JAXA  Special  Publication     JAXA-SP-09-008E82
Extremely Hot Gas in the Most X-ray Luminous Cluster RXJ1347
Naomi Ota1, Kouichi Murase2, Tetsu Kitayama3, Eiichiro Komatsu4, Makoto Hattori5, Hiroshi Matsuo6,Tai Oshima7, Yasushi Suto8, and Yoshikawa9
1 Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan2 Saitama University, Shimo-Okubo 255, Sakura, Saitama 338-8570, Japan3 Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510, Japan4 The University of Texas at Austin, 2511 Speedway, RLM 15.306, Austin, TX 78712, USA5 Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan6 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan7 Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-1805, Japan8 The University of Tokyo, Tokyo 113-0033, Japan9 Center for Computational Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8577, JapanE-mail(NO): ota@rs.kagu.tus.ac.jp
Abstract
We present the results from long Suzaku observations of the most X-ray luminous galaxy cluster,RX J1347.5–1145 at z = 0.451. To understand the gas physics of a violent, cluster merger, we studyphysical properties of the hot (∼ 20 keV) gas clump in the south-east region discovered previously bySunyaev–Zel’dovich (SZ) eﬀect observations.With the joint analysis of the Suzaku and Chandra data, we determine the temperature of the hot gasin the south-east region to be 25.3+6.1−4.5 (statistical; 90% conﬁdence level) +6.9−9.5 (systematic; 90% conﬁdencelevel) keV, which is in an excellent agreement with the previous joint analysis of the SZ eﬀect in radioand the Chandra X-ray data. This is the ﬁrst time that the X-ray spectroscopic observations alone haveenabled a good measurement of such a high temperature gas component, because of Suzaku’s unprecedentedsensitivity over the wide X-ray band. These results indicate strongly that RX J1347.5–1145 has undergonea recent, violent merger. The spectral analysis shows that the SE component is consistent with beingthermal. Therefore the present result conﬁrms the presence of the hottest thermal gas in the Universe. Onthe other hand, no signiﬁcant non-thermal hard X-ray emission is detected and we measure the 3σ upperlimit to the non-thermal ﬂux in the 12–60 keV band, F < 8 × 10−12 erg s−1cm−2. Combining this limitwith the synchrotron ﬂux of a radio mini halo in this cluster, we ﬁnd a lower limit to the strength of thecluster magnetic ﬁeld, such that B > 0.007 µG.Key words: galaxies: clusters: individual: RX J1347.5–1145 – galaxies: intergalactic medium –X-rays: galaxies: clusters – cosmology: observations
1.Introduction
Cluster mergers are the most energetic events in the Uni-verse after the Big Bang and strongly aﬀect the clus-ter structure and the physical properties of intraclustermedium (ICM). The previous multi-wavelength observa-tions have shown that RX J1347.5–1145 has an unusu-ally violent merger activity, which makes this cluster anideal target for probing the gas physics and non-thermalphenomena associated with the merger at high redshifts.
RX J1347.5–1145 is the most X-ray luminous clusterof galaxies on the sky, having the bolometric luminosityof LX,bol ∼ 2×1046 erg s−1 and the global temperature of∼ 12 keV (e.g., Schindler et al. 1997). This cluster wasclassiﬁed as a cooling-ﬂow cluster because of its centrally
peaked X-ray emission and high mass accretion rate atthe center (Schindler et al. 1997; Allen et al. 02).
The most striking feature of RX J1347.5–1145 is thepresence of an extremely hot, ∼ 20 keV, gas clump inthe south-east (SE) region.This component was ﬁrstpointed out by the SZ eﬀect observation at 150 GHzas the prominent substructure, about 20” oﬀ at its thecenter (Komatsu et al. 2001). Kitayama et al. (2004)performed a joint analysis of the SZ data and the Chan-dra X-ray data, and determined the temperature of thehot gas clump to be in excess of 20 keV, which is muchhigher than the average temperature of the ambient gas.
The analysis of the temperature structure in merg-ing cluster is a powerful tool for understanding the gas
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The Energetic Cosmos : from Suzaku to ASTRO-H83
physics in extreme conditions. While X-ray spectroscopyhas been used widely in this type of study, it is noteasy to measure precisely the temperature of very hot(kT ≫ 10 keV) gas.
The X-ray data from the ROSAT satellite failed toidentify the hot component in the SE region. This ex-ample shows the importance of having sensitivity to hightemperatures well in excess of 10 keV. The unprece-dented sensitivity of the Suzaku satellite to hard X-rayemission oﬀers a wonderful opportunity to study violentmerger events. In this paper, we present an analysis ofthe temperature structure and non-thermal, high-energycomponent of the distant cluster RX J1347.5–1145 withthe on-board instruments, XIS and HXD. Combinedwith the Chandra data, we determine the temperatureof the SE clump only from the X-ray spectroscopy, anddiscuss its physical properties.We also constrain, forthe ﬁrst time, the non-thermal hard X-ray emission andestimate the magnetic-ﬁeld strength in the cluster.
Throughout the paper, we adopt ΩM = 0.27, ΩΛ =0.73, and H0 = 72 km s−1 Mpc−1. At the cluster redshift(z = 0.451), 1” corresponds to 5.74 kpc.
2.Observation
Long observation of RX J1347.5–1145 were conducted on2006 June 30 and 2006 July 15. The target was observedat the HXD nominal position. The total exposure timeafter data ﬁltering was 149 ks/122 ks for XIS/HXD.
The XIS spectra were extracted from a circular re-gion within a radius of 5′ centered at the X-ray peakof the cluster emission. The background spectra wereaccumulated from the circular region within a 3′ radius(Figure 1). The HXD spectra were accumulated fromall units of the PIN diodes. The method of backgroundsubtraction is presented in §3.2..
Fig. 1. Suzaku XIS-0 image of RX J1347.5–1145 in the 0.5–10 keVband. The spectral regions for the cluster and the background areindicated with the solid and dashed circles, respectively.
3.Spectral analysis and results3.1.XIS analysis: 0.5–10 keVTo examine the global temperature structure of the clus-ter, we ﬁrst attempted to reproduce the observed XISspectra under the single-temperature model in two dif-ferent ways: (1) by a APEC model ﬁt, and (2) by usingthe intensity ratio of two Fe-K emission lines, as an in-dicator of the “ionization” temperature.For the analysis (1), we ﬁtted the APEC model tothe observed 0.5–10 keV XIS spectra, where the redshiftand Galactic hydrogen column density were ﬁxed at z =0.451 and NH = 4.85 × 1020 cm−2, respectively. We ﬁndthe temperature, the metal abundance, and the spec-tral normalization factor of kT = 12.86+0.08−0.25 keV, Z =0.33+0.03−0.02 solar, and 1.37(1.36−1.39)×10−2, respectively.The unabsorbed ﬂux and luminosity in the 0.5–10 keVband are FX,0.5−10 keV = 1.3 × 10−11 erg s−1 cm−2, andLX,0.5−10 keV = 8.7 × 1045 erg s−1, respectively.For (2), we model the XIS spectra in the 4–5.5 keVband as the sum of (i) the APEC model with the metalabundance reset to 0 for the continuum emission and (ii)two Gaussians for the two major Fe-K lines. This modelprovides the line ratio, (He-like Fe Kα)/(H-like Fe Kα),of 1.05(0.92−1.34). Thus from temperature dependenceof the line ratio predicted by the APEC model, we ﬁndkT = 10.4(9.1 − 11.4) keV. This value is signiﬁcantlydiﬀerent from that found in (1), suggesting that the ICMcannot be explained by the single-temperature model.Therefore, the ICM is more likely to be in the form ofmulti-temperature plasma, which we explore in §3.3..
3.2.HXD analysis: 12–60 keVPrecise modeling of the background is crucial for mea-suring the hard X-ray ﬂux. For the HXD PIN diodes,the background is composed of the following three com-ponents: (i) the non-X-ray background (NXB), (ii) cos-mic X-ray background (CXB), and (iii) bright point-likesources within the detector ﬁeld-of-view. Of these, theNXB dominates the observed ﬂux.For (i), the PIN detector background was subtractedby using the NXB ﬁles provided by the HXD instru-ment team. We adopt the systematic error of 3% due tothe uncertainty of the NXB modeling and propagate thisthrough out our spectral analysis. For (ii), we calculatedthe CXB spectrum by assuming a power-law model withan exponential cut-oﬀ at 40 keV, as previously reportedfrom the HEAO-1 A2 (Boldt 1987). For (iii), we esti-mated the hard X-ray ﬂux of point sources inside theﬁeld-of-view of PIN from the XMM-Newton data of thesame ﬁled and found the contribution was negligible.We then analyzed the PIN spectra with subtractedthe NXB and CXB components under the single APECmodel.The temperature and normalization factorare obtained to be kT=20.2+21.4−8.0 (+7.7−8.7) keV and
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JAXA  Special  Publication     JAXA-SP-09-008E84
1.02+0.95−0.43 (+0.26−0.26) × 10−2, respectively.These parame-ters are consistent with those derived from the XISdata.Therefore, we conclude that the hard X-rayemission observed with PIN does indeed originate inRX J1347.5–1145. The energy ﬂux is estimated to be3.8 ± 0.5 (±3.3) × 10−12 erg s−1cm−2 in the 12–60 keV(The ﬁrst and second errors are the 1σ statistical andsystematic uncertainties).
3.3.Suzaku-Chandra joint analysisNext, we study the physical properties of the very hotcomponent in the SE region by performing a joint analy-sis of the Suzaku and Chandra data. In §3.3.1., the prop-erties of the ambient gas are derived from the spatially-resolved Chandra spectra. In §3.3.2., the spectral prop-erties of the SE clump are derived from the Suzakubroad-band spectra combined with the Chandra data bymeans of a multi-temperature model. In §3.3.3., we ar-gue that the hot component is described more accuratelyby thermal gas rather than non-thermal gas, and derivean upper limit to the possible non-thermal emission.
3.3.1.Modeling the cluster average component withChandraThe deep Chandra ACIS-I data of RX J1347.5–1145(56 ks) was analyzed to derive the average temperatureproﬁle within 5′. The spectra for six radial bins in thenorth-west(NW) region, i.e., the region outside of the SEquadrant were extracted and ﬁtted simultaneously by us-ing the PROJCT model in the XSPEC software. Figure 2shows the de-projected temperature proﬁle in the NWregion. We refer to the best-ﬁt model as the “6APEC”model hereafter.
Fig. 2. Deprojected temperature proﬁle measured from the Chandradata in the NW region.
3.3.2.Temperature measurement of the SE clumpWe subtract the ﬂux inferred by the 6APEC model(§3.3.1.) from the Suzaku data and studied the nature ofthe excess emission in the SE quadrant. We thus addedanother APEC model to describe the SE hot componentto the best-ﬁt 6APEC model and ﬁt the Suzaku XIS
and HXD data simultaneously by the multi-temperaturemodel (i.e., the 6APEC + APEC model).Note thatbecause we found that the Chandra data tended to in-fer systematically higher values for the temperature andnormalization, we took into account the systematic er-ror coming from the uncertainty in the Suzaku-Chandracross-calibration (See Appendix of Ota et al. 2008).Figure 3 shows the result of joint ﬁtting. We found thetemperature of the excess emission in the SE quadrant tobe kTex = 25.3+6.1−4.5 (+6.9−9.5) keV, where the ﬁrst error is sta-tistical and the latter is systematic (both 90% conﬁdencelevels). The reduced χ2 is 1315.1/1219. The absorption-corrected bolometric luminosity of the SE component isestimated as 3 × 1045 erg s−1, which is about 20% of thetotal luminosity.
Fig. 3. 6APEC+APEC model, ﬁt simultaneously to the observed XIS(0.5–10 keV) and HXD-PIN (12–60 keV) spectra (crosses). Thegray dotted and black solid lines show the ambient component(6APEC) and the excess hot component described by an addi-tional APEC model, respectively.
As a consistency check, we analyzed the Chandra spec-trum of the SE quadrant on its own in the same man-ner.The best-ﬁt temperature for the excess compo-nent is kT Chandraex= 31.1+24.1−12.6 keV. Therefore, both theChandra-alone analysis and the joint Chandra+Suzakuanalysis implied that the excess emission had a temper-ature in excess of 20 keV.Compared with the results for the Chandra data alone,the joint Chandra+Suzaku broad-band data analysisyielded a far more accurate determination of kTex. TheSuzaku’s unprecedented sensitivity over the wide X-rayband made it possible to determine, for the ﬁrst time, thetemperature of the hot gas in the ICM solely from theX-ray spectroscopy without the help of SZ eﬀect data.
3.3.3.Constraint on non-thermal emissionWe had been assuming that the excess hard X-ray emis-sion from the SE region is thermal; could it, however, benon-thermal? To address this question, the Suzaku datawere re-analyzed in light of non-thermal emission. While
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the 6APEC thermal model was again used for the ambi-ent gas, the APEC model was replaced with a power-law(PL) model for describing the excess component.
Fig. 4. 6APEC+PL model, ﬁt simultaneously to the observed XISand HXD-PIN spectra (crosses). The gray dotted and black solidlines show the ambient component and the excess hot componentdescribed by a non-thermal power-law model, respectively.
Figure 4 shows the results. We found that the power-law index is given by Γ = 1.45+0.03−0.04 (+0.09−0.04). The reducedχ2 of this model, χ2 = 1317.1/1219, was slightly higherthan that for the thermal model.We examined the non-thermal model more closely. Inthe XIS band below 10 keV, the non-thermal model withΓ = 1.5 and the thermal APEC model with 25 keVcould hardly be distinguished.However, the observedPIN spectrum was far softer than Γ = 1.5, the eﬀec-tive photon index being Γeﬀ ∼ 3. When we ﬁtted the6APEC+PL model to the PIN data alone, the 90% lowerbound on the photon index was found to be Γ > 1.8,which was signiﬁcantly above the 90% upper bound onΓ from the XIS data, Γ < 1.5.Therefore, it appearsdiﬃcult to explain the SE excess component with thenon-thermal PL model.
4.Discussion4.1.Properties of the extremely hot gasFrom the Suzaku broad-band spectroscopy, we havefound that the temperature of the SE clump is kTex =25.3+6.1−4.5 (+6.9−9.5) keV (90% statistical and systematic er-rors). This is an excellent agreement with the previousresult by Kitayama et al. (2004), 28.5 ± 7.3 keV (68%;statistical only). The present result therefore conﬁrmsthe presence of the hottest thermal gas in the cluster.We estimate the gas density and the gas mass by sim-ply assuming that the extremely hot gas is uniformlydistributed within a sphere of r = 25” (because the ex-cess is present in 10” < r < 60” of the Chandra surfacebrightness proﬁle).From the measured normalizationfactor of the APEC model, we obtain ne,ex = (1.6 ±
0.2) × 10−2 cm−3 and Mgas,ex = (5.6 ± 0.8) × 1012 M⊙.We also ﬁnd that the SE clump exhibits the tempera-ture and the density that are higher than the ambientgas in the same radial bins by factors of 1.7 and 2.4, re-spectively. This means that the excess hot componentis over-pressured, and such a region is expected to beshort-lived, ∼ 0.5 Gyr (Takizawa 1999). As already dis-cussed in Kitayama et al. (2004), the gas properties canbe explained by a fairly recent (within the last 0.5 Gyror so), high velocity (∆v ∼ 4500 km s−1) collision of twomassive (5 × 1014M⊙) clusters.The heat energy of the SE clump is also estimatedto be Eth,ex ∼ 6 × 1062 erg. This enormous amount ofenergy cannot be easily produced by a central source inthe cluster. On the other hand, cluster mergers, whichare the most energetic events in the Universe after theBig Bang, will most naturally explain this high energyphenomenon. Therefore, our results support a mergerscenario, on the basis of the X-ray spectroscopic dataalone without the help of SZ data.
4.2.Estimation of the magnetic ﬁeldThe non-thermal hard emission is produced via the In-verse Compton (IC) scattering of relativistic electronsoﬀ the CMB photons, and the same population of elec-trons also produce the synchrotron radiation. From theratio of observed synchrotron and IC ﬂux densities, thestrength of the magnetic ﬁeld can be directly estimated.We ﬁnd SIC < 0.11 µJy from our limit on the non-thermal hard X-ray emission for electrons with the ICemission frequency of 2.9×1018(1+z)−1 Hz. On the otherhand, Gitti et al. (2007) discovered a radio mini halo inthe cluster based on the VLA observations and estimatedthe ﬂux density to be SSyn = 25 mJy at 1.4(1+z)−1 GHz.Combining these numbers, a lower bound to the clustermagnetic-ﬁeld strength is obtained as B > 0.007 µG forthe power-law index of the electron distribution p = 2.This limit, though weak, is consistent with typical val-ues found in other clusters, B ∼ 0.1 − 1 µG (Rephaeliet al. 2008). However, the previous measurements werederived mostly for nearby (z < 0.1) clusters as well assome medium redshift clusters. Our work provides a con-straint on the cluster magnetic-ﬁeld strength at a higherredshift, z = 0.451.
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