)
5) Guharthakurta et al. (1988)
b) Corrected for a distance of 16.1 Mpc.

Table 2. Date, integration time, and source and background count rates of the HRI and PSPC observations.
Notes:
a) Combined HRI observations

Fig. 1 shows the HRI field of view (FOV) of the three combined data sets, convolved with a Gaussian of 5" FWHM. For reasons of identification the 5 sources detected with a maximum likelihood method are numbered in Fig. 1 and listed in Table 3. Several single source detections (no. 9-13) coincide with optical peaks in the galaxy. Source no. 18 can be identified as the QSO 1219+044, which served as the central source of the PSPC pointed observation used for spectral analysis in Sect. 2.2. Source no. 7 coincides spatially with the QSO 1219+047 (Bowen et al. 1996). It turned out that the source is variable the X-rays. Its flux increased by a factor of 3 between the second and third HRI observation (June 1997 and January 1998).

Fig. 1. Total field of view of the combined ROSAT HRI observation of NGC 4303. The detected 5 sources are labeled with numbers. The sources in the center (nos. 9-13) correspond to NGC 4303.

Table 3. EXSAS source detection of X-ray sources in the HRI coinciding with the optical image of NGC 4303.

2.2. PSPC

Since there exist no particular PSPC observations pointed on NGC 4303, we use ROSAT observations from the archive (ROSAT sequence number 701095, PI: R. Staubert). NGC 4303 lies in the FOV at an off-axis distance of 17´ which is still within the inner part of the supporting ring structure of the telescope. The position of the X-ray counterpart of NGC 4303 is given in Fig. 2. The central source is the QSO 1219+044 (no. 18 in Fig. 1). The image has been convolved with a Gaussian of 25" FWHM. Due to strong asymmetry and broadening of the point spread function (PSF) with radial distance from the optical axis (25" FWHM for an on-axis point source at 1 keV; 67" FWHM for a 17´ off-axis point source at 1 keV; Hasinger et al. 1994) we cannot get any useful spatial information from the PSPC data.

Fig. 2. Total field of view of the ROSAT PSPC observation around the QSO 1219+044. The position of NGC 4303 at an off-axis distance of 17´ is marked.

This PSPC exposure was carried out between December 24 and 26, 1992, with an integration time of 8135 sec. The background flux was determined from three circular source-free areas in the field with radii of 83", 88", and 75" and amounts to 5.1410 cts s^-1 arcsec^-2. To analyze the PSPC data we used IDL and the software package XSPEC (Arnaud 1996) for interactively fitting X-ray spectra.

For NGC 4303, we adopt the same distance as of M100, the brightest spiral in the Virgo Cluster (16.1 Mpc; Ferrarese et al. 1996). Then the resolution of the ROSAT detectors of 5" full width at half maximum (FWHM) for the HRI and 67" FWHM for the PSPC at an off-axis distance of 17´ scale to 390 pc and 5.3 kpc, respectively.

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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