🔭 astrophysics

Polarized optical emission reveals a tidal disruption event jet

This paper reports that optical polarimetric observations of the off-nuclear tidal disruption event AT2024tvd, which revealed a high intrinsic polarization of 24.4% coinciding with X-ray and radio flares, provide definitive evidence for the presence of a relativistic jet, demonstrating that polarimetry can uncover hidden jet activity in TDEs.

Original authors: Alberto Floris, Ioannis Liodakis, Karri Koljonen, Kari Nilsson, Elina Lindfors, Odelia Teboul, Alessandro Paggi, Dmitry Blinov, Tapio Pursimo

Published 2026-10-09

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Alberto Floris, Ioannis Liodakis, Karri Koljonen, Kari Nilsson, Elina Lindfors, Odelia Teboul, Alessandro Paggi, Dmitry Blinov, Tapio Pursimo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

When a star wanders too close to a supermassive black hole, the immense gravitational pull tears the star apart. This violent event, known as a tidal disruption event, creates a bright flash of light as the stellar debris spirals inward and heats up. For decades, astronomers have used these flashes to study black holes that are otherwise invisible, watching how they feed on the remains of a star. While most of these events happen quietly at the very center of a galaxy, producing a steady, glowing disk of hot gas, some are far more dramatic. In rare cases, the black hole launches a narrow, high-speed beam of particles, called a jet, that shoots out into space at nearly the speed of light. Detecting these jets is difficult because they are often hidden by the brighter glow of the surrounding gas, and their appearance can be delayed, making it hard to know exactly when or if they have formed.

A team of astronomers recently turned their attention to a specific event, AT2024tvd, which offered a unique opportunity to solve this puzzle. Unlike typical events found in the centers of galaxies, this one occurred nearly a thousand light-years away from the center of its host galaxy, suggesting it was powered by a wandering black hole that had been displaced by a past collision between galaxies. The event was already known to be unusual; it produced a bright optical flare followed by two distinct bursts of radio waves and a peak in X-ray emission. However, the source of these delayed signals remained a mystery. They could have been caused by a jet, a slower outflow of gas, or a shock wave created by debris crashing into surrounding material. To distinguish between these possibilities, the researchers needed to look at the light not just for its brightness, but for its orientation.

The researchers focused on the polarization of the light, a property that describes the direction in which light waves vibrate. Imagine light as a wave moving through space; if it vibrates in all directions equally, it is unpolarized, which is typical for light coming from a hot, round ball of gas. If the light vibrates mostly in one direction, it is polarized, which usually happens when light is emitted by particles moving in a specific, organized way, such as within a magnetic field in a jet. The team used telescopes equipped with special filters to measure this orientation in the light from AT2024tvd at different times. When the event was at its brightest in visible light, the polarization was low, consistent with a standard, spherical cloud of hot gas. But as the visible light began to fade, the team observed a dramatic change.

At a later stage, coinciding with the peak of the X-ray emission and the start of the second radio burst, the polarization of the visible light surged to an extraordinary level of 24.4 percent. This high degree of polarization is far too strong to be produced by a simple, round cloud of gas. Instead, it indicates that a significant portion of the light was coming from a highly structured, asymmetric region where magnetic fields were organizing the particles. By calculating how much of the total light this polarized component represented, the researchers determined that at least one-third of the visible light at this stage was coming from this non-thermal source. This finding, combined with the timing of the X-ray and radio signals, strongly suggests that a relativistic jet had finally emerged.

The data points to a specific sequence of events that explains why the jet was hidden at first. The researchers propose that the jet was initially misaligned with the black hole's spin axis, causing it to be choked by a thick, swirling disk of gas and wind. Over time, the disk and the jet aligned with the black hole's spin, allowing the jet to break free. The first radio flare likely marked the moment the jet began to escape, pushing through the surrounding gas. The second, more intense radio flare and the sudden spike in polarized light occurred later, when the narrow, energetic core of the jet became fully visible as the surrounding thermal glow faded away. This interpretation rules out the idea that the event was powered solely by a spherical explosion or a slow, isotropic outflow, which would not produce such strong, organized polarization.

This discovery highlights the power of measuring light polarization to reveal hidden cosmic machinery. While the initial flash of a tidal disruption event can look like a standard explosion, the later behavior of the light can tell a different story. In this case, the polarization acted as a clear signature, proving that a powerful jet had formed and was now dominating the emission. It also underscores the importance of events like AT2024tvd, which occur away from the centers of galaxies. These off-center events allow astronomers to find and study black holes that are wandering through the outskirts of galaxies, a population that is otherwise extremely difficult to detect. By combining observations across different wavelengths and using the unique clues provided by polarized light, scientists are beginning to piece together the complex life cycle of how black holes launch their most energetic beams.

Technical Summary: Polarized optical emission reveals a tidal disruption event jet

Problem and Context
Tidal disruption events (TDEs) occur when a star is torn apart by a supermassive black hole (SMBH), providing a unique laboratory to study the onset of accretion and the potential formation of relativistic jets. While most confirmed TDEs are located at galactic centers, the event AT2024tvd represents a paradigm shift as the first confirmed optically selected off-nuclear TDE, occurring 0.8 kpc from its host galaxy's center. This event is powered by a wandering SMBH, likely displaced by a past merger.

The physical origin of multi-wavelength emission in TDEs, particularly delayed radio flares and X-ray activity, remains debated. These emissions could arise from relativistic jets, delayed non-relativistic outflows, or shocks driven by debris interacting with the circumnuclear medium. While radio and X-ray data for AT2024tvd suggested continued activity beyond the optical flare, distinguishing between a spherical thermal photosphere and a collimated jet based on photometry and spectroscopy alone is difficult. A spherical thermal source is expected to produce negligible net polarization due to the cancellation of different polarization vectors, whereas asymmetric structures like jets or shocks can generate significant polarization.

Methodology
The authors conducted multi-epoch optical polarimetric observations of AT2024tvd to constrain the geometry and emission mechanisms of the event.

Observations: Data were acquired using the RoboPol polarimeter on the 1.3m Skinakas telescope (g and r bands) near the optical peak, and the ALFOSC instrument on the Nordic Optical Telescope (NOT) in B, V, and R bands during the optical decline phase.
Host Galaxy Correction: A critical step involved correcting for the dilution of polarization by the host galaxy's unpolarized starlight. For NOT observations, the authors modeled and subtracted the host galaxy light directly from polarimetric images using a Sérsic profile convolved with a point spread function (PSF) derived from a nearby field star. For RoboPol observations, an aperture-based dilution correction was applied using pre-TDE Pan-STARRS magnitudes.
Multi-wavelength Analysis: The polarimetric data were correlated with existing optical (ZTF), X-ray (Swift/XRT), and radio (VLA) light curves. X-ray spectral fitting was performed using the Interactive Spectral Interpretation System (ISIS) with a model comprising a blackbody plus a power-law component.
Quantitative Modeling: The authors calculated the minimum fraction of optical flux attributable to a synchrotron (jet) component ( $f_{jet}$ ) by modeling the total flux as a sum of thermal and non-thermal components, accounting for their respective polarization degrees.

Key Results

Temporal Evolution of Polarization: Near the optical peak (MJD 60599), the intrinsic polarization was modest ( $\Pi = 4.12 \pm 1.07\%$ in the r-band), consistent with a thermal photosphere. However, at a later epoch (MJD 60710), coinciding with the X-ray peak and the onset of a second radio flare, the intrinsic B-band polarization rose dramatically to $\Pi = 24.4^{+7.8}_{-5.9}\%$ .
Non-Thermal Contribution: The high polarization degree implies a significant non-thermal contribution to the optical light. Assuming a maximally polarized synchrotron component ( $\Pi_{jet} \approx 70\%$ ) and an unpolarized thermal component, the authors derived a minimum jet contribution of $f_{jet} \simeq 0.35^{+0.11}_{-0.08}$ to the B-band flux. Even with conservative assumptions allowing for a moderately polarized thermal component ( $\Pi_{th} \approx 6\%$ ), the synchrotron fraction remains $\gtrsim 0.29$ .
Spectral and Temporal Correlation: The high-polarization epoch occurred approximately 141 days after the optical peak, contemporaneous with the X-ray spectral hardening (requiring a power-law component) and the radio rebrightening. The polarization angle also shifted between the early and late epochs, indicating an evolving intrinsic polarized component rather than a static foreground effect.
Jet Interpretation: The data favor a scenario where a relativistic jet emerges after a delay. The authors interpret this within the framework of a misaligned, precessing TDE disk-jet system (Teboul & Metzger). In this model, the jet is initially "choked" by a dense, super-Eddington wind envelope due to misalignment with the SMBH spin. The jet escapes only after the disk aligns with the spin axis (estimated at $\sim 80$ days). The first radio flare corresponds to a wide-angle component, while the later high polarization and radio rebrightening correspond to the emergence of a narrow, energetic core.

Significance and Claims
The paper claims that AT2024tvd provides direct evidence for the delayed emergence of a relativistic jet in a TDE, a mechanism that might otherwise remain hidden in photometric or spectroscopic data alone.

Diagnostic Power of Polarimetry: The study demonstrates that optical polarimetry is a critical tool for disentangling emission mechanisms in TDEs, capable of revealing jet activity even when the thermal component dominates the light curve.
Off-Nuclear Black Holes: The event highlights the utility of TDEs in identifying and studying populations of wandering or displaced SMBHs that are otherwise difficult to detect.
Hidden Jet Population: The authors suggest that jet activity in TDEs may be more common than currently observed but is often missed because the polarized component is diluted by thermal emission or host galaxy light, particularly in the early phases of the event. The detection of high polarization only at late times in AT2024tvd underscores the necessity of multi-epoch polarimetry to uncover the full population of jet-producing TDEs.

More like this