4 X-ray analysis

4.1 Abell 901

A ROSAT/HRI image of a 21 $\hbox {$^\prime $ }$ region around A901 shows several sources (Fig. 2). A901 turns out to have a very compact structure contrary to the previous conclusion from the RASS. Ebeling et al. (1996) list A901 as a double cluster. Their so-called "brighter subcluster'' turns out to be a number of point-like sources, while the true cluster emission is what they call the "fainter subcluster''. In Fig. 2a six X-ray point sources with a signal-to-noise ratio of at least 3 are indicated. For three of them optical counterparts can be found on ROE/NRL COSMOS finding charts (see Fig. 2b and Table 4). Unfortunately, for none of these counterparts optical spectra could be taken because the optical observations were carried out before the X-ray observations. At the position of A902 ( $2\hbox{$^\prime$ }$ West of F) no X-ray emission can be detected.

The optical centre of A901 determined by Abell et al. (1989) is located between the X-ray position of A901 and source A. To test the extent of the X-ray emission of A we derive an X-ray profile (see Fig. 4a). The profile is compared with the on-axis PSF of the ROSAT/HRI. The profile of A is only slightly more extended than the on-axis PSF, which is expected for a point source 6 $\hbox {$^\prime $ }$ away from the pointing position. Therefore, we conclude that most likely the X-ray emission from A is point-like and therefore not (sub-)cluster emission, but probably emission from an active nucleus in the centre of a galaxy. The most likely candidate for this AGN is a galaxy of 16.6$^{\rm m}$ in B (see Table 4). This galaxy is located at a distance of 5 $\hbox{$^{\prime\prime}$ }$ from the cluster emission - a distance smaller than the pointing accuracy of ROSAT.

The only cluster emission is coming from the region indicated by "A901'' in Fig. 2a ( $\alpha_{2000}=09^{\rm h} 55^{\rm m} 57.0^{\rm s}$, $\delta_{2000}= -09\hbox{$^\circ$ }58\hbox{$^\prime$ }59\hbox{$^{\prime\prime}$ }$). This emission is shown magnified in Fig. 3 on a scale of 1.4 $\hbox {$^\prime $ }$. The emission is very compact, but not point-like, as can be clearly seen from the comparison of the cluster profile and PSF (see Fig. 4a). A $\beta$-model fit to the cluster profile (Cavaliere & Fusco-Femiano 1976; Jones & Forman 1984) reveals an extremely small core radius of 0.10 $\pm$ 0.03 $\hbox {$^\prime $ }$ or 22 $\pm$ 5 kpc (see also Table 5) reflecting the compactness of the emission.

Figure 2: Region around A901. a) X-ray image taken with the ROSAT/HRI. Apart from the extended emission of A901 six point-like sources (A-F) are visible (see also Table 4). b) Optical image from the Digitized Sky Survey of the same size as a). The X-ray sources A, C and F could be identified with optical counterparts (see Table 4)

Figure 3: ROSAT/HRI image of A901 smoothed with a Gaussian of $\sigma =4\hbox {$^{\prime \prime }$ }$. This image is a zoom of the dashed square in Fig. 2a. The cluster has a very regular and compact structure

The X-ray emission of A901 can be traced out to a radius of almost 2 $\hbox {$^\prime $ }$, corresponding to 430 kpc. Within this radius a countrate of 0.059 $\pm$ 0.002 cts/s is found. If the emission of A901 and the 6 point sources is summed up, the total countrate is at least a factor 1.8 larger than the cluster countrate, i.e. the cluster countrate would be largely overestimated if the point sources were not resolved. For the flux and luminosity shown in Table 5 only the cluster emission of A901 was used.

Estimating a temperature of 4 keV from $L_{\rm X}-T$ relations (Allen & Fabian 1998; Markevitch 1998; Arnaud & Evrard 1999) and assuming hydrostatic equilibrium we estimate the total mass at the outer radius $M_{{\rm tot}}(r<430$ kpc) $\approx 9~10^{13} \left ( {T \over 4~{\rm keV} } \right ) {M}_{\odot}$. The gas mass is $M_{{\rm gas}}(r<430$ kpc) = $1.2~10^{13} {M}_{\odot}$, i.e. the gas mass fraction is about 13%.

Obviously, A901 with a flux of $f_{\rm X}(0.1-2.4$ keV) = 3.0 10^-12 erg/s/cm² is falsely in the RASS X-ray brightest Abell cluster sample of Ebeling et al. (1996) as this sample has a flux limit of 5 10^-12 erg/s/cm². Ebeling et al. list a flux of $5.2\ 10^{-12}$ erg/s/cm² for the "brighter subcluster'', which is in reality not cluster emission. For the "fainter subcluster'', which is the true A901 emission, they list correctly 3.0 10^-12 erg/s/cm², but this value is far below their flux limit.

The compact (but not point-like) nature of the X-ray emission ( $r_{\rm c}=22$ kpc) could be interpreted as emission from a galaxy or from a group of galaxies. But a comparison of X-ray luminosity and blue luminosity of the central galaxy (16$^{\rm m}$ in B) shows that A901 lies far above the $L_{\rm X} - L_{\rm B}$ relation for early-type galaxies found by Eskridge et al. (1995) and Irwin & Sarazin (1998). A group of galaxies can also be excluded, not only because the X-ray luminosity is too high, but also from the gas mass fraction. The gas mass fraction of 13% is typical for a normal cluster (Ettori & Fabian 1999; Schindler 1999), and would be too high for a group of galaxies (e.g. Pildis et al. 1995). An estimate of the central cooling time yields about $t_{{\rm cool}} \approx 10^9$ years. Therefore it is possible, that the compact X-ray emission is caused by a cooling flow.

Figure 4: Radial profiles of two clusters. Left: A901. The filled circles show the cluster profile with the corresponding fit (full line). The crosses show the profile of the source A. For comparison the on-axis PSF of the ROSAT/HRI normalised to the central bin is shown (dashed line). As the source A is not exactly in the centre of the pointing but 6 $\hbox {$^\prime $ }$ away, the profile is expected to be slightly more extended than the dashed line as seen here. Therefore A is probably a point source. Right: A1437. Due to the asymmetry of the cluster, fit curves for different $90^\circ$ sectors are derived, sector $10^\circ - 100^\circ$ (dash-dotted line) (N over E), sector $100^\circ - 190^\circ$ (dotted line), sector $190^\circ - 280^\circ$ (long-dashed line), sector $280^\circ - 370^\circ$ (long dash-dotted line). For clarity only the data points of the profile averaged over all sectors are shown. The corresponding fit curves are the full line for a fit of all data points and the short-dashed line for a fit excluding the two innermost data points. To make these different inner radii visible in the figure, the curves start at the radius that was used as the inner radius for the fit

Figure 5: ROSAT/HRI image of A1437 smoothed with a Gaussian of $\sigma =24\hbox {$^{\prime \prime }$ }$. The point source in the NE is probably not connected with the cluster

4.2 Abell 1437

The cluster A1437 at a redshift z=0.1339 (Struble & Rood 1987) is the most X-ray luminous cluster of this sample. The cluster centre in X-rays (see Table 5) does not coincide with the optical position: Abell et al. (1989) determined a position 45 $\hbox {$^\prime $ }$ in the SE of the X-ray maximum. The emission of the cluster is strongly elongated in SW-NE direction (see Fig. 5). This elongation can be seen as well in the RASS. The RASS distinguishes also easily the point source in the NE, for which an optical counterpart can be found on APM finding charts (see Table 4). Although the cluster shape is not exactly elliptical, we fit ellipses to the isophotes (Bender & Moellenhof 1987) to estimate the elongation. The position angle varies around $55^{\circ}$ (N over E). The minimum axis ratio of 0.38 is reached at 0.01 cts/s/arcmin². At this level the centre of the ellipse is shifted 35 $\hbox{$^{\prime\prime}$ }$ to the west and 36 $\hbox{$^{\prime\prime}$ }$ to the south with respect to the position of the X-ray maximum.

The fit parameters of the surface brightness profile are not well constrained (see Table 5) because of the non-spherical morphology of the cluster. Therefore, radial profiles of the cluster emission are determined in four different sectors using as centre the X-ray maximum listed in Table 3 and subsequently fitted with $\beta$ models (see Fig. 4b). The two central bins show some excess emission. This excess cannot come from a cooling flow because the central cooling time is about 2 10¹⁰ years. It is probably a small contamination by an AGN. Because of this excess we try to fit the overall profile with and without these two bins. The results are listed in Table 3. In both - the fit parameters and the fit curves - it is obvious, that the cluster is very asymmetric.

Such asymmetries can arise during a merger of subclusters. From combined N-body and hydrodynamic simulations it is known that such elongated morphologies are common shortly after the collision of two subclusters, when the intra-cluster gas is squeezed out perpendicular to the collision axis (Schindler & Müller 1993).

 

 

Table 3: Fit parameters of the profile of A1437

region	sector	inner radius	S0	$r_{\rm c}$	$\beta$
	(N over E)	(arcmin [kpc])	(10-2 counts/arcmin2/s)	(arcmin [kpc])
all	$0^\circ - 360^\circ$	0	1.2	2.8 [520]	0.63
all	$0^\circ - 360^\circ$	0.5 [90]	1.2	3.7 [700]	0.80
NE	$10^\circ - 100^\circ$	0	1.2	2.5 [470]	0.57
NE	$10^\circ - 100^\circ$	0.5 [90]	1.1	3.7 [700]	0.77
SE	$100^\circ - 190^\circ$	0	1.5	0.7 [140]	0.35
SE	$100^\circ - 190^\circ$	0.5 [90]	2.4	0.3 [ 50]	0.32
SW	$190^\circ - 280^\circ$	0	1.4	$\infty$	$\infty$
SW	$190^\circ - 280^\circ$	0.5 [90]	1.4	$\infty$	$\infty$
NW	$280^\circ - 370^\circ$	0	1.1	2.7 [500]	0.75
NW	$280^\circ - 370^\circ$	0.5 [90]	1.1	2.8 [530]	0.79

The X-ray emission can be traced out to about 9 $\hbox {$^\prime $ }$. After excluding the point source in the NE a countrate of $0.25\pm0.01$ cts/s is found. This corresponds to a flux of $f_{\rm X}(0.1-2.4$ keV) = $(1.04\pm0.03)~10^{-11}$ erg/s/cm². For A1437, which is the most luminous cluster of this sample, the flux determination from the RASS ( $f_{\rm X}(0.1-2.4$ keV) = 1.02 10^-11 erg/s/cm²(Ebeling et al. 1996) and $f_{\rm X}(0.1\ -$ 2.4 keV) = 1.00 10^-11 erg/s/cm² (Ebeling et al. 1998), respectively is reliable. Also the morphological determination from the RASS is good: the point source in the NE can be distinguished easily and the elongated shape of the cluster is visible in the RASS as well.

4.3 Abell 3570

The cluster A3570 is the nearest cluster of this sample (z=0.037). The X-ray emission is faint and the extent is of the same order as the field-of-view of the ROSAT/HRI. With small smoothing the cluster X-ray emission is hardly visible, because the region is dominated by discrete sources (see Fig. 6a and Table 4). One of the sources (D) is not point-like but has a small extent. This source can be identified with the galaxy ESO 325 - G016 - a cluster galaxy at redshift of z=0.03795(Postman & Lauer 1995). To make the cluster emission visible we remove all point sources, which have a signal-to-noise ratio of at least 3 above the surrounding cluster emission, by fitting a warped surface to the pixels surrounding the point source region and apply a much coarser smoothing (see Fig. 6b). The cluster emission is extended and regular. There is no significant sign of subclustering or merging, i.e. the complex structure seen in the RASS disappears on resolving the discrete sources. Therefore, A3570 is very likely a relaxed cluster. Fitting the profile for this cluster is not possible because the profile is so shallow.

 

 

Table 4: Positions and identification of non-cluster X-ray sources in the pointings

pointing	source	$\alpha$(J2000)	$\delta$(J2000)	HRI count rate	identification
A901	A	09 56 28.2	-09 57 15	0.039	galaxy 17$^{\rm m}$(blue)
	B	09 56 35.4	-10 04 53	0.006	-
	C	09 56 30.6	-10 00 12	0.001	star 14$^{\rm m}$(blue)
	D	09 56 18.1	-09 53 57	0.002	-
	E	09 56 22.3	-09 55 07	0.002	star 15$^{\rm m}$(blue) or galaxy 17$^{\rm m}$(blue)
	F	09 56 35.6	-10 10 08	0.001	-
A1437	A	12 00 55.7	03 26 58	0.007	galaxy 16$^{\rm m}$(red)
A3570	A	13 47 12.5	-37 57 15	0.002	-
	B	13 47 40.7	-37 50 38	0.002	-
	C	13 47 25.6	-38 03 30	0.002	-
	D	13 46 23.3	-37 58 21	0.001	ESO 325 - G016
	E	13 47 15.5	-37 45 06	0.002	-

Because of the large extent of the cluster the image had to be vignetting corrected for the countrate determination (Snowdon 1998). The countrate determination is difficult, because the cluster emission fills probably the whole field-of-view of the HRI. We estimate the countrate to be $1.0^{\rm +0.4}_{\rm -0.7}$ cts/s by counting all the photons within $r=15\hbox{$^\prime$ }$ (corresponding to 250 kpc), excluding the discrete sources and using a standard ROSAT/HRI background. Out to this radius we can clearly trace the X-ray emission, but probably the cluster extends further beyond the field-of-view. Therefore, we estimate the upper limit of the countrate by adding the photons found beyond this radius north and east of the cluster and assume the same number in the south and west, which is not covered by the detector. The lower limit is obtained by using the background at the border of the field-of-view, which is a very conservative estimate. For flux and luminosity see Table 5.

The discrete sources change the cluster morphology drastically by feigning substructure in the RASS image. But they do not contribute significantly to the countrate. We estimate the countrate of the discrete sources by fitting a warped surface to the pixels surrounding the point source region and subtract these fitted counts from the original counts. The discrete sources together have a very small count rate of about 0.01 cts/s, which is negligible compared to the cluster emission.

For the X-ray luminosity $L_{\rm X}(0.1-2.4$ keV) = $(3.2^{\rm +1.1}_{\rm -2.3})\times 10^{44}$ erg/s the velocity dispersion $\sigma=460$ km s^-1 is relatively low. While a temperature of 4 keV is consistent with the $L_{\rm X}-T$relations, the $\sigma-T$ relations predict only 2 keV (White et al. 1997; Mushotzky & Scharf 1997; Wu et al. 1999). The small velocity dispersion confirms the conclusion from the ROSAT/HRI observation, that A3570 is a regular, non-merger cluster.

 

 

Table 5: Summary of the properties of the three clusters. For the flux and luminosity calculation hydrogen column densities from Dickey & Lockman (1990, see last line) were used and temperatures were estimated from $L_{\rm X}-T$ relations (Allen & Fabian 1998; Markevitch 1998; Arnaud & Evrard 1999): $T=4^{\rm +2}_{\rm -1}$ keV for A901 and A3570, $T=6^{\rm +3}_{\rm -2}$ keV for A1437

cluster		A901	A1437	A3570
position (J2000)	$\alpha$	09 55 57.0	12 00 25.7	13 47 16.1
	$\delta$	-09 58 59	+03 20 50	-37 56 28
HRI countrate	[cts/s]	$0.059\pm0.002$	$0.25\pm0.01$	$1.0^{\rm +0.4}_{\rm -0.7}$
luminosity (0.1-2.4 keV)	[1044 erg/s]	$3.6\pm0.1$	$8.5\pm0.3$	$3.2^{\rm +1.1}_{\rm -2.3}$
flux (0.1-2.4 keV)	[10-12 erg/cm2/s]	$3.0\pm0.1$	$10.4\pm0.3$	$5.2^{\rm +1.8}_{\rm -3.8}$
luminosity (bolometric)	[1044 erg/s]	$6.5^{\rm +1.3}_{\rm -0.6}$	$19^{\rm +5}_{\rm -3}$	$5.7^{\rm +3.8}_{\rm -4.3}$
S0	[cts/s/arcmin2]	0.27	0.012	-
$\beta$		$0.50\pm0.03$	$0.80^{\rm +0.75}_{\rm -0.25}$	-
$r_{\rm c}$	[arcmin]	$0.10\pm0.03$	$3.8^{\rm +2.5}_{\rm -1.4}$	-
$r_{\rm c}$	[kpc]	$22\pm5$	$710^{\rm +470}_{\rm -260}$	-
redshift		0.17	0.13	0.037
velocity dispersion	km s-1	-	-	460
$n_{\rm H}$	[1021cm-2]	0.51	0.19	0.44

Figure 6: ROSAT/HRI image of A3570. a) In an image smoothed with a Gaussian of $\sigma =6\hbox {$^{\prime \prime }$ }$ distinct sources dominate over the faint cluster emission. The sources A, B, C, and E are point-like while D has a small extent. These sources are listed in Table 4. b) After subtraction of the five point sources a strongly smoothed image ( $\sigma =36\hbox {$^{\prime \prime }$ }$) shows the cluster emission. The scale is the same as in a). At the upper and lower right corner the edge of the field-of-view is visible