Chandra and XMM-Newton Observations of NGC 6251

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 02/2005; DOI: 10.1111/j.1365-2966.2005.08900.x
Source: arXiv

ABSTRACT We present new X-ray observations of the nucleus, jet and extended emission of the nearby radio galaxy NGC 6251 using the Chandra/ACIS-S camera, together with a reanalysis of archival Chandra/ACIS-I and XMM-Newton/EPIC data. We find that the nuclear X-ray spectrum is well-fitted with an absorbed power-law, and that there is tentative, but not highly significant, evidence for Fe K$\alpha$ emission. We argue that the observed nuclear X-ray emission is likely to originate in a relativistic jet, based on the double-peaked nature, and our synchrotron self-Compton modelling, of the radio-to-X-ray spectral energy distribution. However, we cannot rule out a contribution from an accretion flow. We resolve X-ray jet emission in three distinct regions, and argue in favour of a synchrotron origin for all three; inverse-Compton emission models are possible but require extreme parameters. We detect thermal emission on both galaxy and group scales, and demonstrate that hot gas can confine the jet, particularly if relativistic beaming is important. We show evidence that the radio lobe has evacuated a cavity in the X-ray-emitting gas, and suggest that the lobe is close to the plane of the sky, with the jet entering the lobe close to the surface nearest to the observer. Comment: 27 pages, 17 figures. Accepted for publication in MNRAS. Minor typographical issues updated

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    ABSTRACT: The radio source 3C270, hosted by nearby elliptical galaxy NGC4261, is the brightest known example of counterjet X-ray emission from a low-power radio galaxy. We report on the X-ray emission of the jet and counterjet from 130 ks of Chandra data. We argue that the X-ray emission is synchrotron radiation and that the internal properties of the jet and counterjet are remarkably similar. We find a smooth connection in X-ray hardness and X-ray-to-radio ratio between the jet and one of the X-ray components within the core spectrum. We observe wedge-like depressions in diffuse X-ray surface brightness surrounding the jets, and interpret them as regions where an aged population of electrons provides pressure to balance the interstellar medium of NGC4261. About 20 per cent of the mass of the interstellar medium has been displaced by the radio source. Treating 3C270 as a twin-jet system, we find an interesting agreement between the ratio of jet-to-counterjet length in X-rays and that expected if X-rays are observed over the distance that an outflow from the core would have travelled in ~ 6 × 104 yr. X-ray synchrotron loss times are shorter than this, and we suggest that most particle acceleration arises as a result of turbulence and dissipation in a stratified flow. We speculate that an episode of activity in the central engine beginning ~ 6 × 104 yr ago has led to an increased velocity shear. This has enhanced the ability of the jet plasma to accelerate electrons to X-ray-synchrotron-emitting energies, forming the X-ray jet and counterjet that we see today.
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