2 ROSAT HRI data

2.1 Source detection

In a 5$^\circ$ $\times$ 5$^\circ$ field around RA = 01$^{\rm h}$ 00$^{\rm m}$ 00$^{\rm s}$, Dec = $-73\hbox{$^\circ$ }\ 00\hbox{$^\prime$ }\ 00\hbox{$^{\prime\prime}$ }$ (J2000.0) including the whole SMC a total of 71 pointings were performed by the ROSAT HRI (David et al. 1996) between 1991 and 1998. The total exposure time is shown in Fig. 1 as contours plotted over a grey scale PSPC image (HFPK00). The number of the analyzed HRI pointings with integration times higher than 100 seconds is demonstrated in Fig. 2.

Figure 1: Contours of total exposure times of the HRI pointings plotted on a grey scale PSPC image (0.1 - 2.4 keV). The contours signify regions of exposure times with 1, 20, 40, 60, 80, and 100 ksec

For data analysis and correction of X-ray positions the same procedures as developed for the LMC (see SHP00) were applied to the SMC data using the EXSAS software package (Zimmermann et al. 1994). For each pointing sources were detected in two sliding window methods using the local background and a spline fitted background map. Both detection lists served as input for the following maximum likelihood method. Detections with existence likelihood higher than 10.0 and telescope off-axis angle smaller than 15$^\prime$ were accepted as X-ray sources. In addition pointings with directions within a radius of 1$^\prime$ were merged in order to increase the significance of source signals. For 18 regions in the SMC new deeper data could be created. Further faint sources were found by the source detection procedure in the merged data and augmented the number of detected HRI sources. Finally the source lists from the single and coadded pointings were merged. Multiple detections of one source in different pointings were reduced to one catalogue entry choosing the detection with the highest positional accuracy. In the end we obtained a HRI source catalogue with 121 sources in the SMC region.

This source catalogue was cross-correlated with the PSPC catalogue (HFPK00). We looked for coincidences with objects in the SIMBAD data base operated at the Centre de Données astronomiques de Strasbourg, the TYCHO catalogue from the ESA Hipparcos space astrometry satellite (Hoeg et al. 1997), and the point source catalogue from the Deep Near Infrared Survey of the Southern Sky (DENIS, Cioni et al. 2000, hereafter CLH00) in order to identify X-ray sources. The sources were also compared to objects known from literature like those published by KPFH99 or Schmidtke et al. (1999). Table 5 lists the HRI sources with corrected coordinates (see Sect. 2.2), positional error, existence likelihood, HRI count rate, extent, extent likelihood, the corresponding PSPC source with count rate and hardness ratios (HFPK00).

Figure 2: Histogram for HRI pointing exposure times of SMCobservations

  
2.2 Source position

For ROSAT the systematic positional error (boresight error) is about 7 $^{\prime\prime}$. In cases where an X-ray source in a pointed observation can be identified with an object with accurately known coordinates, the systematic error can be reduced for all detected X-ray sources from the same observation. The remaining systematic error for all sources in that pointing is

$$\sigma_{{\rm sys,p}} = \sqrt{\sigma_{{\rm opt}}^2+\sigma_{{\rm stat,s}}^2} ,$$ (1)

$\sigma_{{\rm opt}}$ being the error of the optical position and $\sigma_{{\rm stat,s}}$ the statistical 90% confidence error of the identified X-ray source used for correction. A systematic error of 7 $^{\prime\prime}$ was used for all other sources of which the X-ray position could not be corrected. The total positional error is finally given by:

$$\sigma_{{\rm tot,X}} = \sqrt{\sigma_{{\rm sys,p}}^2+\sigma_{{\rm stat,X}}^2} .$$ (2)

Therefore the positions of HRI sources identified with TYCHO catalogue sources were corrected to more accurate optical coordinates. For X-ray sources already discussed in the literature and for which the optical counterpart is known, the X-ray positions were also corrected. For each correction the identification of the HRI source with an optical object was confirmed on Digitized Sky Survey (DSS2, red) frames. With the help of the corrected X-ray source positions the coordinates of all the pointings observing these sources were newly computed, and for other sources detected in the corrected pointings again more accurate coordinates were determined. In doing so the X-ray positions of 99 out of 121 sources were improved.

Before the corrections the mean positional error of the 121 HRI catalogue sources was 8 $.\!\!^{\prime\prime}$1. After the boresight correction the mean positional error was reduced to 5 $.\!\!^{\prime\prime}$6, and 4 $.\!\!^{\prime\prime}$9 for corrected sources only.

  
2.3 Flux variability

 

 

Table 1: HRI sources with significant flux variability

1	2	3	4	5	6	7	8
No	Rate	Rate	$\frac{F_{\rm max}}{F_{\rm min}}$	Red. $\chi^2$	DOF	No	Remarks
	HRI	PSPC				PSPC
	[cts s-1]	[cts s-1]
7	1.0e-01	4.4e-01	2.5	112.1	8	176	SSS 1E0035.4-7230
10	4.4e-03	1.6e-02	2.0	8.1	2	562	foreground star HD 3880
23	1.7e-02	7.2e-03	2.9e+01	475.1	3	512	SSS RXJ0048.4-7332
35	1.0e-02		3.7e+02	14.1	3		<foreground star> SkKM 62
44	4.9e-03	2.4e-02	5.5e+03	2973.0	6	453	HMXB Be/X RXJ0052.1-7319, pulsar [LPM99],[ISC99]
46	3.0e-03	1.5e-02	4.5e+02	9.4	9	94	HMXB Be/X RXJ0052.9-7158 [CSM97],[SCC99],[HS00]
51	2.7e-03		3.1e+02	95.0	4	242	HMXB? XTEJ0053-724, pulsar [CML98]
65	4.3e-03	1.9e-02	2.4	10.4	3	508
74	2.2e-03	7.9e-03	2.5e+04	8.7	9	136	HMXB? Be/X [HS00]
79	9.4e-02	3.6e-01	2.6	115.5	5	47	SSS 1E0056.8-7146
93	2.9e-04	<9.1e-03	3.5e+02	9.9	7	132	HMXB Be/X RXJ0101.0-7206 [KP96],[SCB99]
95	2.9e-03	2.4e-02	1.2e+01	8.1	9	159	HMXB? Be/X [HS00]
101	4.3e-03	1.6e-02	2.2	8.3	3	143	HMXB SAXJ0103.2-7209, pulsar [ISC98],[HS94]
114	1.6e-02		6.7e+01	119.6	4
118	3.7e-01	4.9e+00	2.1e+02	24762.9	8	482	HMXB SMC X-1
120	3.4e-02	1.1e-01	2.6	12.1	8	478	foreground star HD 8191 [CSM97]

Notes to Cols. Nos. 2 and 3: Count rates are the mean of output values from maximum likelihood algorithm for single pointings.

Count rate with < is mean upper limit.

Notes to Col. No. 6: Degrees of freedom.

Notes to Col. No. 7: Source number from HFPK00.

Notes to Col. No. 8: Sources classified in this work are put in < >. Abbreviations for references in square brackets are given in the literature list.

Figure 3: The mean of observation-averaged count rates from PSPC pointings versus HRI count rates: All sources observed by both detectors are plotted with small dots. In addition SNRs are marked with squares, XBs with circles, SSSs with asterisks, AGN with triangles, and stars with lozenges. Crossed symbols indicate already known candidates and variable sources are additionally marked with larger dots. The line is drawn for a PSPC/HRI conversion factor of 3

In the maximum likelihood algorithm for source detection, count rates are computed for point sources with intensity distribution peaking in the center. For extended sources the values resulting from the maximum likelihood algorithm are not reliable, because the count rates are determined by fitting a Gaussian to the source intensity profile. Thus for extended sources the total count rate was interactively integrated within a circle around the source, subtracting the background from a ring around the source. For faint sources upper limits were calculated by the maximum likelihood algorithm.

109 HRI sources were detected in more than one ROSAT observation and allowed flux variability studies on time scales of days to years. Lightcurves for these HRI sources were produced in order to test the flux variability. For this purpose not only HRI pointings were used, but also the PSPC pointings of the SMC region. From PSPC count rates and upper limits corresponding HRI values were computed by dividing the PSPC values by 3 which is a typical conversion factor for the ROSAT detectors (see SHP00). Figure 3 shows the correlation between the ROSAT detector count rates for sources detected both by the HRI and the PSPC.

A $\chi^2$-test and the ratio between maximum and minimum flux give information on the significance of variability in the lightcurves (HP99a). Variations with reduced $\chi^2$ higher than 5.0 were accepted as significant. Finally the list of variable sources contains 16 HRI sources, including three supersoft sources (SSSs) and eight X-ray binaries (XBs) and candidates. Table 1 lists parameters indicating variability and the mean count rates both for HRI and for PSPC observations for variable HRI sources.