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

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.

Figures (4)

• Figure 1: The long-term ZTF and $Swift$/UVOT light curves after host subtraction and the Galactic extinction correction. The dotted line marked the time first detected X-ray QPE Hernandez-Garcia25a. The gray shadow marked the period when the X-ray QPE and UV counterpart are simultaneously observed.
• Figure 2: Correlation between the soft X-ray (0.3--2 keV) and UV light curves of Ansky. Left panels: The X-ray light curves are obtained from Swift XRT and XMM-Newton EPIC-pn, while the UVW2 light curves are from Swift UVOT and XMM-Newton OM. The XMM-Newton data are binned to 0.2 days to match the Swift cadence (original data shown in Fig. \ref{['fig:xmm']}). The original Swift UV data and the 0.2-day-binned XMM-Newton data are used for lag measurements (light grey); both are additionally binned to 1 day for clarity. Right panels: Cross-correlation results show the centroid lag (top), peak-lag distribution (bottom), and the corresponding CCF and lag posterior distributions. The maximum correlation coefficient is $r_{\rm max}=0.56$.
• Figure 3: Disk profiles (left panel) and different related timescales for the thin disk model (right panel). The blue line in the left panel shows the disk effective temperature ($T_{\rm disk,eff}$) and the orange line represents the disk surface density $\Sigma$ with an SMBH mass of $M_{\rm BH}=5\times10^{6}M_{\odot}$, disk viscosity $\alpha=0.1$, and accretion rate $\dot{m}=0.03$. Lines of different colors in the right panel represent the dynamical, thermal, viscous, light-crossing, and diffusion timescales calculated using the same set of parameters. The observed delay time $\sim1$ days is shown as the dotted line in the bottom panel.
• Figure S1: XMM-Newton EPIC-pn (0.2-10 keV) X-ray (top) and OM UVW2 UV (bottom) light curves showing two flare episodes. Different colors correspond to four XMM observations (OBSIDs 064540101-0964540401).