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tokens: The model includes galactic and isotropic diffuse emission components and all 3FGL sources within 8° of the position of interest. We optimize the parameters of the model with unbinned likelihood analysis using the gtlike tool. Normalization of the galactic and isotropic diffuse emissions and spectral parameters of the target source were considered as free parameters, while the spectral parameters of all neighboring sources were fixed at the 3FGL values. The spectral form for each source is a power law, a log-parabola, or a power law with exponential cutoff based on the 3FGL data. Next, using the gtsrcprob tool each photon is assigned a weight—the probability that it originates from the given pulsar candidate. For computational efficiency we keep 40,000 events with the highest weights for each source. Finally, the photon arrival times are converted to the barycentric frame using the gtbary tool. 3. METHOD The search for pulsations is performed with the time-differencing technique first proposed by Atwood et al. ( 2006 ) and subsequently refined by Pletsch et al. ( 2012a ). We scan over the pulsar’s frequency f and spin-down rate using the array of barycentric photon arrival times t a and corresponding weights w a . First, the arrival times are corrected to compensate for frequency evolution:where and t 0 = 286,416,002 (MJD 55225) is a reference epoch. The spectral density P m is obtained with a Fourier transform of the time differenceswhere , , and is the rectangular function that takes the value 1 for and 0 otherwise. Technically we consider only positive time differences and bin the time interval into bins. Within the cycle over all event pairs one calculates the sum of the product of weights in each bin. This prepares an input for the fast Fourier transform performed with the open source library fftw (Frigo Johnson 2005 ). 3 3 http://www.fftw.org We use the same value of T as Pletsch et al. ( 2012a ). The Nyquist frequency is , which determines the choice of N . The relatively high value of the Nyquist frequency is required to minimize the artificial binning noise. keeping in mind that some pulsars may be found at the second harmonic of the frequency. We scan over the parameter γ from 0 to with a step equal to . The range corresponds to a characteristic age of the pulsar greater than 1 kyr. The step is selected taking into account the computation time available for the project. The value used introduces a moderate loss of power for pulsations with frequencies higher than 10 Hz; see Equation (3) of Ziegler et al. ( 2008 ). The values of f and corresponding to the highest P m are used as a starting point for the final coherent scan. The latter is performed with maximization of the weighed H -test statistic (Jager et al. 1989 ) defined as follows:where is a Fourier amplitude of the l th harmonic The maximization is performed by a scan over f and γ with steps of and . The steps are chosen to ensure coherence over the whole time interval of