Dust reverberation-mapping of the Seyfert 1 galaxy WPVS48

0.48 MB·

by  F. Pozo Nuñez

Astronomy&Astrophysicsmanuscriptno. wpv_dust (cid:13)cESO2014
January14,2014
LettertotheEditor
Dust reverberation-mapping of the Seyfert 1 galaxy WPVS48
F.PozoNuñez1,M.Haas1,R.Chini1,2,M.Ramolla1,C.Westhues1,K.Steenbrugge2,3,L.Kaderhandt1,H.Drass1,R.
Lemke1,andM.Murphy4
1 AstronomischesInstitut,Ruhr–UniversitätBochum,Universitätsstraße150,44801Bochum,Germany
2 InstitutodeAstronomia,UniversidadCatólicadelNorte,AvenidaAngamos0610,Casilla1280Antofagasta,Chile
3 DepartmentofPhysics,UniversityofOxford,KebleRoad,OxfordOX13RH,UK
4 DepartamentodeFísica,UniversidadCatólicadelNorte,AvenidaAngamos0610,Casilla1280Antofagasta,Chile
Received;accepted
4
1 ABSTRACT
0
2 UsingrobotictelescopesoftheUniversitätssternwarteBochumnearCerroArmazonesinChile,wemonitoredthez=0.0377Seyfert-
n 1 galaxy WPVS48 (2MASX J09594263-3112581) in the optical (B and R) and near-infrared (NIR, J and Ks) with a cadence of
twodays. Thelightcurvesshowunprecedentedvariabilitydetails. TheNIRvariationfeaturesofWPVS48areconsistentwiththe
a
correspondingopticalvariations,butthefeaturesappearsharperintheNIRthanintheoptical,suggestingthattheopticalphotons
J
undergomultiplescatterings.TheJandKsemission,tracingthehot(∼1600K)dustecho,lagstheBandRvariationsbyonaverage
3 τ=64±4daysand71±5days,respectively(restframe). WPVS48liesontheknownτ−M relationship. However,theobserved
V
1 lagτisaboutthreetimesshorterthanexpectedfromthedustsublimationradiusr inferredfromtheoptical-UVluminosity,and
sub
explanationsforthiscommondiscrepancyaresearchedfor. ThesharpNIRechosargueforaface-ontorusgeometryandallowusto
] putforwardtwopotentialscenarios: 1)aspreviouslyproposed,intheequatorialplaneoftheaccretiondisktheinnerregionofthe
A
torusisflattenedandmaycomeclosertotheaccretiondisk. 2)Thedusttoruswithinnerradiusr isgeometricallyandoptically
sub
G thick,sothattheobserveronlyseesthefacingrimofthetoruswall,whichliesclosertotheobserverthanthetorusequatorialplane
andthereforeleadstoanobservedforeshortenedlag. Bothscenariosareabletoexplainthefactorthreediscrepancybetweenτand
.
h r .Longer-wavelengthdustreverberationdatamightenableonetodistinguishbetweenthescenarios.
sub
p
- Keywords. galaxies:active–galaxies:Seyfert–galaxies:individual:2MASXJ09594263-3112581
o
r
t
s
1. Introduction spectral energy distribution of type 1 AGN might be attributed
a
[ toapuffed-upinnerrimofthetorus(Krolik2007).
Inthestandardparadigm,activegalacticnuclei(AGNs)arecom- WPVS48isanearbySeyfert-1galaxy(z = 0.0377)located
1
posed of a central black hole with an accretion disk (AD). A atadistanceof161Mpc(Véron-Cetty&Véron2010). Single-
v
broadlineregion(BLR)surroundstheAD,andfartheroutthere epoch UBVRI multi-aperture photometry by Winkler (1997)
4
is a (clumpy) molecular dust torus (Antonucci 1993). These showed evidence of a luminous nucleus (V = 14.78). Here we
3
componentscannotbespatiallyresolvedbyconventionalimag- present results for the dust torus geometry of WPVS48. Based
8
2ing,butreverberationmapping(RM,Blandford&McKee1982) on a dust RM campaign, we determine inner size of the dust
.can separate them. In RM, continuum brightness variations of torusandthehost-subtractedAGNluminosity. Thesharpnear-
1
theADareechoedbytheBLRemissionlinesandbythetorus infrared(NIR)variationinWPVS48allowustofavortwogeo-
0
dust emission. The time delay τ gives the characteristic size metricmodelsthatareabletoexplainthefactorofthreediscrep-
4
(R = τ·c, wherecisthespeedoflight)andinfavorablecases ancybetweenR =τ·candR inSeyfert1galaxies.
1 in sub
:thegeometryoftherespectiveregions.
v
Xi The geometry of AGN dust distributions is still under de- 2. Observationsanddata
bate.DustRMofSeyfert1galaxies(Glass2004,Minezakietal.
ar2004,Suganumaetal.2006)revealedarelationRin =τ·c∝ LU0.5V TheopticalJohnsonbroad-bandB(4330Å)andR(7000Å)ob-
betweentheinnerradiusRin ofthedusttorusandtheUVlumi- servations were carried out between January 12 and May 13 in
nosityLUV (Oknyanskijetal.1999,Kishimotoetal.2007). For 2013,withtherobotic25cmBerlinExoplanetSearchTelescope-
abagel-shapeddusttorus, however, theobservedRin isabouta II(BEST-II),locatedattheUniversitätssternwarteBochum,near
factor three smaller than the sublimation radius Rsub predicted Cerro Armazones, Chile1. As in Pozo Nuñez et al. (2013), we
fromthedustsublimationtemperatureTsub ∼1600K(Barvainis performedstandarddatareductionincludingcorrectionsforbias,
1992). SomemodifieddustgeometriesinvolveBLRassociated darkcurrent, flatfield, astrometryandastrometricdistortionbe-
dust due to winds/outflows from the accretion disk (Konigl & forecombiningthenineditheredimagesofeachnightandfilter.
Kartje 1994, Elitzur & Shlosman 2006, Czerny & Hryniewicz A7(cid:48).(cid:48)5diameteraperturewasusedtoextractthephotometryand
2011). Non isotropic illumination of the torus by the AD has tocreateflux-normalizedlightcurvesrelativeto17nonvariable
beenproposedaswell(Kawaguchi&Mori2010),allowingthe stars located in the same images, within 30(cid:48) around the AGN,
dust to come closer in the AD’s equatorial plane without being
sublimated. Ontheotherhand, the3µmemissionbumpinthe 1 http://www.astro.ruhr-uni-bochum.de/astro/oca/
Articlenumber,page1of4
Fig.1. Left: B-andR-bandlightcurvesobtainedin2013January-May. TheRlightcurveisverticallyshiftedforclarity. Thestraightlines
indicatethelong-termbrightnesstrends. Thedottedlinesshowthedetrendedlightcurves. Middle: J-andK-bandlightcurvesobtainedin2013
April-May. TheK lightcurveisverticallyshiftedforclarity. Thedottedlightcurvesdepictthestatebeforehostgalaxysubtraction. Right: B-
andK-bandlightcurves(blueandred)afterhostgalaxysubtraction. TheK-bandvariationpattern(red),whenshiftedbackby∼72days(black),
largelycorrespondstotheB-bandvariationpatterninFebruary2013;seetextfordetails.
andofsimilarbrightnessastheAGN.Absolutecalibrationwas
performed using standard reference stars from Landolt (2009)
observedonthesamenightsastheAGN.Wealsocorrectedfor
atmospheric(Patatetal.2011)andGalacticforeground extinc-
tion(Schlafly&Finkbeiner2011).
AfterobservingthepronouncedopticalvariationsinFebru-
ary 2013, we performed the NIR J and K (hereafter denoted
s
as K) observations between April 10 and May 13 in 2013 us-
ing the 0.8m Infrared Imaging System (IRIS) telescope (Ho-
dapp et al. 2010) at the Universitätssternwarte Bochum. Im-
ages were obtained and reduced in the standard manner. Pho-
Fig.2. FluxvariationgradientdiagramsofWPVS48intheoptical
tometry was extracted using a 7(cid:48).(cid:48)0 diameter aperture, slightly (left)andNIR(right). Thedataarerepresentedbytheblackdots. The
smaller than for the optical light curves, because the seeing in solidbluelinesrepresentthebisectorfit,yieldingtherangeoftheAGN
the NIR images is slightly better and smaller pixel sizes were slopes. The dashed red lines indicate the range of host slopes deter-
used. Light curves were calculated relative to six non variable minedintheopticalbySakataetal.(2010)andtheNIRbySuganuma
stars located in the same field that have a similar brightness as etal.(2006). TheintersectionbetweenthehostgalaxyandAGNslope
the AGN. The photometric calibration was achieved using four (redarea)givesthehostgalaxyfluxintherespectivebands.
high-quality flag (AAA) 2MASS stars in the same field as the
AGN. Absolute fluxes were corrected for Galactic foreground
(2012). As noted by Glass (2004), the K-band light curve may
extinction(Schlafly&Finkbeiner2011).
becontaminatedbyslower-varyinglonger-wavelength(Lband)
emission, and we address this effect in Sect.3.2. Power-law
3. Resultsanddiscussion extrapolationoftheAGN BandRfluxes(presumablyfromthe
AD) to longer wavelengths shows that the contribution of the
3.1. Opticalandinfraredlightcurves
ADtotheJandK fluxesisnegligible(<10%).
The light curves of WPVS48 are shown in Fig 1. A summary We correlated both the B- and K-band light curves and the
of the photometric results and the fluxes in all bands is listed B- and J-band light curves using the discrete correlation func-
in Table1. The light curves are published in electronic format. tion (DCF, Edelson & Krolik 1988). The DCF centroid yields
The two optical and the two NIR light curves show a similar a time delay τ = 65.8 days of B/J and τ = 72.1 days of B/K
variabilitybehavior. Theopticallightcurvesexhibitalong-term (Fig3).Theuncertaintiesofτwerecalculatedusingthefluxran-
trendthatwecorrectedfor,followingWelsh(1999)andDenney domization and random subset-selection method (FR and RSS,
et al. (2010), to avoid a potential bias in the cross-correlation Petersonetal.2004).Themedianofthisprocedureyieldsτcent=
analysis. TheNIRlightcurvesaretooshortfordetrending. 66.5+−34..81daysandτcent=73.5+−45..42daysforB/JandB/K,respec-
Using the flux variation gradient (FVG) method tively. Correcting for the time delation factor, we obtain a rest
(Choloniewski 1981; Winkler et al. 1992, Glass 2004, Sakata frametime-delayτrest =64.1±3.81daysandτrest =70.8±4.63
et al. 2010), we determined the host galaxy contribution to the daysforB/JandB/K,respectively.
lightcurves(Fig.2). Inbrief,itwasobservationallyestablished When the K light curve is shifted back by 72 days (black
that the optical and near-infrared flux ratios (B/R and J/K) of curve in Fig 1, right), the variation features (the steep decline,
the variable component remain constant with time, allowing thevalley,andthesubsequentpeak)areroughlyconsistentwith
us to separate the AGN flux through the use of a well-defined the Bfeatures. However, the K declineprecedesthe Bdecline,
range of host galaxy colors (Glass 2004, Sakata et al. 2010). thesubsequentpeakissharperinK thaninB,andthetimespan
MoredetailsontheFVGmethodaregiveninPozoNuñezetal. oftheKvalley(betweenthedeclineandthesubsequentpeak)is
Articlenumber,page2of4
F.PozoNuñezetal.:DustreverberationmappingofWPVS48
Fig.3. Discretecorrelationfunctionofthe Band J andthe Band
K lightcurves. Thedottedlinesindicatetheerrorrange(±1σ)around
the cross correlation. The black shaded area marks the range used to
calculatethecentroidofthelag(verticalline).
longerthanforB.ThesameholdsfortheRandJcurves,respec-
tively. Anexplanation(privatecommunicationbyRalfSieben-
morgenandEndrikKrügel;seealsoKrügel2009)mightbethat
on the way to the observer a significant fraction of the optical
Fig.4. Lag–luminosityrelationshipbasedonthedataofSuganumaet
photons are scattered multiple times in the kpc-scale narrow-
al.(2006).ThesymbolsforindividualSeyfert1galaxiesarethesameas
lineregion. Thisleadstoalongertravelingpathtotheobserver, inSuganumaetal.(2006).Thesolidlineisthebest-fitregressionfrom
hence a delayed arrival. The net effect is that we observe the Suganumaetal.(2006). WPVS48(reddot)liesclosetotheregression
ADvariabilityconvolvedwithanasymmetricdelaykernel. The line.Thedashedlineindicatesthedustsublimationradiusr expected
sub
longer-wavelength NIR photons may be scattered in the torus, atagivennuclearluminosityMV (fromKishimotoetal.2007).
butwithsmallpath-lengthchanges,andtheyareonlyweaklyaf-
fectedbyscatteringintheNLR,sothattheirobservedvariation
featuresremainsharp,evensharperthantheopticalfeatures. with the height z above the equatorial plane increasing radially
OnemightexpectthatADbrighteningleadstodustdestruc- withrtoacoveringhalf-angleof∼45◦.Inthispicture,illustrated
tion and the receding of the dust sublimation front, and vice inFig.5,thehotdustemissioncomesfromageometricallythin
versa. Then after a bright NIR light curve plateau a decline inner dust torus with a remarkably small dust covering angle,
shouldbedelayedandariseafteralowstateshouldoccurear- andalongthedustsublimationrim.
lier, but the variations of the current NIR data are too small to SpecificallyforWPVS48,thedustmayapproachintheAD’s
allowanidentificationofsuchsignatures. equatorialplane,withoutsublimation,andthehotdustwillpro-
Dust-reverberation studies of ten Seyfert 1 galaxies have duceasharpechoatmeanlag∼72days,butthelonginnertorus
shown that the lag τ of the dust K-band emission is propor- rimwilladdasmeared-outechoofalongerlagrangingbetween
tionaltothesquarerootoftheopticalluminosity(τ∝ L0.5;Sug- 80d >τ<100d(Fig.5). ThebulkofthedustechoofWPVS48
anumaetal.2006andreferencestherein). Weinterpolated MV is as sharp as the triggering continuum variations in the J and
ofWPVS48fromour BandRdata. Figure4showsitsposition K light curves with a rapid decline in J and a two-step (rapid
intheτ−MV diagramclosetotheregressionline. and subsequent shallow) decline in K (Fig.1, middle). In the
cross correlation B/K a faint tail clearly extends to ∼100 days,
but not in B/J (Fig.3). Therefore the current data of WPVS48
3.2. Innergeometryofthedusttorus
appeartobenotfullyconsistentwiththeexpectationsfromthe
Photometric Hα reverberation-mapping of WPVS48 revealed KM2010model: Firstly, ifthedustsublimationfrontisshaped
that the echo of the Hα BLR has a mean lag of 25 days (Pozo bytheanisotropicilluminationfromtheADalone,asproposed
Nuñez et al. in prep). In consequence, because of the different byKM2010, onewouldexpectthatthehottest J-emittingdust
lags of Hα and dust echo, BLR-related dust probably plays a exists along the entire rim and that its signatures seen in the K
minorrole. lightcurveandtheB/KcrosscorrelationarealsoseeninJ. Sec-
If the dust distribution is spread far along the line-of-sight, ondly, this interpretation of the slow shallow K decline leaves
onewouldexpectthattheechoissmearedoutintime.Therefore no room for K-band variations that are associated in part with
the sharp dust echo of WPVS48 argues in favor of a face-on longer-wavelength L-bandflux(Glass2004). Thirdly,while,in
torusgeometry. Wefocusedonthebasicgeometryoftheoverall additiontotheradiationfields,AGNwindsmighthelptoshape
absorbing dust, and considered smooth dust distributions. The thetorusrim,thegeometryoftheAD(thindiskvs. diskbecom-
topicofaclumpyduststructureisleftforfuturemodeling. ingthickerwithincreasingradius)isunknown,anditisnotclear
A general puzzle from dust RM is that with reasonable howstrongtheanisotropyactuallyis(e.g. Antonucci2013and
assumptions for graphite grains of a sublimation temperature referencestherein).
∼1600K and size 0.05µm in radius (Barvainis 1992) and for a We here also considered an alternative scenario, illustrated
bagel-shaped torus, the inner radius R = τ · c is about three inFig.6. Inthispicture,forsimplicity,weassumedthattheAD
in
timessmallerthanthedustsublimationradiusr inferredfrom essentiallyradiatesisotropically. Thedusttorusisgeometrically
sub
theoptical-UVluminosity(Kishimotoetal.2007). thickwithinnerradiusr andanopeninghalfangle∼45◦ and
sub
To solve this puzzle, Kawaguchi & Mori (2010, hence anopticallythickatNIRwavelengths,sothattheobserveronly
KM2010, 2011) have considered a model where the accretion seesthefacingrimoftheentireinnerdusttoruswall. Thestrik-
disk emits relatively little radiation in its equatorial plane. The ing feature is that the rim lies at height z, that is, closer to the
anisotropic illumination allows the torus inner region in the observerthantheequatorialplane.Thiszcontributionforeshort-
equatorialplanetoapproachtothecentralblackholeandshapes enstheobservedlag, namelyτ·c = (r2 +z2)0.5 −z ≈ r −z.
sub
theinnertoruswallsothatitisconcavefromtheobserversview, Iftherimliesatanangle45◦ fromtheequatorialplane,oneob-
Articlenumber,page3of4
Fig.5. SchematicillustrationofthedusttorusmodelbyKawaguchi& Fig. 6. Schematic illustration of a torus with constant sublimation
Mori. BluedenoteshotT ∼1600Kdustvisibleat1-2µm,reddenotes radiusforinclinations0◦ < i < 45◦,opticallythickat1-2µm. ZoneA
coolerT ∼800Kdustvisibleat∼4µm. Thesingleblacknumbersleft (blue)markshotT ∼1600Kdustvisibleat1-2µm,zoneB(red)cooler
andrightoftherimgivethelagsalongthetorusrimat1-2µmandat T ∼800Kdustvisibleat∼4µm;zoneC(blackshaded)markeshotdust
∼4µm, respectively. Note the foreshortening of the lags at the high- thatisheavilyobscured(A >1)intheobserver’slineofsight,sothat
K
altitude part of the torus rim. The mean lags of the equatorial zones itbecomesdimat1-2µm,butisclearlyvisibleat∼4µm.Themeanlags
(AandC)andtheentire2πintegratedtorusrim(BandD)aregiven, foreachzonearegiven.
adoptingaconstantdustdensitydistributionalongtherim.
Table 1. Photometry range in mag and mean flux densities with er-
rors in mJy. The values are as observed in 7(cid:48).(cid:48)5 (B,R) and 7(cid:48).(cid:48)0 (J,K)
apertures.
tains z = cos(45◦)·r , hence τ·c = 0.3·r . This scenario
sub sub
canexplainthefactorthreediscrepancybetweenτandr asan B R J K
sub
observationalforeshorteningeffect.TheJandKlightcurvesare 14.74-14.94 13.93-14.06 12.33-12.36 10.61-10.66
consistentwiththispicture: therapiddeclinein J and K arises fB fR fJ fK
from dust in the blue zone of Fig. 6 and the subsequent shal- 4.90±0.04 7.04±0.07 18.36±0.46 36.56±1.10
low decline in the K light curve might be explained by dust in
thetransitionzonebetweenthebluezoneandtheredandblack
zones, which are not warm enough (red zone) or too much ab-
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