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On "Nonstationarity of AGN variability" -- selection effect
A new paper accepted by ApJL, arXiv:2001.04471 "Nonstationarity of AGN
variability: The only way to go is down!" by Caplar et al, concludes that visible quasar luminosity is linearly decreasing due to cosmological effects, and that this change is visible over a 15-year timeline. It seems to me that a selection effect is a more likely cause. Their data sample is 5919 SDSS-DR7 quasars which were re-observed in the recent HSC survey, with a redshift-dependent magnitude decrease observed over that time (14.85 years). But what is omitted is that SDSS-DR7 quasars needed to be of a certain minimum brightness for a spectrum to be obtained by them. This means that of a specified complete population of quasars, only the brighter ones would get the DR7 spectrum -- therefore you'd expect a subsequent average decrease in brightness. The fainter members of that population (too faint for SDSS-DR7 to take a spectrum) may well brighten in that 15-year interval, but HSC doesn't take their spectrum and so they are not included in the sample. For the sample to be unbiased, HSC should take spectra in the same way as SDSS did, and so classify the quasars, and then the sample can be selected from both ends to see the unbiased behaviour across that 15-year baseline. The corollary to their methodology would be that if HSC did classify quasars from their data, and you selected those HSC-found quasars and then matched them to all SDSS-DR7 pipeline optical data (including those which were not classified by SDSS-DR7), that you'd likely find that most of those objects were *brightening* in the 15-year interval. Let's see that test as a control. The authors use, as a control sample, a group of non-variable stars and observe that they don't vary in the same way as the quasars. Well, why did they use non-variable stars to compare to quasars which are known to vary? Why didn't they use variable stars? So I think that the paper has not made its case. Eric Flesch |
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