Leveraging Impact Parameter to Mitigate the Transit Light Source Effect: Early Insights from TRAPPIST-1

@misc{glidden2026leveraging,
  title = {{Leveraging Impact Parameter to Mitigate the Transit Light Source Effect: Early Insights from TRAPPIST-1}},
  author = {Ana Glidden and Alexander I. Shapiro and Sara Seager and Nadiia Kostogryz and Valeriy Vasilyev and Roeland P. van der Marel and Julien de Wit and Benjamin V. Rackham and Prajwal Niraula and Natalie H. Allen and Jingcheng Huang and Nikole K. Lewis and Zifan Lin and Jacob Lustig-Yaeger and Ryan J. MacDonald and Brett M. Morris and Elijah Mullens and Kevin B. Stevenson and Jeff A. Valenti and Daniel Valentine and Hannah R. Wakeford and C. Matt Mountain},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2608.20450},
  eprint = {2608.20450},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2608.20450},
}

TY  - UNPB
TI  - Leveraging Impact Parameter to Mitigate the Transit Light Source Effect: Early Insights from TRAPPIST-1
AU  - Ana Glidden
AU  - Alexander I. Shapiro
AU  - Sara Seager
AU  - Nadiia Kostogryz
AU  - Valeriy Vasilyev
AU  - Roeland P. van der Marel
AU  - Julien de Wit
AU  - Benjamin V. Rackham
AU  - Prajwal Niraula
AU  - Natalie H. Allen
AU  - Jingcheng Huang
AU  - Nikole K. Lewis
AU  - Zifan Lin
AU  - Jacob Lustig-Yaeger
AU  - Ryan J. MacDonald
AU  - Brett M. Morris
AU  - Elijah Mullens
AU  - Kevin B. Stevenson
AU  - Jeff A. Valenti
AU  - Daniel Valentine
AU  - Hannah R. Wakeford
AU  - C. Matt Mountain
PY  - 2026
DA  - 2026/09/08
PB  - arXiv
DO  - 10.48550/arxiv.2608.20450
UR  - https://arxiv.org/abs/2608.20450
AB  - Stellar activity complicates exoplanet transmission spectra, particularly for smaller planets around M dwarfs with JWST. The transit light source (TLS) effect, the imprinting of spectral differences between the average stellar disk and the occulted transit chord onto the transmission spectrum, makes it challenging to directly use the out-of-transit spectrum to correct for stellar contamination. Theory and observations suggest that spots may concentrate towards higher latitudes when the Coriolis force is substantial relative to buoyancy, leaving the equatorial region relatively quiet. Here, we evaluate how the latitudinal distribution of active regions shapes the strength of the TLS effect for planets spanning a range of impact parameters (b), using TRAPPIST-1 as a testbed. We first construct a fiducial model to illustrate two distribution regimes. With our model, the moderate-b outer TRAPPIST-1 planets (f, g, h) occult a more typical region of the stellar disk than the inner planets and are thereby less affected by the TLS effect, though their bias may vary more from visit-to-visit as these active regions evolve with time. More generally, our results imply an impact parameter "sweet spot" for atmospheric characterization, independent of the sign of the active region temperature contrast, whose location depends on the distribution of active regions. The distribution may be revealed by transit residuals as multiple planets probe different latitudes, while longitudes are sampled in time, such that the variance and frequency of the correlated scatter could constrain active-region filling factors, sizes, and separations.
ER  -