Light curve analysis of X Aur

I was doing some light curve analysis on the long period Mira star X Aur. I used Peranso to download the last 10 years of data from AAVSO. Then I did an ANOVA analysis of it.

The strongest period was at 327.89 days, which is about twice the published period of 163.79 days. Here are the phase diagrams t the two periods.

To my eye the 327.89 period looks cleaner and it looks like there is a clear difference in the alternating peaks.

How is the period determined when alternating peaks differ? Is 327.89 the correct period or is 163.79?

Thanks,
Bill (TBOB)

Hi Bill.

The pulsation period is 164 d. Miras change in amplitude from cycle to cycle but there is no such thing like a double period like, e.g. in RV Tauri stars, where you see alternating minima that more or less remain stable (especially in the shorter period stars).
If you look at the JD light curve of miras (including X Aur), you will see that maxima and minima always vary.
A different thing are the double maxima miras like R Cen or R Nor, but they have longer periods and maxima are different both in magnitude and color behaviour.
In X Aur I only see typical mira star behaviour.

I have updated the VSX entry to show the most representative period for all the data. Period has changed ove the decades so it is impossible to fold all the observations with a single period.

Cheers,
Sebastian

2 Likes

Sebastian,

Thanks for the explanation.

Bill

Hi Bill,
I was wondering why your peak for the double period is stronger than the real period peak. I used VStar to analyze the V Band data from the database and the real peak is by far stronger both with ANOVA as well as with DC DFT.
Then I did the same for the visual data (which cover obviously a much longer history) and indeed, the difference goes down in magnitude in the ANOVA case, but with DC DFT the real period peak is still much stronger.
ANOVA is in my opinion not the best analysis tool for periodicity. I always use DC DFT or in some cases lomb scargle which I have not tried here as there are so many data covering all the time.
Next I used the WWZ tool to identify changes in periodicity over time for the visual data. There is some fluctuation, but no clear trend.
image
Matthias

Here are O-C and period diagrams from my database with Mira Maxima. My impression is that the period are quite stable. Maybe it has a small tendency to increase over its whole timespan, but I think that tendency drowns in the random cycle to cycle-behavior all miras have.

Thomas Karlsson

XAur_OC
XAur_per

Matthais,

You may be right that ANOVSA is not the best choice here. I am using Peranso with has a wide variety of methods to analyze periodicity.

The author has this to say about which method to choose:

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Peranso offers a wide variety of methods to analyse periodicities. An obvious question therefore is : which method should I use for what type of object (e.g., variable star). Are some period analysis methods better suited than others for specific types of variable stars or asteroids ?

This is a very difficult question to answer, and unfortunately there is no such thing as the “universal” period analysis method, that is the best choice for whatever type of object. Below, we present some simple guidelines, that may be helpful in answering the question.

There’s an excellent paper by M.J. Graham et al, which compares several period finding algorithms, and addresses the above question too. It analyses the accuracy, reliability, performance etc of the algorithms.

Whatever method of Peranso you decide to use, always take following two remarks into consideration:

all period determination methods are dependent on the quality of the observations. Low quality light curves (few data points, noisier data) will produce results with lower reliability.

Use your eyes and your brain first before starting a period analysis. Draw a light curve of the observations, and inspect its shape, signature, characteristics, etc. We call this visual inspection. That by itself usually reveals very significant information. Unfortunately, our brain is not faultless, so we have to rely on other approaches too, using mathematical techniques. That’s where Peranso enters the picture.

The selection of a period analysis method may be influenced by many things :

the amount of observations

their spread in time (equally or unequally spaced)

type of variations (regular-shaped or not)

expected physical properties of the system (can it be multi-periodic)

etc

As a rule of thumb :

Delta Cepheids and RR Lyrae variables in general can be quite well analysed with the Lafler-Kinman method

Eclipsing binaries may produce dominant periods at half-period values. Lomb-Scargle and Generalized Lomb-Scargle are more sensitive to this, and hence are not recommended for such variables. CE is very successful in handling these stars.

Regular periodic variables (T Tau stars, Cepheids, Delta Scuti stars, Mirae, RR Lyrae, …) are well suited for CE.

If you expect the system to be multi-periodic, use CLEANest.

If the light curve is highly non-sinusoidal, use ANOVA. Otherwise, you may consider DCDFT or CLEANest.

PDM is also well suited for highly non-sinusoidal data with only a few observations over a limited period of time.

FALC is a de facto standard for asteroid period analysis. Try that one first.

If you’re studying exoplanet transits, use EEBLS.

If you suspect that the period of your variable is not constant over time, then use WWZ

In developing Peranso, I have studied hundreds of light curves of many different objects. Although there is no “universal” period analysis method, there is one that - in my humble opinion - comes pretty close, and that’s ANOVA. I have been amazed by its power to improve peak detection sensitivity and to damp alias periods. Try it out yourself, and see if it suits your data. If not, there are many others to experiment with.

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Note the final paragraph where he says ANOVA is the most universal of analysis methods.

I tried all the methods in Peranso using the last 10 years of V data from AAVSO. Here are the ones that give an ~164 day period and those that give an ~328 day period.

164 Day

Lomb-Scargle
Generalized Lomb-Scargle
Bloomfield
DFT (Deeming)
DCDFT (Ferraz-Mello)
CLEANest (Forster)
Jurkewich
WWZ
PDM
Phase-binned AoV
CE

328 Day

ANOVA
FALC (Harris)
Dworetsky (both peaks almost even)
Renson (both peaks almost even)
Lafler-Kinman (both peaks almost even)

So yeah, more methods come up with a 164 day period. But still the author seems to think ANOVA is a good first choice. In general, it works well for me.

I have also a good experience of ANOVA in Peranso. Note that the method has a parameter in Peranso called “Nbr harmonics”. I think the default is 2. That value give a stronger peak for 328 days for X Aur. If you set Nbr Harmonics=1, then the 328 day peak disappear and the only strong peak is 164.

I have tried some different values for this parameter and noticed for another star that only 1 or 2 gave good results. For X Aur 1 seems best. Have anyone explored the function of this parameter?