HomePlanetFalse Positives: W-shaped transits | Planet Hunters

False Positives: W-shaped transits | Planet Hunters

Computer systems are superb, however generally they do one thing sudden. On this weblog submit, we clarify the rationale you generally discover a W-shaped transit on Planet Hunters NGTS.

On Planet Hunters NGTS, we present you phase-folded mild curves that are pictures produced by a pc algorithm that has calculated the most effective guess on the interval of the transiting object (if there may be one!). You may learn extra about how that is completed on this weblog submit. Nonetheless, generally the pc algorithm would possibly calculate this era incorrectly. This could occur for quite a few causes however very often it may be as a consequence of a deep secondary transit in an eclipsing binary system that the algorithm misinterprets as one other major transit. Within the picture under, the first transits are the deep transits when the yellow star is totally blocked (occulted) by the redder star. The secondary transit is the shallower dip within the centre of the plot when the smaller star is obstructing a part of the big, purple star. Word that this plot reveals each the uncooked knowledge factors in gray and the binned knowledge in purple (defined under).

Determine 1: Diagram of an eclipsing binary system and a piece of its mild curve

Within the instance above, the first transits are at 0.5 and three.0 days subsequently the true orbital interval is the distinction between them, 2.5 days. If the pc accurately identifies this then you definately could be offered with a picture zoomed in on the first transit. Nonetheless, if the algorithm doesn’t accurately discover the distinction in depths then it could calculate the interval to be 1.25 days, half of the true interval, because it believes the shallow transit within the center to be one other major transit. As soon as the algorithm has decided a interval, it “bins” the uncooked knowledge factors (gray) to create the purple knowledge factors. Which means that we calculate the typical flux of all of the factors in a set time window, which is known as a bin. Every purple level within the plot above corresponds to a 30 minute bin and can comprise round 140 uncooked knowledge factors on common because the NGTS telescopes take a picture of the sky each 13 seconds. (The telescope cameras use 10 second exposures, adopted by a 3 second delay earlier than the following publicity whereas the shutter closes and the digicam CCD reads out the flux on every pixel).

If the algorithm has folded the first and secondary transit factors on high of one another, as proven under, then the binning course of will mix the uncooked fluxes into a mean worth someplace in between the true flux values. This leads to the inaccurate W-shaped transit!

Determine 2: An eclipsing binary folded utilizing half the true orbital interval

The following determine is a picture from the Planet Hunters NGTS web site that reveals this W-shape, though it’s solely a really slight impact. On this case, the topic gained sufficient votes to be pushed via to the odd/even transit depth verify. That is anticipated since we but don’t have a classification possibility for W-shaped transits.

Determine 3: W-shaped transit on Planet Hunters NGTS

The odd/even transit verify permits us to immediately spot the totally different depths of the first and secondary transit. We will see that the magenta factors correspond to the deeper major transit whereas the inexperienced factors present the shallower secondary transit. The distinct V-shape of each transits, in addition to the rising flux after the transit are stereotypical of eclipsing binary methods.

Determine 4: Odd/Even transit verify for a “W-shaped” transit

Should you spot one thing like this, then attempt to classify the form of the transit as greatest you possibly can into both U or V-shape. Within the case of the instance above, I’d select V-shaped, however should you want extra assist then try this weblog submit. Additionally, be at liberty to mark it as #w-shaped on the Speak channel, it helps us to verify why this occurs when our laptop processes the info. As soon as we now have analysed the speed at which this type of transit form happens, we could introduce a W-shaped classification class to the interface, permitting you to extra simply assist us filter out these false positives.



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