A recently discovered young, high-velocity giant star J01020100-7122208 is a good candidate of hypervelocity star ejected from the Galactic center, although it has a bound orbit. If we assume that this star was ejected from the Galactic center, it can be used to constrain the Galactic potential, because the deviation of its orbit from a purely radial orbit informs us of the torque that this star has received after its ejection. Based on this assumption, we estimate the flattening of the dark matter halo of the Milky Way by using the Gaia DR2 data and the circular velocity data from Eilers et al. (2019). Our Bayesian analysis shows that the orbit of J01020100-7122208 favors a prolate dark matter halo within $sim$ 10 kpc from the Galactic center. The posterior distribution of the density flattening $q$ shows a broad distribution at $q gtrsim1$ and peaks at $q simeq 1.5$. Also, 98.5% of the posterior distribution is located at $q>1$, highly disfavoring an oblate halo.
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