Giant Lobes of Centaurus a Radio Galaxy Observed With the Suzaku X-Ray Satellite

Abstract

We report on Suzaku observations of selected regions within the Southern giant lobe of the radio galaxy Centaurus A. In our analysis we focus on distinct X-ray features detected with the X-ray Imaging Spectrometer within the range 0.5 − 10 keV, some of which are likely associated with fine structure of the lobe revealed by recent high-quality radio intensity and polarization maps. With the available photon statistics, we find that the spectral properties of the detected X-ray features are equally consistent with thermal emission from hot gas with temperatures kT > 1 keV, or with a power-law radiation continuum characterized by photon indices Γ ∼ 2.0 ± 0.5. However, the plasma parameters implied by these different models favor a synchrotron origin for the analyzed X-ray spots, indicating that a very efficient acceleration of electrons up to 10 TeV energies is taking place within the giant structure of Centaurus A, albeit only in isolated and compact regions associated with extended and highly polarized radio filaments. We also present a detailed analysis of the diffuse X-ray emission filling the whole field-of-view of the instrument, resulting in a tentative detection of a soft excess component best fitted by a thermal model with a temperature of kT ∼ 0.5 keV. The exact origin of the observed excess remains uncertain, although energetic considerations point to thermal gas filling the bulk of the volume of the lobe and mixed with the non-thermal plasma, rather than to the alternative scenario involving a condensation of the hot intergalactic medium around the edges of the expanding radio structure. If correct, this would be the first detection of the thermal content of the extended lobes of a radio galaxy in X-rays. The corresponding number density of the thermal gas in such a case is n g ∼ 10 −4 cm −3 , while its pressure appears to be in almost exact equipartition with the volume-averaged non-thermal pressure provided by the radio-emitting electrons and the lobes' magnetic field. A prominent large-scale fluctuation of the Galactic foreground emission, resulting in excess foreground X-ray emission aligned with the lobe, cannot be ruled out. Although tentative, our findings potentially imply that the structure of the extended lobes in active galaxies is likely to be highly inhomogeneous and non-uniform, with magnetic reconnection and turbulent acceleration processes continuously converting magnetic energy to internal energy of the plasma particles, leading to possibly significant spatial and temporal variations in the plasma β parameter around the volume-averaged equilibrium condition β ∼ 1.

Mass entrainment and turbulence-driven acceleration of ultra-high energy cosmic rays in Centaurus A

“…The discrepancy between the figure for the synchrotron age and the sound-crossing timescale is not surprising: using spectral breaks (and inferred synchrotron age) to estimate the physical age of a source can be misleading, especially if in situ re-acceleration is taking place (e.g. Pacholczyk & Scott 1976;Alexander 1987;Eilek et al 1994Eilek et al , 1997Bicknell 1995;Feretti et al 1998;Katsuta et al 2013), as envisaged for the giant lobes of Cen A (Abdo et al 2010;Feain et al 2011;Stawarz et al 2013;Stefan et al 2013, and this paper).…”

“…Christiansen (1989), Kronberg et al (2004), Benford & Protheroe (2008) and Stawarz et al (2013) have proposed in situ magnetic reconnection as a viable mechanism for particle acceleration in galaxies' lobes. Lazarian & Vishniac (1999) have shown that the magnetic reconnection is determined by the degree of field wandering induced by magnetic turbulence, and we consider the physical conditions in Cen A's giant lobes (see Sect.…”

“…The fields of view of Chandra and XMM-Newton are too small to map the giant lobes of Cen A and also the distribution of group gas surrounding them. ASCA did detect thermal emission from hot gas in a region associated with the northern giant lobe (Isobe et al 2001), and most recently, Stawarz et al (2013) have claimed thermal and non-thermal X-ray detection with Suzaku of parts of the southern giant lobe, though they do not detect inverseCompton emission. INTEGRAL hard X-ray observations of Cen A's giant lobes by Beckmann et al (2011) are consistent Feain et al 2011), showing the giant lobes and the vertex and vortex filaments.…”

Centaurus A: constraints on the nature of the giant lobe filaments fromXMM-Newtonobservations

“…However, the reported detection could also originate from the Galactic X-ray foreground (Stawarz et al 2013;Wykes et al 2013). Deep INTEGRAL-SPI observations (spatial resolution 2.5…”

Application of a multiscale maximum entropy image restoration algorithm to HXMT observations

“…As the nearest radio galaxy to us, its proximity (D=3.8±0.1 Mpc [17]) makes it an attractive target for comprehensive physical studies. The most prominent features of Cen A are its two large radio lobes, which extend for ∼600 kpc (∼10 degrees) in the northsouth direction and are ∼250 kpc (∼4 degree) wide [18]. Their γ-ray glow emission has been detected by the Fermi Gamma-ray Space Telescope [19], providing insights into radiating high-energy particles in the lobes.…”