Ansky’s galaxy shows signs of black-hole activity before its 2019 brightening

The galaxy SDSS1335+0728 hosts Ansky, a nuclear brightening first detected in December 2019 and subsequently associated with unusually energetic, recurring X-ray flares. These quasi-periodic eruptions, or QPEs, repeat at roughly regular but changing intervals. Their physical cause remains unresolved.

A study led by P. Sánchez-Sáez investigates a related question: what was the central black hole doing before the brightening? First submitted to arXiv on 1 July 2026 and revised on 5 November 2026, the paper is available as a preprint and is listed as accepted for publication in Astronomy & Astrophysics. The authors conclude that the observations favour earlier accretion—the process in which material falls towards a black hole and releases energy. But they cannot determine whether this came from a persistently faint active galactic nucleus or material left by an earlier stellar disruption.

Mapping the surrounding galaxy

The team combined optical observations from the Multi Unit Spectroscopic Explorer, or MUSE, on the Very Large Telescope with mid-infrared observations from the James Webb Space Telescope’s MIRI instrument. Both provide spectra across a field of view, allowing researchers to map physical conditions rather than blend the entire galaxy into one measurement.

The MUSE observations were taken between May and July 2025; Webb observed in July 2025. By fitting stellar spectra and gas emission lines across the images, the researchers estimated stellar ages, motions and the sources of gas ionisation. Ionisation occurs when radiation removes electrons from atoms.

The maps reveal three broad zones. The nucleus has emission-line ratios characteristic of a hard radiation field powered by accretion. Around it lies a ring of ongoing star formation, roughly 0.5–1.4 kiloparsecs from the centre. A kiloparsec is about 3,260 light-years. Farther out, the gas shows low-ionisation emission that young stars alone cannot readily explain, although evolved stars remain a possible contributor.

Most of the galaxy’s stellar mass is in old stars. Its motions also preserve evidence of a complicated past: the inner and outer stellar regions rotate in opposite directions, while the gas rotates smoothly without the same reversal. The gas has a low velocity dispersion, meaning relatively little spread in its local velocities, generally no more than about 60 kilometres per second.

Together, these features are consistent with an earlier minor merger involving externally supplied gas. They do not establish when that interaction happened or demonstrate that it caused Ansky.

Gas as a record of earlier activity

To investigate the nucleus’s history, the authors used hydrogen emission lines as a form of light-echo evidence. Gas illuminated by an energetic source can absorb ionising photons and subsequently emit characteristic radiation as electrons recombine with ions. Its brightness can therefore constrain the radiation needed to power it.

The analysis excludes the star-forming ring when interpreting nuclear luminosity, because local young stars supply many of its photons. It also accounts approximately for the instruments’ spatial blurring when identifying emission too far from the nucleus to be explained by the 2019 event.

For gas classified as consistent with accretion-driven ionisation beyond that region, the inferred minimum ionising luminosity is about 3 × 10^40 ergs per second. The authors interpret this as evidence of nuclear activity before December 2019, rather than a previously dormant black hole turning on for the first time.

This is not a dated reconstruction of the black hole’s output. The gas’s unknown three-dimensional arrangement means projected distance cannot be translated directly into a light-travel delay. The analysis cannot establish how long the precursor activity lasted, whether it brightened or faded, or its true luminosity above the estimated lower limit. The possible contribution of evolved stars to outer emission also complicates interpretation.

Dust leaves two explanations open

Webb detected strong silicate-dust emission near wavelengths of 9.7 and 18 micrometres. These features indicate directly illuminated dust seen through a low effective mid-infrared optical depth, meaning relatively little absorption along the relevant paths.

The features are unusually strong compared with most active galactic nuclei and closer to those observed in some infrared-selected tidal disruption events, in which a black hole tears apart a passing star. They are not a unique fingerprint, however. Both broadly transparent dust and a clumpy arrangement of opaque clouds can produce the observed emission.

The authors therefore retain two explanations for the earlier activity: a low-luminosity active nucleus or a long-lived accretion disc remaining from a previous tidal disruption. Compact infrared lines from highly ionised neon confirm a hard, accretion-powered radiation field, but cannot separate the contribution of Ansky from earlier activity.

Neither scenario requires a once-brilliant nucleus that subsequently faded away. Nevertheless, this single galaxy cannot settle the history of other QPE hosts, identify Ansky’s trigger or resolve the eruption mechanism itself.