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tokens: between 323 and 608 MHz (black solid-line). Radio flux density at 323 MHz across the source extension (red dashed-line). The data-points correspond to the regions displayed in Fig. 1d . Error-bars on the -axes correspond to the region size. Table 2 Galaxy cluster properties. Notes. (a) Coordinates of the BCG. (b) Rescaled with spectral index from 323 MHz. (c) k-corrected with spectral index 0.7. 4. Discussion A prediction from simple DSA theory is a relation between the Mach number of the shock and the injection spectral index of the accelerated electrons: Therefore, a spectral index implies an unphysical Mach number. In the case of this specific relic the injection spectral index appears extremely flat. If we assume the outer region as representative for the injection index we find: . This value seems to point towards a stronger Mach number shock than what is usually found for radio relics. However, we note that Eq. ( 2 )has been shown not to hold in at least the well-studied case of the “toothbrush” cluster ( van Weeren et al. 2016 ). Also the integrated spectral index (extracted from -harmonised images at 323 and 608 MHz) appears quite flat () with respect to the average spectral index of radio relics (; Feretti et al. 2012 ). As already noted for another radio relic with a flat integrated spectral index (Abell 2256; van Weeren et al. 2012b ; Trasatti et al. 2015 ), if the stationary shock conditions do not hold. As a consequence, a possible way to reconcile the flat spectrum with shock acceleration is that the relic must have been produced very recently (0.1 Gyr ago). This means that the time for energy losses is longer than the age of the shock at some observed energies and that the integrated spectrum of the relic will still evolve, getting steeper. We note that using the total flux density at 148, 323, 608, and 1400 MHz ( mJy; from NVSS), we obtain a steeper integrated spectral index of . However, given the different -coverages, these maps can be sensitive to different spatial scales. The X-ray luminosity extracted from ROSAT data is erg/s. From this luminosity, we derived the associated cluster mass: ( Pratt et al. 2009 ). The cluster is amongst the least massive clusters hosting a radio relic discovered so far (see Fig. 7b in de Gasperin et al. 2014 ). Only two other less-massive systems are known to host radio relics: Abell 3365 (, , from X-ray; van Weeren et al. 2011 ) and Abell 3376 (, , from SZ; Kale et al. 2012 ). Both galaxy clusters host a double radio-relic system. In de Gasperin et al. (2014) , we discussed the presence of a scaling relation between radio-relic power and hosting-cluster mass. Given that relation, Abell 3527-bis is almost an order of magnitude over-luminous for the hosting cluster mass. Nonetheless, Abell 3527-bis provides a rare example of low-mass cluster hosting a radio relic at . Nuza et al. (2012) investigated the predicted abundance of radio relics in current and future radio surveys. They estimated the