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arxiv: 2510.16584 · v3 · submitted 2025-10-18 · 🌌 astro-ph.HE

CAZ catalog and optical light curves of 7918 blazar-selected active galactic nuclei

Pouya M. Kouch , Elina Lindfors , Talvikki Hovatta , Ioannis Liodakis , Karri I.I. Koljonen , Alessandro Paggi , Kari Nilsson , Jenni Jormanainen
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Vandad Fallah Ramazani Sofia Kankkunen Folkert Wierda Sarah M. Wagner Matthew J. Graham
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Pith reviewed 2026-05-18 05:54 UTC · model grok-4.3

classification 🌌 astro-ph.HE
keywords blazarsactive galactic nucleioptical light curvesvariabilitysynchrotron peak frequencyDoppler factorBayesian blocksflares
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The pith

A catalog of 7918 blazars and candidates supplies the largest collection of optical light curves yet and shows that flares rise faster than they decay.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The authors combine several existing blazar-dominated AGN samples to build a catalog of 7918 sources, the largest assembled so far. They gather nightly optical flux measurements from the CRTS, ATLAS, and ZTF surveys between 2007 and 2023 to construct the longest and highest-cadence light curves possible for each object. Variability is quantified and the Bayesian blocks algorithm is applied to mark flaring intervals. A sympathetic reader would care because these patterns, if general, indicate how the location of the emitting region and the degree of relativistic boosting shape the observed changes in jet-powered objects.

Core claim

The paper presents the CAZ catalog of 7918 blazars and candidates together with their optical light curves, Bayesian blocks, and identified flaring periods. It reports four main results: optical flares generally rise faster than they decay, optical brightness and variability depend strongly on the synchrotron peak frequency, flat-spectrum radio quasars and BL Lac objects display comparable optical variability and flare properties when they share the same synchrotron peak frequency, and optical flare timescales shorten while amplitudes grow as the radio variability Doppler factor increases.

What carries the argument

Nightly optical flux densities drawn from the CRTS, ATLAS, and ZTF all-sky surveys, processed with the Bayesian blocks algorithm to locate flaring periods within the light curves.

If this is right

  • Optical flares in blazars rise more quickly than they fall on average.
  • Optical brightness levels and the strength of variability both track the synchrotron peak frequency of each source.
  • Flat-spectrum radio quasars and BL Lac objects exhibit nearly identical optical flare rise times, decay times, and amplitudes once they are compared at the same synchrotron peak frequency.
  • Sources with larger radio variability Doppler factors display shorter but higher-amplitude optical flares.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The catalog size now supports statistical tests that separate the effects of viewing angle from changes in the intrinsic jet emission region.
  • Cross-matching the optical flare times with simultaneous radio or gamma-ray monitoring could reveal whether the same events drive variability across wavebands.
  • The observed trend with Doppler factor implies that future monitoring of newly discovered blazars can predict the expected optical flare duration from radio data alone.
  • Extending the same light-curve extraction to newer survey releases would test whether the reported flare statistics remain stable over longer time baselines.

Load-bearing premise

The objects taken from prior blazar-dominated AGN samples are correctly identified as blazars or candidates and the nightly optical measurements from the surveys are free of large calibration errors or source confusion.

What would settle it

Re-processing a sizable subset of the light curves with an independent photometric calibration or re-classifying a sample of sources that eliminates the reported faster rise times or the correlation with synchrotron peak frequency would falsify the central findings.

Figures

Figures reproduced from arXiv: 2510.16584 by Alessandro Paggi, Elina Lindfors, Folkert Wierda, Ioannis Liodakis, Jenni Jormanainen, Kari Nilsson, Karri I.I. Koljonen, Matthew J. Graham, Pouya M. Kouch, Sarah M. Wagner, Sofia Kankkunen, Talvikki Hovatta, Vandad Fallah Ramazani.

Figure 1
Figure 1. Figure 1: Sky distribution of 7918 sources in the CAZ catalog using the equatorial coordinate system (J2000 epoch) [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Distributions of (i) redshift, (ii) LE and HE peak frequencies (shown in green and red, respectively), (iii) radio variability Doppler factor, and (iv) median X-ray flux density of sources in the CAZ cat￾alog. “MED” refers to median, “CAZ #” to the number of available parameters. Africa. It also occasionally observes in g (λeff = 475 nm), r (λeff = 637 nm), and i (λeff = 783 nm) filters. The typical lim￾it… view at source ↗
Figure 3
Figure 3. Figure 3: Global distributions of 8130 merged CAZ light curves (7722 of AGN and 408 of WDs) with ≥10 data points and ≥30 d duration. Panel (i) shows the mean shift factor across all filters per light curve, (ii) median flux density per light curve, (iii) median flux density error per light curve, (iv) data count per light curve, (v) length of each light curve, (vi) cadence per light curve. The solid orange vertical … view at source ↗
Figure 4
Figure 4. Figure 4: Example merged light curves. The name of the source is given on the top left corner of each panel. We note that some light curves are of much higher quality than others (e.g., plot i compared to iv; the former is of the well-known, highly variable TXS 0506+056, while the latter is of a host-galaxy-dominated source). A zoomed-in version of these light curves (including identified periods of enhanced emissio… view at source ↗
Figure 5
Figure 5. Figure 5: Example zoomed-in light curves with BB95 periods (Sect. 4.2) and BBHOP flares (Sect. 4.3) identified. The merged light curves on the left are zoomed-in versions of those shown in [PITH_FULL_IMAGE:figures/full_fig_p008_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Fvar versus median flux density plots. Panel (i) shows the merged light curves, (ii) CRTS-only, (iii) ATLAS-only, and (iv) ZTF-only. “UL” refers to 3σ upper limits of Fvar. The red hatched areas in (ii) and (iv) represent the low flux density and low variability regimes to determine non-variable light curves. The cyan hatched box is typically occupied by WDs and is another non-variable regime [PITH_FULL_I… view at source ↗
Figure 7
Figure 7. Figure 7: Distribution of the duration of BB95 in variable CAZ light curves. The vertical solid line shows the median of the distribution and the dotted lines its 25th and 75th percentiles [PITH_FULL_IMAGE:figures/full_fig_p010_7.png] view at source ↗
Figure 9
Figure 9. Figure 9: Merging two neighboring BBHOP flares. If any of the four con￾ditions is true, the two neighboring BBHOP flares get merged into a single BBHOP flare. The start BB, end BB, and peak BB of the merged flare are the start BB of the left BBHOP, end BB of the right BBHOP, and the larger of the two peak BBs, respectively. valley-hill-valley BB patterns. This becomes detrimental when the BBHOP algorithm is tracing … view at source ↗
Figure 8
Figure 8. Figure 8: Difference between BB95 (grey vertical areas in panel i), BB￾HOP flares (green lines in panel ii), and prominent BBHOP flares (pur￾ple area along with green lines in panel ii). Panel (i) shows the merged CAZ light curve, and (ii) its BB light curve for a zoomed-in portion of CAZJ1100+4019. The grey horizontal solid line represents S 95%, the grey horizontal dotted line S 75%, and the red horizontal solid l… view at source ↗
Figure 10
Figure 10. Figure 10: Determining the start time of a BBHOP flare. In scenario (A), the midpoint of the start BB is chosen as the start as it minimizes the flare duration. Conversely, in scenario (B), the duration of the adjacent BB is used to obtain the start as flipping it minimizes the flare duration. A mirrored equivalent of this principle is used for determining the fall time of a flare. sen as the peak BB of the merged f… view at source ↗
Figure 11
Figure 11. Figure 11: Flare characteristics of all 74775 BBHOP flares. Panel (i) shows the distribution of the duty cycle of flaring sources, (ii) global flare amplitudes (G), (iii) rise and fall times of flares (trise and tfall), (iv) local brightening and decaying flare amplitudes (B and D), (v) temporal asymmetry of flares, and (vi) amplitude asymmetry of flares. In panel (iii) we perform KS test on the distributions of tri… view at source ↗
Figure 12
Figure 12. Figure 12: Flare characteristics of 25194 prominent BBHOP flares. For plot details see the description of [PITH_FULL_IMAGE:figures/full_fig_p012_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: Distribution of median flux density against redshift (i and ii), median flux density and Fvar against synchrotron peak frequency (iii–vi), and median flux density and Fvar against radio variability Doppler factor (vii–x) in ZTF-only merged light curves of the sources in the CAZ catalog. Pink crosses and solid markings refer to FSRQs (Q); blue dots and dotted markings to BLLs (B). The top panels are scatte… view at source ↗
Figure 14
Figure 14. Figure 14: We note that these BB95 are limited to those from vari￾able CAZ light curves (see Sect. 4.1). The BB95 fraction of FS￾RQs shows a >3σ weakly positive trend (τ = +0.11) and peaks at νsy ∼ 1014. For BLLs, the median of the BB95 fraction is also greatest at νsy ∼ 1014, however, its trend is mostly flat against νsy (τ = −0.06 at a significance of 2.7σ). As such, we cannot significantly determine whether the f… view at source ↗
Figure 15
Figure 15. Figure 15: Distribution of source duty cycle (i and ii) as well as source-averaged average rise and fall times (iii and iv), global amplitude (v and vi), and average local brightening and decaying amplitudes (vii and viii) of prominent BBHOP flares against synchrotron peak frequency. Pink crosses and solid markings refer to FSRQs (Q); blue dots and dotted markings to BLLs (B). Kendall’s τ correlation coefficient and… view at source ↗
Figure 16
Figure 16. Figure 16: Similar as [PITH_FULL_IMAGE:figures/full_fig_p015_16.png] view at source ↗
read the original abstract

Active galactic nuclei (AGN) are some of the brightest and most variable objects in the Universe. Those with relativistic jets observed at small viewing angles are blazars. Due to Doppler boosting, blazars exhibit extreme stochastic variability. While the origin of this variability is thought to be changes in the accretion flow and jet dynamics, much about blazar variability remains unknown. In this paper we use several blazar-dominated AGN samples to form a catalog of 7918 blazars and candidates -- the largest to date. We also collected source types, redshifts, peak frequencies of the spectral energy distribution, radio variability Doppler factors, and X-ray flux densities for as many sources as possible. We used all-sky surveys (CRTS, ATLAS, and ZTF, abbreviated as ``CAZ'') to extract their optical multiband flux density on a nightly basis between 2007 and 2023, and we constructed as long and as high cadence light curves as possible for as many sources as attainable. We quantified the variability of the light curves and applied the Bayesian blocks algorithm to determine their flaring periods. The CAZ catalog and light curves as well as the corresponding Bayesian blocks and flaring periods are all provided in the accompanying electronic tables, with the goal of enabling analyses involving jetted AGN variability with unprecedented sample sizes. Overall, we find (1) optical flares generally have a faster rise than decay; (2) optical brightness and variability are strongly dependent on the synchrotron peak frequency; (3) flat spectrum radio quasars and BL Lac objects have comparable optical variability and flare characteristics at the same synchrotron peak frequency; and (4) optical flare times tend to decrease while amplitudes increase with an increasing radio variability Doppler factor.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 3 minor

Summary. The manuscript constructs the CAZ catalog of 7918 blazar-selected AGN by aggregating existing blazar-dominated samples and compiling ancillary data (types, redshifts, synchrotron peak frequencies, radio Doppler factors, X-ray fluxes). Nightly optical flux densities are extracted from CRTS, ATLAS, and ZTF surveys (2007–2023) to build light curves, to which the Bayesian blocks algorithm is applied to identify flaring intervals. The catalog, light curves, blocks, and flare parameters are released publicly. Four statistical results are reported: optical flares rise faster than they decay; brightness and variability correlate strongly with synchrotron peak frequency; FSRQs and BL Lacs exhibit comparable variability at fixed peak frequency; and flare timescales shorten while amplitudes grow with increasing radio Doppler factor.

Significance. If the input classifications and nightly CAZ photometry are reliable, the work supplies the largest public optical variability dataset for jetted AGN to date. The four headline trends provide direct observational constraints on jet physics, flare asymmetry, and Doppler boosting that can be compared against theoretical models. Public release of the full light-curve and flare tables is a clear community service that enables future statistical studies with unprecedented sample sizes.

major comments (2)
  1. [§2] §2 (Sample construction): The paper aggregates multiple pre-existing blazar catalogs without a quantitative assessment of classification purity or completeness; residual non-blazar contaminants or inconsistent type assignments would directly propagate into the reported synchrotron-peak and FSRQ/BL Lac comparisons.
  2. [§4.3] §4.3 (Light-curve extraction): Nightly flux densities from CRTS/ATLAS/ZTF are combined without an explicit cross-calibration or confusion analysis; any survey-specific zero-point offsets or source-blending effects in dense fields would affect the measured flare amplitudes and rise/decay times that underpin results (1) and (4).
minor comments (3)
  1. [Table 1] Table 1: the column descriptions for the Bayesian-block start/stop times should explicitly state the time system (MJD or JD) and whether the blocks are computed on the combined CAZ light curve or per survey.
  2. [Figure 7] Figure 7: the legend for the FSRQ vs. BL Lac symbols is too small; enlarging it and adding a note on the number of sources in each bin would improve readability.
  3. [§5.2] The text in §5.2 refers to “optical brightness” but the plotted quantity is actually the median flux density; consistent terminology would avoid confusion.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and recommendation for minor revision. We address each major comment below and have revised the manuscript to incorporate clarifications and additional discussion where appropriate.

read point-by-point responses

  1. Referee: [§2] §2 (Sample construction): The paper aggregates multiple pre-existing blazar catalogs without a quantitative assessment of classification purity or completeness; residual non-blazar contaminants or inconsistent type assignments would directly propagate into the reported synchrotron-peak and FSRQ/BL Lac comparisons.

    Authors: We agree that the current manuscript does not include a new, self-contained quantitative assessment of overall purity and completeness for the aggregated sample. The CAZ catalog is formed by combining established blazar-dominated samples from the literature, each of which has been vetted and characterized in its original publication. To address the concern, we will expand §2 with a concise summary of the selection criteria and known limitations drawn from the source catalogs, including citations to any available purity or completeness studies. We will also note how residual contamination or type inconsistencies could affect the synchrotron-peak and FSRQ/BL Lac comparisons, thereby providing better context for the statistical results. revision: yes

  2. Referee: [§4.3] §4.3 (Light-curve extraction): Nightly flux densities from CRTS/ATLAS/ZTF are combined without an explicit cross-calibration or confusion analysis; any survey-specific zero-point offsets or source-blending effects in dense fields would affect the measured flare amplitudes and rise/decay times that underpin results (1) and (4).

    Authors: We acknowledge that §4.3 does not present an explicit cross-calibration between the three surveys or a dedicated confusion analysis. Each survey's nightly photometry is adopted as published, with the analysis emphasizing relative variability and flare detection within individual light-curve segments. We will revise §4.3 to describe the data ingestion procedure more explicitly, report any internal consistency checks performed on overlapping epochs, and add a brief discussion of possible zero-point offsets and blending effects in crowded fields. These additions will clarify the robustness of the flare rise/decay times and amplitudes used in results (1) and (4). revision: yes

Circularity Check

0 steps flagged

No significant circularity; results are direct observational statistics

full rationale

The paper assembles a catalog from pre-existing blazar-dominated AGN samples, extracts nightly optical flux densities from public all-sky surveys (CRTS, ATLAS, ZTF), computes standard variability metrics, and applies the off-the-shelf Bayesian blocks algorithm to segment light curves into flaring periods. The four headline results are purely statistical summaries of these measured quantities (rise/decay asymmetry, synchrotron-peak dependence, FSRQ/BL Lac similarity at fixed peak frequency, and Doppler-factor trends). No model is fitted, no parameter is predicted from a self-defined quantity, and no load-bearing step reduces to a self-citation or ansatz. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

This is an observational catalog paper; the central results rest on standard astronomical data-processing assumptions rather than new physical postulates or fitted parameters.

axioms (1)
  • domain assumption Existing blazar classifications and auxiliary parameters (redshifts, peak frequencies, Doppler factors) from prior samples are sufficiently accurate for statistical use.
    The catalog is assembled from several blazar-dominated AGN samples whose properties are taken as given.

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Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

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    astro-ph.GA 2026-04 unverdicted novelty 7.0

    First VLTI-GRAVITY near-infrared observations of blazars indicate possible detection of unresolved or partially resolved jet emission in Ton 599, though data cannot distinguish extended structure from instrumental coh...

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