[AstroPy] timeing analysis(scipy.fftpack)

[gonghang.naoc]

Hi Matteo,
L-S and FFT should get a similar result  if we 'treat' the data correctly?
Using the same regularly sampled data, I meet artifacts generated by some
method, but this kind of artifact does not necessarily appear in some other
method.

Hang

On Thu, Oct 22, 2015 at 3:20 PM, Matteo <matteo at matteobachetti.it> wrote:

> Hi Hang,
> that plot does not measure significance but just the value of the FFT. In
> the real world, significance depends also on the uncertainties of the time
> series. For example, for event data (and Poisson-distributed light curves)
> people often calculate the power density spectrum (|FFT|^2) and normalize
> it by 2 / <number of photons>. Then, significances are calculated based on
> the chi^2 statistics (Leahy et al. '83).
>
> For the comparison between L-S and fft, it's difficult to answer without
> at least some example data. Do you take the square modulus of the FFT? Do
> you use an irregular sampling? Do you have gaps in data? How do you
> normalize the periodograms?  Etc.
> e.g.
> http://stackoverflow.com/questions/20658318/my-power-spectra-are-believable-a-comparison-between-lomb-scargle-and-fft-scip
>
> Cheers,
>
> Matteo
>
>
> PS: for X-ray timing analysis, you can take a look at
> http://maltpynt.readthedocs.org
>
>
> On Thu, Oct 22, 2015 at 6:21 AM, gonghang.naoc <ghang.naoc at gmail.com>
> wrote:
>
>> Hi all,
>> Does anyone use scipy.fftpack to do power spectrum for time series?
>> Its doc file is here:
>> http://docs.scipy.org/doc/scipy-0.14.0/reference/tutorial/fftpack.html
>>
>> What does the y axis in figure 1 and figure 2 mean? In Figure 1, it seems
>> y axis does not  stand for the sifnificane at least.
>>
>> Besides FFT, I use Lomb-Scargle to do timing analysis, and find there is
>> a big difference between the significance given by L-S and fft. Significant
>> peaks which appear in L-S do not necessarily appear in FFT.
>> Which method should I believe then?
>>
>> Thank you.
>> regards
>> Hang
>>
>>
>>
>>
>>
>>
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>>
>
>
> --
> *********************************************
> Matteo Bachetti
>
> www.matteobachetti.it
>
> "I would love to change the world,
> but they won't give me the source code"
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