COBALT-BLUE: a fast and accurate model for gravitational self-lensing binary systems
Abstract
We present COBALT-BLUE, a fast public model for the optical light curve of a luminous star in a non-accreting binary with a compact object. The model includes gravitational self-lensing, Doppler beaming, ellipsoidal variations, eccentric orbital geometry, and wavelength-dependent limb darkening. In sufficiently edge-on systems, it predicts sharp periodic self-lensing flares together with phase-locked orbital modulation outside the flare. We improve upon earlier models by including non-linear limb darkening laws based on stellar atmosphere simulations to calculate the self-lensing flare profile. This enables light curves to be self-consistently calculated in multiple wavebands from one set of physical parameters, thus offering the prospect of constraining binary parameters such as compact object mass from photometry alone if multiple self-lensing flares are observed. We also include the option to utilise simpler limb darkening prescriptions to save computational expense. As a proof of principle, we fit our model to ZTF data of 14 candidate self-lensing sources. The model describes the flare morphology well for most candidates, but we find that the best fitting parameters are not physically plausible. We conclude that the observed flares are most likely stellar flares and not self-lensing events. In future, we plan to use the model to systematically search optical survey data for signatures of binary motion.
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BibTeX
@misc{dixon2026cobaltblue,
title = {{COBALT-BLUE: a fast and accurate model for gravitational self-lensing binary systems}},
author = {Danny J. Dixon and Adam Ingram and Matthew J. Middleton and Adam McMaster and Grzegorz Wiktorowicz and Hugh Dickinson and Allison Crossland and Eric C. Bellm},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.21708},
eprint = {2609.21708},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.21708},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - COBALT-BLUE: a fast and accurate model for gravitational self-lensing binary systems
AU - Danny J. Dixon
AU - Adam Ingram
AU - Matthew J. Middleton
AU - Adam McMaster
AU - Grzegorz Wiktorowicz
AU - Hugh Dickinson
AU - Allison Crossland
AU - Eric C. Bellm
PY - 2026
DA - 2026/09/18
PB - arXiv
DO - 10.48550/arxiv.2609.21708
UR - https://arxiv.org/abs/2609.21708
AB - We present COBALT-BLUE, a fast public model for the optical light curve of a luminous star in a non-accreting binary with a compact object. The model includes gravitational self-lensing, Doppler beaming, ellipsoidal variations, eccentric orbital geometry, and wavelength-dependent limb darkening. In sufficiently edge-on systems, it predicts sharp periodic self-lensing flares together with phase-locked orbital modulation outside the flare. We improve upon earlier models by including non-linear limb darkening laws based on stellar atmosphere simulations to calculate the self-lensing flare profile. This enables light curves to be self-consistently calculated in multiple wavebands from one set of physical parameters, thus offering the prospect of constraining binary parameters such as compact object mass from photometry alone if multiple self-lensing flares are observed. We also include the option to utilise simpler limb darkening prescriptions to save computational expense. As a proof of principle, we fit our model to ZTF data of 14 candidate self-lensing sources. The model describes the flare morphology well for most candidates, but we find that the best fitting parameters are not physically plausible. We conclude that the observed flares are most likely stellar flares and not self-lensing events. In future, we plan to use the model to systematically search optical survey data for signatures of binary motion.
ER -