A&A 392, 13–17 (2002) Astronomy DOI: 10.1051/0004-6361:20020696 & c ESO 2002 Astrophysics A BeppoSAX view of the Centaurus Cluster S. Molendi1, S. De Grandi2, and M. Guainazzi3 1 Istituto di Fisica Cosmica, CNR, via Bassini 15, 20133 Milano, Italy 2 Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy 3 XMM-Newton SOC, VILSPA ESA, Apartado 50727, 28080 Madrid, Spain Received 4 October 2001 / Accepted 6 May 2002 Abstract. We present results from the analysis of a BeppoSAX observation of the Centaurus Cluster. The radial metal abun- dance profile shows evidence of a large enhancement in the core, where Ab > 1 (sol. units). The temperature map indicates that the cluster is characterized by a gradient oriented in the NW/SE direction, with cooler gas in the NW and hotter gas in the SE (∆kT ∼ 1 keV). In hard X-rays, where the PDS detects emission with a flux of 2:2 × 10−11 erg/cm2/s in the 20–200 keV band, the spectrum lies above the extrapolation of the cluster thermal emission. We discuss several possible interpretations for the hard excess finding that none is completely satisfactory. Key words. X-rays: galaxies: clusters – general: individual: Centaurus 1. Introduction one interesting parameter, unless otherwise stated. We assume H = 50 km s−1 Mpc−1. The Centaurus Cluster is amongst the nearest (z = 0:0114) 0 and brightest clusters in the X-ray sky. It has been looked at with all major X-ray missions. In recent years, observations 2. Observations and data preparation with the ASCA satellite have produced a number of interesting BeppoSAX observed the Centaurus Cluster between 2000 results. Analysis of the core of the cluster has provided evi- July 3 (15:03:31 UTC) and July 5 (11:21:44 UTC). All the dence for a high metal concentration, the measured abundance instruments were operated in standard photon-counting di- exceeds the solar value (Fukazawa et al. 1994), and for a cool rect modes, the exposure times were respectively 70 ks for emission component (Fukazawa et al. 1994; Finoguenov et al. MECS and 31 ks for PDS. Data reduction followed standard 2001; Furusho et al. 2001). Churazov et al. (1999) and Furusho procedures as described, e.g., in Guainazzi et al. (1999). In et al. (2001) have detected asymmetric temperature variations this paper only data from the Medium Energy Concentrator on the few hundred kpc scales. The fact that the hot region Spectrometer (MECS, 1.8–10.5 keV) and from the Phoswitch located SE of the cluster is roughly coincident with the posi- detector System (PDS, 13–200 keV) will be presented. tion of NGC 4709, which is the dominant galaxy of the Cent45 We have analyzed the data from the MECS2 and MECS3 subcluster, indicates that the cluster is probably undergoing a separately. The SAXDAS package under FTOOLS environ- merger event. In this paper we present results from the anal- ment has been applied to produce equalized and linearized ysis of a BeppoSAX (Boella et al. 1997a) observation of the MECS event files. Using the information contained in the Centaurus cluster. We use the MECS experiment (Boella et al. housekeeping files we have rejected all events which have oc- 1997b), which is characterized by a spatial resolution a factor curred at times when the instantaneous pointing direction dif- of ∼2 better than that of the ASCA instruments, to investigate 00 fered by more than 10 from the mean pointing direction. the temperature and metal abundance structure of Centaurus. The PDS data have been reduced using a variable Rise Time We also use the PDS experiment (Frontera et al. 1997), to threshold method, which allows an increase of the signal-to- study the hard X-ray emission. The paper is organized as fol- noise (S=N) ratio for sources fainter than 1 mCrab. The corre- lows: in Sect. 2 we describe the data reduction. In Sect. 3 we sponding normalization factor (0.8; Fiore et al. 1999) has been present the spatially resolved spectroscopy performed with the applied in all the spectral fits discussed in this paper. MECS instrument. In Sect. 4 we describe the PDS spectrum. In Sect. 5 we discuss our results and compare them to pre- vious findings. Confidence intervals are at the 68% level for 3. Spatially resolved spectroscopy Send offprint requests to: S. Molendi, Spectra have been prepared for analysis following the e-mail: [email protected] guidelines described in De Grandi & Molendi (2001) and Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20020696 14 S. Molendi et al.: A BeppoSAX view of the Centaurus Cluster De Grandi & Molendi (2002), below we summarize the vari- ous operations which have been performed. Spectral distorsions introduced by the energy dependent PSF are taken into account using the Effarea program publicly available within the latest Saxdas release. The temperature and metal abundance radial profiles are truncated at the last bin where the source counts exceed 30% and 40% respectively of the total (i.e. source+background) counts. The energy range used for the spectral fitting is 2– 10 keV with the following exceptions. In the outermost bins, if the source counts drop to less than 50% with respect to the total counts, we restrict the energy range to 2–8 keV, to avoid possi- ble distortions from the hard MECS instrumental background. A strongback correction has been applied to the effective area for the 80–120 annulus, in this bin we restrict our analysis to the range 3.5–10 keV to avoid the low energy part of the spec- trum where our correction is less reliable. All other regions of the detector covered by the strongback have been appropriately masked and the data rejected. The background subtraction has been performed using Fig. 1. Top panel: projected radial temperature profile for the spectra extracted from blank sky events files in the same region Centaurus Cluster. Filled circles indicate temperatures obtained from BeppoSAX MECS data. Open circles and open triangles represent the of the detector as the source. temperature profile derived from ASCA data respectively by White (2000) and by Finoguenov et al. (2001). Bottom panel: projected ra- 3.1. Radial profiles dial abundance profile. Symbols as for Top Panel. The cluster has been divided into concentric annuli centered on the X-ray emission peak. Out to 80 we accumulated spectra Given the limitations of the MECS instrument both in terms of from 4 annular regions each 20 wide, beyond this radius we spatial resolution and of band coverage, the MECS data does accumulated spectra from annuli 40 wide. not allow a more detailed analysis of the temperature structure We have fitted each spectrum with a single temperature in the core of Centaurus. model (MEKAL code in XSPEC) absorbed by the galactic line In Fig. 1 we show the temperature and abundance pro- 20 −2 of sight hydrogen column density, NH,of8:06 × 10 cm de- files obtained from the spectral fits with single temperature rived from HI maps (Dickey & Lockman 1990). All spectra, models. The values reported for the innermost bin are respec- with the exception of the one associated to the innermost bin, tively the lower temperature and the metal abundance obtained can be adequately fit with the above model. In the innermost with the 2 temperature model discussed above. region the single temperature model leaves a rather large ex- The temperature profile in this, as in other centrally peaked cess above ∼5 keV. Since ASCA (e.g. Makishima et al. 2001) clusters, is found to rise with increasing radius, roughly level- 0 and XMM-Newton (Molendi & Pizzolato 2001) studies of the ing off at the cooling radius , rc ∼ 4 (Peres et al. 1998). Beyond cores of centrally peaked clusters have shown that the spectra rc the profile shows a drop. When fitting data for r > 4arcmin, accumulated from these regions are generally better fitted by a declining line gives a slightly better fit than a constant (the multi temperature models than by single temperature models, improvement is significant only at the 75% level according to we have performed a new fit with a model including a sec- the F-test). Note however that if we concentrate only on the ond MEKAL component. The metal abundace of this second last 4 data points (r > 6 arcmin) the improvement is found to component has been linked to the metal abundance of the first. be significant at more than the 99% level. Since the temperature of the hotter component is not well con- The metal abundance profile shows a striking excess in the strained we have fixed it to 4 keV, which is approximatively the innermost bin. We remark that the measurement in this bin largest temperature we measure in our profile (see Fig. 1) and is not particularly sensitive to the spectral model, indeed the in rough agreement with a maximum temperature of 4.4 keV single temperature and 2 temperature models described above found by Ikebe et al. (1999) from the analysis of ASCA data. give very similar results. Metal abundance excesses in the core The two temperature model yields a substantially better fit than of so-called cooling-flow clusters have been found in virtu- the one temperature model, the improvement is statistically sig- ally all BeppoSAX observations (see De Grandi & Molendi nificant at more than the 99% level according to the F-test. Note 2001), however the value we find for Centaurus is the largest however that the two temperature modeling should be regarded measured so far and the only one to exceed the solar value.