A century of change: new changing-look event in Mrk 1018's past

TL;DR

The study reconstructs a 100-year light curve for the changing-look AGN Mrk 1018 by combining DASCH archival photographic plates with modern photometry, uncovering a historic CL-like brightening around $1935$–$1960$ that matches the contemporary Type 1 phase. Through colour- and host-flux corrections, a Generalised Lomb-Scargle analysis reveals a broad low-frequency feature with a characteristic timescale of $P \approx 29$–$47$ years, suggesting long-term modulation rather than a strictly periodic cycle. The modern data constrain the latest turn-off to $< 1.9$ years, supporting rapid CL transitions and a changing-mode accretion scenario. The results favor Chaotic Cold Accretion as the driver of long-term stochastic variability in Mrk 1018 and provide a predictive window for future activity (roughly $2033$–$2045$) while highlighting the value of historic datasets for understanding AGN variability in the LSST era.

Abstract

We investigate the long-term variability of the known Changing Look Active Galactic Nuclei (CL AGN) Mrk 1018, whose second change we discovered as part of the Close AGN Reference Survey (CARS). Collating over a hundred years worth of photometry from scanned photographic plates and five modern surveys we find a historic outburst between ~1935-1960, with variation in Johnson B magnitude of ~0.8 that is consistent with Mrk 1018's brightness before and after its latest changing look event in the early 2010s. Using the combined modern and historic data, a Generalised Lomb-Scargle suggests broad feature with P = 29-47 years. Its width and stability across tests, as well as the turn-on speed and bright phase duration of the historic event suggests a timescale associated with long-term modulation, such as via rapid flickering in the accretion rate caused by the Chaotic Cold Accretion model rather than a strictly periodic CL mechanism driving changes in Mrk 1018. We also use the modern photometry to constrain Mrk 1018's latest turn-off duration to less than ~1.9 years, providing further support for a CL mechanism with rapid transition timescales, such as a changing mode of accretion.

Figures (6)

• Figure 1: Example plate scan taken from 1935 by the 3-inch Ross Fecker telescope. Despite the small aperture, Mrk 1018 is clearly detected (red circle). This plate was exposed for 8160s, slightly longer than the average of 5847s for the other plates with resolved detections taken by the same telescope.
• Figure 2: Subplot a: Mrk 1018 AGN + host galaxy historic data (in Johnson B band magnitude). Subplot b: five nearby, similarly bright and coloured reference stars. Subplot c: residuals of the mean magnitude of the reference stars at each epoch (weighted by error) subtracted from Mrk 1018 magnitude. This shows the variability in Mrk 1018 is not associated with plate systematics.
• Figure 3: Mrk 1018 AGN + host 100 year Johnson B band light curve with DASCH photographic data and modern photometry (SDSS, CRTS, ASAS-SN, ATLAS and ZTF). Marker colour and shape differentiates historic data per telescope. Horizontal dashed lines and grey bands indicate Type 1 (SDSS) and Type 2 (ASAS-SN) mean magnitudes and 1 $\sigma$ ranges. Vertical dashed lines and shaded regions show spectroscopic CL events in Mrk 1018's past.
• Figure 4: Mrk 1018's extreme event peaking around 1944. AGN + host flux contribution from historic DASCH Johnson B band magnitudes. The boxcar rolling mean (3 years width) is shown by the solid black line, with a 1$\sigma$ grey band. Plates taken by telescopes larger than 8 inches have larger markers, whereas smaller telescopes are indicated with 55% opacity. Horizontal lines indicate Type 1 magnitude (SDSS; 2000--2010) and Type 2 magnitude (ASAS-SN; 2013--2018) with 1$\sigma$ bands.
• Figure 5: Mrk 1018 AGN + host modern light curve (Johnson B). The CL event can be photometrically discerned by the drop in brightness between SDSS/CRTS and ASAS-SN/ATLAS/ZTF around 2012. ATLAS and ZTF include additional host light contribution calculated from ASAS-SN measurements. All data is binned into 3 day epochs and cleaned with two iterations of 3$\sigma$ clipping.