arxiv: 2604.02529 · v1 · submitted 2026-04-02 · 🌌 astro-ph.GA

Recognition: no theorem link

The South Pole Telescope AGN Monitoring Campaign: First Release of SPTpol Bright AGN Light Curves

J. C. Hood II , P. A. R. Ade , A. J. Anderson , M. Archipley , J. E. Austermann , J. A. Beall , A. N. Bender , B. A. Benson

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F. Bianchini L. E. Bleem J. E. Carlstrom C. L. Chang P. Chaubal H. C. Chiang T-L. Chou R. Citron C. Corbett Moran T. M. Crawford A. T. Crites T. de Haan M. A. Dobbs W. Everett A. Foster J. Gallicchio E. M. George N. Gupta N. W. Halverson G. C. Hilton G. P. Holder W. L. Holzapfel J. D. Hrubes N. Huang J. Hubmayr K. D. Irwin E. Jarvela L. Knox A. T. Lee D. Li A. Lowitz T. J. Maccarone M. Malkan J. J. McMahon S. S. Meyer J. Montgomery T. Natoli J. P. Nibarger G. Noble V. Novosad S. Padin S. Patil K. A. Phadke C. Pryke C. L. Reichardt J. E. Ruhl B. R. Saliwanchik K. K. Schaffer C. Sievers A. Simpson G. Smecher A. A. Stark C. Tandoi C. Tucker T. Veach J. D. Vieira G. Wang N. Whitehorn W. L. K. Wu V. Yefremenko J. A. Zebrowski

Authors on Pith no claims yet

Pith reviewed 2026-05-13 20:13 UTC · model grok-4.3

classification 🌌 astro-ph.GA

keywords active galactic nucleilight curvesSouth Pole TelescopeSPTpolvariabilityspectral indexradio astronomyAGN monitoring

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The pith

SPTpol releases the first public catalog of 158 active galactic nuclei light curves at 90 and 150 GHz.

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

The paper makes available 158 AGN light curves extracted from repeated scans of a 500 square degree sky patch by the SPTpol camera between 2012 and 2016. These data come from an instrument built for cosmic microwave background observations but now repurposed to track brightness changes in active galactic nuclei at radio frequencies. The release includes full details on how the flux densities were measured from the survey data plus an example analysis that finds a strong link between how much an AGN varies and its average spectral index. This dataset opens the door for broader studies of AGN behavior using existing CMB survey archives.

Core claim

The paper releases 158 AGN light curves and associated data from SPTpol observations between 2012 and 2016, obtained from the 500 deg² survey at 90 and 150 GHz, along with a description of the processing methods and an initial analysis finding a greater than 10 sigma correlation between fractional intrinsic variance and mean spectral index.

What carries the argument

The extraction of total intensity light curves from repeated survey scans using the SPTpol camera's 90 and 150 GHz detectors, which isolates AGN flux densities from the broader sky maps.

If this is right

Researchers can now access and analyze these light curves for studies of AGN variability.
The data enable searches for correlations between variability properties and other AGN characteristics like spectral index.
Future releases will incorporate polarization measurements and additional frequencies from SPT-3G.
These light curves provide a new resource for multi-wavelength AGN monitoring campaigns.

Where Pith is reading between the lines

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

Combining these radio-frequency light curves with optical or X-ray data could reveal emission mechanisms across wavelengths.
Longer-term monitoring with SPT-3G might detect rarer, high-amplitude flares in the sample.
The processing pipeline could be adapted for other CMB experiments to extract transient sources.

Load-bearing premise

The assumption that the data processing methods can accurately separate AGN signals from atmospheric noise, instrumental effects, and other sky sources in the survey data.

What would settle it

Comparison of the released light curves with simultaneous observations from another telescope at similar frequencies showing significant discrepancies in flux measurements.

Figures

Figures reproduced from arXiv: 2604.02529 by A. A. Stark, A. Foster, A. J. Anderson, A. Lowitz, A. N. Bender, A. Simpson, A. T. Crites, A. T. Lee, B. A. Benson, B. R. Saliwanchik, C. Corbett Moran, C. L. Chang, C. L. Reichardt, C. Pryke, C. Sievers, C. Tandoi, C. Tucker, D. Li, E. Jarvela, E. M. George, F. Bianchini, G. C. Hilton, G. Noble, G. P. Holder, G. Smecher, G. Wang, H. C. Chiang, J. A. Beall, J. A. Zebrowski, J. C. Hood II, J. D. Hrubes, J. D. Vieira, J. E. Austermann, J. E. Carlstrom, J. E. Ruhl, J. Gallicchio, J. Hubmayr, J. J. McMahon, J. Montgomery, J. P. Nibarger, K. A. Phadke, K. D. Irwin, K. K. Schaffer, L. E. Bleem, L. Knox, M. A. Dobbs, M. Archipley, M. Malkan, N. Gupta, N. Huang, N. W. Halverson, N. Whitehorn, P. A. R. Ade, P. Chaubal, R. Citron, S. Padin, S. Patil, S. S. Meyer, T. de Haan, T. J. Maccarone, T-L. Chou, T. M. Crawford, T. Natoli, T. Veach, V. Novosad, V. Yefremenko, W. Everett, W. L. Holzapfel, W. L. K. Wu.

**Figure 1.** Figure 1: Orthographic projection of the SPTpol 500 deg2 (green, dashed), SPT-SZ 2500 deg2 (gray, dot-dashed) and the SPT3G 1500 deg2 (orange, solid) survey fields, where the background is the Planck dust map (Planck Collaboration et al. 2014b). scan, resulting in an effective high-pass filter in the scan direction with a cutoff in angular multipole of ℓ = 300. Areas within a 5’ radius of point sources that are bri… view at source ↗

**Figure 3.** Figure 3: Angular separation to the nearest AT20G counterpart shown as a function of SPTpol 150 GHz flux density for all SPT-SZ sources within the footprint defined by the SPTpol 25% weight contour. The dashed horizontal line marks the 1 arcmin (60 arcsec) association radius used to define accepted matches; sources below this threshold are retained, while sources above it are treated as non-associations. above our… view at source ↗

**Figure 2.** Figure 2: Source selection takes the SPT-SZ point source catalog (Everett et al. 2020), extracts sources with a S/N greater than 10, cross matches the RA and DEC to the AT20G catalog, then, takes those with a pixel value with a weight value above 25% of the maps mean weight. The initial step involves identifying all known sources in our survey field that meet our threshold for signalto-noise (S/N) ratio. For this, … view at source ↗

**Figure 4.** Figure 4: Fraction of median map weight values at the location of each source in the 150 GHz coadded map (blue) and the 90 GHz coadded map (orange). The dashed vertical line indicates the threshold for inclusion in the catalog (25% weight). 4. BUILDING LIGHTCURVES The next step towards making our catalog is extracting the flux density from each of the bundled maps at the location of each of our sources. In this sec… view at source ↗

**Figure 5.** Figure 5: Azimuthally averaged matched filter for point source extraction at 150 GHz. but if we zoom in, as shown in the top image of Figure 6, we can see that there are point sources visible but sitting on the CMB fluctuations in the unfiltered maps. Filtering largely removes the typical CMB patches and structures from the maps. In the bottom image of Figure 6, we can see that the filtering has worked well; we se… view at source ↗

**Figure 6.** Figure 6: Top: 5’x5’ cutout of SPTpol coadd map with long wavelength features still visible. Bottom: Matched filtered version of the same map cut out shown above. or 96.24 × 109 Hz (the effective band centers discussed in Section 2.1), and Ωb is the solid angle of the beam measured on Venus as described in Section 4.1. All of the sources that are included in this study were masked during the TOD filtering mentioned … view at source ↗

**Figure 7.** Figure 7: Top: Distribution of source four-year average S/N values per bundle in both 90 GHz (blue) and 150 GHz (orange) bands. Bottom: Distribution of mean flux densities for each source. Each reported redshift value is the result of a NED query for sources with a redshift. Appendix B contains a table with information about all sources in our current catalog. We note that 56 of our sources have a measured redshift… view at source ↗

**Figure 8.** Figure 8: 150 GHz fractional intrinsic variance vs. weighted mean spectral index. Red points indicate sources with log νLν ≥ 43, blue points indicate sources with values lower than 43, and black points indicate sources with no measured redshift. in Edelson et al. (2002): FIV = MSD − σ 2 ⟨F⟩ 2 (9) σ(FIV) = s 2 Npoints · MSD ⟨F⟩ 2 (10) where σ 2 is the noise squared deviation (measure of the variability in our data du… view at source ↗

**Figure 9.** Figure 9: Delta of spectral indexes between flaring and non flaring states for each of our 40 brightest AGN light curves. we did not find any significant detection [PITH_FULL_IMAGE:figures/full_fig_p013_9.png] view at source ↗

**Figure 10.** Figure 10: Sample 90 and 150 GHz light curves from the STRAWHAT catalog [PITH_FULL_IMAGE:figures/full_fig_p015_10.png] view at source ↗

**Figure 11.** Figure 11: Sample 90 and 150 GHz light curves from the STRAWHAT catalog [PITH_FULL_IMAGE:figures/full_fig_p016_11.png] view at source ↗

read the original abstract

The South Pole Telescope (SPT) collaboration has recently embarked upon a campaign to monitor the brightness of a sample of active galactic nuclei (AGN), both in real time and in archival SPT data. The original design of the SPT was optimized for observations of the cosmic microwave background (CMB) at arc-minute and larger angular scales, and it has been used for this purpose for nearly twenty years, using three generations of CMB cameras. Recently it has been recognized that data from CMB experiments have the potential to be used for AGN monitoring. In this paper, we present the first public release of data from a full sample of SPT-monitored AGN, comprising 158 AGN light curves and associated data from the SPTpol camera, which was operational from 2012-2016. These light curves were created using observations from the SPTpol 500 deg$^{2}$ survey, in which the instrument was used to scan a 500 deg$^2$ patch of the sky several times per day with detectors sensitive to radiation in bands centered at 90 and 150 GHz. We provide a comprehensive description of the observations, the data processing methods, and the resulting light curve catalog. As an example of analyses that these data enable, we searched for a correlation between variability and spectral index, and we looked for ``bluer-when-brighter'' trends in the sample. Our analysis finds $> 10 \sigma$ correlation between fractional intrinsic variance and mean spectral index in the sample, but no significant evidence for bluer-when-brighter trends. The datasets from this study can be accessed through the SPT Treasury Record of AGN With Historical Activity and Time-Series or STRAWHAT catalog. This initial data release includes SPTpol light curves at 90 and 150 GHz, focusing on total intensity. In later updates, SPTpol polarization data and new observations from the SPT-3G instrument at 90, 150, and 220 GHz will be included.

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.

Desk Editor's Note private letter to a colleague

This is a clean data release of 158 SPTpol AGN light curves at 90 and 150 GHz from archival survey scans, plus one solid example correlation.

read the letter

The main point is that the team has turned existing SPTpol 500 deg² survey data into a public catalog of 158 AGN light curves at 90 and 150 GHz covering 2012-2016. That is genuinely new and fills a gap for millimeter-wavelength variability work without new telescope time. They describe the scanning strategy, the map-making steps adapted from CMB processing, and how they extract the time series, then show an example analysis that finds a >10σ correlation between fractional intrinsic variance and mean spectral index, with no clear bluer-when-brighter trend. The correlation looks real for the sample and gives a quick sense of what the data can do. The processing follows standard CMB practices for atmosphere removal and point-source handling, so the extraction method itself is not exotic. The release includes the light curves and some metadata through the STRAWHAT catalog, which is the practical deliverable. The soft spots are modest. Full validation plots and tests against contamination or systematics sit in the paper rather than the abstract, so a referee will want to check those carefully. This release is total intensity only, with polarization and SPT-3G data promised later, and the sample is limited to the survey footprint. No external cross-checks with other mm monitoring programs appear yet, but that is reasonable for a first release. The work is aimed at AGN variability researchers who need 90/150 GHz coverage for multi-band studies or time-domain statistics. It is not a theoretical advance, but the catalog is usable and the example result is straightforward. I would send it to peer review. The methods need a close look, but once cleared the data release itself is worth archiving.

Referee Report

0 major / 2 minor

Summary. The paper presents the first public release of 158 AGN light curves at 90 and 150 GHz from SPTpol observations of the 500 deg² survey (2012-2016). It describes the observations, data processing pipeline adapted from CMB map-making, and the resulting catalog, with an example analysis showing a >10σ correlation between fractional intrinsic variance and mean spectral index but no significant bluer-when-brighter trends. Data are released via the STRAWHAT catalog.

Significance. If the processing pipeline performs as described, this data release is significant for AGN variability research at millimeter wavelengths. It efficiently repurposes existing high-quality CMB survey data into a large public sample, enabling cross-wavelength studies and model tests. The public catalog and example analysis with a robust statistical detection are strengths that enhance reproducibility and community utility.

minor comments (2)

Abstract: The description of the correlation analysis would benefit from a brief note on the sample statistics (e.g., number of AGN with sufficient epochs) to contextualize the >10σ result for readers.
Data Release section: Clarify the precise file formats, column definitions, and any quality flags included in the STRAWHAT catalog to facilitate immediate use by the community.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript, the recognition of the data release's significance for AGN variability research, and the recommendation for minor revision. No specific major comments were provided in the report beyond the overall summary and significance evaluation.

Circularity Check

0 steps flagged

No significant circularity identified

full rationale

This is an observational data-release paper presenting 158 SPTpol AGN light curves at 90 and 150 GHz extracted from the 500 deg² survey scans (2012-2016). The central claim is the public catalog itself together with a description of standard map-making and flux-extraction steps. No predictions, derivations, or fitted parameters are presented whose outputs reduce by construction to the inputs. The single example analysis (a >10σ correlation between fractional variance and spectral index) is a post-hoc statistical test performed on the released measurements and does not feed back into the data products. No self-citation chain, ansatz smuggling, or uniqueness theorem is invoked to justify the core result. The manuscript is therefore self-contained against external benchmarks with no load-bearing circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

This is an observational data release paper. No free parameters, axioms, or invented entities are invoked in the central claim of releasing the light curves; standard radio astronomy calibrations are assumed but not detailed as fitted in the abstract.

pith-pipeline@v0.9.0 · 6059 in / 1023 out tokens · 39445 ms · 2026-05-13T20:13:17.224244+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

5 extracted references · 5 canonical work pages

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work page doi:10.5281/zenodo.831784 2021
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