VLTI-GRAVITY observations of blazars

¹ Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, University of Turku, FI-20014 Turku, Finland
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
³ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Espoo, Finland
⁴ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg, Germany
⁶ National Centre for Radio Astrophysics (NCRA) – Tata Institute of Fundamental Research (TIFR), S. P. Pune University Campus, Ganeshkhind, Pune 411007, Maharashtra, India
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁸ European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁹ Nordic Optical Telescope, E-38700 Santa Cruz de La Palma, Santa Cruz de Tenerife, Spain
¹⁰ Department of Physics and Astronomy, Aarhus University, Munkegade 120, 8000 Aarhus C, Denmark

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 December 2025
Accepted: 17 April 2026

Abstract

Aims. Parsec-scale jets of blazars have so far been spatially resolved only in millimeter and submillimeter wavelengths, where very long baseline interferometry can be used to obtain milliarcsecond-scale images of the jets. We have attempted to spatially resolve the near-infrared emission in jet-dominated blazars for the first time.

Methods. We used the VLTI-GRAVITY instrument to obtain milliarcsecond-scale near-infrared interferometric observations of a flaring blazar Ton 599. Additionally, we observed four non-flaring blazars using the GRAVITY-wide mode, where a nearby bright star is used as a fringe tracker.

Results. We modeled the squared visibilities of Ton 599 and found that they are incompatible with a single unresolved point source unless there is a significant amount of additional unknown coherence loss in the instrument. With the present data, we cannot distinguish between a model with an unresolved point source and extended emission or coherence loss and a model with a single Gaussian component. This suggests that we are seeing the unresolved or only partially resolved jet-base in near-infrared wavelengths. The wide-field mode of GRAVITY was challenging for the additional relatively faint targets, resulting in either non-detections or poor-quality data that could not be modeled.

Conclusions. Our observations demonstrate that it is possible to detect the compact jet emission in blazars with near-infrared interferometry, suggesting that with the improved GRAVITY+ instrument it will be possible to spatially resolve and image the near-infrared emission of blazar jets.