On "Nonstationarity of AGN variability" -- selection effect

[Eric Flesch]

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

[Steve Willner]

In article <5e20f65d....@news.aioe.org>,

Eric Flesch <er...@flesch.org> writes:
> 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.

> 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 is a version of "Eddington bias," and it would have been my
first thought to explain the effect too. The source selection
details are at
https://iopscience.iop.org/article/10.1088/0004-6256/139/6/2360

Having only glanced through the paper -- and I realize I'm late
responding to this -- I don't see how Eddington bias can give the
redshift dependence in Fig 1. Also, Fig 3 shows the effect is
greatest for the brightest objects, not the faintest ones.

Bias questions are difficult, and I'm not sure the effect isn't an
observational bias, but it's not obvious to me it is.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA

[Eric Flesch]

On Thu, 20 Feb 2020, Steve Willner <wil...@cfa.harvard.edu> wrote:
>This is a version of "Eddington bias," and it would have been my

>first thought to explain the effect too. ...

>Bias questions are difficult, and I'm not sure the effect isn't an
>observational bias, but it's not obvious to me it is.

Well, the paper didn't include the following 55 SDSS quasars which
have brightened from the POSS-I epoch (1960-ish) to the SDSS epoch
(2005-ish) . This is a very partial list, I have over a thousand of
these. So I'm seeing data bias in the paper. Eric

Rmag SDSS SDSS POSS POSS
(Cousins) 2005 2005 1960 1960
# name change rmag gmag rmag omag
-- ------------------------ ----- ----- ----- ----- -----
1 PKS 1222+21 -2.17 16.10 15.88 18.21 18.36
2 SDSS J101340.06+104116.0 -2.11 18.49 18.80 20.45 21.07
3 SDSS J002021.19+180708.3 -2.03 18.33 18.58 20.22 20.46
4 SWIREJ160947.05+550533.3 -1.98 18.42 18.16 20.35 20.53
5 SDSS J133904.14+161831.6 -1.96 18.81 19.13 20.61 21.08
6 SDSS J020406.98-052842.8 -1.93 18.26 18.38 20.07 20.62
7 SDSS J163210.13+440705.9 -1.87 18.72 18.61 20.51 20.74
8 SDSS J101337.31+613234.9 -1.81 18.31 18.49 19.99 21.00
9 SDSS J131706.97+473358.0 -1.80 18.17 18.01 19.90 19.97
10 SDSS J013447.29+212907.5 -1.79 18.94 19.17 20.59 21.08
11 SDSS J212246.17-011314.6 -1.77 18.66 18.74 20.32 20.38
12 2QZ J140000.7+015827 -1.73 18.48 18.52 20.11 20.55
13 SDSS J084542.82+151642.0 -1.71 18.70 18.70 20.31 20.72
14 SDSS J105210.02+165543.7 -1.69 16.41 16.63 17.96 18.40
15 SDSS J135520.97+385058.2 -1.68 17.64 17.71 19.21 19.81
16 SDSS J154439.17+354819.1 -1.68 18.84 19.22 20.35 21.20
17 SDSS J023602.76-051526.9 -1.68 18.92 19.02 20.48 20.77
18 7C 0912+2532 -1.68 17.77 17.89 19.33 19.57
19 SDSS J024001.70-054559.3 -1.67 18.39 18.28 19.98 20.24
20 SDSS J152108.00+144438.1 -1.66 18.70 18.78 20.25 21.74
21 SDSS J024302.64-021546.0 -1.65 19.32 19.47 20.85 21.30
22 SDSS J154612.90+385349.8 -1.64 18.88 18.95 20.41 21.12
23 SDSS J120933.13+401246.6 -1.64 18.84 19.03 20.35 20.83
24 FBQS J0130-1019 -1.63 17.83 18.02 19.33 20.04
25 SDSS J174251.80+542949.2 -1.63 18.83 19.12 20.31 20.62
26 SDSS J022101.06-072628.9 -1.62 18.50 18.65 20.00 20.24
27 TEX 1147+022 -1.61 18.59 18.67 20.09 21.38
28 SDSS J115506.79+123326.2 -1.59 18.68 18.87 20.14 21.82
29 4C 12.35 -1.59 18.04 18.24 19.50 20.78
30 SDSS J141819.79+212910.3 -1.59 18.69 18.77 20.17 20.93
31 SDSS J162919.58+190530.3 -1.59 18.86 19.13 20.30 20.36
32 PKS 1502+106 -1.59 18.22 18.53 19.66 19.75
33 SDSS J134031.02+570129.6 -1.58 18.54 18.65 20.00 21.29
34 SDSS J012108.88+224558.6 -1.58 18.79 18.82 20.27 21.08
35 SDSS J103111.30+043211.4 -1.57 18.42 18.44 19.89 20.36
36 SDSS J131437.74+042644.0 -1.56 18.42 18.52 19.86 20.56
37 SDSS J144249.81+053228.8 -1.56 19.26 19.26 20.72 21.12
38 F864:072 -1.55 18.83 18.91 20.27 21.30
39 SDSS J072952.43+441146.0 -1.55 18.54 18.56 19.99 20.35
40 SDSS J025420.59-073745.5 -1.55 18.62 18.76 20.05 20.31
41 SDSS J085538.58+235005.4 -1.55 18.73 18.96 20.14 20.50
42 SDSS J155829.23+530704.1 -1.54 18.91 18.85 20.36 21.44
43 2QZ J125309.5+012830 -1.54 18.23 18.46 19.63 20.46
44 SDSS J162329.86+255531.7 -1.53 18.88 19.11 20.27 21.37
45 SDSS J094915.33-005921.3 -1.53 18.93 18.99 20.35 20.84
46 SDSS J023439.04+010740.1 -1.52 18.77 19.06 20.14 21.77
47 SDSS J113111.14+373709.1 -1.52 18.20 17.98 19.66 20.29
48 SDSS J023805.37-001652.1 -1.52 18.49 19.08 19.80 21.45
49 SDSS J080132.80+073413.5 -1.52 18.58 18.82 19.96 20.79
50 SDSS J120849.25+303332.9 -1.52 18.05 17.95 19.49 19.62
51 SDSS J095617.02+022901.7 -1.52 19.36 19.66 20.73 21.14
52 SDSS J084422.46+351034.9 -1.51 17.90 17.83 19.33 19.69
53 SDSS J133808.74+304113.7 -1.51 18.96 19.00 20.37 20.46
54 SDSS J233402.44+061302.4 -1.51 18.64 18.63 20.05 20.09
55 2QZ J142343.2+014306 -1.50 18.33 18.54 19.69 20.53