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Register a new userConstraint of Non-thermal X-ray Emission from the On-going Merger Cluster Abell 3376 with Suzaku
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Clusters of galaxies are among the best candidates for particle acceleration sources in the universe, a signature of which is non-thermal hard X-ray emission from the accelerated relativistic particles. We present early results on Suzaku observations of non-thermal emission from Abell 3376, which is a nearby on-going merger cluster. Suzaku observed the cluster twice, focusing on the cluster center containing the diffuse radio emission to the east, and cluster peripheral region to the west. For both observations, we detect no excess hard X-ray emission above the thermal cluster emission. An upper limit on the non-thermal X-ray flux of $2.1times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ (15--50 keV) at the 3$sigma$ level from a $34times34$ arcmin$^2$ region, derived with the Hard X-ray Detector (HXD), is similar to that obtained with the BeppoSAX/PDS. Using the X-ray Imaging Spectrometer (XIS) data, the upper limit on the non-thermal emission from the West Relic is independently constrained to be $<1.1times10^{-12}$ erg s$^{-1}$ cm$^{-2}$ (4$-$8 keV) at the 3$sigma$ level from a 122 arcmin$^2$ region. Assuming Compton scattering between relativistic particles and the cosmic microwave background (CMB) photons, the intracluster magnetic field $B$ is limited to be $>0.03mu$G (HXD) and $>0.10mu$G (XIS).
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We present an X-ray spectral analysis of the nearby double radio relic merging cluster Abell 3376 ($z$ = 0.046), observed with the $Suzaku$ XIS instrument. These deep ($sim$360 ks) observations cover the entire double relic region in the outskirts of the cluster. These diffuse radio structures are amongst the largest and arc-shaped relics observed in combination with large-scale X-ray shocks in a merging cluster. We confirm the presence of a stronger shock (${cal M}_{rm{W}}$ = 2.8 $pm~0.4$) in the western direction at $rsim26$, derived from a temperature and surface brightness discontinuity across the radio relic. In the East, we detect a weaker shock (${cal M}_{rm{E}}$ = 1.5 $pm~0.1$) at $rsim8$, possibly associated to the notch of eastern relic, and a cold front at $rsim3$. Based on the shock speed calculated from the Mach numbers, we estimate that the dynamical age of the shock front is $sim$0.6 Gyr after core passage, indicating that Abell 3376 is still an evolving merging cluster and that the merger is taking place close to the plane of the sky. These results are consistent with simulations and optical and weak lensing studies from the literature.
We present the results from Suzaku observations of the hottest Abell galaxy cluster A2163 at $z=0.2$. To study the physics of gas heating in cluster mergers, we investigated hard X-ray emission from the merging cluster A2163, which hosts the brightest synchrotron radio halo. We analyzed hard X-ray spectra accumulated from two-pointed Suzaku observations. Non-thermal hard X-ray emission should result from the inverse Compton (IC) scattering of relativistic electrons by the CMB photons. To measure this emission, the dominant thermal emission in the hard X-ray band must be modeled in detail. To this end, we analyzed the combined broad-band X-ray data of A2163 collected by Suzaku and XMM-Newton, assuming single- and multi-temperature models for thermal emission and the power-law model for non-thermal emission. From the Suzaku data, we detected significant hard X-ray emission from A2163 in the 12-60 keV band at the $28sigma$ level (or at the $5.5sigma$ level if a systematic error is considered). The Suzaku HXD spectrum alone is consistent with the single-T thermal model of gas temperature $kT=14$ keV. From the XMM data, we constructed a multi-T model including a very hot ($kT=18$ keV) component in the NE region. Incorporating the multi-T and the power-law models into a two-component model with a radio-band photon index, the 12-60 keV energy flux of non-thermal emission is constrained within $5.3 pm 0.9 (pm 3.8)times 10^{-12}~{rm erg, s^{-1} cm^{-2}}$. The 90% upper limit of detected IC emission is marginal ($< 1.2times 10^{-11}~{rm erg, s^{-1} cm^{-2}}$ in the 12-60 keV). The estimated magnetic field in A2163 is $B > 0.098~{rm mu G}$. While the present results represent a three-fold increase in the accuracy of the broad band spectral model of A2163, more sensitive hard X-ray observations are needed to decisively test for the presence of hard X-ray emission due to IC emission.
The brightest cluster radio halo known resides in the Coma cluster of galaxies. The relativistic electrons producing this diffuse synchrotron emission should also produce inverse Compton emission that becomes competitive with thermal emission from the ICM at hard X-ray energies. Thus far, claimed detections of this emission in Coma are controversial (Fusco-Femiano et al. 2004; Rossetti & Molendi 2004). We present a Suzaku HXD-PIN observation of the Coma cluster in order to nail down its non-thermal hard X-ray content. The contribution of thermal emission to the HXD-PIN spectrum is constrained by simultaneously fitting thermal and non-thermal models to it and a spatially equivalent spectrum derived from an XMM-Newton mosaic of the Coma field (Schuecker et al. 2004). We fail to find statistically significant evidence for non-thermal emission in the spectra, which are better described by only a single or multi-temperature model for the ICM. Including systematic uncertainties, we derive a 90% upper limit on the flux of non-thermal emission of 6.0x10^-12 erg/s/cm^2 (20-80 keV, for photon index of 2.0), which implies a lower limit on the cluster-averaged magnetic field of B>0.15 microG. Our flux upper limit is 2.5x lower than the detected non-thermal flux from RXTE (Rephaeli & Gruber 2002) and BeppoSAX (Fusco-Femiano et al. 2004). However, if the non-thermal hard X-ray emission in Coma is more spatially extended than the observed radio halo, the Suzaku HXD-PIN may miss some fraction of the emission. A detailed investigation indicates that ~50-67% of the emission might go undetected, which could make our limit consistent with these detections. The thermal interpretation of the hard Coma spectrum is consistent with recent analyses of INTEGRAL (Eckert et al. 2007) and Swift (Ajello et al. 2009) data.
We present results from X-ray analysis of a Galactic middle-aged supernova remnant (SNR) G156.2+5.7 which is bright and largely extended in X-ray wavelengths, showing a clear circular shape (radius about 50). Using the Suzaku satellite, we observed this SNR in three pointings; partially covering the northwestern rim, the eastern rim, and the central portion of this SNR. In the northwestern rim and the central portion, we confirm that the X-ray spectra consist of soft and hard-tail emission, while in the eastern rim we find no significant hard-tail emission. The soft emission is well fitted by non-equilibrium ionization (NEI) model. In the central portion, a two-component (the interstellar medium and the metal-rich ejecta) NEI model fits the soft emission better than a one-component NEI model from a statistical point of view. The relative abundances in the ejecta component suggest that G156.2+5.7 is a remnant from a core-collapse SN explosion whose progenitor mass is less than 15 M_solar. The origin of the hard-tail emission is highly likely non-thermal synchrotron emission from relativistic electrons. In the northwestern rim, the relativistic electrons seem to be accelerated by a forward shock with a slow velocity of about 500 km/sec.
Wide-band Suzaku data on the merging cluster Abell 3667 were examined for hard X-ray emission in excess to the known thermal component. Suzaku detected X-ray signals in the wide energy band from 0.5 to 40 keV. The hard X-ray (> 10 keV) flux observed by the HXD around the cluster center cannot be explained by a simple extension of the thermal emission with average temperature of ~7 keV. The emission is most likely an emission from a very hot (kT > 13.2 keV) thermal component around the cluster center, produced via a strong heating process in the merger. In the north-west radio relic, no signature of non-thermal emission was observed. Using the HXD, the overall upper-limit flux within a 34x34 field-of-view around the relic is derived to be 5.3e-12 erg s-1 cm-2 in the 10-40 keV band, after subtracting the ICM contribution estimated using the XIS or the XMM-Newton spectra. Directly on the relic region, the upper limit is further tightened by the XIS data to be less than 7.3e-13 erg s-1 cm-2, when converted into the 10--40 keV band. The latter value suggest that the average magnetic field within the relic is higher than 1.6 uG. The non-thermal pressure due to magnetic fields and relativistic electrons may be as large as ~20% of the thermal pressure in the region.
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