Gentle reenergization of electrons in merging galaxy clusters

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Science Advances  04 Oct 2017:
Vol. 3, no. 10, e1701634
DOI: 10.1126/sciadv.1701634
  • Fig. 1 Composite image showing optical, radio, and x-ray emission of the galaxy cluster Abell 1033.

    The background image shows optical data from the Sloan Digial Sky Survey (i, r, and g filters). In purple, we show the x-ray surface brightness (0.5 to 4 keV, from the Chandra X-ray Observatory) tracing thermal gas, and in blue, we show the radio emission [from Low-Frequency Array (LOFAR)] tracing CR. The source labeled GReET is the main topic of this work. The source labeled Southern source is a probable radio phoenix (23). The image size is around 1 Mpc × 1 Mpc.

  • Fig. 2 Radio images at 608, 323, 148, and 142 MHz. The telescope resolution is shown in the lower left corner of each panel.

    Contours are at (1, 2, 4, 8, 16, 32, 64) × 5 σ (solid lines) and −5σ (dashed lines), with noises as in Table 1. The GReET is not detected at 608 MHz down to sensitivity of 38 μJy per beam because of its very steep spectrum (α ≃ − 4). The low resolution of the Giant Metrewave Radio Telescope (GMRT) image at 148 MHz does not allow to separate the two trails of the GReET.

  • Fig. 3 Spectral index map of the radio emission in Abell 1033.

    (Left) Spectral index values (142 to 323 MHz) of the radio emission. Contours are from LOFAR observation at 142 MHz as in the last panel of Fig. 2. To trace the evolution of the spectral index along the tail of the WAT radio galaxy, we defined 20 beam-sized regions. (Right) Each point in the plot is associated with one region defined in the left panel. Lines show the spectral index and the 142-MHz flux density predicted by a model, as explained in the text, assuming three possible values for the magnetic field. The position of point 1 has been shifted on the x axis to match the model spectral index for the 2.3-μG case, and the age axis has been stretched to fit the spectral index data (reflecting the unknown galaxy speed). Star markers in the plot show the expected spectral index and flux density if a gentle reacceleration starts at 450 My. Green stars assume no projection effect, while yellow stars assume a 40° inclination of the tail with respect to the line of sight.

  • Fig. 4 Spectral index error map.

    Each pixel shows the standard deviation of the bootstrapped spectral index distribution.

  • Table 1 Radio observations.
    TelescopeFrequency (MHz)Observation dateTotal observation time (hours)Bandwidth (MHz)ResolutionRMS noise (μJy per beam)
    LOFAR14224 November 201584810.3″ × 4.9″180
    GMRT14826 December 2014816.422.2″ × 17.9″800
    GMRT3232 November 2014532.811.2″ × 7.4″42
    GMRT6082 December 2014432.85.8″ × 3.7″38

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