X-Ray Measurements of Nonthermal Emission from the Abell 1367 ...

THE ASTROPHYSICAL JOURNAL, 553 : 84È89, 2001 May 20 ( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

X-RAY MEASUREMENTS OF NONTHERMAL EMISSION FROM THE ABELL 1367 GALAXY CLUSTER USING THE ROSSI X-RAY T IMING EXPL ORER MARK HENRIKSEN Joint Center for Astrophysics, Physics Department, University of Maryland, 1000 Hilltop Circle, Baltimore, MD 21250 ; henrikse=umbc.edu

AND RICHARD MUSHOTZKY Laboratory for High Energy Astrophysics, NASA/GSFC, Code 661, Greenbelt, MD 20771 ; mushotzky=lheavx.gsfc.nasa.gov Received 2000 September 5 ; accepted 2001 January 18

ABSTRACT Observations with the Rossi X-Ray T iming Explorer (RXT E), the Advanced Satellite for Astrophysics and Cosmology, and ROSAT have been used to search for X-ray emission produced by the inverse Compton process in the Abell 1367 galaxy cluster. The three data sets provide a high-quality spectrum which extends from 0.4 to 20 keV, allowing accurate separation of thermal and nonthermal components. In the cases of both the clusterÏs radio halo relic and radio galaxy 3C 264, the detection of nonthermal emission is model dependent. Nonthermal emission from the relic is detected using the RXT E Proportional Counter Array with a Ñux of D0.010È0.019 photons cm~2 keV~1 s~1 at 1 keV, when the thermal emission is modeled with a single thermal component. However, modeling the thermal emission with two thermal components provides a better Ðt to the data and obviates the need for a nonthermal power-law component. We also Ðnd that thermal emission is a physically plausible origin for the second component. Using two thermal components to model the spectrum gives an upper limit of 3.3 ] 10~3 photons cm~2 keV~1 s~1 on nonthermal X-ray emission from the radio relic region. We derive an average intracluster magnetic Ðeld of º0.84 kG for this region. This value is consistent with the radial Ðeld derived from Faraday rotation studies of noncooling Ñow clusters. For the central region of the intracluster medium, we Ðnd an upper limit of 1.08 ] 10~3 photons cm~2 keV~1 s~1 at 1 keV for nonthermal emission. Joint Ðtting of the data sets gives a detection of nonthermal emission for 3C 264 of 1.21 ] 10~4 to 2.45 ] 10~4 photons cm~2 keV~1 s~1 at 1 keV, using a single thermal component. However, as with the radio relic region, two thermal components provide a much better Ðt to the spectrum and give an upper limit of less than 5.3 ] 10~5 photons cm~2 keV~1 s~1 at 1 keV. Combining the X-ray upper limit with the radio spectrum gives an average magnetic Ðeld greater than 0.41 kG. Subject headings : galaxies : clusters : individual (Abell 1367) È galaxies : magnetic Ðelds È intergalactic medium È X-rays : galaxies 1.

INTRODUCTION

In this paper, we present the results based on an analysis of Rossi X-Ray T iming Explorer (RXT E), ASCA, and ROSAT observations of the A1367 cluster to constrain X-ray emission due to inverse Compton from the brightest radio sources in the cluster : the radio relic and 3C 264. A1367 is a relatively cool cluster, and nonthermal emission should be more easily detected with the RXT E Proportional Counter Array (PCA) as the thermal continuum declines exponentially. Our analysis combines the hard X-ray band, where RXT E may detect the nonthermal emission directly, with the soft X-ray band, where the nonthermal emission from the steep spectrum radio relic may also appear, and ROSAT is most sensitive. Also, Ðtting di†erent data sets with the same models allows consistency checks between data and minimizes errors due to poor calibration in any part of a single data set. All parameter ranges and errors are reported in this paper with 90% conÐdence and use H \ 0 50 km s~1 Mpc~1.

Radio, optical, and X-ray observations of the Abell 1367 (A1367) galaxy cluster (z \ 0.022) reveal it to be an extremely interesting cluster. On the largest spatial scale it shows subclustering in the X-ray (Grebenev et al. 1995), while the temperature map obtained from the Advanced Satellite for Astrophysics and Cosmology (ASCA ; Donnelly et al. 1998) shows mild shock heating, presumably associated with the collision of subclusters. One of the most interesting features of the cluster is the large number of both radio sources (Gavazzi & Contursi 1994) and small extended X-ray sources, which may be linked to the merger. In addition, the cluster contains a radio relic at (J2000) 11h43m2s. 98, ]20¡02@40A. 32 (Gavazzi & Trinchieri 1983) that extends approximately 8@ in radius. The relic is 24@ from the cluster center at (J2000) 11h44m29s. 59, ]19¡50@2A. 0 (Abell, Corwin, & Olowin 1989). The radio relic has a Ñux of 590 ^ 88 mJy at 610 MHz and an energy spectral index of 1.9 ^ 0.27. The radio source associated with NGC 3862, 3C 264, is the strongest nonrelic source and is composed of three components : a core, a jet, and an extended plateau. The plateau emission, 3.1 ^ 0.6 Jy at 1465 MHz, has a spectral index of 0.55 and dominates both the core and the jet (Bridle & Vallee 1981). Both the radio relic and 3C 264 may be sources of nonthermal X-ray emission produced from inverse Compton scattering of cosmic microwave photons o† of the relativistic electron population.

2.

OBSERVATIONS

The PCA observation of A1367 produced 28,848 s worth of good data after standard Ðltering. The top layer of the PCA is used where the signal-to-noise ratio is highest, giving a background subtracted count rate of 15.44 ^ 0.05 counts s~1 in the 2È20 keV energy band. The RXT E pointing coincides with the optical cluster center. The PCA has a half-width at half-maximum of 30@. The ASCA Gas Imaging 84

A1367 : NONTHERMAL EMISSION TABLE 1 JOINT FIT NORMALIZATIONS Model

Data Set

Norm : High T

Norm : Low T

Cluster : 2RS . . . . . .

PSPC GIS PCA PCA SIS/GIS

0.026È0.037 0.042È0.050 0.057È0.068 0.044È0.063 0.0016È0.0029

0.0071È0.019 0.0133È0.022 0.0351È0.048 0.034È0.047 0.00028È0.0018

3C 264 : 2RS . . . . . .

Spectrometer (GIS) spectrum consists of 37,364 s of good data with a background subtracted count rate of 0.86 ^ 0.0066 counts s~1 in the 0.7È10 keV band. Preparation of the GIS spectrum and related calibration issues are discussed in detail in Henriksen (1998), where these data were Ðrst used. The ROSAT Position Sensitive Proportional Counter (PSPC) exposure is 18,745 s in duration with a count rate of 2.2 ^ 0.01 counts s~1 in the 0.4È2 keV band. The PSPC and GIS spectra are taken from the same region of the cluster : a circle centered at (J2000) 11h44m57s. 55, ]19¡41@46A. 95, the approximate X-ray brightness peak, with a radius of 16@. The PSPC background is obtained from source-free regions of the PSPC Ðeld and are corrected for vignetting and uneven exposure. The ASCA Solid-State Imaging Spectrometer (SIS) data are not used to analyze

85

the radio relic region since the cluster region spans 4 CCD chips, which grossly complicates modeling the instrument spectral response. An X-ray spectrum for the radio source 3C 264 is taken from a 3@ region centered on the optical coordinates of the NGC 3862 galaxy : (J2000) 11h45m>5s. 0, ]19¡36@22A. 67 (Clements 1983). The radius is chosen to minimize the e†ect of the point-spread function (PSF) of the GIS and SIS, while still keeping the contribution of cluster gas to the galaxy spectrum as small as possible. 3.

ANALYSIS AND RESULTS

The models Ðt to the data consist of one and two Raymond & Smith (1977) thermal components, with and without a power-law component. For the thermal components, all data groups share the following free parameters : column density, temperature, and abundance. The normalizations of the thermal components for all data sets are not tied but left as free parameters except in the case of modeling the radio galaxy ; in this case the added SIS data shares normalizations with the GIS. This is because they have the same region and similar energy bands. Even though the GIS and PSPC spectra are taken from the same region, they have di†erent energy bands and di†erent spectral sensitivity. For multiple temperature components, the

TABLE 2 RESULTS OF SINGLE AND JOINT FITS FOR A1367 CLUSTER EMISSION

Model

Data Set

n ]1022 H (cm~2)

kT (keV)

0.013È0.014 0.0È0.011 0.0È0.007 ... ... ... 0.3È4.1

2.45È2.89 3.81È3.90 3.35È3.60 ... ... ... 4.00È4.37 0.63È1.22 3.61È4.95 0.68È1.19 ... ... 3.78È3.86 3.58È3.67 ... ... 4.09È4.27 0.94È1.23 ... 1.11È1.55 4.35È4.72 ...

1RS . . . . . . . . . . . . . . . . 1RS . . . . . . . . . . . . . . . . 1RS . . . . . . . . . . . . . . . . 1RS . . . . . . . . . . . . . . . . 1RS ] POW . . . . . . 1RS ] POW . . . . . . 2RS . . . . . . . . . . . . . . . .

PSPC PCA GIS PSPC PCA GIS PCA

2RS . . . . . . . . . . . . . . . .

GIS

2RS ] POW . . . . . . 2RS ] POW . . . . . . 1RS . . . . . . . . . . . . . . . . 1RS . . . . . . . . . . . . . . . . 1RS ] POW . . . . . . 1RS ] POW . . . . . . 2RS . . . . . . . . . . . . . . . .

PCA GIS GIS ] PCA PSPC ] GIS ] PCA GIS ] PCA PSPC ] GIS ] PCA GIS ] PCA

... ... 0.0È0.0019 0.0060È0.0095 ... ... 0.0È0.11

2RS ] POW . . . . . . 2RS . . . . . . . . . . . . . . . .

GIS ] PCA PSPC ] GIS ] PCA

... 0.0075È0.012

0.0È0.17

2RS ] POW . . . . . .

...

Abundance 0.12È0.25 0.14È0.17 0.23È0.41 ... ... ... 0.16È0.20 ...

NT Normalizationa ... ... ... \6.1 ] 10~4 0.010È0.019 0.0015È0.0066 ...

259.9/226 141.2/41 253.4/464 262.6/226 77.58/40 243.4/463 66.95/40

...

236.56/462

... ... 0.16È0.18 0.18È0.20 ... ... 0.16È0.19

\3.3 ] 10~3 \6.3 ] 10~3 ... ... 0.0039È0.0095 0.0012È0.0023 ...

... ... 423.1/509 623.56/680 355.37/508 600.91/679 306.41/506

... 0.16È0.19

\1.7 ] 10~3 ...

495.48/677

\1.08 ] 10~3

...

...

a Nonthermal upper limit in photons cm~2 s~1 keV~1 at 1 keV. TABLE 3 NONTHERMAL CONSTRAINTS FOR 3C 264 Model

Data Set

1RS ] POW . . . . . . 1RS ] POW . . . . . . 2RS . . . . . . . . . . . . . . . . 2RS . . . . . . . . . . . . . . . .

GIS ] PCA PSPC ] GIS ] PCA GIS ] PCA PSPC ] GIS ] PCA

s2/dof

NT Normalizationa

s2/dof

5.5 ] 10~4È2.4 ] 10~4 1.21 ] 10~4È2.45 ] 10~4 \5.5 ] 10~5 \5.3 ] 10~5

315/502 472.86/372 223.2/502 338.17/369

a Nonthermal upper limit in photons cm~2 s~1 keV~1 at 1 keV.

FIG. 1.ÈSequence of Ðgures showing the results of model Ðtting to the PSPC, GIS, and PCA spectra. The models are a single thermal component (top), a power law added to a thermal component (middle), and two thermal components (bottom). The s2 vs. energy plots in the lower panels show residual emission around 1È1.5 keV in both the PSPC and the GIS for the single thermal-component models. There is also residual emission in the PCA in the 7È10 keV range in the single thermal-component models. The addition of a second thermal component successfully models both of these features and is preferred over the addition of a nonthermal component.

A1367 : NONTHERMAL EMISSION

FIG. 2.ÈBest-Ðtting model with two thermal components is shown with the 90% conÐdence upper limit on a nonthermal component and a composite model. One can see the relative contribution of the various components to the broadband spectrum.

GIS will be more sensitive to the hotter component as can be seen by comparing normalizations in Table 1. The power-law component has a free normalization. The spectral index of the inverse Compton component is identical to the spectral index of the synchrotron component. Thus, it is Ðxed at a photon index of 2.9 ( \ a ] 1, where a is the r r radio energy spectral index) for the extended relic (Gavazzi & Trinchieri 1983) and 1.55 for brightest radio component in the 3C 264 complex (Bridle & Vallee 1981). We also tried Ðtting the data with a free spectral index, but it is not constrained. The nonthermal emission has the same column density as the thermal components. When multiple data sets are used, the normalization on the nonthermal components are tied. Thus, for the joint data Ðts, there are seven free parameters for the model consisting of a single thermal component plus a power law and 11 free parameters for two thermal components and a power law. The cosmic X-ray background Ñuctuations in the PCA are modeled as described in Henriksen (1999). Three regions were modeled : (1) the radio relic region observed with PCA ; (2) the radio galaxy observed with PCA, GIS, SIS, and PSPC ; and (3) the central region of the cluster observed with PCA, GIS, and PSPC. The results of Ðtting these models to each data set alone as well as the combination of all data sets for the radio relic region are shown in Table 2. For thermal emission, the joint Ðt of the data sets gives an emission-weighted temperature of 3.58È 3.67 keV. The two temperature components from the joint Ðt are 1.11È1.55 keV and 4.35È4.72 keV. For nonthermal emission, a power law added to a single temperature component improves the Ðt. However, the addition of a second temperature component provides a much better Ðt than the additional power law, obviating the need for a power law. This is independently found in the joint Ðts of various combinations of data sets. The best-Ðt data, model, and s2 are shown in a series of three panels in Figure 1 for the sequence of a single thermal component, an added power law, and two thermal components. The second thermal component models residual soft X-ray emission in the range 1È1.5 keV and 7È10 keV over the single thermal component, which the power law does not. Figure 2 shows the best-Ðt model com-

87

ponents and the resulting spectral model. It is worth noting that the nonthermal component would partially model the residual soft and hard (7È10 keV) emission over the single thermal component and would add substantial emission in the 15È20 keV energy range that is not evident in the PCA spectrum. The upper limit on the nonthermal Ñux from the radio relic obtained with the PCA is 2.5 ] 10~13 ergs cm~2 s~1 in the 20È60 keV band. The upper limit on the nonthermal Ñux from the central cluster region obtained from the joint Ðt, 7.95 ] 10~14 ergs cm~2 s~1 in the 20È60 keV band, is D35 times better than that obtained by the GIS and HEAO 1 A-2 (Henriksen 1998). The radio source associated with 3C 264 consists of a nuclear region, a jet, and a plateau. The plateau has the largest radio luminosity at 3.3 ] 1041 ergs s~1 and is the most extended, at 13@@ ] 9@@ (Bridle & Vallee 1981). The upper limit on nonthermal X-ray emission from 3C 264 is 0.42 ] 10~12 ergs cm~2 s~1 in the 20È60 keV band (see Table 3). The total emission from this galaxy using the PSPC image is 4.0 ] 1042 ergs s~1. Our upper limit on the nonthermal Ñux is 2.1 ] 1041 ergs s~1 in the 0.5È3 keV band, or less than 5% of the total soft X-ray emission. A spatial separation of thermal and nonthermal emission is not possible with ROSAT PSPC data since the plateau is smaller than the PSPC PSF. The Chandra ACIS would be ideally suited for this study because of its combination of high spatial resolution, broad band, and good spectral resolution. Observations with this instrument will be capable of isolating the plateau within the galaxy to separate extended nonthermal emission from both thermal and possible nuclear emission. 4.

DISCUSSION

4.1. Physical JustiÐcation for the T wo T hermal Component Model The two thermal component model for the di†use cluster gas is strongly preferred statistically to a single component plus a power law, based on the results of the spectral analysis we presented in ° 3. Here we provide a physical justiÐcation of the two thermal component model. Cluster X-ray spectral observations often show nonisothermality originating from several sources : cooling Ñows, cD or elliptical galaxies, di†use cool gas in the cluster atmosphere, and shock heating in the atmosphere. There are several possibilities for the source of the cool gas in A1367, including the small extended structure and mild shock heating in the atmosphere. The two-dimensional temperature map obtained with ASCA (Donnelly et al. 1998) shows nonisothermality. We can use the map to identify the temperatures of the regions which contribute to the PSPC/GIS/PCA spectra to determine if the temperatures of the two thermal components are consistent with the ASCA map. The greater than 4 keV gas in the cluster is generally in regions 5, 6, and 7 of the map, while the cooler gas is in regions 1È4. Regions 5È7 have an average temperature, not weighted by emission, of 4.4 keV, while regions 1È4 have an average temperature of 3.4 keV. The PCA pointing contains all of the regions, 1È7, which would give an average temperature of 3.95 keV, while the GIS contains gas from regions 1È4 only and thus should be closer to 3.5. The emission-weighted temperature of the GIS (3.35È3.60 keV) is consistent with the prediction from the temperature map. The emission-weighted tem-

88

HENRIKSEN & MUSHOTZKY

perature of PCA (3.81È3.90 keV) is lower than the predicted average value of 3.95 keV. This is expected, as regions 1È4 should have a more signiÐcant e†ect on the PCA spectrum (containing regions 1È7) since they have a higher density (and therefore emission measure) than the hotter regions. Within the central region of the PSPC image is an unusually large number of small extended sources (Grebenev et al. 1995). Wavelet analysis of the central part of the PSPC observation has shown that there are 20 sources in the PSPC region r \ 16@ from which the spectrum was taken. These sources have a total count rate of 3.3 counts s~1 using the data given in Grebenev et al. (1995). They are converted to a Ñux using the spectral parameters for the soft component of the two thermal component Ðt, giving a total Ñux of 3.7 ] 10~12 ergs cm~2 s~1. The cool component Ñux measured by the PSPC is 7.2 ] 10~12 ergs cm~2 s~1. Thus, the small-scale structure can account for about 1 of the total soft component in the two-component 2 model. It is likely that the remaining emission is from the intracluster medium. A Ðne resolution temperature map in Donnelly et al. (1998) divides each of the large regions discussed above into three smaller regions. Several of the smaller regions have temperatures in the 2.5È3 keV range. This is higher than the temperature range of the cool component, of 1.1È1.55 keV. However, the temperature map also utilizes a harder band ([1.5 keV) than we use here ([0.4 keV), so the PSF can be more accurately modeled in producing the two-dimensional map. This could account for the higher temperature and make it plausible that all of the remaining cool gas is identiÐed with cooler, di†use intracluster gas. 4.2. Calculation of B The average magnetic Ðeld, B, is calculated from the radio spectrum for both the radio relic and 3C 264 and the X-ray Ñux upper limit using the equations in Henriksen (1998). This procedure combines the expressions for the synchrotron Ñux and the inverse Compton Ñux to eliminate the relativistic electron density and to obtain an expression that is independent of the size of the emitting region or the distance to the cluster. The calculated values of B are given in Table 4. The lower limit for the average relic magnetic Ðeld is 0.84 kG, while the lower limit for 3C 264 is 0.41 kG. 4.3. Nondetections versus Detections of Nonthermal Emission Giovannini (1999) found that the frequency of radio halos and relics is higher in X-ray luminous clusters with high TABLE 4 NONTHERMAL EMISSION PARAMETERS

Model 3C 264 : 1RS . . . . . . 3C 264 : 2RS . . . . . . Cluster : 1RS . . . . . . Cluster : 2RS . . . . . . Relic : 1 RS . . . . . . . Relic : 2RS . . . . . . . .

Data Set Joint Joint Joint Joint PCA PCA

NT Fluxa

B (kG)

0.99È2.01 \0.42 0.09È0.17 \0.0795 0.76È1.43 \0.25

0.15È0.26 [0.41 ... ... 0.46È0.57 [0.84

a Nonthermal Ñux : ]10~12 ergs cm~2 s~1 in 20È60 keV band.

Vol. 553

temperature and no cooling Ñow (e.g., clusters likely to have undergone a recent merger), providing support for the hypothesis that radio halos are made by cluster mergers. This would suggest that the clusters that are the best cases for active mergers (i.e., those with twisted X-ray isophotes or substructure and evidence of shock heating) would be most likely to have nonthermal radio and X-ray emission. However, this is not observed. Recent broadband X-ray searches for nonthermal emission from galaxy clusters have produced mixed results. Detections of strong nonthermal emission have been reported for the Coma Cluster (FuscoFemiano et al. 1999 ; Rephaeli et al. 1999), A2199 (Kaastra et al. 1999), and A2256 (Fusco-Femiano et al. 2000), while tight upper limits on nonthermal inverse Compton emission have been found for A754 (Valinia et al. 1999), A2256 (Henriksen 1999), A496 (Valinia et al. 2000), and A1367 (this paper). Two of the clusters, A496 and A2199, both appear to be dynamically relaxed and have no radio halos or relics, yet one has nonthermal emission (A2199). Clusters A754, A2256, A1367, and Coma all have radio halos or relics ; A2256 and Coma have detections of nonthermal emission. If one looks for direct X-ray evidence of a merger, only A754 seems to be undergoing a major merger. The X-ray temperature maps of A2256, Coma, and A1367 indicate either a dynamically quiet state by a lack of shocked gas (A2256) or minor mergers (A1367, Coma). The detection of a large nonthermal component for A2199 (Kaastra et al. 1999) and A2256 also implies a very small average magnetic Ðeld (0.05 kG), if it is a result of inverse Compton (Kempner & Sarazin 2000). Such small Ðelds appear to be inconsistent with the typical cluster Ðelds derived from Faraday rotation studies (Kim, Tribble, & Kronberg 1991 ; Clarke, Kronberg, & Bohringer 1999). In contrast, the upper limits found for A2256, A1367, and A754 imply average magnetic Ðelds of 0.4È0.8 kG. Magnetohydrodynamical simulations by Dolag, Bartelmann, & Lesch (1999) show that a small, primordial Ðeld of 10~3 kG can be ampliÐed to an average Ðeld of several kG during a cluster merger, so that larger Ðelds than those implied by the recent detections are expected. Alternative models such as nonthermal bremsstrahlung (Blasi 2000 ; Sarazin & Kempner 2000) have also been suggested, which alleviate some of these problems (e.g., lack of observable radio emission and very small magnetic Ðelds) associated with an inverse Compton interpretion of the nonthermal X-ray component. 5.

CONCLUSIONS

A1367 is perhaps one of the best clusters to detect nonthermal emission using data less than 20 keV because it is a cool cluster and has a steep radio spectral index. Our analysis shows that the detection of nonthermal emission produced via the synchrotron inverse Compton process is model dependent in the 0.4È20 keV energy band. More accurate modeling of the thermal component obviates the need for nonthermal emission. For A1367, two thermal components are physically plausible when compared to the temperature map. The small-scale structure seen in ROSAT most likely accounts for much of the cool component. The calculated lower limit on the average magnetic Ðeld for the radio relic is 0.84 kG. This lower limit is consistent with Faraday rotation studies of background radio sources in cluster atmospheres in noncooling Ñow clusters. The best-Ðt model for 3C 264 has two thermal components and pro-

No. 1, 2001

A1367 : NONTHERMAL EMISSION

vides an upper limit of 4.2 ] 10~13 ergs cm~2 s~1 in the 2È10 keV band, and a calculated Ðeld of greater than 0.41 kG.

89

This work has received partial support from NSF grant AST-9624716. We thank Dr. Eric Perlman and the referee for comments on improving the manuscript.

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