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MethodsArchival photometry / time-series statisticsMatched-comparison century-slope test for DASCH light curves
Method · archival photometry / time-series statistics

Matched-comparison century-slope test for DASCH light curves

Settle whether a claimed century-long fade (Schaefer 2016) exceeds the plate-photometry systematic floor (Hippke et al. 2016, Lund et al. 2016) using the same, current data release for target and comparisons.

Archival photometry / time-series statisticsVersion 1.0Licence MIT1 runUsed on 1 mystery

Questions it answers

Code Python

Download module (.py)analyze_dasch.pyView source

Runs and results 1 run

RunMysteryMeasuredResultAgainstDate
RUN0DDYGPP634AATabby'sTarget slope (mag per century) and its percentile and z among 49 matched comparison stars; plate-differential slope and its matched null; post-1969 minus 1930-1953 offset{'tight_z': 2.38, 'target_n': 895, 'target_z': 1.96, 'matched_n': 49, 'matched_sd': 0.068, 'pre1953_err': 0.046, 'gap_offset_z': 2.02, 'matched_mean': 0.034, 'target_theil': 0.19, 'pre1953_slope': 0.019, 'differential_z': 2.04, 'target_wls_err': 0.02, 'tight_matched_n': 18, 'differential_err': 0.021, 'target_bin_slope': 0.208, 'target_wls_slope': 0.166, 'gap_offset_target': 0.088, 'target_percentile': 0.98, 'bootstrap_raw_ci95': [0.131, 0.201], 'differential_slope': 0.133, 'differential_null_sd': 0.069, 'gap_offset_matched_sd': 0.034, 'differential_null_mean': -0.008, 'gap_offset_matched_mean': 0.02, 'bic_linear_minus_step_target': -6.9}hippke2016 0.12-0.14 +/- 0.02 with systematics floor 0.15-0.2; break at 1962 · schaefer2016 0.164 +/- 0.013 (check-star rms 0.043, 4 sigma) · matched_null_this_run mean 0.034, sd 0.068

Applied to

Tabby's star (KIC 8462852)
1 datasetDASCH DR7 light curves of KIC 8462852 and 184 comparison stars within 30 arcmin

References

  1. Schaefer, B. 2016. KIC 8462852 Faded at an Average Rate of 0.164 ± 0.013 Magnitudes per Century from 1890 to 1989. The Astrophysical Journal Letters 822(2), L34.
  2. Hippke, M., Angerhausen, D., Lund, M. et al. 2016. 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. The Astrophysical Journal 825(1), 73.
  3. arXiv:1605.02760 (preprint, May 2016). DASCH light curves of 644 F stars (337 Tycho-2 F3 IV/V in APASS B, 307 KIC analogues in g). The population's slope statistics are over-dispersed until a break at 1970 (the Menzel gap) is modelled; the target's full-series B slope of 0.129 +/- 0.026 mag per century splits into two flat segments and a jump; 11 of 644 stars still exceed 5 sigma after the split. Concludes that several apparent F-star fades are artefacts of the gap and that KIC 8462852 is consistent with constant flux.
  4. Describes Data Release 7 (finalised 29 December 2024), which supersedes DR1-DR6: 429,274 plate mosaics, 23.6 billion B magnitudes for 252 million sources calibrated to APASS DR8; typical light-curve rms about 0.15 mag. The release Schaefer and Hippke used in 2016 is no longer served, so DR7 is the only reproducible basis for the century-fade question. See also https://dasch.cfa.harvard.edu/dr7/ for the known-issues list, colour terms and the daschlab API.
  5. Schaefer's public rebuttal (with a follow-up post of 24 May 2016): Hippke's comparison stars mixed red and yellow plates and included flagged data, and crowded stars are merged by SExtractor on the coarse dnb plates; his own uncrowded, colour-matched check stars have a mean slope of -0.007 with rms 0.044 mag per century, making the fade 4 sigma. There is no separate journal rebuttal; v2 of arXiv:1601.03256 carries the same critique.
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