4 The X-ray emission and the gas distribution

Abell 970 is an X-ray source, first observed with Einstein IPC in June 1980 (Ulmer et al. 1981). It was also observed during the ROSAT all-sky survey (Voges 1992), in 1990, and is included in the X-ray Brightest Abell type Cluster catalogue (XBACs; Ebeling et al. 1996).

Some properties of the X-ray emission of Abell 970 were derived by White et al. (1997) from its Einstein IPC image. They applied a deprojection analysis that, to constrain the cluster gravitational potential, requires an X-ray temperature ($T_{\rm X}$) and velocity dispersion ($\sigma$) for the cluster. Due to the absence of temperature measurements for this cluster, these authors estimated it from a $\sigma$-$T_{\rm X}$ relation, assuming $\sigma = 832$ kms^-1. The result of the analysis is $T_{\rm X} = 4.1^{+0.2}_{-0.6}$ keV. Moreover, White et al. (1997), using this same data, suggests that Abell 970 has a weak cooling-flow, with a mass deposition rate $\dot{M} = 20^{+32}_{-20}~M_\odot$yr^-1(see also Loken et al. 1999). Ebeling et al. (1996) determined the X-ray flux, luminosity and temperature using an iterative method running roughly as follows: assuming an initial X-ray temperature of 5 keV, the bolometric luminosity was computed. Then, with this luminosity and using the $T_{\rm X}{-}L_{\rm X}$ relation from White et al. (1997), a new estimate of the temperature was made which, in turn, was used to compute a new luminosity and so on. Thus, with ROSAT data and the above mentioned $T_{\rm X}{-}L_{\rm X}$relation, Ebeling et al. (1996) quoted a flux equal to $9.9 \times 10^{-12}$ ergcm^-2s^-1, luminosity of $1.50 \times 10^{44}$ ergs^-1 (both in the 0.1-2.4 keV band), and gas temperature $k T_{\rm X} = 3.6$ keV. By applying this same iterative method to the Einstein IPC data, Jones & Forman (1999) derived a X-ray luminosity of $2.13 \times 10^{44}$ erg s^-1 in the [0.5-4.5 keV] band and a bolometric luminosity equal to $3.79 \times 10^{44}$ ergs^-1, corresponding to a temperature in agreement with the one given by Ebeling et al. (1996).

Figure 2: Projected density map of the galaxies brighter than $b_{\rm J}^{\rm cosmos} = 19.75$ in the field of Abell 970, together with the positions of the 15 brightest ones. The peak density of this map corresponds to about 210 galaxies Mpc -2 h502. All density levels in this map are significant. Other symbols are as follows- dots: galaxies kinematically linked to the cluster; pluses: galaxies projected in this field but not belonging to the cluster; crosses: galaxies with no measured velocities. The central rectangle displays the area covered by the X-ray image of Fig. 4. The substructure at NW, at $\sim$(7,3), is centered near galaxy number 40 of Table 1 (see text).

However, since the Einstein IPC detector has some spectroscopic capability, it is possible to estimate its temperature by a direct fitting of the available spectra. We have thus obtained both the "events'' and image (in the 0.2-3.5 keV band, rebinned to 24 arcsec per pixel) files from the HEASARC Online Service. The spectrum of Abell 970 was extracted from the events file with XSELECT and analysed with XSPEC using the PI channels 4-12 (0.5-4.5 keV) within a region of 9.6 arcmin (corresponding to 1 h₅₀^-1 Mpc). The X-ray emission was fitted with a single temperature, absorbed MEKAL model (Kaastra & Mewe 1993; Liedahl et al. 1995). We have also used the recipe given by Churazov et al. (1996) for computing the weights (available in XSPEC), based on the smoothed observed spectrum. With these weights, one can still use the least-square minimisation and the $\chi ^2$ statistics to estimate the confidence interval of the fitted parameters.

With only 9 bins covering the 0.5-4.5 keV band, it is impossible to constrain the metallicity. Therefore we have fixed Z to the "canonical'' value of $\sim$ $0.3~Z_\odot$, which is the mean value obtained for 40 nearby clusters by Fukazawa et al. (1998). Also, with only 3 bins with energy less than 1 keV, it is difficult to constrain independently the temperature and the hydrogen column density. This happens because they are anti-correlated (e.g. Pislar et al. 1997). Therefore, we have also fixed the hydrogen column density at $N_{\rm H} = 5.3\times 10^{20}$ cm^-2, which is the galactic value at the Abell 970 position (Dickey & Lockman 1990). Figure 3 shows the fitting of the X-ray spectrum using the 9 available energy bins. The results are summarised in Table 2. The fitting shown in Fig. 3 presents large residuals at low energies ( 1 keV), but beyond $\sim$2 keV, which is the region of the spectra most important for temperature determination, the residuals are acceptable. Nevertheless, it is worth stressing that the estimated temperature is strongly dependent on the number of energy bins employed in the analysis. For instance, keeping only the 8 bins with energy 0.7 keV, the estimated temperature increases to 4.9_-2.1^+2.7 kev, with $\chi^{2\strut}/{\rm dof}=11.6/6$, while with the 7 bins with energy larger than $\sim$1 keV the temperature is essentially unconstrained: $k T = 15.0_{-12.2}^{+\infty}$, with a $\chi^{2\strut}/{\rm dof}=4.2/5$.

   

Table 2: X-ray spectral fitting results. kT is the gas temperature, $N_{\rm H}$ is the hydrogen column density, Z is the metallicity, $L_{\rm X}$ is the [0.5-4.5 keV] non-absorbed luminosity inside 1  h^-1₅₀  Mpc, and the last column gives the $\chi ^2$ and number of degrees of freedom. Errors are at a confidence level of 68%.

kT	$N_{\rm H}$	Z	$L_{\rm X}$	$\chi^{2\strut}$/dof
(keV)	(1020 cm-2)	($Z_\odot$)	(1044 ergs s-1)
3.3-0.7+1.1	5.3$^\star$	0.3$^\star$	1.74-0.10+0.09	18.1/7

Notes: $^\star$Fixed values. Varying the metallicity from 0.1 to 0.5 $Z_\odot$ produces a change in temperature of less than 0.3 keV, increasing towards $Z=0.1~Z_\odot$.

Figure 3: Fit of the Abell 970 IPC X-ray spectrum. Both the metallicity and hydrogen column density are fixed ( $0.3~Z_\odot$ and $5.3 \times 10^{20}$ cm-2, respectively).

The value of the gas temperature in Table 2, kT = 3.3 keV, is well below the IPC upper energy cutoff at 4.5 keV. Although slightly cooler, this value of temperature is in agreement with those derived by Ebeling et al. (1996) and White et al. (1997), because its error is large ($\sim$1 keV at the 68% confidence level; at the 90% confidence level our standard solution gives an upper limit for kT of 5.4 keV), due mainly to the small number of energy bins. We find no evidence from the data for systematic errors due to unusual background (e.g., solar flares). Using the $\sigma{-}T_{\rm X}$ relation given by Wu et al. (1999), the temperature in Table 2 corresponds to $\sigma$ in the range 640-720 kms^-1.

In Fig. 4 we display the X-ray isophotes of an Einstein IPC image in the [0.2-3.5 keV] band. This image has 24 arcsec per pixel. As it can be seen, the X-ray isophotes are also regular but, interestingly, their peak is not coincident with the peak of the projected density distribution, being slightly displaced towards the NW substructure associated to the cluster brightest galaxy (see Fig. 2). Figure 4 also suggests that there is no X-ray emission excess near this substructure. To verify whether this is indeed true, we have performed a wavelet multi-scale reconstruction on the IPC image. We have used the package MVM, "Modèle de Vision Multi-Échelle'', described in detail by Rué & Bijaoui (1997; see also Slezak et al. 1994 for an application to X-ray cluster images). With this method, we can remove (spatial) high frequency noise while retaining small-scale genuine (with $3 \sigma$ confidence level) objects. The wavelet image restoration technique is able to locate structures at various scales simultaneously and superposed objects may be revealed. The result of this analysis is also shown in Fig. 4. No emission excess is seen near the substructure. This is also consistent with the hypothesis that the galaxies in this substructure are members of a group recently captured by the cluster, whose X-ray emission is much lower than that of the cluster.

Since the relaxation time of the hot gas is expected to be much lower than that of the galaxies, the non-coincidence between the peak of the galaxy distribution and the X-ray emission may be evidence of a state of non-equilibrium in the galaxy distribution, as expected if the substructure associated with the brightest galaxy has only recently fallen into the cluster. We will explore this point further in next sections.

Figure 4: Left: X-ray isophotes from an Einstein IPC image of Abell 970 (obtained from the HEASARC Online Service) superinposed on a DSS image of the cluster. Upper and left axes give the offset in arcmin from the galaxy number 1 in Table 1. The X-ray levels are linearly spaced. Right: same image but restored with wavelets (see text).

The X-ray isophotes (even in the restored image) also show a small peak SE of the cluster centre, at the position of galaxies number 37 and 38 in Table 1, identified in the COSMOS catalogue as just one galaxy with magnitude $b_{\rm J}^{\rm cosmos} =16.07$. However, an examination of the optical image of this object using POSS indicates that it indeed corresponds to two merging galaxies. A butterfly-shape due to the tidal currents induced by the merger can be noticed in the image and the spectrum of object number 38 has emission lines. It is somewhat surprising that this system is not catalogued as an IRAS source. The excess of X-ray emission associated with this galaxy pair may be evidence of an active nucleus excited by the merger. The magnitudes given in Table 1 were estimated from the COSMOS magnitude assuming that both galaxies have the same luminosity.