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Abstract: We confirm a 23day periodicity during a large flare in 1997 for X-ray data of X-ray satellite RXTE all sky monitor (ASM), 2TeV gamma ray data from Utah Seven Telescope and HEGRA, with a Fourier analysis. We found the three results to be the same with a newly estimated error. We confirm the presence of a frequency dependent power (1/f noise) in a frequency–power diagram. Further, we calculated a chance probability of the occurrence of the 23day periodicity by considering the 1/f noise and obtained a chance probability 4.88×10−3 for the HEGRA data: this is more significant than the previous result by an order. We also obtained an indentical periodicity with another kind of timing analysis–epoch folding method for the ASM data and HEGRA data. We strongly suggest an existence of the periodicity. We divided the HEGRA data into two data sets, analyzed them with a Fourier method, and found an unstableness of the periodicity with a 3.4 sigma significance. We also analyzed an energy spectra of the X-ray data of a RXTE proportional counter array and we found that a combination of three physical parameters—a magnetic field, a Lorentz factor, and a beaming factor—is related to the periodicity. [Copyright &y& Elsevier]