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release_readme.txt
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release_readme.txt
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K2SC-detrended K2 light curves
------------------------------
Overview
--------
K2SC (K2 Systematics Correction, Aigrain et al. 2016) is a K2 light
curve (LC) detrending tool that uses Gaussian processes (GPs) to
robustly model the systematics due to the Kepler telescope pointing
jitter together with the astrophysical variability. The K2SC-detrended
light curves are especially suited for studying variable stars in K2
photometry (by allowing us to remove the position-dependent
systematics while keeping time-dependent variability intact), and
searches for transiting planets (by allowing us to remove both the
systematics and astrophysical variability).
The detrended light curves available here use either SAP or PDC-MAP
photometry as a starting point (stored in separate binary table
extensions in the same fits file), and consist of three components:
position-dependent trend, time-dependent trend, and residual
(detrended) flux.
Description of Data Products
----------------------------
The detrended light curves are stored in a subdirectory structure with
the format
c<##>/<####00000>
where
- c<##> is the zero-padded two-digit K2 Campaign number
- <####00000> are the first four digits of the target's EPIC ID
The light curves are stored as fits-files with the names
hslp_k2sc_k2_llc_<#########>-c<##>_v<#>.fits
where
- <#########> is the nine-digit EPIC ID
- c<##> is the zero-padded two-digit K2 Campaign number
- v<#> is the file release version
The fits files have two binary table extensions: the first contains a
k2sc-detrended PDC-MAP light curve, and the second a k2sc-detrended
SAP light curve. The binary table columns are
- quality : Original K2 photometry pipeline quality flags
- cadence : observation cadences
- time : observation times
- flux : k2sc-detrended flux
- trend_p : k2sc-estimated position-dependent trend
- trend_t : k2sc-estimated time-dependent trend
- mflags : k2sc outlier flags
The PDC-MAP and SAP extensions are identical, the only difference being
the flux for which the detrending was applied to. The detrended flux
has both the position- and time-dependent trends removed, and can be
directly used for transit and flare searches. The `trend_t` column
contains the time-dependent trend, and can be combined with the
detrended flux in the `flux` column for variability studies as
flux_v = flux + trend_t - median(trend_t)
Finally, the original flux can be recovered by combining the three
components as
flux_r = flux + trend_t - median(trend_t) + trend_p - median(trend_p)
Finally, the outliers are tagged in a bit field, where zero value
corresponds to a good-quality point, and bits
- 2**0 : one of the K2 quality flags on
- 2**1 : flare (reserved but not currently used)
- 2**2 : transit (reserved but not currently used)
- 2**3 : upwards outlier
- 2**4 : downwards outlier
- 2**5 : nonfinite flux
- 2**6 : a periodic mask applied manually by k2sc (not used)
are used to flag different reasons for masking.