Investigating the AGN variability timescale -- black hole mass relationship with Gaia, SDSS and ZTF

TL;DR

The paper investigates the empirical relationship between AGN variability timescales and black hole mass by leveraging the Gaia GLEAN catalog, SDSS DR17 spectroscopy, and ZTF DR21 light curves. It evaluates the capability of damped random walk modelling to recover reliable timescales from Gaia data and demonstrates that well-sampled ZTF light curves are essential for robust estimates of variability amplitudes and timescales. From cross-matches and DRW fits, the authors assemble a catalog of 127 GLEAN AGNs with spectroscopic virial masses from H$\alpha$ and measured DRW parameters, finding no $M_{\rm BH} < 10^{6.7} M_{\odot}$ objects but confirming a positive $\tau_{\rm DRW}$–$M_{\rm BH}$ relation consistent with previous studies. The work highlights the necessity of dense, long-baseline photometric monitoring to extend the variability–mass relation to the low-mass end and provides a valuable data resource for calibrating BH mass estimators in the high-mass regime. Overall, it demonstrates the feasibility of using large variability catalogs to constrain SMBH demographics and AGN physics, while clarifying the limitations imposed by sampling on DRW-derived timescales.

Abstract

Active galactic nuclei (AGNs) exhibit variability in their luminosities with timescales that correlate with the mass of the black hole at the centre of the AGN. Presently, the empirical correlation lacks sufficient precision to confidently convert these timescales into black hole masses, especially at the low-mass end. To find more AGNs with timescale measurements, we study a very large catalog of AGNs from the Gaia Data Release 3 called GLEAN (Gaia variabLE AgN; 872228 objects). We identify GLEAN objects with optical spectra from the Sloan Digital Sky Survey DR17 and light curves from the Zwicky Transient Facility (ZTF) DR21. After fitting the light curves with a damped random walk model, we find that the GLEAN light curves have insufficient sampling to extract reliable amplitude and timescale measurements outside the range of 50-100 days. On the other hand, well-sampled ZTF light curves allow more accurate estimations of these parameters. The fractional variability amplitude is an effective, model-independent metric for measuring variability amplitude, but only when derived from high-quality light curves. We provide a catalog of 127 GLEAN AGNs with spectroscopic virial black hole masses, and variability amplitudes and timescales. Though we do not find any low-mass black holes in this AGN sample, we confirm a relationship between the damped random walk timescale and the black hole mass that is consistent with previous studies.

Figures (9)

• Figure 1: Light curve of the GLEAN object with Source ID 1268847039908643840 in Gaia DR3. The green dots represent the $G$-band, the blue dots the $G_{\mathrm{BP}}$-band and the red dots the $G_{\mathrm{RP}}$-band. BJD in TCB is the Barycentric Julian Date in Barycentric Coordinate Time, or the number of days elapsed since January 1st, 2010. The isolated blue point is representative of the outliers that appear in many Gaia light curves and is likely instrumental rather than physical. The sampling is uneven and is different for each object.
• Figure 2: Color-magnitude diagram of the $872~228$ GLEAN variable AGNs. The faintest magnitude in GLEAN is $G \sim 21$. Fainter objects tend to be redder.
• Figure 3: Histogram of fractional variability amplitude $f_G$ in the $G$-band of GLEAN variable AGNs. Most objects have $f_G \sim 0.08$, and 93.9% of the distribution has 0 $\lesssim f_G \lesssim$ 0.2.
• Figure 4: Histogram of mean $G$ magnitude of GLEAN variable AGNs. Most objects have $G \sim 20.2$, and 84.2% of the distribution has 19 $\lesssim G \lesssim$ 21.
• Figure 5: SDSS $i$-band model magnitude versus redshift (1 + $z$) for the $203~915$ GLEAN-SDSS objects. The colors represent different classes: the red density contours for QSOs ($201~631$ objects), the blue squares for GALAXYs ($2050$ objects) and the green stars for STARs ($234$ objects). The 127 AGNs of the final sample are shown in pink salmon diamonds.