A multi-epoch VLBI study of compact symmetric objects as contaminants in the search for milli-lenses
Abstract
The Lambda Cold Dark Matter model predicts a high abundance of low-mass dark matter halos, which can be probed through milliarcsecond-scale gravitational lensing (milli-lensing) of background radio-loud active galactic nuclei (AGNs). The Search for MIlli-LEnses (SMILE) project aims to find these rare systems with Very Long Baseline Interferometry (VLBI). However, compact symmetric objects (CSOs) can closely resemble milli-lenses in VLBI images and are the main contaminant in such searches. We aim to determine the nature of the nine remaining milli-lens candidates of the SMILE pilot study and to characterize their physical properties. We analyzed all available multi-epoch, multi-frequency archival VLBI data, new HSA and EVN observations, and single-dish and connected-element interferometer measurements. We tested the lensing hypothesis through the morphology and spectra of the sources, and characterized their kinematics, spatially resolved spectral indices, magnetic fields, and integrated spectra, using synchrotron self-absorption (SSA) and free-free absorption (FFA) models. We identify eight of the nine candidates as CSOs, four of them for the first time. Spectral index asymmetries between components and previously undetected emission connecting them rule out the lensing hypothesis. Only J0923-3435 remains ambiguous. For the CSOs with known redshifts, the lobes have equipartition magnetic fields of ∼20-320 mG and projected separations of ∼60-240 pc, and show no measurable expansion, implying kinematic ages of at least ∼50-950 yr. The integrated spectra of J0210-2213 and J2347-1856 favor an inhomogeneous external free-free absorber over homogeneous and inhomogeneous SSA models. Since CSOs will be the main contaminant in the full SMILE survey, applying this framework will provide the same constraints for a much larger sample of compact radio-loud AGNs.
Related
- Impact of the Host Galaxy on the Search for Gravitational-Wave Microlensing by Compact Objects
- Near-infrared periodicity in the SPICY catalog. True young stellar objects versus contaminants
- Near-infrared periodicity in the SPICY catalog. True young stellar objects versus contaminants
- The kinematic Kelvin-Helmholtz instability-driven dynamo: implications for merging compact objects
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BibTeX
@misc{alvarezortega2026multiepoch,
title = {{A multi-epoch VLBI study of compact symmetric objects as contaminants in the search for milli-lenses}},
author = {Diego Álvarez-Ortega and Felix M. Pötzl and Carolina Casadio},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.07029},
eprint = {2611.07029},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.07029},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A multi-epoch VLBI study of compact symmetric objects as contaminants in the search for milli-lenses
AU - Diego Álvarez-Ortega
AU - Felix M. Pötzl
AU - Carolina Casadio
PY - 2026
DA - 2026/11/04
PB - arXiv
DO - 10.48550/arxiv.2611.07029
UR - https://arxiv.org/abs/2611.07029
AB - The Lambda Cold Dark Matter model predicts a high abundance of low-mass dark matter halos, which can be probed through milliarcsecond-scale gravitational lensing (milli-lensing) of background radio-loud active galactic nuclei (AGNs). The Search for MIlli-LEnses (SMILE) project aims to find these rare systems with Very Long Baseline Interferometry (VLBI). However, compact symmetric objects (CSOs) can closely resemble milli-lenses in VLBI images and are the main contaminant in such searches. We aim to determine the nature of the nine remaining milli-lens candidates of the SMILE pilot study and to characterize their physical properties. We analyzed all available multi-epoch, multi-frequency archival VLBI data, new HSA and EVN observations, and single-dish and connected-element interferometer measurements. We tested the lensing hypothesis through the morphology and spectra of the sources, and characterized their kinematics, spatially resolved spectral indices, magnetic fields, and integrated spectra, using synchrotron self-absorption (SSA) and free-free absorption (FFA) models. We identify eight of the nine candidates as CSOs, four of them for the first time. Spectral index asymmetries between components and previously undetected emission connecting them rule out the lensing hypothesis. Only J0923-3435 remains ambiguous. For the CSOs with known redshifts, the lobes have equipartition magnetic fields of ∼20-320 mG and projected separations of ∼60-240 pc, and show no measurable expansion, implying kinematic ages of at least ∼50-950 yr. The integrated spectra of J0210-2213 and J2347-1856 favor an inhomogeneous external free-free absorber over homogeneous and inhomogeneous SSA models. Since CSOs will be the main contaminant in the full SMILE survey, applying this framework will provide the same constraints for a much larger sample of compact radio-loud AGNs.
ER -