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The central engine of gamma-ray bursts (GRBs) is poorly constrained. Thereexist two main candidates: a fast-rotating black hole and a rapidly spinningmagnetar. Furthermore, the X-ray plateaus are widely accepted by the energyinjection into the external shock. In this paper, we systematically analyze the\emph{Swift}/XRT light curves of 101 GRBs having plateau phases and knownredshifts (before May 2017). Since a maximum energy budget($\sim2\times10^{52}$ erg) exists for magnetars but not for a black hole, thisprovides a good clue to identify the type of GRB central engine. We calculatethe isotropic kinetic energy $E_{\rm K,iso}$ and the isotropic X-ray energyrelease $E_{\rm X,iso}$ for individual GRB. We identify three categories basedon how likely a black hole harbor at central engine: 'Gold' (9 out of 101, both$E_{\rm X,iso}$ and $E_{\rm K,iso}$ exceed the energy budget), 'Silver' (69 outof 101, $E_{\rm X,iso}$ less than the limit but $E_{\rm K,iso}$ is greater thanthe limit), and 'Bronze' (23 out of 101, the energies are not above the limit).We then derive and test the black hole parameters with the Blandford Znajekmechanism, and find that the observations of the black hole candidate('Gold'+'Silver') samples are consistent with the expectations of the blackhole model. Furthermore, we also test the magnetar candidate ('Bronze') samplewith the magnetar model, and find that the magnetar surface magnetic led($B_{p}$) and initial spin period ($P_{0}$) are consistent with theexpectations of the magnetar model. Our analysis indicates that, if themagnetar wind is isotropic, a magnetar central engine is possible for 20\% ofthe analyzed GRBs. For most GRBs a black hole is most likely operating.

Constraining the Type of Central Engine of GRBs with Swift Data