Long term periodicity analysis of the spectral index of 2251+158

We analyze the radio spectral index of blazars from the University of Michigan Radio Astronomy Observatory database,and find that there exists quasiperiodic activity in 2251+158.The long-term periodicity analysis is accomplished by three methods,which are the Jurkevich method(Jurkevich),the discrete correlation function(DCF),and the Periodogram method(P).The results show that 2251+158 has two strong periodicities,which are P1=6.3±1.1 yr and P2=3.8±1.2 yr.