Constraint of Non-thermal X-ray Emission from the On-going Merger Cluster Abell 3376 with Suzaku

Publications- Astronomical Society of Japan (Impact Factor: 2.44). 06/2008;
Source: arXiv

ABSTRACT 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 $2.1\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ (15--50 keV) at the 3$\sigma$ level from a $34\times34$ 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 $<1.1\times10^{-12}$ 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 $>0.03\mu$G (HXD) and $>0.10\mu$G (XIS).

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    ABSTRACT: New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z=0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentified diffuse radio sources (`arcs'), symmetrically located about 2.6 h_{50}^{-1} Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativistic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capable of accelerating cosmic ray particles upto ultra-high energies (UHECR) of E_{max} ~ 10^{18-19} eV. Thus this cluster is an excellent test-bed for understanding the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtunities for further multi-wavelength observations with ground and space-borne observatories.
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    ABSTRACT: In the current paradigm of cold dark matter cosmology, large-scale structures are assembling through hierarchical clustering of matter. In this process, an important role is played by megaparsec (Mpc)-scale cosmic shock waves, arising in gravity-driven supersonic flows of intergalactic matter onto dark matter-dominated collapsing structures such as pancakes, filaments, and clusters of galaxies. Here, we report Very Large Array telescope observations of giant ( approximately 2 Mpc by 1.6 Mpc), ring-shaped nonthermal radio-emitting structures, found at the outskirts of the rich cluster of galaxies Abell 3376. These structures may trace the elusive shock waves of cosmological large-scale matter flows, which are energetic enough to power them. These radio sources may also be the acceleration sites where magnetic shocks are possibly boosting cosmic-ray particles with energies of up to 10(18) to 10(19) electron volts.
    Science 12/2006; 314(5800):791-4. · 31.20 Impact Factor
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    ABSTRACT: X-ray spectra of groups of galaxies, obtained with the GIS instrument onboard ASCA, were investigated for diffuse hard X-rays in excess of the soft thermal emission from their inter-galactic medium (IGM). In total, 18 objects with the IGM temperature of 0.7--1.7 keV were studied, including HCG 62 in particular. Non X-ray backgrounds in the GIS spectra were carefully estimated and subtracted. The IGM emission was represented by up to two temperature thermal models, which was determined in a soft energy band below 2.5 keV mainly by the SIS data. When extrapolated to a higher energy range of 4--8 keV, this thermal model under-predicted the background-subtracted GIS counts in HCG 62 and RGH 80 by > 2 sigma significance, even though the background uncertainties and the IGM modeling errors are carefully accounted. A hard excess could be also present in NGC 1399. The excess was successfully explained by a power-law model with a photon index ~ 2, or a thermal emission with a temperature exceeding ~ 3 keV. In HCG 62, the 2--10 keV luminosity of the excess hard component was found to be 5.5E41 erg/s at 2--10 keV, which is ~ 30% of the thermal IGM luminosity in 0.7--2.5 keV. Non-thermal and thermal interpretations of this excess components are discussed.
    Publications- Astronomical Society of Japan 01/2007; · 2.44 Impact Factor

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