AN X-RAY COOLING-CORE CLUSTER SURROUNDING A LOW-POWER COMPACT STEEP SPECTRUM RADIO SOURCE 1321+045

The X-ray diffuse emission covers a large part of the ACIS-S CCD with the radio source located in the center (Figure 1). The smoothed X-ray image overlaid with radio contours shown in Figure 1 seems to suggest a broad enhanced X-ray emission with two peaks in the vicinity of the core. Offsets between the radio core and the enhancements are consistent with the astrometric uncertainty of Chandra and we did not attempt to apply any additional adjustments. The adjustment to the aspect solution might be possible with a deeper observations in the future if there are additional point sources detected in this field.

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The X-ray cluster emission extends outside the field of view of the CCD. However, we measured the extent along the CCD using the surface brightness profiles toward the north and south from the center. We used dmextract and extracted one-dimensional radial profiles assuming 18 annuli located between 3'' and 100'' within position angles (P.A.s) of 70°–150° and 260°–335°. The cluster emission is detected at 3σ level out to about ∼60'' (∼240 kpc) from the radio source. We fit the two profiles in Sherpa assuming a beta1d model and obtained the core radius of $r_c = 9.4^{+1.1}_{-0.9}$ arcsec and β-parameter of 0.58$^{+0.03}_{-0.02}$. The extrapolation of the beta1 model into the circular (r = 2'') region centered on the radio source shows the excess X-ray emission. We associate this emission with the radio source.

We used specextract tool to extract the X-ray spectra assuming 125 radius circle for the radio source and an annulus with inner and outer radii equal to 15 and 10'', respectively, for the local background. This region encloses the radio core and a part of the innermost radio structure. The X-ray spectrum contains 36.1 ± 8.3 net counts (53 total) in the 0.5–7 keV energy range. The cluster emission contributes to this spectrum and we need to take it into account in the further modeling of the source spectrum. In the modeling we kept the absorption parameter at the Galactic value of N_H = 2.04 × 10²⁰ cm⁻² (Dickey & Lockman 1990).

We first fit the background spectrum assuming the APEC model in Sherpa (this is the cluster emission within 15 and 10'' annulus). We set the metal abundance to 30% of the solar values and z = 0.263 for this model and fit the spectrum containing 1054 counts in 0.5–7 keV range. The resulting best-fit temperature and normalization are $kT_b = 4.4^{+0.5}_{-0.3}$ keV and 8.8 ± 0.3 × 10⁻⁴, respectively, and represent the average observed values of the central region of the cluster. We expect only about 16 ± 4 counts from the cluster to contribute to the Chandra spectrum of the radio source. These background model parameters remained unchanged in fitting the spectrum of the radio source described below.

We assumed an absorbed power-law emission model for the radio source and the APEC model with the fixed parameters to the above best-fit values to account for the cluster emission. The resulting best-fit power-law photon index is equal to $\Gamma = 2.35^{+0.39}_{-0.36}$ and the normalization to $6.2^{+1.4}_{-1.2}\times 10^{-6}$ photons cm² s⁻¹ at 1 keV. The model unabsorbed flux of F_{0.5–2 keV} = 1.4 ± 0.3 × 10⁻¹⁴ erg cm⁻² s⁻¹ corresponds to the luminosity L_{X(0.5–2 keV)} ∼ 3 × 10⁴² erg s⁻¹ typical for a low-luminosity active galactic nucleus (AGN). The photon index, however, is quite steep and may indicate a presence of soft thermal emission often observed in low-luminosity AGN and explained as a result of hot interstellar medium (ISM) of the host galaxy or a jet emission (Hardcastle et al. 2009; LaMassa et al. 2012). A higher quality spectrum is needed to understand the origin of this emission.

We detected 2882.2 ± 55.4 net counts within the circular region with 382 radius in the Chandra 9.7 ks observation. The cluster is bright in X-rays and we attempted to obtain the cluster's temperature profile by fitting the spectra of three annuli centered on the radio source. The three annuli span the cluster emission between 2'' and 28'' and the fourth one between 28'' and 35'' accounts for the background (see Table 1).

Table 1. Best-fit Model Parameters for the X-Ray Cluster^a

R b	Range	Total Counts c	Net Counts c	kT d	Norm e	ne	S f	tc g
(arcsec)	(arcsec)			(keV)		(10−2 cm−3)	(keV cm−2)	(109 yr)
5.3	2.0–8.6	815.0 ± 28.5	761.5 ± 29.5	3.9$_{-0.6}^{+0.7}$	0.0186 ± 0.0012	3.9 ± 0.6	33.9 ± 6.6	1.6 ± 0.3
11.9	8.6–15.2	696.0 ± 26.4	575.9 ± 28.5	4.1$_{-0.7}^{+0.9}$	0.0043 ± 0.0003	1.9 ± 0.4	57.6 ± 13.8	3.3 ± 0.8
21.8	15.2–28.4	924.0 ± 30.4	606.4 ± 35.2	5.4$_{-0.5}^{+0.6}$	0.0014 ± 0.0001	1.1 ± 0.1	109.2 ± 12.9	6.7 ± 0.7

Notes. ^aListed uncertainties are at 68% for one interesting parameter. ^bThe assumed annuli are circular with the mean radius listed in the "R" column and ranges in "Range" column. ^cA number of counts within the 0.5–7 keV energy range. ^dDeprojected temperature. ^eNormalization for APEC thermal model defined as Norm = (10⁻¹⁴)/(4π[D_A(1 + z)]²)∫n_en_HdV with the abundance table set to Anders & Grevesse (1989). Note that the "Norm" values given by deproject are normalized to a total volume given by the outermost sphere. ^fEntropy. ^gCooling time.

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We assume the APEC model and fit the cluster spectrum in each annulus. We account for the cluster three-dimensional volume effects using the deproject model⁴ in Sherpa (for model details, see Fabian et al. 1981; Kriss et al. 1983; Siemiginowska et al. 2010). The best-fit model provides the cluster temperatures, normalizations, and densities listed in Table 1.

These numbers suggest that the cluster has a cooling core, although this result has to be confirmed with better quality data in the future. We used the exposure corrected image given by fluximage tool and assumed an elliptical region with 35'' and 60'' radii to estimate the X-ray luminosity of the cluster to be L_{(0.5–2 keV)} = 3 × 10⁴⁴ erg s⁻¹. The mass of the cluster enclosed by a sphere with 60'' radius is about 1.5 × 10¹⁴ M_☉ assuming the average cluster temperature of 4.4 keV, and β = 0.58, which is in a broad agreement with the clusters scaling relations in Eckmiller et al. (2011).

The radio source 1321+045 (R.A. = $13^{\rm h}24^{\rm m}19\buildrel{\mathrm{s}}\over{.}7$, decl. = +04°19'072 (J2000.0)) belongs to a class of young CSS radio sources (Fanti et al. 1995). It has been observed with MERLIN at 1.6 GHz in 2007 as a part of large sample of low-luminosity compact (LLC) sources (Kunert-Bajraszewska et al. 2010a). The position of the central component visible in 1.6 GHz MERLIN image is well correlated with the position of the optical counterpart suggesting it is a radio core (C). The two lobes (E and W) are located on the opposite sides of the core. There is no evidence of jets or hotspots; however, this needs to be confirmed by observations at higher frequency. A total projected length of the source is equal to ∼17 kpc and its radio luminosity, L_5GHz ∼ 10²⁵ W Hz⁻¹ (<10⁴² erg s⁻¹), places it in the FR I–FR II transition region.

Studies of compact radio sources suggest that they exhibit periodic activity on timescales of 10⁴–10⁵ yr (Reynolds & Begelman 1997). On shorter timescales, the radio source is not able to escape from the host galaxy and starts to recollapse within the ISM (Czerny et al. 2009). Our analysis of the whole sample of LLC sources suggests that they can represent a population of short-lived objects and undergo this phase of activity many times before they become large-scale FR I or FR II (Kunert-Bajraszewska et al. 2010a, 2010b). What is more, the evolution of the radio source and its radio morphology is determined by the properties of the central engine: strength, accretion mode, excitation level of the ionized gas, and the ISM. Optically many of them belong to the class of low excitation galaxies (LEGs), which are thought to be powered by the accretion of hot gas (Hardcastle et al. 2007; Buttiglione et al. 2010) and can be progenitors of large-scale LEGs (Kunert-Bajraszewska et al. 2010b). The Ninth SDSS Data Release (Ahn et al. 2012) gives the fluxes of the emission lines visible in spectrum of 1321+045 and the calculated line ratios log [O ii] λλ3727,3729/[O iii] λ5007 = 1.15 and log [O iii]λ5007/Hβ = −0.41 (see Buttiglione et al. 2010 for definitions) are consistent with 1321+045 being an LEG.

We used ITERA (Groves & Allen 2010) to generate emission-line diagnostic (or BPT, after Baldwin et al. 1981) diagrams and to compare the emission-line ratios of 1321+045 with predictions for starburst (Leitherer et al. 1999; Dopita et al. 2006; Levesque et al. 2010), dusty and dust-free AGN (Groves et al. 2004a), and shock (Allen et al. 2008) models. We found that the emission line ratios of 1312+045 are consistent with AGN-dominated photoionization with small contributions from star formation models. None of the shock models (with and without precursor) reproduce the flux ratios, suggesting that jet-induced shocks are not present in the ISM of 1321+045 or their contribution to the ionization of the optically emitting gas is negligible.

The presence of distortions and cavities in the X-ray image of a cluster can signal interactions between the central galaxy (the radio source) and the ICM (Hlavacek-Larrondo et al. 2012). Some clusters have multiple pairs of cavities filled with low-frequency radio emission, probably caused by the previous radioactivity phase. The high-resolution radio observations show that the reborn radio source can reside inside the cluster core and multiple cavities provide the evidence for the previous outbursts (O'Sullivan et al. 2012; Clarke et al. 2009; Tremblay et al. 2012). Figure 1 shows X-ray emission from the 1321+045 cluster in 0.5–7 keV energy range overlaid with radio contours from the MERLIN 1.6 GHz observations. There is no presence of any ripples and discontinuities in the current X-ray image of a 1321+045 cluster but rather uniform emission without indications of an interaction with the radio jets, or any signature of the previous activity.

Inspection of the VLSS image of 1321+045 at 74 MHz (Cohen et al. 2007) shows that the low-frequency emission extends out to ∼120 kpc. This low-frequency radio emission is uniform and there is no trace of more extended components which could indicate the older phase of radioactivity.

The synchrotron spectrum of 1321+045 consist of four points and is steep from 74 MHz to 5 GHz with the index α = 0.95 (defined as S∝ν^−α). We used a simple model of synchrotron emission (Kunert-Bajraszewska et al. 2009) to reproduce the observed spectrum in order to find the value of the magnetic field. We took the size of the radio lobe, assumed an equipartition between the particle energy and the magnetic field energy and a value of the Doppler factor δ = 1. We obtained the best-fit value of B = 1.5 × 10⁻⁴ G which gives the magnetic pressure in each radio lobe to be ∼9 × 10⁻¹⁰ dyn cm⁻². Based on the cluster central density and temperature, we estimate a central thermal pressure of ∼5 × 10⁻¹⁰ dyn cm⁻². Taking into account the uncertainties in determining the deprojected temperature and density (only three annuli) and the value of the thermal pressure, we conclude that the cluster environment could limit the growth of the weak radio source.

Given the value of the magnetic field and the spectrum break frequency, ν_br, we can estimate the synchrotron age of the source 1321+045. The break frequency indicates the critical point in which the radio spectrum changes its spectral index and the value of ν_br depends on the elapsed time since the source formation (Murgia et al. 1999). In the case of older objects, the break frequency is moved toward lower frequencies. The synchrotron spectrum of 1321+045 does not show the self-absorption peak or the break frequency in the range 74 MHz–5 GHz. We suggest that the well-known electron self-absorption process modifies the index of the synchrotron emission of 1321+045 below 74 MHz. However, since this assumption is based on a small number of spectral points, we consider two cases in our calculation of the spectral age of the source: the break frequency is higher than 5 GHz or lower than 74 MHz. This gives us a synchrotron age of 1321+045 in the order of ∼10⁵ and ∼10⁶ yr, respectively. The ν_br lower than 74 MHz indicates that the population of electrons has cooled down to low energies and the source after a typical for CSS sources lifetime (10⁴–10⁵ yr) started to fade away. As we already suggested, the evolutionary paths of young radio AGNs are probably determined by the properties of their central engines, namely, the high excitation galaxy (HEG)/LEG path (Kunert-Bajraszewska et al. 2010b). However, in some objects the surrounding environment could be also an important factor influencing the evolution.