A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC 8462852

Abstract: We present a statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades. In our examination of archival Johnson B photometry from the Harvard DASCH archive, we find a median RMS scatter of lightcurves of order 0.15mag over the range B~9-17 for all calibrations. Slight underlying systematics (trends or flux discontinuities) are on a level of ~<=0.2mag per century (1889-1990) for the majority of constant stars. These historic data can be unambiguously used for processes that happen on scales of magnitudes, and need to be carefully examined in cases approaching the noise floor. The characterization of these limits in photometric stability may guide future studies in their use of plate archives. We explain these limitations for the example case of KIC8462852, which has been claimed to dim by 0.16mag per century, and show that this trend cannot be considered as significant.

• The paper concludes from a statistical analysis that digitized historical plates have an inherent photometric stability limit around 0.1 magnitude per century.
• The analysis of KIC 8462852 suggests previous claims of century-long dimming lack statistical significance when considering the identified systematic errors in the plate data.
• These findings underscore the importance of carefully accounting for systematic uncertainties when using historical photographic plate data for precise long-term astrophysical analyses.

Statistical Evaluation of Long-Term Photometric Stability in Century-Long Observations

The paper "A statistical analysis of the accuracy of the digitized magnitudes of photometric plates on the time scale of decades with an application to the century-long light curve of KIC 8462852" by Hippke et al. provides a rigorous quantitative assessment of the photometric precision achievable using digitized historical photographic plates. These plates, archived in the Digital Access to a Sky Century @ Harvard (DASCH) project, hold data spanning over a century, collected primarily in the Johnson B band.

Summary

The authors conducted a thorough statistical analysis aimed at gauging the fidelity and systematic variances present in the photometry derived from these archival sources. Using data from the DASCH project, the exploration began with a reported median root mean square (RMS) scatter of 0.15 mag covering a magnitude range of B ~9 to 17. Notably, the paper outlines systematic trends and potential flux discontinuities on the scale of 0.2 mag per century. Such findings indicate that despite the impressive breadth of the data, caution is warranted when using these plates for precise, long-term photometric studies.

A case study involving KIC 8462852 is detailed, where claims of significant dimming across the century were statistically challenged by the authors based on data from DASCH. They conclude that previous assertions of a 0.16 mag per century dimming lack statistical significance when confronted with the inherent systematic errors identified in the broader dataset.

Methodology

The authors deployed several methodologies to scrutinize the DASCH data rigorously:

1. Selection of Benchmark Stars: Candidates for comparison were selected based on their photometric and spectral similarity to KIC 8462852. The selection involved cross-matching photometric properties to ensure robust benchmarking.
2. Data Retrieval and Cleaning: The data underwent stringent quality control—excluding plates with significant photometric deviations due to non-optimal conditions or known errors. Three separate calibrations of the DASCH data were considered, utilizing KIC, GSC, and APASS standards.
3. Statistical Tests: Tests for normality and analyses for linear trends and potential structural breaks (flux discontinuities) were conducted. Notably, they utilized robust regression to mitigate the effects of statistical anomalies.

Findings and Implications

The study found consistent slopes across hundreds of stars, including KIC 8462852. Such consistency implies a photometric stability ceiling of approximately 0.1 mag per century. This sets a benchmark for the long-term reliability of photometric data extracted from Harvard plates, suggesting that studies requiring precision below this threshold should proceed with caution.

Furthermore, the evidence presented challenges prior projections of dramatic photometric characteristics for KIC 8462852, casting doubt on hypotheses like the Dyson sphere scenario around this star. With a careful consideration of systematic uncertainties, the narrative around purported variability for KIC 8462852 has been grounded to levels explained plausibly by instrumental factors or transient cosmic events.

Future Prospects

The revelations in this paper underscore the need for meticulous approaches in analyzing historical datasets. Future research can expand this statistical framework to other historical archives or integrate additional data refinement techniques, possibly leveraging contemporary machine learning methods to disentangle systematic biases from astrophysical signals.

The rigorous framework established by Hippke et al. inspires confidence in the continued use of historical photometric data, albeit with carefully set expectations. As contemporary missions such as TESS and PLATO continue to supplement these datasets, the need for such robust cross-validation becomes all the more critical, ensuring that the legacy of past astronomical endeavors continues to illuminate current and future questions in astrophysics.