# The 'Big Dipper': The nature of the extreme variability of the AGN SDSS J2232-0806

## TL;DR

The paper investigates the extreme variability of the galaxy SDSS J2232-0806, revealing rapid luminosity changes in its active nucleus that challenge standard accretion disc models.

## Contribution

It provides detailed spectral and multiwavelength analysis of the 'Big Dipper' AGN, demonstrating significant luminosity variability over years and supporting intrinsic luminosity change as the cause.

## Key findings

- Detected a major dimming and rise over four years.
- Evidence of previous variability in archival data.
- Luminosity changes are likely intrinsic, not due to obscuration.

## Abstract

SDSS J2232-0806 (the 'Big Dipper') has been identified as a 'slow-blue nuclear hypervariable': a galaxy with no previously known active nucleus, blue colours and large-amplitude brightness evolution occurring on a timescale of years. Subsequent observations have shown that this source does indeed contain an active galactic nucleus (AGN). Our optical photometric and spectroscopic monitoring campaign has recorded one major dimming event (and subsequent rise) over a period of around four years; there is also evidence of previous events consistent with this in archival data recorded over the last twenty years. Here we report an analysis of the eleven optical spectra obtained to date and we assemble a multiwavelength data set including infrared, ultraviolet and X-ray observations. We find that an intrinsic change in the luminosity is the most favoured explanation of the observations, based on a comparison of continuum and line variability and the apparent lagged response of the hot dust. This source, along with several other recently-discovered 'changing-look' objects, demonstrate that AGN can exhibit large-amplitude luminosity changes on timescales much shorter than those predicted by standard thin accretion disc models.