James O. Chibueze
We report on the discovery of an interaction between relativistic electrons and intracluster magnetic field around radio galaxy in a violent merging galaxy cluster Abell 3376. Abell 3376 hosts a density contact discontinuity at the central region, where the radio galaxy is located. A contact discontinuity is formed during merging activities by cool and dense plasmacore run into hot ambient plasma. The cool core swept intracluster magnetic field and generate ordered and amplified field. The radio galaxy was observed by a new-generation radio telescope MeerKAT at 1.4 GHz. Contrary to the typical radio galaxies whose jets bent along with plasma motion, the jet from radio galaxy shows almost 90-degree sudden bending against plasma flow at the density contact discontinuity. We performed simulations and found that ordered magnetic field may playsignificant role to change the direction of the jet propagation. The overall morphology of the bended jet bears remarkable similarities with high-resolution numerical simulations, which greatly strengthens our understanding of the interaction between relativistic electrons and intracluster magnetic field.
MeerKAT observations and reduction:
MeerKAT L-band (856 ~ 1712 MHz) observations was carried out on the 1st June, 2019 (PI: James Chibueze)
60 of 64 antennas of the MeerKAT array participated in the observations
Bandwidth: 856 MHz
Number of channels: 4096 (~209 kHz per channel)
Primary flux and bandpass calibrator: J0408-6545
Gain calibrator: J0538-4405
OXKAT (https://ascl.net/code/v/2627) used of calibration and imaging
XMM-Newton observations:
Observed with EPIC instruments on XMM-Newton X-ray Observatory (OBSID:0151900101)
0.5-2.0 keV images produced after proper processing
MeerKAT 1283 MHz image (Figure1 pseudo color) of radio galaxy in Abell 3376 cluster overlaid on the XMM-Newton x-ray image (gray scale background). A and B radio galaxies can been observed to have bent lobes feature.
Focusing on radio galaxy A, we constructed the spectral index map (Figure 2 left panel) using sub-band images created from the MeerKAT observation. Extracting the flux densities and corresponding spectral indices in the encircled points, we explored the trend in the changes in intensities of the radio emission and spectral indices.
Three distinct trends (R1, R2 and R3) can be observed in Figure 2 (right panel). R1 show a sharp drop in both intensities and spectral indices, R2 stayed fairly flat in both, while R3 show a slow decay in both intensities and spectral indices.
To understand the reason for the observed trend, our x-ray observation and numerical simulation hold key pieces of the puzzle.
Figure 3 show an image similar to Figure 1 but with an indication of a cold front (with clear temperature discontinuity) observed in x-ray. This is an indication of increased magnetic field created by the compression of the magnetic field around the merging cluster. Forming of such cold front and magnetic field layer have been shown to be possible by Asai et al. 2004, 2007 (see also Figure 4).
Figure 4 shows our numerical simulation results (a. 3-D view of the bent jet lobe due to relativistic electron interaction with the compressed magnetic field of the merging cluster, Abell 3376, (b) initial conditions of the simulations) and (c) 2-D view of the same results). These results confirm the observed R1, R2, R3 trends in intensities, especially the brightening observed in R2, the point of interaction between relativistic electron and compressed magnetic field.
Conclusions
We reported the first ever convincing observational evidence of an interaction between relativistic electron and compressed magnetic field of a merging cluster (Abell 3376)
Numerical simulations supported the observed features and showed a magnetic field of ~ 65 microGauss is required for such interaction.
Multi-wavelength view of observed features in radio galaxies is undoubtably the best way to unveil the underlying physics behind them.
These results will completely change our current view of intracluster interactions.