An alternative interpretation of the exomoon candidate signal in the combined Kepler and Hubble data of Kepler-1625. (arXiv:1902.06018v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Heller_R/0/1/0/all/0/1">René Heller</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Rodenbeck_K/0/1/0/all/0/1">Kai Rodenbeck</a> (1,2), <a href="http://arxiv.org/find/astro-ph/1/au:+Bruno_G/0/1/0/all/0/1">Giovanni Bruno</a> (3) ((1) Max Planck Institute for Solar System Research, Göttingen (GER), (2) Institute for Astrophysics Göttingen, Georg August University Göttingen (GER), (3) INAF, Astrophysical Observatory of Catania (ITA))
Kepler and Hubble photometry of a total of four transits by the Jupiter-sized
Kepler-1625b have recently been interpreted to show evidence of a Neptune-sized
exomoon. The profound implications of this first possible exomoon detection and
the physical oddity of the proposed moon, that is, its giant radius prompt us
to re-examine the data and the Bayesian Information Criterion (BIC) used for
detection. We combine the Kepler data with the previously published Hubble
light curve. In an alternative approach, we perform a synchronous polynomial
detrending and fitting of the Kepler data combined with our own extraction of
the Hubble photometry. We generate five million MCMC realizations of the data
with both a planet-only model and a planet-moon model and compute the BIC
difference (DeltaBIC) between the most likely models, respectively. DeltaBIC
values of -44.5 (using previously published Hubble data) and -31.0 (using our
own detrending) yield strongly support the exomoon interpretation. Most of our
orbital realizations, however, are very different from the best-fit solutions,
suggesting that the likelihood function that best describes the data is
non-Gaussian. We measure a 73.7min early arrival of Kepler-1625b for its Hubble
transit at the 3 sigma level, possibly caused by a 1 day data gap near the
first Kepler transit, stellar activity, or unknown systematics. The radial
velocity amplitude of a possible unseen hot Jupiter causing Kepler-1625b’s
transit timing variation could be some 100m/s. Although we find a similar
solution to the planet-moon model as previously proposed, careful consideration
of its statistical evidence leads us to believe that this is not a secure
exomoon detection. Unknown systematic errors in the Kepler/Hubble data make the
DeltaBIC an unreliable metric for an exomoon search around Kepler-1625b,
allowing for alternative interpretations of the signal.
Kepler and Hubble photometry of a total of four transits by the Jupiter-sized
Kepler-1625b have recently been interpreted to show evidence of a Neptune-sized
exomoon. The profound implications of this first possible exomoon detection and
the physical oddity of the proposed moon, that is, its giant radius prompt us
to re-examine the data and the Bayesian Information Criterion (BIC) used for
detection. We combine the Kepler data with the previously published Hubble
light curve. In an alternative approach, we perform a synchronous polynomial
detrending and fitting of the Kepler data combined with our own extraction of
the Hubble photometry. We generate five million MCMC realizations of the data
with both a planet-only model and a planet-moon model and compute the BIC
difference (DeltaBIC) between the most likely models, respectively. DeltaBIC
values of -44.5 (using previously published Hubble data) and -31.0 (using our
own detrending) yield strongly support the exomoon interpretation. Most of our
orbital realizations, however, are very different from the best-fit solutions,
suggesting that the likelihood function that best describes the data is
non-Gaussian. We measure a 73.7min early arrival of Kepler-1625b for its Hubble
transit at the 3 sigma level, possibly caused by a 1 day data gap near the
first Kepler transit, stellar activity, or unknown systematics. The radial
velocity amplitude of a possible unseen hot Jupiter causing Kepler-1625b’s
transit timing variation could be some 100m/s. Although we find a similar
solution to the planet-moon model as previously proposed, careful consideration
of its statistical evidence leads us to believe that this is not a secure
exomoon detection. Unknown systematic errors in the Kepler/Hubble data make the
DeltaBIC an unreliable metric for an exomoon search around Kepler-1625b,
allowing for alternative interpretations of the signal.
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