<p>Ground-based radial velocity surveys can be affected by strong aliasing because the cadence of the observations is 1 per day. This can produce alias frequencies in the periodogram at <span class="math-container">$1$</span> day<span class="math-container">$^{-1}$</span> <span class="math-container">$\pm$</span> the true frequency. You can think of these as beat frequencies between the true frequency and the observation frequency.</p>
<p>If you have a true frequency of <span class="math-container">$(1/4.23) = 0.236$</span>, then you could get aliases at frequencies of <span class="math-container">$\sim 1.236$</span> day<span class="math-container">$^{-1}$</span> or <span class="math-container">$0.764$</span> day<span class="math-container">$^{-1}$</span>. The latter corresponds to a period of 1.31 days and is I suspect what you have found.</p>

CC BY-SA 4.0
2022-12-13T03:36:06.517Z
Thank you, I came across a paper that offers a similar explanation: https://arxiv.org/abs/1502.01344. However I'm still not entirely satisfied with this answer since the raw data is far from evenly sampled: https://exoplanetarchive.ipac.caltech.edu/data/ExoData/0113/0113357/data/UID_0113357_RVC_005.tbl

Please let me know what your thoughts are. Thank you once again.
chuffyduffy
CC BY-SA 4.0
2022-12-13T06:55:38.870Z
@chuffyduffy why not generate some fake data at the same observation times, but with a different real period and see if the second peak shifts accordingly?
ProfRob
CC BY-SA 4.0
2022-12-11T17:02:07.893Z
2022-12-11T19:39:20.730Z
2022-12-11T19:39:20.730Z
ProfRob
<p>I was messing around with some radial velocity data from NASA's Exoplanet Archive and the Lomb-Scargle periodogram for Pegasi 51 yielded two very sharp peaks. One corresponds to about 4.23 days which is the reported value for Pegasi 51b while the other is about 1.3 days. Existing archives and paper only account for one orbiting planet. Would anyone happen to know why or what caused this extra signal to be picked up? Thank you!
<a href="https://i.sstatic.net/dm3L3.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/dm3L3.png" alt="Lomb-Scargle Periodogram Peg51" /></a></p>

<observational-astronomy><exoplanet><radial-velocity>
Why is there an extra peak in the Lomb-Scargle periodogram?
chuffyduffy
# Why is there an extra peak in the Lomb-Scargle periodogram?

(Asked by: username_0 on 2022-12-11)

I was messing around with some radial velocity data from NASA's Exoplanet Archive and the Lomb-Scargle periodogram for Pegasi 51 yielded two very sharp peaks. One corresponds to about 4.23 days which is the reported value for Pegasi 51b while the other is about 1.3 days. Existing archives and paper only account for one orbiting planet. Would anyone happen to know why or what caused this extra signal to be picked up? Thank you!
[![Lomb-Scargle Periodogram Peg51](https://i.sstatic.net/dm3L3.png)](https://i.sstatic.net/dm3L3.png)

**Tags:** observational-astronomy, exoplanet, radial-velocity

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## 1 answers

### Answer by username_1 on 2022-12-11. Score: 5

Ground-based radial velocity surveys can be affected by strong aliasing because the cadence of the observations is 1 per day. This can produce alias frequencies in the periodogram at $1$ day$^{-1}$ $\pm$ the true frequency. You can think of these as beat frequencies between the true frequency and the observation frequency.

If you have a true frequency of $(1/4.23) = 0.236$, then you could get aliases at frequencies of $\sim 1.236$ day$^{-1}$ or $0.764$ day$^{-1}$. The latter corresponds to a period of 1.31 days and is I suspect what you have found.

> **Comments:**
> **username_0**: Thank you, I came across a paper that offers a similar explanation: https://arxiv.org/abs/1502.01344. However I'm still not entirely satisfied with this answer since the raw data is far from evenly sampled: https://exoplanetarchive.ipac.caltech.edu/data/ExoData/0113/0113357/data/UID_0113357_RVC_005.tbl  Please let me know what your thoughts are. Thank you once again.  
> **username_1**: @chuffyduffy why not generate some fake data at the same observation times, but with a different real period and see if the second peak shifts accordingly?  

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