Temporal and Spatial Variation of Synchrotron X-ray Stripes in Tycho's Supernova Remnant.

The synchrotron X-ray "stripes" discovered in Tycho's supernova remnant (SNR) have been attracting attention since they may be evidence for proton acceleration up to PeV. We analyzed Chandra data taken in 2003, 2007, 2009, and 2015 for imaging and spectroscopy of the stripes in the southwestern region of the SNR. Comparing images obtained at different epochs, we find that time variability of synchrotron X-rays is not limited to two structures previously reported but is more common in the region. Spectral analysis of nine bright stripes reveals not only their time variabilities but also a strong anti-correlation between the surface brightness and photon indices. The spectra of the nine stripes have photon indices of \Gamma = 2.1--2.6 and are significantly harder than those of the outer rim of the SNR in the same region with \Gamma = 2.7--2.9. Based on these findings, we indicate that the magnetic field is substantially amplified, and suggest that particle acceleration through a stochastic process may be at work in the stripes.