In the recent paper by Mielczarek et al. (Ref. [8]), an idea of the method which can be used to put some constraint for the reheating phase was proposed. Another method of constraining the reheating temperature has been recently studied by Martin and Ringeval (Ref. [17]). Both methods are based on observations of the cosmic microwave background (CMB) radiation. In this paper, we develop the idea introduced in this first article to put constraint on the reheating after the slow-roll inflation. We restrict our considerations to the case of a massive inflaton field. The method can be, however, easily extended to the different inflationary scenarios. As a main result, we derive an expression on the reheating temperature $T_{\mathrm{RH}}$. Surprisingly, the obtained equation is independent on the unknown number of relativistic degrees of freedom $g_{*}$ produced during the reheating. Based on this equation and the WMAP 7 observations, we find $T_{\mathrm{RH}}=3.5\times {10}^6\mathrm{GeV}$, which is consistent with the current constraints. The relative uncertainty of the result is, however, very high and equal to $\sigma (T_{\mathrm{RH}})/T_{\mathrm{RH}}\approx 53$. As we show, this uncertainty will be significantly reduced with future CMB experiments.

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