Large scale X-ray image

Figure 1: Adaptively smoothed three-colour Chandra/ACIS-S image of the 3C 401 cluster overlaid with contours of 20cm radio-emission from MERLIN. Note the cross-like structure displayed by the ICM. See text for a discussion of how this image was constructed and the robustness of the cross-like structure. North is up in this image.

Figure 1 shows an adaptively smoothed 3-color Chandra image of the general region surrounding 3C 401, overlaid with contours of 20cm emission from MERLIN. To produce this X-ray image, we first extracted and exposure-corrected images from the cleaned filtered level-2 events file in three bands; 0.3-0.8keV (soft band), 0.8-2keV (medium band), and 2-10keV (hard band). The total image (i.e., the sum of the three bands) was adaptively smoothed using a $3-4\sigma$ smoothing kernel, and the resulting map of smoothing lengths was then applied to each of the images in the three bands separately. The three images are then overlaid to form the 3-color image, with red, green and blue denoting the soft, medium and hard band images respectively. At the distance of 3C401, this image covers a square region 680kpc on a side.

As can be seen from Fig. 1, we clearly detect an extended ICM halo centered on 3C 401, although we fail to detect any spatial variations in the hardness of the emission. Interestingly, the ICM appears to possess a cross-like structure, with four almost perpendicular spurs extending in the NNE/ESE/SSW/WNW directions. The NNE/SSW axis is exactly that defined by the radio-jets of 3C 401. Given that this is an adaptively smoothed image, one might be concerned that these spurs extend approximately in the direction of the point sources also seen in the field. However, precisely the same spur pattern is observed if one first removes the point sources prior to producing the adaptive smoothing map. One might also be concerned about the validity of the exposure map used to produce these images. However, the spurs are seen in both exposure corrected and non-exposure corrected images, and the amplitude of variations across the spurs (30-50%) is substantially greater than can reasonably be explained by any exposure map effect. Thus, we conclude that these spurs are real. We return to this issue in Section 3.4 where we further verify the reality of these spurs through surface brightness profiles derived from unsmoothed data.

We now proceed to discuss the core of this system and, in particular, evidence for radio-galaxy/ICM interaction.