Investigating the radio emission in the Perseus cluster with LOFAR sub-80 MHz LBA

Abstract

The Perseus cluster is a nearby cool-core galaxy cluster that hosts an archetypal radio mini-halo. Recent Low Frequency Array (LOFAR) High Band Antenna (HBA) observations at 120 - 168 MHz have revealed the presence of a giant radio halo within the cluster with a size of 1.1 Mpc enveloping the mini-halo. By exploring the spectral properties of the radio emission at low frequencies, we can gain deeper insights into the nature of this emission and improve our understanding of its origin. Here we present LOFAR Low Band Antenna (LBA) images of the cluster between 30.0 - 57.7 MHz, with a resolution of 19.2'' x 15.0'' and a r.m.s. noise of 3.7 mJy/beam . In our images, we detect both the mini-halo and giant radio halo. We measured the spectral indices between 44 and 144 MHz of the mini-halo and giant radio halo to be -1.34 +- 0.10, and -1.01 +- 0.11, respectively. An alternative and more direct measurement of the spectrum of the giant radio halo results in a spectral index of -1.28 +- 0.15. The discrepancy between both values is caused by the poor ionospheric conditions. In addition, we study two X-ray 'ghost cavities' in the cluster. These cavities are thought to have been produced by an older outburst from the central AGN 3C 84. We measure a spectral index between 44 and 144 MHz for the radio plasma in these cavities of -1.86 +- 0.12 and -1.90 +- 0.12 for the northwest and southern ghost cavities, respectively. Furthermore, by including VLA 352 MHz data, we find that the spectrum steepens at higher frequencies. These results are consistent with the ghost cavities being filled with old and aged radio plasma. We also detect the tailed radio galaxies NGC 1265 and IC 310. In our analysis, these sources show signs of spectral steepening along their tails.

Figures (16)

• Figure 1: Full-resolution image from the LOFAR LBA (30.0--57.7 MHz of the core of the Perseus cluster. The restoring beam is shown in the bottom left (19.2$\times$15.0 ). The colour range is displayed on a logarithmic scale.
• Figure 2: LOFAR LBA image of the Perseus Cluster (30.0--57.7 MHz) tapered to 67 (left) and 131 (right). The tailed radio galaxies NGC 1265 and IC 310 are marked. For image details, see Table \ref{['tab:details']}.
• Figure 3: Radial surface brightness profile of the diffuse radio emission in the Perseus cluster. The dashed lines represent two exponential profiles that add up to the continuous line (Eq. \ref{['eq:profile']}). The model was fitted using the extracted radial profile of the Perseus image, with point sources subtracted and residual sources masked out, starting from 43 kpc from the centre to 650 kpc. We note that the error bars do not include the 10% uncertainty on the adopted flux density scale, as this uncertainty is identical between all data points. In the bottom panel, the spectral index between LOFAR LBA (43 MHz) and LOFAR HBA (144 MHz) is shown with error bars (which do not include the 10% uncertainty). We note that at larger radial distances from 3C 84, the error on the spectral index drastically increases.
• Figure 4: Comparison between radio emission detected with LOFAR LBA (native resolution) and X-ray emission (with a radially smooth profile subtracted). Radio contours are drawn at the levels labelled in the image.
• Figure 5: Spectral index map of the minihalo of the Perseus cluster between 44 and 144 MHz. The HBA map has been smoothed to the resolution of the native LBA map. The associated spectral index uncertainty map is displayed in Fig. \ref{['fig:spix_err']}. We note that 3C 84 is located at the centre, and it has been blanked out for clarity. The red circles refer to the ghost cavities whose spectra are shown in Fig. \ref{['fig:cavity spectrum']}. The noise in this image is higher than the noise reported in Table \ref{['tab:details']} due to the proximity of 3C 84.