Evidence for a Delayed UV Counterpart to X-ray Quasi-periodic Eruptions in Ansky

TL;DR

This paper reports the first detection of a UV response linked to X-ray quasi-periodic eruptions in the galaxy Ansky, revealing a delayed UV emission that provides new insights into the physical mechanisms of QPEs.

Contribution

It presents the first observation of a UV counterpart to X-ray QPEs, with a measured delay, in the galaxy Ansky, offering new constraints for QPE models.

Findings

UV emission lags X-ray eruptions by about 1 day.

UV response shows coherent periodic modulation over five cycles.

The delay may be due to diffusion or light-crossing time in the system.

Abstract

X-ray quasi-periodic eruptions (QPEs) represent a novel population of extreme, repeating nuclear transients whose physical origins remain debated. A defining characteristic of QPEs has been their exclusive detection in the X-ray band, with a notable absence of correlated multi-wavelength counterparts. Here we report the first detection of a recurrent UV response temporally coupled to the X-ray QPE signal in the source Ansky/ZTF19acnskyy. The UV emission displays coherent periodic modulations over five consecutive cycles, systematically lagging the X-ray eruptions by $0.96^{+0.38}_{-0.39}$ days, with a cross-correlation coefficient of $r_{\rm max} \sim 0.6$. We suggest that the detectability of this corresponding signal may be enabled by Ansky's unusually long recurrence timescale, which could reduce the temporal smearing of the UV response seen in more rapid QPEs. The observed delay may correspond to a diffusion timescale associated with heated blobs. However, we cannot exclude the possibility that the lag corresponds to the light-crossing time associated with X-ray irradiation that originates near the central black hole and propagates to the outer UV-emitting region. While numerous QPE models have been proposed, any viable model for Ansky must be able to simultaneously explain the presence of a UV counterpart, its measured time lag, and the previously observed steadily increasing recurrence period.