  In a range of citation networks, the in-degree distributions boast
time-periodicity---the distributions of citations per article published each
year present similar scale-free tails. This phenomenon can be regarded as a
consequence of the emergence of hot topics and the existence of the "burst"
phenomenon. With this inference considered, a geometric model based on our
previous study is established, in which the sizes of the influence zones of
nodes follow the same power-law distribution and decrease with their ages. The
model successfully reproduces the time-periodicity of the in-degree
distributions of the empirical data, and accounts for the presence of citation
burst as well. Moreover, a reasonable explanation for the emergence of the
scale-free tails by regarding the citation behavior between articles as a
"yes/no" experiment is presented. The model can also predict the
time-periodicity of the local clustering coefficients, which indicates that the
model is a good tool in researches on the evolutionary mechanism of citation
networks.
