NASA Technical Reports Server (NTRS) 20120013038: Kepler Observations of Rapid Optical Variability in Active Galactic Nuclei

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KEPLEROBSERVATIONSOFRAPIDOPTICALVARIABILITYINACTIVEGALACTICNUCLEI
R.F.MUSHOTZKY1,2,5,R.EDELSON1,W.H.BAUMGARTNER2,3,P.GANDHI4
(Submitted9Sep2011;Accepted31Oct2011)
ABSTRACT
Overthreequartersin2010–2011,Keplermonitoredopticalemissionfromfouractivegalacticnuclei(AGN)
with∼30minsampling,>90%dutycycleand(cid:2)0.1%repeatability. ThesedatadeterminedtheAGNoptical
fluctuation power spectral density functions (PSDs) over a wide range in temporal frequency. Fits to these
PSDs yielded power law slopes of −2.6 to −3.3, much steeper than typically seen in the X-rays. We find
1
evidencethatindividualAGNexhibitintrinsicallydifferentPSDslopes. ThesteepPSDfitsareachallengeto
1
0 recent AGN variability models but seem consistent with first order MRI theoretical calculations of accretion
2 diskfluctuations.
Subjectheadings:Accretion,accretiondisks—Blackholephysics—Galaxies: active—Galaxies: Seyfert
v
o
N 1. INTRODUCTION 109M(cid:2)typicalforAGN,thesenaturaltimescalesrangefrom
hours to years. Previous data have been unable to constrain
2 Theopticalcontinuumfromactivegalacticnuclei(AGN)is
theopticaltimevariabilityoverthiswiderangeforanyindi-
believedtobedominatedbyemissionfromanaccretiondisk
vidualAGN.
] surroundingasupermassiveblackholeandcanbeadequately
A The Kepler mission (Borucki et al. 2010) provides a so-
modeled as radiation from a simple Shakura-Sunyaev disk
lution to these observational difficulties. Kepler has been
G (Edelson & Malkan 1986). Because this region is too small
observing a ∼115 square degree region of sky, monitoring
to image (except via gravitational lensing; Kochanek 2004),
. ∼165,000sourcesevery29.4minuteswithunprecedentedsta-
h indirectmethodsmustbeusedtoprobeitsstructureandphys-
p ical conditions. One of the best probes is provided by the bility((cid:2)0.1%fora15thmagnitudesource)andhighdutycy-
- strong variability seen throughout the optical/ultraviolet/X- cle(>90%)overaperiodofyears.DuringQ6(Quarter6:UT
o 24June–22September2010),Q7(23September–22Decem-
ray bands in most AGN. However, limitations with many
r ber 2010) and Q8 (22 December 2010–24 March 2011), the
t ground-based optical observations have made it difficult to
s KeplertargetlistincludedatleastfourvariableAGNfromour
obtainaccurate,denselyandregularlysampleddatasetscov-
a
guest observer program. This paper reports initial results of
[ eringthelargerangeoftimescalesnecessarytoconstraindisk
physicsandsearchforcharacteristictimeswhichmaybere- Q6–Q8(andinoneinstanceQ4)observationsoftheseKepler
1 latedtoorbital,dynamicorotherexpectedtimescales. Inpar- AGN,focusingonfluctuationpowerspectraldensityanalysis.
v Thesourceselection,datacollectionandreductionaregiven
ticular,diurnalandweatherrelatedinterruptionscanseverely
2 inSection2,thetimeseriesanalysisandresultsarereported
degrade the ground based sampling pattern and atmospheric
7 inSection3,implicationsarediscussedinSection4,andbrief
seeingintroducesphotometricerrorsthataremuchlargerthan
6 conclusionspresentedinSection5.
theKepleruncertaintiesandoftenareaslargeasorlargerthan
0
. the intrinsic short timescale optical source variability. How- 2. DATA
1 evergroundbaseddatahavesampledmuchlongertimescales
2.1. SourceSelection
1 thanareavailableinthepresentKeplerdatasets.
1 The natural timescales for a disk—light-crossing (t), dy- Because it lies at low galactic latitudes not systematically
l
1 namical (t ), and thermal (t ) timescales—are set by covered by major extragalactic or AGN surveys the Kepler
dyn th
v: the black hole mass and the accretion processes (Frank, field (∼0.3% of the sky) currently contains only a few cata-
i King & Raine 2002). The order of magnitude estimates loguedAGN6.Targetsmustbeidentifiedandwindowschosen
X for these timescales are: t = 2.6 M R hours, t = beforeKeplerdatacanbedownloaded. Thuswehaveunder-
l 7 100 dyn
ar w10heMre7MR1700is3/t2hdeabylsa,caknhdotltehm=a0s.s46inMun7itRs1o00f31/207αM0.0(cid:2)1−,1Ry1e0a0riss, tTahkiesnsteaxrtteendswiviethefafodratstabtoasiedesnetairfcyhAtoGfiNndinptrheeviKouespllyeridfieenltdi-.
fiedAGN.WethenappliedthemethodofStockeetal. (1983)
theemissiondistanceinunitsof100timestheSchwarzschild
radius 2GM/c2, and α0.01 is the Shakura-Sunyaev viscosity totheRosatallskysurvey(RASS;Vogesetal. 1999)tose-
lectAGNcandidatesbasedontheirX-raytoopticalfluxratio.
parameter(Shakura&Sunyaev1973)dividedby100. Foras-
Wealsousedthe2MASSallskysurveycatalog(Strutskieet
sumedEddingtonratiosof0.01–0.1andmassrangesof106–
al. 2006)toidentifyAGNcandidatesbasedoninfraredcolors
(Malkan2004)andassociationwithaRASSsource.
1DepartmentofAstronomy,UniversityofMarylandCollegePark,Col- Table1givesdetailsoftheKeplerAGNwhoselightcurves
legePark,MD20742 are presented in this paper, a sample of four variable AGN
2NASA/GoddardSpaceFlightCenter,AstrophysicsScienceDivision, thatKeplerhasbeenobservingsinceQ6. Ofthesefour,only
Greenbelt,MD20771 Zw229−15(z=0.0275, Falcoetal. 1999, Proust1990)had
3Joint Center for Astrophysics, University of Maryland Baltimore
County,Baltimore,MD21250 beenidentifiedasanAGNpriortothelaunchofKepler. Are-
4InstituteofSpaceandAstronomicalScience,JapanAerospaceExplo- centreverberationmappingcampaignfoundithadanHβ lag
rationAgency, 3-1-1Yoshinodai, chuo-ku, Sagamihara, Kanagawa252-
5210,Japan 6 However, a portion of the Kepler field is covered by SDSS/SEGUE,
5Correspondingauthor:[email protected] http://www.sdss.org/segue/
2 MUSHOTZKYETAL
remindthereaderthatsystematicerrorsofthissortcouldstill
Table1 bepresentinthesedata.
KeplerAGNReferenceInformation
3. POWERSPECTRALDENSITYFUNCTIONS
SourceName KeplerID RA(J2000) Dec(J2000) z RASS
3.1. PSDMeasurement
Zw229−15 6932990 190526.0 +422740 0.028 0.450
KA1925+50 12158940 192502.2 +504314 0.067 0.170 The optical flux variations in AGN are aperiodic. A stan-
KA1858+48 11178007 185801.1 +485023 0.079 0.210 dardtoolforcharacterizingsuchbroadband(intemporalfre-
KA1904+37 2694185 190458.7 +375541 0.089 0.023 quency) variability is the periodigram, which measures the
Note. — Columns 1 and 2 give the source name (KA refers to newly fluctuation power spectral density (PSD) function. AGN
discoveredKeplerAGNfirstreportedinthispaper)andKeplerIDnumber, PSDs have been best studied in the X-rays, where the PSDs
columns3and4givetheposition,column5theredshift,andcolumn6the show a broad shape that has been simply characterized as a
Rosatallskysurvey(RASS)countrateincountss−1. doublepowerlawthatbreaksfromasteeprednoisehighfre-
of ∼4 days and estimated its black hole mass at ∼107 M(cid:2) quency slope of αH ∼−2 (S∝ fα, where α is the slope, S
is the spectral density and f is the temporal frequency) to a
(Barthetal. 2011). TheotherthreeAGNinTable1wereall
flatter low frequency slope of α ∼−1, at a break frequency
discoveredasaresultofthesearchdescribedabove.(Thepre- L
f that typically corresponds to timescales of order a week,
fix“KA”isusedtodesignatenewlyidentifiedKeplerAGN.) b
but scales with the mass of the black hole (e.g., Edelson &
Spectraofthesethree,plustenothernewlydiscoveredKepler
Nandra1999,Uttleyetal. 2002,Markowitzetal. 2003).
AGNaregiveninEdelson&Malkan(2012).
We used the Kepler SAP data to measure PSDs for all of
theseKeplerAGN.Currently,largephotometricoffsetsintro-
2.2. KeplerSAPLightCurves
duced by quarterly spacecraft rolls prevent data from being
The Kepler standard data processing pipeline (Jenkins et combinedacrossquarters,sothesePSDsonlycoverindivid-
al. 2010), operates on original spacecraft data to produce ual quarters. This problem should eventually be solved, so
calibrated pixel data (Quintana et al. 2011). The next step, wewillproducePSDscoveringlongertimescalesinafuture
PA, uses simple aperture photometry to extract SAP_FLUX paper.
count rates from these 2-dimensional images (Twicken et al. For each light curve, a first order function was subtracted
2011). The spacecraft downloads not full CCD frames but off so that the first and last points of the light curve were
only “postage stamp” images for the targets. Only a frac- equal. This“end-matching”removesspuriouslowfrequency
tionofthedownloadedpixelsareusedintheextraction. The powerintroducedbythecyclicnatureofthePSDwhichtends
next step in the standard pipeline, SAPPDC, conditions the toflattenthePSDs. (SeeFougere1985fordetails.) Thiscor-
lightcurvesfortransitsearches,outputtingPDC_FLUXlight rection steepens the slopes by a mean value of 0.7, 0.3, 0.8
curves. However, no conditioning occurred for sources pre- and0.7forZw229−15,KA1925+50,KA1858+48andKA
sentedinthispaper(theSAP_FLUXandPDC_FLUXdataare 1904+37,respectively. Fractionalnormalizationwasused,so
identical to within a constant offset), so this and all further theresultingpowerdensityhasunitsofrms2Hz−1.
stepsarenotrelevanttothecurrentwork.WeuseSAP_FLUX The resulting PSDs (see Figure 3), fitted with a single
count rates for our AGN light curve analyses. These light powerlaw(S∝ fα)plusnoisemodelontemporalfrequencies
curvesarepresentedinFigures1and2. of∼4×10−7 to∼4×10−5 Hz(correspondingtotimescales
Kepler,withits(cid:2)0.1%repeatability,>90%dutycycleand of ∼6 hours to ∼1 month), are very steep with slopes from
durationsofyears,exploresalevelofdataqualitysuperiorto α=−2.6to−3.3.
anything previously obtained. Thus one must be concerned
about other sources of error, especially systematic errors, in 3.2. Erroranalysis
this relatively young mission. An independent check of the
Kepler data is available for Zw229−15 since in 2010, it was ThesePSDsalsoallowacheckofthetruenoiselevelinthe
lightcurves. Thefractionalerror,err =(cid:4)err(cid:5)/(cid:4)flux(cid:5),isre-
observed by both Kepler and the ground based Lick AGN dir
portedinColumn4ofTable2.Anindependentmethodofde-
Monitoring Program (LAMP). These light curves, shown in
terminingtheerrorfrom(cid:2)thePSDusestheformulaofVaughan
Figure1,indicateaverygoodagreementbetweenKeplerand
independentgroundbasedLAMPdata,wellwithintheLAMP etal. (2003): errind = (cid:4)err2(cid:5)/(cid:4)flux(cid:5)2,an(cid:3)disgiveninC(cid:4)ol-
∼1% errors and so, at least in this case, the systematic and umn 5. This reduces to the same quantity (cid:4)err(cid:5)/(cid:4)flux(cid:5) in
other errors in the Zw 229−15 data are generally no larger thelimitofsmallfluctuationsinthefluxesanderrors,asisthe
thanthe∼1%LAMPerrors. case with these data. The errors derived from the PSD anal-
However, the quoted Kepler errors are much smaller, and ysis are typically ∼25% larger than the quoted light curve
thereiscurrentlynowaytobesurethatsystematicerrorsare errors. Thisindicatesthequotederrorsareslightlyunderesti-
notaffectingthedataatthelevelbetween∼0.1%and∼1%. mated,andthatnoothersourceofsystematicsdominatesthe
Indeed,Figure2showsthatsmall,shortterm(1–2day),dis- quotederrors.
continuities are sometimes observed following monthly data The PSD slopes for each quarter (listed in Table 2) show
downloads or safe mode events. This is believed to arise small scatter for individual objects. It is difficult to directly
from thermally induced focus changes as the solar illumina- measure reliable errors on derived PSD slopes, but an esti-
tion changes during spacecraft slews7. Both our group and mateisprovidedbytheobserveddispersionforindividualob-
theKeplerteamareworkingtocorrectforthisinfutureanal- jects. Forthetwosourceswiththemostdata,Zw229−15and
yses. While our understanding will undoubtedly improve as KA 1925+50, the mean slope and associated standard devi-
themissionprogresses, allthatcanbedoneatthistimeisto ations are (cid:4)α(cid:5)=−3.11±0.15 and −2.67±0.08. These dif-
ferby∼2.5standarddeviations,suggesting,atverymarginal
7 http://archive.stsci.edu/kepler/release_notes/ significance,thattheintrinsicdifferencebetweenthederived
release_notes5/Data_Release_05_2010060414.pdf slopes for these objects is larger than the associated errors.
KEPLEROBSERVATIONSOFAGN 3
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Figure1. KeplerQ6(top)andQ7(bottom)lightcurvesofthenarrowlineSeyfert1galaxyZw229−15(inblack). Eachpanelcontainsover4,200cadences,
gatheredoneevery∼30min,withaprecisionof(cid:2)0.1%.AtypicalerrorbarisseenintheoutlieratTJD∼539.Therearemonthly∼1daydatadownloadgaps
(e.g.,TJD∼431and524),buttheoveralldutycycleis>90%.Notethe∼8%fluxdiscontinuitybetweenQ6andQ7asthequarterlyspacecraftrollmovesthe
sourceontoadifferentchipandanewSAPapertureisused.NotealsotheexcellentagreementwithsimultaneousgroundbasedLAMPdata(showninred;Barth
etal.2011),scaledtoaccountfordifferentaperturesizes.
4 MUSHOTZKYETAL
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Figure2. Q6–Q8lightcurvesforfourvariableKeplerAGN.A1%barisshownforscale.Q8datawerenotobtainedforKA1858+48becauseitfellondefective
Module3.KeplerobservationsofKA1904+37didnotbeginuntilQ7.Arbitraryoffsetshavebeenappliedtomatchlightcurvesacrossquarterlytransitions(the
dottedlinesatTJD∼462and552).Notethe16daygapduetoasafemodeeventatthebeginningofQ8;thismakestheoffsetforthatquarterhighlyuncertain.
Notealsothatlightcurvesoccasionallyshow∼1%discontinuitiesimmediatelyfollowingmonthlydatadownloadsorsafemodeevents(e.g.,TJD∼568and
586inKA1925+50,andTJD∼432inZw229−15andKA1858+48)duetothermallyinducedfocuschanges.
ThesmallbottompanelisthesameasFigure2abutfortheZw229−15Q4data.
KEPLEROBSERVATIONSOFAGN 5
(The quoted uncertainties are standard deviations of the dis-
(cid:16)(cid:11)(cid:17)(cid:18)(cid:19)(cid:20)(cid:21)(cid:22) (cid:18)(cid:28)(cid:28)(cid:5)(cid:7)(cid:4) (cid:23)(cid:11)(cid:6)(cid:2)(cid:3)(cid:18)(cid:30)(cid:18)(cid:7)(cid:18)(cid:18)(cid:24)(cid:31)(cid:25)(cid:18)(cid:25)(cid:26) (cid:27)!(cid:18)"(cid:28)(cid:27)(cid:29) (cid:26)"% (cid:23)(cid:11)(cid:6)(cid:2)(cid:3)(cid:18)(cid:30)(cid:18)(cid:7)(cid:18)(cid:18)(cid:24)(cid:31)(cid:25)(cid:18)(cid:25)(cid:26)(cid:18)(cid:27)!(cid:18)(cid:24)(cid:28)$% $% *(cid:11)(cid:20)(cid:2)(cid:3)(cid:18)(cid:26)(cid:30)(cid:24)(cid:7)(cid:18)(cid:18)(cid:31)(cid:27)(cid:18)-(cid:25)(cid:27)(cid:18)"!$(cid:18)(cid:28)#% twriibthuotiuotntshoefrethdenPoSisDelseloakpecsofrorercdtiifofne,rethnetqsutaanrdtearrsd.)dNevoitaetitohnast
(cid:7)(cid:10)(cid:11)(cid:12)(cid:13)(cid:14)(cid:18)(cid:15)(cid:8) (cid:6)(cid:8) (cid:29)$ (cid:29) ftiovretlhy,esseotowuorscoourrreccetsiownosuulcdcheasvsfeublleyenre0p.r5o8duacneds0s.i2m2i,larersPpSecD-
(cid:7)(cid:8)(cid:18)(cid:9) (cid:7) (cid:18) (cid:18) slopes between the various quarters for each source. Since
(cid:3)(cid:4)(cid:27)(cid:5)(cid:3)(cid:6) (cid:2)(cid:27) (cid:2)(cid:8) (cid:2)(cid:9) (cid:2)(cid:6) (cid:25)$ (cid:25)(cid:27) (cid:25)(cid:29) (cid:25)$ (cid:25)(cid:27) (cid:25)(cid:29) PteSmDatiacnsa(lyseseese.agr.e,Vnaoutogrhioanuselytaslu.s2c0e0p3ti)balendtothaenraelyistitchaelpsyoss--
(cid:2)
(cid:16)(cid:11)(cid:17)(cid:18)(cid:19)(cid:20)(cid:21)(cid:22) (cid:18)(cid:26)"% (cid:23)(cid:11)(cid:6)(cid:2)(cid:3)(cid:18)(cid:30)(cid:18)(cid:7)(cid:18)(cid:18)(cid:24)(cid:31)(cid:25)(cid:18)(cid:25)(cid:26) (cid:27)!(cid:18) (cid:28)(cid:26)$ (cid:26)"% *(cid:20)(cid:11)(cid:2)(cid:18)(cid:3)(cid:26)(cid:30)(cid:24)(cid:7)(cid:18)(cid:18)(cid:27)(cid:31)-(cid:18)(cid:25)(cid:27)(cid:18)"!(cid:18)$(cid:28)$ (cid:26)"% *(cid:11)(cid:20)(cid:2)(cid:3)(cid:18)(cid:26)(cid:30)%(cid:7)(cid:27)(cid:18)(cid:31)%(cid:18)-(cid:25)(cid:29)(cid:18)%!(cid:24)(cid:18)(cid:28)## stfhriobemisleitqynuetwahratKterceuptorlreeqrnudtalayrttaeur(nspkerneooSvwiedncets.sy2as.t2de)me,gtarhteieecaeogrfrreoceromsncfieondutelnidnceasfltfohepcaett
(cid:7)(cid:10)(cid:11)(cid:12)(cid:13)(cid:14)(cid:18)(cid:15)(cid:8) (cid:29)$ (cid:29)$ (cid:29)$ theobservedsteepslopesareaccurate.
(cid:9)
(cid:6)(cid:7)(cid:8)(cid:18) (cid:18) (cid:18) (cid:18) 4. DISCUSSION
(cid:4)(cid:27)(cid:5)(cid:3) (cid:25)# (cid:25)$ (cid:25)(cid:27) (cid:25)(cid:29) (cid:25)# (cid:25)$ (cid:25)(cid:27) (cid:25)(cid:29) (cid:25)# (cid:25)$ (cid:25)(cid:27) (cid:25)(cid:29) 4.1. ComparisontoPreviousResults
(cid:3)
(cid:2)
(cid:10)(cid:11)(cid:12)(cid:13)(cid:14)(cid:18)(cid:15)(cid:8)(cid:16)(cid:11)(cid:17)(cid:18)(cid:19)(cid:20)(cid:21)(cid:22) (cid:18)(cid:26)(cid:26)"(cid:18)$% (cid:23)(cid:11)(cid:6)(cid:2)(cid:3)(cid:18)(cid:30)(cid:18)(cid:7)(cid:18)(cid:18)(cid:24)(cid:31)(cid:25)(cid:18)(cid:25)(cid:26) (cid:27)!(cid:18)(cid:26)(cid:28)(cid:29)# (cid:26)"(cid:29)$% *(cid:11)(cid:20)(cid:2)(cid:3)(cid:18)(cid:26)(cid:30)(cid:24)(cid:7)(cid:18)(cid:18)(cid:31)(cid:27)(cid:18)-(cid:25)(cid:27)(cid:18)"!#(cid:18)(cid:28)(cid:27)# (cid:26)"(cid:29)$% *(cid:11)(cid:20)(cid:2)(cid:3)(cid:18)(cid:26)(cid:30)(cid:24)(cid:7)"(cid:18)(cid:31)(cid:29)(cid:18)-(cid:25) (cid:18)#!#(cid:18)(cid:28)(cid:29)# rleaytKeeetthpalaenlr.pl(irg2eh0vt0ioc9uu)srtvhdeeastaha4.ri.1geF.h1ooe.frsmOetxppuathcimchoatpholliDmeg:aheitetnarritcqhueqaudlaiatlytiatayunsidsedsfarbmoympKlitenhlge-
(cid:7)(cid:8)(cid:18)(cid:9)(cid:7) (cid:29) (cid:18) (cid:18) MACHO survey of Geha et al. (2003) which has ∼5% pho-
(cid:2)(cid:3)(cid:4)(cid:27)(cid:5)(cid:3)(cid:6) (cid:18)(cid:25)#(cid:2)(cid:3)(cid:4)(cid:27)(cid:25)&$(cid:8)(cid:7)’((cid:7)(cid:25)(cid:10)(cid:27))(cid:14)(cid:18)(cid:15)(cid:20)(cid:25)(cid:21)(cid:29)(cid:22) (cid:25)#(cid:2)(cid:3)(cid:4)(cid:27)(cid:25)&$(cid:8)(cid:7)’((cid:7)(cid:25)(cid:10)(cid:27))(cid:14)(cid:18)(cid:15)(cid:20)(cid:25)(cid:21)(cid:29)(cid:22) (cid:25)#(cid:2)(cid:3)(cid:4)(cid:27)(cid:25)&$(cid:8)(cid:7)’((cid:7)(cid:25)(cid:10)(cid:27))(cid:14)(cid:18)(cid:15)(cid:20)(cid:25)(cid:21)(cid:29)(cid:22) t7o.5myeteraicrse,rarnodrstahnuds6sa0m0pgloeosdapth∼o1topmoeitnrticevmeeryas4u.r5emdaeynstscoovme-r
paredtothe0.1%Keplererrorsand1datapointroughlyevery
30minutes. PreviousattemptstoderivethePSDoverawide
Figure3. OpticalPSDsandpowerlawpluswhitenoisefitsforthe4AGN rangeoftimescaleshavehadtocombinethedatafrommany
inselectedquartersovertemporalfrequencies∼10−6.5 to10−3.5 Hz. The objectsandseveralsurveys(Hawkins2002)orhavereliedon
fitsareshowningreen,andthenoiselevelinred.Sourcename,quarter,and
fittedpowerlawslope(α)aregivenintheupperrightofeachplot. relatively sparsely sampled data, from several different tele-
scopes(Breedtetal. 2010).
Previous results (e.g. Kelly et al 2009) tend to find best
fitting PSDs with slopes of ∼ −1.8 for the collective sam-
ple, rather flatter than what we have found. Since the Ke-
plerPSDscannotcontinuetoverylowfrequencieswithsuch
steep slopes without implying very large variability ampli-
tudes, theremustbeabreakattimescales>1month, which
maymaketheKeplerPSDsconsistentwithpreviouswork. It
isnotsurprisingthattheresultsofourobservationsarerather
differentthanwhathasbeenpublishedpreviously—theother
observationscouldnotseetheeffectswearedetecting. While
there is a formal overlap in sampled timescales between our
Keplerandotherdata,themuchlargererrorbarsfortheprevi-
Table2 ousPSDs(e.g. Breedtetal. 2010,)atcharacteristicfrequen-
KeplerAGNObservations ciesaboveafew×10−5Hzmakescomparisondifficult. How-
ever, for at least one object, NGC 4051 (Breedt et al 2010),
SourceName Quarter 103ctss−1 err_dir err_ind α the observed PSD in the 10−6– 10−8 Hz range is well deter-
Zw229−15 Q4 12.1 0.047% 0.065% −3.05 minedandisflatterthanourKeplerresultsforallofourob-
Zw229−15 Q6 12.0 0.051% 0.068% −3.31 jects. Onepossibleexplanationforthedifferencesmayliein
Zw229−15 Q7 12.9 0.046% 0.062% −3.14
Zw229−15 Q8 10.4 0.052% 0.055% −2.96 the different luminosities or Eddington ratios of the objects,
KA1925+50 Q6 4.2 0.071% 0.084% −2.60 since NGC 4051 is significantly less luminous and probably
KA1925+50 Q7 3.8 0.065% 0.081% −2.75 lessmassivethantheobjectsinoursample.
KA1925+50 Q8 4.1 0.075% 0.078% −2.67
KA1858+48 Q6 2.1 0.117% 0.159% −2.87 4.1.2. X-rayData
KA1858+48 Q7 1.3 0.128% 0.207% −2.97
KA1904+37 Q7 5.8 0.071% 0.097% −2.74 AlthoughtheparticularSeyfert1sinoursampledonothave
KA1904+37 Q8 5.5 0.080% 0.087% −2.95 measured X-ray PSDs, many other Seyfert 1s have had X-
Note.—Columns1and2givethesourcenameandquarter,column3the ray PSDs measured over these timescales. These are always
meanSAP_FLUXcountrateinunitsof103ctss−1,andcolumn4theratioof muchflatter, typicallyhavinghighfrequencyslopesof−1to
themeanquotederrorsdividedbythemeanflux. Column5givestheerror −2 (Edelson & Nandra 1999, Uttley et al. 2002, Markowitz
ratederivedfromthePSDfitsasdiscussedinSection3.2. Column6gives etal. 2003). ThusourmeasurementofsteepopticalPSDson
thefittedPSDslopes(α)foreachquarter.
shorttimescalesissomewhatsurprisingbecauseitissodiffer-
ent from that measured in the X-rays, and because Seyfert 1
opticalandX-raylightcurvesappeartotrackwell,atleaston
longertimescales(Uttleyetal. 2003).
4.2. PhysicalImplicationsforAccretionDisks
6 MUSHOTZKYETAL
The characteristic timescales of the fluctuations should 5. CONCLUSIONS
correspond to different physical mechanisms which may
Power spectral analysis of four AGN observed by Kepler
be related to the size of the system, the dynamical during Q6-Q8 show very steep (α ∼ −2.6 to −3.3) slopes,
timescales, epicyclic frequencies, g-modes or other charac-
considerablysteeperthanthatseenintheX-rays. ThePSDs
teristic timescales which could influence the source of vari-
foreachsourceareconsistentfromquartertoquarterand,at
ance. Since the source of the accreting material in AGN is >2σ confidence, are different from each other. Analysis of
notknown,itisunclearifthesourcesoftheperturbationsare
thesehighquality lightcurvesindicatesthatthe influenceof
changesintheaccretionflow,theturbulenceduetophysicsin
systematicerrorsisrathersmall;additionally,directcompar-
thediskitself(fromthemagnetorotationalinstabilitymecha- ison of Kepler and LAMP monitoring of Zw 229−15 shows
nism (MRI), e.g. Miller & Reynolds 2009, Noble & Krolik
excellent agreement. Comparison with analytic models of
2009),orperhapsotherphysics. AsshownbyMcHardyetal.
AGN variability shows steeper than predicted slopes; how-
(2006),thecharacteristictimescaleseenintheX-rayPSDsis
ever, comparisonwithMHDsimulationsseemstoshowbet-
relatedtotheAGNmassandtheaccretionrate. However,itis
teragreement. Furtheranalysisofothercharacteristicsofthe
notknownifthisisalsotruefortheopticalPSDs(MacLeod
light curve, longer time series, the analysis of more objects
etal. 2010).
andthecomparisontosemi-analyticmodelsoftimevariabil-
Recentresultsfromgroundbasedopticalobservations(e.g.
ity will be the subject of future papers. We hope that these
Kellyetal. 2009,MacLeodetal. 2010)findthattheirresults
newhighqualityKeplerdatawillstimulatethecalculationof
are consistent with a “damped random walk model”. How-
thetimeseriesfromaccretiondisks.
ever,theirlightcurvesareirregularlyandmoresparselysam-
pledcomparedtoKeplerdata(seeFigure2inKozlowskietal.
2010). Ourdatadonotfindthepredicted f−2powerspectrum WethanktheKeplerteamfortheireffortstomakethedata
at high frequencies predicted by this model. However, since accessibleandtractableandtheKeplerGOprogramforfund-
thereisverylittleoverlapinfrequenciesandoursamplesize ing, Matt Malkan for extensive contributions to the identi-
is much smaller, direct comparison is difficult. Our data are fication of new Kepler AGN, Simon Vaughan for valuable
justcapableofreachingthelighttraveltimesizeofthedisks helpwithPSDmeasurementsandAaronBarthandtheLAMP
onoursampledAGN.Theeffectivesizeoftheregionemitting teamforearlyaccesstotheirdata.
radiationatagivenfrequencyis(Baganoff&Malkan1995):
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