The curtain remains open: NGC 2617 continues in a high state

0.72 MB·

by  V. L. Oknyansky

MNRAS000,1–9(2016) Preprint28March2017 CompiledusingMNRASLATEXstylefilev3.0
The curtain remains open: NGC 2617 continues in a high state
V. L. Oknyansky,1⋆ C. M. Gaskell,2 N. A. Huseynov,3 V. M. Lipunov,1 N. I. Shatsky,1
S. S. Tsygankov,4 E. S. Gorbovskoy,1 Kh. M. Mikailov,3 A. M. Tatarnikov,1
D. A. H. Buckley,5 V. G. Metlov,1 A. E. Nadzhip,1 A. S. Kuznetsov,1 P. V. Balanutza,1
7 M. A. Burlak,1 G. A. Galazutdinov,6 B. P. Artamonov,1 I. R. Salmanov,3
1
0 K. L. Malanchev,1 R. S. Oknyansky7
2

r 1SternbergAstronomicalInstitute,M.V.LomonosovMoscowStateUniversity,119234,Moscow,Universitetskypr-t,13,RussianFederation
a 2DepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064,USA
M
3ShamakhyAstrophysicalObservatory,NationalAcademyofSciences,Pirkuli,AZ5626,Azerbaijan
4TuorlaObservatory,DepartmentofPhysicsandAstronomy,UniversityofTurku,Va¨isa¨la¨ntie20,FI-21500,Piikkio¨,Finland

7 5TheSouthAfricanAstronomicalObservatory,POBox9,Observatory7935,SouthAfrica
2 6InstitutodeAstronomia,UniversidadCatolicadelNorte,Av.Angamos0610,Antofagasta,1270709,Chile
7BenGurionUniversityofNegev,POBox653,8410501,Beer-Sheva,Israel

]
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Received...Accepted...
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- ABSTRACT
o Opticalandnear-infraredphotometry,opticalspectroscopy,andsoftX-rayandUVmonitoring
r
of the changing look active galactic nucleus NGC 2617 show that it continues to have the
t
s appearance of a type-1 Seyfert galaxy. An optical light curve for 2010–2016indicates that
a
thechangeoftypeprobablyoccurredbetween2010Octoberand2012Februaryandwasnot
[
relatedtothebrighteningin2013.In2016NGC2617brightenedagaintoalevelofactivity

5 closetothatin2013April.Wefindvariationsinallpassbandsandinboththeintensitiesand
v profilesofthebroadBalmerlines.AnewdisplacedemissionpeakhasappearedinHβ.X-ray
2 variationsarewellcorrelatedwithUV–opticalvariabilityandpossiblyleadby∼2–3d.The
4
KbandlagstheJbandbyabout21.5±2.5d.andlagsthecombinedB+Jfiltersby∼25d.J
0
lagsBbyabout3d.ThiscouldbebecauseJ-bandvariabilityarisesfromtheouterpartofthe
5
accretiondisc,whileK-bandvariabilitycomesfromthermalre-emissionbydust.Wepropose
0
. thatspectral–typechangesarearesultofincreasingcentralluminositycausingsublimationof
1
theinnermostdustinthehollowbiconicaloutflow.Webrieflydiscussvariousotherpossible
0
reasonsthatmightexplainthedramaticchangesinNGC2617.
7
1 Key words: line: profiles – galaxies: active – galaxies: individual: NGC 2617 – galaxies:
:
v Seyfert–infrared:galaxies–X–rays:galaxies.
i
X
r
a
1 INTRODUCTION Oknyanskiietal. 1991). Recent reviews by Shappeeetal. (2014)
andKoayetal.(2016)givelistsofobjectsandreferences.
Activegalacticnuclei(AGNs)canbeclassifiedonthebasisoftheir
Some time between 2003 and 2013 the type 1.8 Seyfert
opticalspectrainto‘type1’AGNs(Sy1),showingprominentbroad
NGC2617 underwent adramatic change toappear likeaSeyfert
Balmer lines, and ‘type 2’ AGNs (Sy2), lacking obvious broad
1 galaxy (Shappeeetal. 2013, 2014). Before 2013 optical spec-
Balmer lines (Khachikyan&Weedman 1971). Designations such
tra of the object were obtained only twice: in 1994 (Moranetal.
as ‘Seyfert 1.8’ are used for intermediate cases (see Osterbrock
1996) andin2003 (the6dFGS spectrum). TheSDSSubgrizdata
1981).Rarecasesofso-called‘changing-look’AGNs(CLAGNs)
for NGC 2617 in 2006 showed a very low level of brightness.
–AGNsthatshowextremechangesofspectraltype–provideim-
Shappeeetal.(2013,2014)thereforesuspectedthatNGC2617had
portant tests of theories of the Sy1/Sy2 dichotomy. The first de-
beeninalowstateandhadaspectraltypeclosetotype2forthe
tailedinvestigationsofchangesfromtype2totype1andbackto
decadebeforethatbutthenchangeditsappearancetotype1,pre-
type 2 were for Mrk 6 (Pronik&Chuvaev 1972; Chuvaev 1991)
sumablyin2013April.
and for NGC 4151 (Lyutyjetal. 1984; Penston&Perez 1984;
CL AGNs such as NGC 2617 are rare. There are currently
only some tens of cases known. However, the small number of
⋆ E-mail:[email protected] knownCLAGNsiscomparabletothenumberofAGNsthathave
(cid:13)c 2016TheAuthors
2 V. L. Oknyanskyet al.
hadmanyyearsofspectralmonitoring.Itisthereforereasonableto
suspect, thatperhaps eachstrongly variableAGNcouldbefound
to be a CL AGN if observed long enough. This assumption is
supportedbyrecentresultsofRuncoetal.(2016)thatabout 38%
of 102 Seyferts changed type and about 3% of the objects have
disappearing Hβon time–scalesof 3−9 yr.AlsoMacLeodetal.
(2016)estimatethat> 15%ofstronglyvariableluminousquasars
exhibit a changing-look behaviour on rest-frame times–scales of
3000−4000d.
Keel (1980) discovered that orientation plays a key role in
whetheranAGNisseenastype1ortype2.Thisledtothesimplest
unifiedmodelofAGNactivity(the‘straw-personmodel’or‘SPM’
of Antonucci (1993)), in which all thermal AGNs have a broad-
line region (BLR), but in type-2 AGNs we cannot readily see it
becauseourviewisblockedbyobscuringgasanddustperpendic-
ular tothe axis of rotational symmetry. Despite thepopularity of
theSPM,therehasalwaysbeen adiscussion ontheroleof other
factors(seeAntonucci1993,2012).CLAGNsprovideachallenge
to the simplest unified models and would seem to require either
thattheobscuringdustisveryclumpyorthatitisbeingsublimated
astheluminosityincreasesand reformingastheluminosityfalls.
ThedramaticchangeofSeyferttypeandtheoutburstsobservedin
NGC2617canthusgiveusvaluableinformationforunderstanding
thegeometry,physicalnatureandevolutionofAGNs.
2 OBSERVATIONS
Wecommenced thespectroscopic and photometricmonitoring of
NGC2617in2016Januarytoseeifitstillappearedtobeatype-1
AGNthreeyearsaftertheintensive2013monitoringcampaignof
Shappeeetal.(2014).OurobservationsincludedIR(JHK)andop-
tical(BVR I )photometryandspectroscopy.Additionally,wehave
c c
used unfiltered optical monitoring by the MASTER robotic net-
workfrom2010to2016.WefoundthatthenucleusofNGC2617
remainsinahighstateandcanstillbeclassifiedasatype-1AGN
(Oknyanskyetal.2016a).TheopticalphotometryandIRphotom-
etryshow that theactivityof NGC2617 iscontinuing andthat it
underwent another series of outbursts in 2016 April–June. These
outburstsarecomparable, inlevel,tothosewhenNGC2617 was
observedbyShappeeetal.(2014)in2013May(Oknyanskyetal.
2016b,c).WesubsequentlyappliedforsoftX-rayandUVobserva-
tionswiththeSwift/XRT.Thesebeganon2016May17andcontin-
uedtill2016June23.
Figure 1. Top panel: our mean spectra for four dates normalized to the
continuum and offset for clarity. Bottom panel: comparison between the
Shappeeetal.(2014)ApachePoint Observatory 2013April25spectrum
2.1 Opticalspectroscopy
oftheHβregionofNGC2617andourspectrafrom2016February4(1),
Weobtained optical spectra covering 4100–7000 Å withthe2×2 March4(2)andApril9(3).Spectrahavebeennormalizedtothecontin-
uumlevel,andthenrelativecalibrationhasbeenperformedbyassuminga
prismspectrographanda4KCCD(spectralresolution3–7Å)on
constantfluxin[Oiii]λ5007andλ4959.Spectraareoffsetforclarity.The
the2-mZeisstelescopeoftheShamakhyAstrophysicalObserva-
arrowsshowthelocationsofthedisplacedemissionpeakintheredwingof
tory(ShAO)onthefournightsof2016February3and4,March4
Hβ
andApril9.ExamplesofmeanspectraoftheHβregionforthree
ofthenightscanbeseeninFig.1together withaspectrumfrom
Shappeeetal.(2014).Itcanbeseenfromallour2016spectrathat
2.2 IRphotometry
NGC2617canbeclassifiedwithoutanydoubtasatype-1AGN.
Inourspectraonecanseeadisplacedemissioncomponentin TheJHKphotometrywasobtainedfrom2016January20toMay
theredwingofHβatarelativevelocityof∼+2500kms−1which 13 with the new 2.5-m telescope of the SAI Caucasus Mountain
wasnotapparentinspectraobtainedin2013.Wecouldnotverify Observatory equipped with the ASTRONIRCAM instrument (an
thatthisnewcomponent isalsopresentintheHαprofilebecause MKOphotometricsystemwiththeHAWAII2-RGdetector)operat-
oftheinferiorresolutionoftheprismspectrograph atlongwave- ingintheJHandKbands.Thedatawerecalibratedusing2MASS
lengths.Theemissioncomponentcannotbeidentifiedconfidently starswithin1.5arcminofNGC2617andthenconvertedintothe
intheprofileofHγasthereisstrong[Oiii]λ4363emission. MKOsystemfollowingLeggettetal.(2006).Weperformedpho-
MNRAS000,1–9(2016)
NGC2617 ina highstate 3
tometryusing2.5and5arcsecradiusaperturesandfoundthatthe werereduceduniformlytotracetheevolutionofthebehaviourof
signal(ratioofrealvariationstonoise)wasbetterforthesmaller NGC2617onalongertime–scale.
aperture.Measurementswitha5arcsecradiusapertureareuseful, The XRT telescope observed NGC 2617 both in pho-
however,forcomparisonwiththeIRphotometryofShappeeetal. ton counting and in windowed timing modes depending
(2014)whousethesamesizephotometricaperture.Wemeasured on its brightness. The spectra were reduced using the on-
thebackground withanannulus of radii 30–60 arcsec. The JHK line tools provided by the UK Swift Science Data Cen-
lightcurvesforthe2.5arcsecradiusaperturecanbeseeninFig.2. tre (http://www.swift.ac.uk/user_objects/; Evansetal.
Aswediscussbelow,therearesomedifferenceinthevariationsof 2009). The spectra obtained were grouped to have at least one
differentIRbandsbecauseoftimelags. count per bin and were fitted in the 0.3–10 keV band with the
XSPEC package using Cash statistics (Cash 1979). Following
Shappeeetal.(2014)weusedasimpleabsorbedpower-lawmodel
2.3 OpticalBVRcIcobservations forthefittingprocedurewiththeabsorptionparameterfrozenatthe
Galacticvalue(N =3.64×1020cm−2;Kalberlaetal.2005).
WeobtainedopticalBVR I CCDdatawiththeAZT-5(aMaksutov H
c c The Swift Ultraviolet/Optical Telescope observes in parallel
50-cm meniscus telescope equipped withan Apogee AltaU8300
withtheXRTtelescopethusmakingitpossibletogetabroad–band
CCDcamera)attheMSUCrimeanObservatoryandwiththeZeiss-
viewfromtheopticaltoX-rays.Duringourprogrammetheobser-
600usinga4KCCDatShAOon17nightsin2016March–May.
vationsweremadewiththe‘filter-of-the-day’.Analysisofimages
The data were calibrated using SDSS stars within 1.5 arcmin of
hasbeendonefollowingtheproceduredescribedontheweb-page
NGC2617 andtransformedintotheJohnson–Cousins magnitude
ofUKSwiftScienceDataCentre.Photometrywasperformedwith
systeminthesamemannerasShappeeetal.(2014).Wemeasured
theuvotsourceprocedurewithsourceaperturesofradii5and10
thebackgroundwithinanannulusofradii35–45arcsec.
arcsec for the background (with the centre about 1 arcmin away
TheBVR I lightcurvesareshowninFig.2.Themagnitudes
c c
fromthegalaxy)forallfilters.
areforanapertureof5arcsecondsradius.NGC2617canbeseen
Given thevery highcorrelation between variations indiffer-
tohavebrightenedbyabout0.3magin BduringMarchandthen
entUVbandsinpreviousdatawereducedthenewUVdatatoone
decreasedby0.1maginthemiddleofApril.AttheendofAprilit
band,UVW1,andweshowthecombinedUVlightcurvetogether
begantobrightenagainandreachedamaximumof∼14.6inBon
withnewX-rayfluxvariationsinFig.2.TheoldX-rayflux,UVW1
May19.Onemoremaximumofthesimilarmagnitudewasreached
and BvariationsincombinationwithunfilteredMASTERoptical
nearJune15.TheamplitudeofvariationsinV wasabouthalfthe
data(2010–2015) areshown inFig.3.Ascan beseen inthefig-
amplitudein B.VariationsinR and I weresynchronizedwith B
c c
ure,allvariationsintheoptical,UVandX-rayarecorrelated.The
bandvariationsbutstillwithsmalleramplitudes.
amplitude is largest for the X-ray and decreases with increasing
wavelength. Possible lags between these variations are discussed
2.4 ObservationswiththeMASTERnetwork below.
Wealsogiveanopticallightcurvefor2010–2016fromtheMAS-
TER Global Robotic Network (Lipunovetal. 2010). The white
3 TIMELAGS
light (unfiltered) magnitudes, ∆W, relative to two nearby com-
parison stars, correspond to a photometric aperture of 7.5 arcsec Wemeasuredlagsbetweencontinuumvariationsindifferentbands
(4 pixels) radius. The variations are similar to those shown by using two independent methods: MCCF, a modification of tradi-
Shappeeetal. (2014) for 2012–2013. NGC 2617 had about the tional cross-correlation techniques (see Oknyanskii 1993) and a
samebrightnessin2012andin2016February,andin2016,ithas BayesiananalysisusingtheJAVELINsoftware
atype1spectrum,sowesuggestthatthedramaticchangeintype
ofNGC2617wasnotrelatedtothebrighteningobservedin2013
butprobablyhappenedbetween2010Octoberand2012February. 3.1 MCCF
FromtheendofAprilweobtainedobservationsofNGC2617as
Cross-correlation analysis of astronomical time series is compli-
oftenaspossible.Thelightcurvefortheunfilteredphotometrycor-
catedbecausethesamplingisusuallyuneven.Asarule,seriesof
relateswellwithourBVphotometry(seeFig.2).From2016May
astronomicalobservationsinevitablyhavegapsbecauseofseasons
16westartedobservationsintheBandV bands,mostlyusingthe
whentheobjectisinvisibleandinterruptionsduetothefullmoon,
MASTERtelescopes located in SouthAfrica (SAAS)inorder to
weather, and observing schedules etc. Classical cross-correlation
beabletocontinuewithourBVlightcurves.Thesemeasurements
analysismethodsweredevelopedonlyforuniformlysampledtime
weremadewiththesameapertureparametersasforourprevious
series. The analysis of non-uniform astronomical series requires
BVR I photometry. Wehavecombined theseestimatesinFig.2.
c c special techniques. Inpractice, anumber ofmethods canbesim-
SincevariationsinBVR I andtheunfilteredMASTERphotome-
c c ply divided into two types: (1) interpolation or approximation of
tryarewellcorrelated,wereducedW magnitudestotheBsystem
theseriesinthetimedomain,followedbytheapplicationofclas-
andcalledthemB (seeFig.2and3).
w sical methods of cross-correlation analysis. This is the approach
ofthewidely-usedinterpolationmethodICCF(Gaskell&Sparke
1986); and (2) methods that do not interpolate in the time do-
2.5 SwiftToOoptical,ultraviolet,andX-rayobservations
main but bin the correlation function by lag. A typical example
TheSwiftobservatorymonitoredNGC2617moreorlessregularly ofthistypeofmethods isthediscretecorrelationfunction(DCF;
for a year from 2013 May. Some of these observations (those of Edelson&Krolik1988).
2013April–June)werepublishedbyShappeeetal.(2014).Inad- In order to improvethe methodology of ICCFand DCF,we
ditiontousingtheavailabledataintheSwiftarchiveweappliedfor usedanintermediatemethod,whichisbetweentheICCFandDCF
aToOprogrammefor2016May17throughJune202016.Alldata methods,andwecallitMCCF(Oknyanskii1993).Attheheartof
MNRAS000,1–9(2016)
4 V. L. Oknyanskyet al.
Figure2.Near–IR(bottompanel),optical(middlepanel)andUV–X-ray(top)photometricobservationsofNGC2617overthe∼5-monthperiodfrom2016
January30toJune29.ThesolidcirclesinthemiddlepanelareBVRcIcdataobtainedwithAZT-5whiletheopencirclesareobservationswiththeZeiss-600.
OpenboxesarefilteredBVdataobtainedbytheMASTERnetwork,andsolidsquares(Bw)arefromunfilteredMASTERdatareducedtotheBsystem.Inthe
toppanelthesolidcirclesarethecombinedSwiftUVphotometryreducedtotheUVW1systemwhilsttheX-rayfluxisshownbytheopentriangles.Error
barsareshown,buttheyaregenerallysmallerthantheplottingsymbols.Thedatesoftheopticalspectraareindicated.
MNRAS000,1–9(2016)
NGC2617 ina highstate 5
Figure3.Bottompanel:unfilteredMASTERopticalphotometryofNGC2617reducedtotheBsystem(largeopencircles)andBdataobtainedbySwift
(closedcircles).Middlepanel:UVphotometricobservationsinUVW1.Toppanel:X-rayfluxobtainedbySwift(filledcircles).Theperiodofobservationsof
Shappeeetal.(2014)isindicated.
ourmethodistheICCFmethod,butwestrivetoreducethecontri- forming 10,000 independent simulations the same way as above,
butionoftheerrorinterpolationbyintroducingamaximuminter- but for twosetsof datawithout any correlationand lag. Wethen
val,MAX,usedforinterpolatingpoints.Weuseonlythoseinterpo- estimatesingle-tail97.5%confidencelevelsfornegativeandposi-
latedpointsthatareseparatedintimefromthenearestobservation tivevariationsofthecorrelationcoefficients.
points by no more than the value of MAX. Accordingly, at each UsingtheMCCFmethodwegetcross-correlationfunctions,
lag we will typically have a different number of pairs of points r(τ),asafunction of thelag,τ, for thetimeseries X(t) andY(t).
to calculate the correlation while in the ICCF method this num- Fromr(τ) thebest lagbetween X(t)andY(t)canbeestimatedin
berremainsfixed.ThevalueofMAXischosentobeassmallas at least two ways: from τ corresponded to r , and from the
peak max
possibleandyetmakeitpossibletocalculatethecross-correlation centroid,definedasτcent = R τr(τ)dτ/R r(τ)dτforvaluesofr ≥
functionintherangeoflagsweareinterestedin(formoredetails 0.8r .
max
seeOknyanskyetal.2014).
To estimate the errors in the derived lags, we applied the
3.2 JAVELIN
same Monte Carlo procedure as before (Oknyanskijetal. 1999;
Oknyanskyetal. 2014). We simulated artificial series (10000 We also used the reverberation mapping package JAVELIN
times) with the same temporal distribution, power spectrum and (http://bitbucket.org/nye17/javelin; Zuetal. 2011,
measurementerrorsastheobserveddata.Atthesametimewein- 2013); formerly known as SPEAR). Gaskell&Peterson (1987)
troducedthesametimelagbetweenthetimeseriesasfoundfrom showed that AGN light curves can be represented by a damped
the real observations. We then plotted the distribution of lags. In random walk. JAVELIN models the continuum light curve as
addition,asbefore,asacheck,theerrorwasestimatedusingthean- a damped random walk, assumes a transfer function, and then
alyticalformulagiveninGaskell&Peterson1987(seetheirequa- fits this 104 times to both time series. In place of the usual
tion 4). As has been noted before (see Koratkar&Gaskell 1991; cross-correlation function JAVELIN produces a histogram of
Maozetal. 1991 and Oknyanskijetal. 1999), error estimates for lags. These histograms can be used for the estimation of optimal
theMonteCarlomethodandthisanalyticalformulaareinagree- mean lags τ and its 1σ intervals (see thre details in e.g.,
JAVALIN
ment.Wealsoestimatea95%confidencelevelforeachlagbyper- Shappeeetal.2014;Zuetal.2013).
MNRAS000,1–9(2016)
6 V. L. Oknyanskyet al.
3.3 Results
Results of our reverberation mapping analyses for IR, optical,
UV andX-Raydatausing MCCFand JAVELINarepresented in
Figs4–6andTable1.AquickinspectionofFigs4–6confirmsthat
the results obtained with these two different methods are mostly
consistent witheach other. Somedifferences in the lags obtained
with the two different methods are, in most of the cases, in the
limit of estimated errors (see e.g., Liraetal. 2015). The differ-
enceandadvantagesofpeakandcentroidlagshavebeendiscussed
many times in the literature (see e.g. Koratkar&Gaskell 1991;
Petersonetal.2004).Anadvantageofthecentroidisthatitisbet-
terdefinedwhenthecross-correlationfunction(CCF)hasabroad
peak.AscanbeseenfromFigs4–6,inmostofthecaseswehave
principalpeaksinthecross–correlationfunctionsthatareisolated
and well defined. In these cases we prefer the peak lags that are
less dependent on free parameters. In what follows we will talk
mostlyaboutthepeaklags,butthecentroidsfromMCCFandthe
JAVELINvaluesforlagscanbeseeninTable1.
Our analysisshows [seeFigs4(a)and (e)and Table1]that
variationsinthe J bandlagthoseofthe Bbandbyabout3d.We
take thislaginto account and use alinear regression relationship
to reduce the J fluxes to the B system and then combine the re-
duceddatawith B.Thiscombineddatasetwewillrefertoasthe
‘B+J’data. Thevariationsof the H band lagthoseof B+J by
about 18 d. [seeFigs4 (b) and(f) and Table1].The K band lags
theBbandbyabout21.5d.andlagtheB+Jbyabout25.3d.(see
Figs.4(c),(d),(g)and(h)).Weinterpretthenear-infrared(NIR)lag
oftheKbandasbeingduetothermalreradiationofshorterwave-
length fluxbyhot dust. Wesuggest that theshorter lag of ∼3d.
forthe Jband,comparedwiththeBband,isprobablybecauseof
thestrongcontributionofemissionfromtheoutermostpartofthe
Figure 4. Left–hand panels: CCFs calculated by the MCCF method of
accretiondisc.Thelagfor K behind Bisthereforeabout25±3d.
Oknyanskii(1993)forour2016IR(JHK)andoptical(Bband)photom-
ThisismorethantwicethelagfoundbyShappeeetal.(2014)of
etryThedashedlinesare97.5%single-tailconfidencelevelsestimatedvia
about 9d, butan examination of theIRdata showsthat thelight
MonteCarlosimulations(seethetext).Right–handpanels:thecorrespond-
curveforK in2013isjustlinetrendataconfidencelevelof0.99
inglagsobtainedfromtheJAVELINmethodofZuetal.(2011).Thehis-
andisnotappropriateforreverberationmappinganalysis.Therea- togramsarenormalizedbydividingbytheirmaximumvalues.Thevertical
son why lags of about 8–10 d. were found by Shappee et al. is dashedlinesindicatezerolag.
relatedtothepropertiesoftheopticalandUVlightcurvesin2013.
Theyhave anapproximately linear riseand then aroughly linear
decrease.Themaximumoftheselightcurvesisshiftedrelativeto The lagsweobtain areconsistent withthe light-traveltimes
theendoftheIRdatabyabout8–10d.So,forlagsmorethan8–10 expected when UV and optical radiation are driven by the repro-
d.thecorrelationisveryhigh.Forsmallerlagsthecorrelationwill cessing of extreme UV and soft X-ray emission. If some of the
belower.Shappeeetal.forcedJAVELINtoreturnonlylagsofless changesinlagsbetween2013–2014and2016arereal,theycould
than10d.Giventhelimitationsofthedatawecanonlysaythatthe beduetochangingphysicalconditionsintheobject.
thelagfor K fromUVand Bvariationsisnot shorter thanabout
8d.Soour analysisdoesnot confirmtheresult ofShappee etal.
thatthelagforK relativetotheUVisabout9d.Itisquitelikely
4 DISCUSSION
thatthelagchangedduringthethreepreviousyears,butavailable
datacannotestablishthisyet. NGC2617isoneoftheclearestcaseswhereappearances,bothas
Ourcross-correlationanalysesforoptical,UVandX-raydata type-1andastype-2AGNshavebeenmanifestedinthesameobject
are presented in Fig. 5 for the new data for 2016 and in Fig. 6 at different epochs. What must happen to make such a dramatic
forthedatafrom2013to2014. Wefindthatin2016(seeFig.5) change possible?CLAGNslikeNGC2617 present problemsfor
optical variations probably lag the UV by about one day or less. the simplest ‘SPM’ unification models where type–2 and type–1
TheUVvariationsthemselveshavealagabout2–3d.behindthe AGNs are the same and the difference in type is only due to the
X-rayfluxandabout1d.behindthephotonindex,Γ .Fortheolder orientationoftheobserver.Inthismodelweseeatype-2AGNif
X
2013–2014 data, UV variations in different bands correlate very theBLRandaccretiondiscareblockedfromourviewbyobscuring
wellwitheachother(correlationcoefficientsareabout0.98andthe dustsurrounding theAGNperpendicular totheaxisofsymmetry
lagsareconsistent withzero)andhaveverysimilarMCCFswith (Keel1980).Orientationobviouslycannotchangeonthetimescale
X-rayvariabilityshowingalagofabout2.5±0.5d.InFigs6(a) oftheobservedtypechangesandhencesomeotherexplanationis
and(c)wethereforeshowresultsonlyforUVW1.Variabilityinthe needed.
BbandcorrelateswellwithUVvariabilitywithlagsofabout0.5d. Itisdifficulttofitourobservationswithtransienteventssuch
MNRAS000,1–9(2016)
NGC2617 ina highstate 7
Table1.LagsfromthereverberationmappinganalysiswiththeMCCFand
JAVELINmethodsindays.MCCFandJAVELIN1σconfidencelimitsare
presented.
MCCF JAVELIN Time
τpeak τcent τJAV
JfromB 2.8+1.2 3.0+1.0 2.7+1.2 2016
−0.8 −1.2 −1.4
HfromJ+B 18.2+3.0 13.9+4.0 10.7+1.2 –
−4.0 −3.0 −1.2
KfromJ 21.5+2.4 20.5+2.2 19.9+1.4 –
−2.6 −2.4 −1.6
KfromJ+B 25.3+3.4 24.5+3.1 26.0+2.1 –
−2.3 −2.6 −2.1
UVfromXray 2.0−+00..57 1.5−+00..57 2.7−+00..24 –
UVfromΓX 0.8−+00..55 0.9−+00..24 0.8−+00..33 –
BWfromUV 1.0−+00..55 0.5−+00..55 0.6−+00..53 –
UVfromXray 2.5−+00..75 2.9−+00..85 2.5−+00..86 2013–2014
BS fromUV 0.4−+00..44 0.6−+00..33 0.4−+00..22 –
as the tidal disruption of a star (Eracleousetal. 1995) – a so-
called ‘tidal disruption event’ (TDE) – or a supernova in the nu-
cleusofNGC2617(aswasconsideredfortheCLAGNNGC7582
byAretxagaetal.(1999)).Supernovaedonotrepeatandproduce
Figure5.Leftpanels:CCFsforourMayandJune2016Swiftdata.CCFs outburstslastingseveralyears.Theyalsoproducesmooth,mono-
havebeencalculated bytheMCCFmethod.Panel(a)showstheMCCFs tonically decaying light curves that are quite different from what
forUVW1laggingtheX-rayflux,(b)showsUVW1laggingtheX-raypho- we find for NGC 2617. The changing appearance of NGC 2617
tonindex,ΓX,and(c)showstheunfilteredMASTERdatalaggingUVW1. isobviously different fromNGC 7582 and ismore similar tothe
Thedashedlinesare97.5%single-tailedconfidencelevelsobtainedusing
sortsofchangesseeinNGC863(Denneyetal.2014))andMrk6
Monte-Carlosimulations.Rightpanels:thecorrespondinglagsobtainedus-
(Pronik&Chuvaev1972;Chuvaev1991),whichspentalongtime
ingJAVELIN.Thehistogramsarenormalizedbydividing bytheirmaxi-
intype-1statesaftertransitionsfromtype-2states.Themainprob-
mumvalues.Theverticaldashedlinesindicatezerolag.
lemwithinvokingaTDEisthattheyareextremelyrare.Thepres-
ence of a secondary supermassive black hole could significantly
increase the tidal disruption rate(see, e. g., (Ivanovetal. 2005)).
However,atpresentthereisnoevidenceofasecondaryblackhole
inNGC2617.Likesupernovae,aTDEnearaquiescentblackhole
producesasmooth,decayinglightcurve.ThecaseofaTDEneara
blackholeinanAGNcouldbedifferentbecauseofthepreexisting
accretiondisc.ATDEcouldperhapsproducerecurrentbrightening
forsometimebyperturbingthediscandaddingmaterialtoit.
WeconsideritmorelikelythatAGNtypechangesarethere-
sult of normal AGN processes. It has also long been argued that
AGN type depends on the accretion rate (Dibai 1981) and hence
changesintypearisefromvariationsintheluminosityofthecen-
tralengine(Lyutyjetal.1984).Themultiwavelengthobservations
ofNGC2617supportthispicture.Toexplainchangesintheopti-
calobscurationofAGNsandrelatedchangesinX-rayspectraithas
beenproposedthattheobscuringmaterialhasapatchydistribution
(seeTurner&Miller2009forareview).
We propose that AGN type changes arise from a combina-
tionofachangeincentralluminosityandarelatedchangeinthe
Figure 6. Left–hand panels: CCFs for archival Swift data from 2013 to obscuration.Heard&Gaskell(2016)showthatthesubstantialex-
2014.(a)showstheMCCFsforUVW1laggingtheX-rayfluxand(b)shows tinctionseeninalargefractionofAGNsmostlycomesfromdust
theCCFfortheB–bandlaggingUVW1.Thedashedlinesare97.5%single- betweenthenarrow-lineregionandtheBLR.Thesurprisingwave-
tailconfidencelevelsobtainedfromMonteCarlosimulations.Right–hand lengthindependenceofIRlagsinreverberationmappingstudiesof
panels: corresponding lags obtained from JAVELIN. The histograms are manyAGNs(Oknyanskyetal.2015)canbeexplainedbythehot
normalizedbydividingbytheirmaximumvalues.Theverticaldashedlines dustbeinginahollow,biconicaloutflow.Whenthelevelofactiv-
indicatezerolag.
ityincreases,theextraradiationwillsublimatethenearestdustin
thebiconicaloutflow.SuchchangesinlagsintheNIRhavebeen
seeninseveral AGNs(seeOknyanskyetal.2015).Asaresult of
thedestructionofinnerdust,wecangetalessobstructedviewof
MNRAS000,1–9(2016)
8 V. L. Oknyanskyet al.
theBLRandtheAGNwillbeseenasatype1.Soonerorlaterthe related components in the X-ray and UV/optical variations could
levelofactivitywilldecreaseagain,thedustwillre-form,andthe beexplainedbysomevariableobscurationonthelineofsightwith
AGNwillagainbeseenatype2. differences in the locations and sizes of the regions. Some previ-
Barvainis(1992)hasconsidered‘reformation’and‘survival’ ousstudies(e.g.Shappeeetal.2014;Nodaetal.2016)havenoted
models where dust grains are located in the clouds. These mod- thatthelagsfoundfromcorrelationsoftheX-ray–UV/opticalvari-
elscanbereconciled withobservations by introducing anaxially ations are longer by a factor of a few than can be expected from
symmetricdistributionofthesecloudswithastrongconcentration thestandardthindiscmodel.Gaskell(2017)hasproposedthatthis
totheequatorialplane(see,e.g.,Rowan-Robinson1995).Forthis isbecausetheluminosityhasbeenunderestimatedbecauseofthe
typemodelwecanuseimprovementsfromSitkoetal.(1993): neglect of reddening. Other possible explanations havebeen pro-
posed to explain the discrepancy (see, e.g., Buissonetal. 2017)
r =136L0.5 [T/1500K]−2.8[0.05/A ]0.5e−τ/2light-days (1)
evap UV,44 µ including the effect of emission lines and the Balmer continuum
wherer isthesublimationradius, A isthegraphitegrainsize (Korista&Goad2001).
evap µ
inµ,andτistheopticaldepthofthecloudstoUVradiation.Fol- Observationspresentedheremostlysupportascenariowhere
lowingSitkoetal.,Oknyanskij&Horne(2001)usethesamepa- thevariabilityacrossseveralwavebands(spanningX-rays–NIR)is
rameters:T = 1700 K, A = 0.15, τ = 1andfindagood agree- drivenbyvariableilluminationoftheaccretiondiscbysoftX-rays
µ
ment with observed IR lags for 11 AGNs. If for NGC 2617 we (Shappeeetal.2014).However,weprefertointerprettheIRradia-
take L = 7×1043 erg/s ( ∼ 10% of L for M =4×107M tion(near2.2µ)asbeingdominatedbythermalre-radiationofthe
uv Edd BH ⊙
Shappeeetal. 2014) we get a rough estimate for the sublimation dustincloudslocatedfartherawayfromtheblackholethantheAD
radiusatNGC2617ofabout32light-days.NIRreverberationradii andtheouteredgeoftheBLR.
havebeenshowntoscalewiththecentralengine’sluminosityLas
∼ L1/2 (Oknyanskij&Horne2001;Suganumaetal.2006),which
suggeststhatthepropertiesoftheinnermostdustgrainsareindeed 5 SUMMARY
similar in different objects (Barvainis 1992). Theradius does not
Wehaveshownusingspectroscopyandmulti-wavelengthphotom-
necessarily show the expected L1/2 dependence for an individual
etrythatNGC2617continuestobeinahighstateandtoappearto
object.Forexample,forNGC4151reverberationmappingshows
beatype-1AGN.Thedurationofthehighstateandthecontinu-
that the changes in radii have a lag of several years relative to
ingvariabilityarenotreadilyconsistentwiththetypechangebeing
changes in L (Oknyanskijetal. 2008;Kishimotoetal. 2013). So,
duetoatidaldisruptioneventorasupernova.Weproposethatthe
if theluminosity inNGC 2617 increased sharply in2013 theex-
changeoftypeisaresultofincreasedluminositycausingthesubli-
pectedchangeintheNIRreverberationradiuscouldbeobserveda
mationofdustintheinnerpartofthebiconicaldustyoutflow.This
fewyearslater.
leadstoamuchmoredirectviewofthecentralregions.
The appearance and disappearance of emission compo-
nents in the profiles of broad Balmer lines, such as we re-
port here for NGC 2617, are common in type-1 AGNs includ-
ACKNOWLEDGEMENTS
ing CL AGNs (e.g., NGC 5548 Bonetal. 2016; NGC 4151
Oknyanskij&vanGroningen1999).Thereisnoconsensusyeton WearegratefultoB.Shappeeforsendingusdatainadigitalfor-
thecauseofthesefeatures.Possibilitiesconsideredincludebinary mat.WethanktheSAIdirector,A.Cherepashchuk,andShAOdi-
supermassiveblackholes(seeGaskell2010),ellipticaldiscs(which rector,N.Jalilov,forgrantingusDirector’sDiscretionaryTimefor
couldarisewhenastarisdisrupted,Eracleousetal.1995),regions observations, and the staff of the observatories. We also express
ofenhancedemissioninadisc(Zhengetal.1991),andoff-axisil- ourthankstoN.GehrelsforapprovingtheSwiftToOobservation
lumination(Gaskell2011;Goosmannetal.2014). requestsandtheSwiftToOteamforpromptlyschedulingandexe-
Previous studies (Breedtetal. 2008; Shappeeetal. 2014; cutingourobservations.WearegratefultoP.B.Ivanovforuseful
Connollyetal. 2015; Nodaetal. 2016; Buissonetal. 2017) con- discussions and to the referee for useful comments that have im-
firmthatstrongX-rayUV/opticalcorrelationswithshort(nomore provedthepresentationofthispaper.Thisworkwassupportedin
thanfewdays)lagsforlongerwavelengthsarethenorminradio- part by the M. V. Lomonosov Moscow StateUniversity Program
quietAGNsratherthantheexception,whichseemstosupportsus- of Development, in part by the National Research Foundation of
picions that the UV/optical variations are primarily produced by SouthAfrica,inpartbytheRussianFoundationforBasicResearch
the reprocessing of variable EUV/soft X-ray emission by the ac- throughgrant 14-02-01274 andinpart byRussianScienceFoun-
cretiondisc.However,inseveralcasescorrelationsarepresenton dationthroughgrant16-12-00085.
short time–scales but absent on long time–scales. For some ob-
jects, it is known that the level and type of correlation can be
changed significantly from one epoch to another (for example,
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