Polarization echoes from past nuclear activity in the quasi-periodic eruption source GSN 069

TL;DR

This study investigates whether optical polarization echoes can record GSN 069's past nuclear activity and help discriminate between a faded AGN and a past elevated TDE rate as the engine behind its QPEs. By combining VLT/FORS2 imaging polarimetry and spectropolarimetry with existing EELR maps, the authors map polarization to ~2 kpc and show a radial increase in polarization with distance from the nucleus, along with a rotation of the polarization angle. The nucleus is effectively unpolarized, while polarized flux in the outskirts highlights scattering off an extended EELR; no broad Balmer lines are detected in polarized light. The polarization geometry favors a faded AGN origin and relic polarization cones, suggesting a torus-like past structure, though a past TDE-driven scenario cannot be completely excluded. Overall, optical polarization echoes provide a geometrical diagnostic of the duty cycle and fossil records of nuclear activity in QPE hosts like GSN 069.

Abstract

Context. X-ray quasi-periodic eruptions (QPEs) are repeating, high-amplitude, soft X-ray bursts observed from the nuclei of a dozen nearby low-mass galaxies. Their origin remains a puzzle in the physics of accretion variability. Observational data indicate that X-ray and/or optical tidal disruption events (TDEs) may precede QPE detections. Although both kinds of outburst are driven by supermassive black holes, they are more frequently detected in faded active galactic nuclei (AGNs), when the TDE is not happening in a dormant galaxy. In the case of the QPE discovery source, GSN 069, it remains debated whether its past activity arose from a previous AGN phase or from an enhanced TDE rate. Aims. We investigated the origin of the past nuclear activity in GSN 069. Methods. Past AGN activity imprints detectable polarization in optical light, due to the expected delay between direct and scattered light. On 6 September 2019, we targeted GSN 069 with VLT/FORS2 in both imaging polarimetry and spectropolarimetry modes so that its optical polarization could be investigated while the first detected QPE phase was still active. Results. We measured a rising polarization, from ~0% to ~1.5%, as moving away from the nucleus of GSN 069. This rise is probed to be intrinsic to the central engine, confirming the already detected extended emission line region (EELR) by integral field unit data. Conclusions. The increasing radial polarization demonstrates a switched-off nucleus. The polarization angle traces an axis aligned with elongated [OIII], [NII], and Hα gas distributions, revealing an EELR that may be consistent with relic polarization cones, therefore suggesting the presence of a torus-like structure in the past. Thus, optical polarization echoes geometrically favor a faded AGN as the origin of the EELR rather than a past elevated TDE rate, although the latter cannot be excluded.

Figures (11)

• Figure 1: Acquisition image of GSN 069. Flux contours follow the shown color scale. The green dashed circles mark IPOL aperture areas with radii of $\sim3\sigma$ (2.1"), $\sim5\sigma$ (3.5"), $\sim7\sigma$ (4.9"), and $\sim9\sigma$ (6.3"), where $\sigma$ is the seeing value during IPOL observations, which remained stable at $\sim$ 0.7. The white boxes indicate the north, nucleus, and south regions used for PMOS extractions (see text), while the red box show the southern spot extraction region. The blue dashed box outlines the same $12"\times12"$ area as the Chandra image of GSN 069 shown in panel d of Extended Data Fig. 1 in Miniutti2019Natur. The spot is not visible in our optical FORS2 IPOL images, but appears in those Chandra observations and in the MUSE continuum-subtracted H$\alpha$ image (Fig. \ref{['fig:muse']} and Appendix \ref{['append:Spot']}).
• Figure 2: Polarization degree (p), polarized flux ($p\times F$in arbitrary units), and polarization angle ($\theta$) measured in the V band, through an aperture of 4 pixels of diameter (1) centered on the nucleus, and in 4 pixel wide annuli of increasing radius (1$\simeq$ 380 pc). The polarization degree is not corrected for the bias. The first bin is circular (nucleus) showing $p$ compatible with zero and an undefined $\theta$.
• Figure 3: Flux density in arbitrary units ($F_{density}$), debiased polarization degree ($p_{0}$), polarized flux ($F_{density}\times p_{0}$), and polarization angle ($\theta$) for the nucleus (upper panel) and southern spot (lower panel).
• Figure 4: Polarization degree and angle measured in spatial bins (subslits) along the slit. The polarization degree is not corrected for the bias. The bin centered on the nucleus is 5 spatial pixels (1.25) long. The subsequent bins are 3, 3, 5, 6, and 7 pixels long. Positive distances are toward the north, negative ones toward the south (1$\simeq$ 380 pc). Measurements integrated over the continuum wavelength ranges 3800-5000 Å < and 5400-6600 Å are shown in blue and red, respectively. The bin at $-$4.1 contains the southern spot.
• Figure 5: Direct spectra of the nucleus of GSN 069 (1.25 extraction, upper panel) and the southern spot (2.25 extraction, lower panel). The starlight model (magenta) has been subtracted from the observed spectrum (black) to generate the emission-line spectrum (dashed line).