I obtained the optical light curve of 3C 273 QSO from the MAST repository off the K2 mission and plotted it by making the FITS into a CSV file through Pandas dataframe and plotting the obtained columns using lightkurve package in Python.

The SAP_FLUX and the PDCSAP_FLUX vs the Time gives a gradual steep slope but when the steepness is over and the slope is climbing up, the steepness is very dramatic, in the sense, that it's very precipitous and not gradual like how it should be.

I am attaching the image of a part of the light curve. My teacher was quick to spot that but I totally missed it while including it in my project. Thanks.

EDIT: Since my question is not clear as pointed out in the comments (apologies for it!), I will reformulate it. My question is that why are the curves rising so precipitously (as seen in the image) and when they are coming down from the peak the change is gradual and slow. Shouldn't both of them be rising and falling gradually? enter image description here

  • 2
    $\begingroup$ Can you edit your question to actually provide a concrete question you'd like someone to answer? As is, you have no actual question for someone to address in your post. $\endgroup$
    – zephyr
    Commented Oct 14, 2021 at 15:24
  • $\begingroup$ If your question is about the origin of this, you'd habe to understand whether such brightness - changes would be normal when symmetric and what processes drive brightness variations in such objects. If neither seems to provide an answer look for instrumental effects, possible only affecting this observations or a few around that time or region $\endgroup$ Commented Oct 14, 2021 at 17:26
  • $\begingroup$ I made an edit to address the question directly. Thanks for pointing out. $\endgroup$
    – mnuizhre
    Commented Oct 15, 2021 at 14:04
  • $\begingroup$ I see that the light curve has been smoothed out to some extent, how separated are the individual data points? Is the steep jumps transition from one data point to another? $\endgroup$
    – Justin T
    Commented Oct 15, 2021 at 16:20
  • $\begingroup$ QSOs show variability in flux in the observer's frame the spikes indicate the variability in the optical regime. The gradual fall show the decrease and the rise show the increase in the observed fluxes. $\endgroup$
    – mnuizhre
    Commented Oct 16, 2021 at 3:38

1 Answer 1


While I am not really familiar with Kepler, there is the following paper by Vanderburg & Johnson, where they explain:

Extracting raw photometry and centroid positions from the K2 images results in flux and position time series characterized by jagged features occurring several times a day. Figure 1 shows an example of a “raw” light curve and image centroid positions for a 10th magnitude K2 target. The position of the star on the detector drifts on time scales of hours and then is quickly corrected back to its starting point when the spacecraft thrusters fire. The changes in position introduce significant artifacts into the raw K2 photometry, which must be corrected in order to achieve the photometric precision characteristic of the original Kepler Mission.

enter image description here

So it seems that this is just a result of the reduced pointing accuracy introduced after Keppler lost two of its four reaction wheels. The paper also goes into detail how to correct for this behavior.


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