In the panels, the fitting function with $n$=3 does not reproduce the data and a higher $n$th-order polynomial function is required [^4]. Thus, the remaining signal could be corrupted by the foreground residuals. In order to reduce the residual contamination, we attempt to fit with $n=7$ and the reproduction of the data is better than $n=3$. However, the $7$-th polynomial fitting slightly improves the results discussed following section. [Thus, we adopt $n=7$ for the foreground removal in this work.]{} Note that the higher order polynomial fitting function could fit out the EoR signal.

Generally, the expected spectral index of the synchrotron emission is negative. However, as shown in the right panel of Fig. \[fig:fit\], the data shows a positive index, which is understood as below. The average intensity is subtracted from actual intensity since the interferometer cannot observe the visibility at $(u,v) = (0,0)$. Then, for example, the intensity of image is [$F (\nu, \,\theta)-F_{\rm ave}(\nu)$]{}, where true flux of a LOS is [$F(\nu, \,\theta)=F_0\nu^{\alpha}$]{} and averaged flux is $F_{\rm ave}(\nu) = F_{\rm ave,0} \nu^{\overline{\alpha}}$. If the index $\alpha$ is larger than the $\overline{\alpha}$, the index of observed intensity can be positive.

Fig. \[fig:FGRM\] shows the image at 171 MHz 