Extracted radii from the fits of $\pi-\pi$ correlations as a function of $m_{\rm T}$ for three different multiplicity classes. From left to right and bottom: $1 \leq N_{\mathrm{ch}} \leq 18$, $19 \leq N_{\mathrm{ch}} \leq 30$ and $N_{\mathrm{ch}} \geq 31$. The reduced $\chi^2$ for the obtained radii is shown in the lower panels and evaluated in the range of $6 < k^{*} < 100$ MeV/$c$. The assumption that a second-order polynomial background shape exhibits either greater or equivalent compatibility with the data compared to a first-order polynomial.