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Astron. Astrophys. 353, 129-134 (2000)

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2. Observations and data analysis

2.1. X-ray data

The ROSAT archive contains several pointed PSPC and HRI observations centered close to the field of AX J0105-722. More details on the ROSAT mission can be found in Tr"umper (1982). The two nearest ROSAT PSPC (energy range 0.1 - 2.4 keV) observations which have best angular resolution were used in this study. For more details of the PSPC observations see Table 2. In Fig. 1 we show the ROSAT PSPC image ("hard" energy range 0.5 - 2.4 keV) of the AX J0105-722 region. It was obtained from combining the two PSPC observations, binned to 5" pixels and smoothed for better representation. Several ROSAT HRI observations were performed within the field of AX J0105-722 but larger offsets from the source and lower exposure times did not allow deeper quantitative study than was obtained from the PSPC observations.

[FIGURE] Fig. 1. ROSAT PSPC (energy range 0.5 - 2.4 keV) image (gray-scale) of AX J0105-722. Contours represent the radio continuum image at 843 MHz. The synthesized beam of the MOST observations is 45" [FORMULA] 45" (upper right corner) with r.m.s. noise (1[FORMULA]) of 0.4 mJy. Contours are 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 4, 5, 7, 10, 12 and 15 mJy


Table 2. Summary of new X-ray and radio-continuum observations of the region of AX J0105-722.

The ROSAT PSPC catalogue of the SMC of Haberl et al. (1999) contains six sources in the AX J0105-722 region which are marked in Fig. 1 and listed in Table 3 with properties taken from the catalogue. Source positions, RA and Dec, are given in J2000 coordinates in Column 2 and Column 3, respectively. The statistical (90%) positional error is given in Column 4 (a systematic error of 7" should be added). Count rate (0.1 - 2.4 keV, Column 5), source extent (Column 8), hardness ratios (HR1) and (HR2) (Columns 6 and 7, respectively) are extracted from Haberl et al. (1999). HR1 is defined as HR1=(hard-soft)/(hard+soft) and HR2 is defined as HR2=(hard2-hard1)/(hard2+hard1) where soft, hard, hard1 and hard2 denote the PSPC count rates in the energy bands 0.1 - 0.4 keV, 0.5 - 2.0 keV, 0.5 - 0.9 keV and 0.9 - 2.0 keV.


Table 3. Measured ROSAT PSPC properties of objects near AX J0105-722. Suggested source types are discussed in the text (Sect. 3); BG correspond to background object, Be/X to Be/X-ray binary and SNR to supernova remnant.

2.2. Radio-continuum data

The region around AX J0105-722 was observed as part of the ATCA mosaic observations of the SMC with a baseline of 375 m at frequencies of 1.42 and 2.37 GHz with corresponding angular resolution of [FORMULA] and 45" (Table 2). More information about these observations can be found in Staveley-Smith et al. (1997) and Filipovi & Staveley-Smith (1998; hereafter FS98). The MOST data used in this study were part of the SMC survey at 843 MHz (Turtle et al. 1998). Amy & Ball (1993) studied the SMC SNR 1E 0102.2-7219 which is [FORMULA]10´ north-west from the region AX J0105-722. We used a subset of their observations and more details about the observing procedure can be found in their paper. Radio contours of these observations are shown in Figs. 1 and 2.

[FIGURE] Fig. 2. Map of the [S II ]/H[FORMULA] ratios overlaid with contours of the MOST 843 MHz image of AX J0105-722 as in Fig. 1. The darker areas indicate [S II ]/H[FORMULA]0.5 (shocked gas), while the lighter areas indicate ratios [FORMULA]0.5 (photo-ionized gas)

We list in Table 4, for each detected radio object, positions (RA and Dec) with positional errors ([FORMULA]), integrated flux densities at the given frequency, the spectral index and suggested source type. The position and flux density for each source at a given radio frequency were determined using the two-dimensional elliptical Gaussian-fitting algorithms in the MIRIAD software package (Sault & Killeen 1999). The radio emission from DEM S128 is extended (Fig. 1) and we fitted one source in that area (J0105.4-7209) with a radius of 90". That includes the bright knot at RA(J2000)=01h05m30s and Dec(J2000)= -72o 10´ 27" which could be a contaminating background object. Estimates of the spectral index ([FORMULA]) of each radio feature/source are based on flux densities obtained from corresponding radio-frequencies. The spectral index [FORMULA] is defined by the relation [FORMULA], where [FORMULA] is the integrated flux density and [FORMULA] is frequency. The errors in spectral index ([FORMULA]) have been deduced given the scatter in flux density. In Fig. 3, we plot the spectral indices of the radio sources listed in Table 4.

[FIGURE] Fig. 3. Radio spectral index plots of the radio-continuum sources found within the area of AX 0105-722. The steep spectrum of SNR DEM S128 ([FORMULA]=-0.48[FORMULA]0.06) is distinctively different from those of the other three radio-continuum objects which are likely background objects (AGNs, quasars or radio-galaxies)


Table 4. Measured radio-continuum properties of objects within AX J0105-722. Listed positions (RA and Dec) and positional error [FORMULA] are from 843 MHz observations. Source types are discussed in the text (Sect. 3); BG correspond to background object and SNR to supernova remnant.

2.3. Optical data

Optical images of this part of the SMC (field centre RA(J2000)=00h43m30s and Dec(J2000)= -72o 45´) were obtained on 26 July 1998 using the STIS CCD camera (2048 [FORMULA] 2048 pixels; FOV 1.3o) on the 0.6/0.9 m Curtis Schmidt Telescope at CTIO. The two filters used were H[FORMULA] ([FORMULA] nm [FORMULA]=6.8 nm) and [S II ] ([FORMULA]=674.4 nm [FORMULA]=5.0 nm) and the exposure times were 600 sec. Also, a red continuum filter ([FORMULA] nm [FORMULA]=9.5 nm) was used to obtain images of the continuum background (mostly stars). The 2.3" pixels, though large for point sources, are much smaller than the structures we are studying here.

Optical U and B band images of the investigated area were obtained from ESO Schmidt plates digitized at the Centre de Donnees astronomiques de Strasbourg (CDS) using the Aladin service which provides data produced by the CAI-MAMA facility. In Fig. 4 the images are overlaid with X-ray contours. The brightest objects in the X-ray error circle of RX J0105.1-7211 are a pair of stars [FORMULA]10" away from the X-ray position.

[FIGURE] Fig. 4. Optical images (gray-scale) of the SMC region AX J0105-722 in the B-band (left) and U-band (right) obtained from ESO Schmidt plates. Contours represent the ROSAT PSPC image of Fig. 1. Contours are: 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4 and 1.7 cts ksec-1

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Online publication: December 8, 1999