Astronomer
I am a German-Swiss astronomer currently based at the Canadian Institute for Theoretical Astrophysics (CITA) of the University of Toronto, Canada.
Sebastiano D.
von Fellenberg, PhD
Research Interests
I am interested in the Galactic Center, the black hole it harbors and the stars surrounding it.
Fascinated by the technology required and developed to study it.
Contact me at:
s.fellenberg at utoronto.ca
or take a look at my CV
I'm an active member of both the GRAVITY Collaboration, which studies Sgr A* in the near-infrared as well as a member of the Event Horizon Telescope Collaboration (EHTC), which studies Sgr A* and other black holes at radio frequencies. I joined the EHTC in 2023.
In both collaborations, I am heavily involved in data calibration. I develop calibration strategies, both on a more conceptual level and in producing data products that can be used for interpretation.
I've contributed substantially to a novel method called dual-beam astrometry, which allows the tracking of stars in the Galactic Center over large distances. This was instrumental in the detection of the so-called Schwarzschild Precession of the star S2, a prediction of Einstein's General Relativity.
In the EHTC, I've been leading efforts to integrate of the NOEMA into the VLBI array, which joined observations for the first time in 2021. This will allow us to obtain even sharper observations of the event horizons of Sgr A* & M87. For this work, I have won the EHT Early Career Award - twice. The M87* results have been published in Fall 2025.
In January 2025, an international team of researchers and I published a paper on the first detection of a flare from Sgr A* in the mid-infrared using the James Webb Space Telescope. The paper was published in Astrophysical Journal Letters, and was first presented at the 245th AAS Meeting in Maryland. Several press releases are available, including from my home institute. Some more details on these results are available here.
Since April 2025, I am an Alexander von Humboldt Feodor Lynen Fellow at the Canadian Institute for Theoretical Astrophysics (CITA) at the UofT in Toronto to work on observations and models of accretion flows like that of Sgr A*.
Publications
S. D. von Fellenberg et al. 2025
"First Mid-infrared Detection and Modeling of a Flare from Sgr A*"
S. D. von Fellenberg et al. 2025
Details
In this paper we use JWST/MIRI–MRS observations to measure the mid‑infrared extinction toward the Galactic Centre by modeling dust emission and comparing hydrogen recombination lines. We derive a refined MIR extinction law for Sgr A* and its surroundings. Applying this law yields residual uncertainties of ∼0.2–0.3 mag across 5–22 μm, allowing for the first measurement of the MIR SED of Sgr A* published in a companion study (Michail et al. 2025).
Michail & von Fellenberg et al. 2025
Details
In this paper we present the first fully extinction‑corrected mid‑infrared spectral energy distribution of a Sgr A* flare, applying new Galactic‑centre extinction measurements to the JWST/MIRI/MRS detection reported in Paper I. We recover a maximum mid‑infrared spectral index of ~ 0.45 and identify a rapid spectral‑index jump at flare onset that marks the transition to a hard electron power‑law.
S. D. von Fellenberg et al. 2022
Details
In this paper we studied the distribution of young stars in the Galactic Center. We found a new disk of young stars!
GRAVITY Collaboration et al. 2021
GRAVITY Collaboration et al. 2021
S.D. von Fellenberg et al. 2023
"General relativistic effects and the near-infrared and X-ray variability of Sgr A* I"
S.D. von Fellenberg et al. 2024
Details
In this paper we took a closer look a the temporal asymmetry of Sgr A* - we found that the source is symmetric in time: you could flip the time axis an nothing would happen!
This implies that we likely look at face-on accretion flow
S.D. von Fellenberg et al. 2018
Details
A paper that reports on the first detection of Sgr A* at 160 and 100 micron!
Student-led work
Saurabh et al. 2025
"Probing jet base emission of M87* with the 2021 Event Horizon Telescope observations"
Details
In this student‑led paper we use the enhanced 2021 EHT array to search for jet‑base emission in M87* at 230 GHz, enabled by two new intermediate baselines. We detect a faint Gaussian component offset from the ring by ∼5500 AU with a flux of only ∼60 mJy, showing that most of the previously inferred “missing flux” must originate on larger scales. This feature aligns with the jet direction, but limited baseline coverage means we treat it as an upper limit, motivating future EHT observations with improved intermediate‑scale sensitivity.
Roychowdhury et al. 2025
"Photometric Constraints on Intermediate-mass Black Holes in the Galactic Centre"
Details
In this student‑led paper we use JWST/MIRI time‑series observations to search for mid‑infrared variability from a putative intermediate‑mass black hole in the IRS 13E complex near the Galactic Centre. We detect no such variable emission and place stringent limits of ≲1–2 mJy, ruling out IMBHs of ≳10³ M⊙ accreting at ≳10⁻⁶ Eddington within IRS 13E or the central 6″×6″ region. These results demonstrate the power of mid‑IR variability monitoring for constraining accreting black holes in dense Galactic‑centre environments.
Non-Galactic Center Publications
von Fellenberg et al. 2025
Details
This really quite techincal paper details how we calibrated the EHT data recorded at the NOEMA telescope in France
von Fellenberg et al. 2025
"Radio properties of the quasi-periodic eruption source RXJ1301.9+2747 at parsec scales"
Details
In this paper we present new 5 GHz HSA observations of RXJ 1301.9+2747, confirming an extremely compact, steep‑spectrum radio source. We detect radio–optical and inter‑frequency positional offsets likely caused by dust or calibration uncertainties. Our results favor a remnant jet or outflow origin over star formation or a jet‑core scenario.
von Fellenberg et al. 2023
Details
This paper discusses a peculiar 7-year precession in the radio jet of M81*, the second closest AGN to us! While temporal baseline of observations is to short to know for sure this could be explained by the presence of dynamical pace-maker: a IMBH companion.
Publications with important contributions
GRAVITY Collaboration et al. 2022
Details
In this work led by the MPE Galactic Center Team, I contributed the 2021 astrometry of stars zipping by the black hole.
GRAVITY Collaboration et al. 2019
"A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty"
Details
In this work led by the MPE Galactic Center Team, Felix Widman and I helped re-reduce all SINFONI spectroscopic observations. This led to a 46% improvement of the radial velocities.
Y. Dalillar et al. 2022
"Flaremodel: An open-source Python package for one-zone numerical modelling of synchrotron sources"
Details
I intiated this project led by Yigit Dalillar, and contributed to the code.
