A Systematic Survey for z < 0.04 Changing-Look AGNs
Madhooshi R. Senarath, Michael J.I. Brown, Michelle E. Cluver, Thomas H. Jarrett, Christian Wolf, Nicholas P. Ross, John R. Lucey, Vaishali Parkash, Wei J. Hon
aa r X i v : . [ a s t r o - ph . GA ] F e b MNRAS , 1–15 (2020) Preprint 16 February 2021 Compiled using MNRAS L A TEX style file v3.0
A Systematic Survey for z < Madhooshi R. Senarath, ★ Michael J. I. Brown, Michelle E. Cluver, , Thomas H. Jarrett, Christian Wolf, Nicholas P. Ross, John R. Lucey, Vaishali Parkash, Wei J. Hon, School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria, 3122, Australia Department of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Road, Bellville, 7535, South Africa Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch, 7701, South Africa Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, United Kingdom Centre for Extragalactic Astronomy, University of Durham, Durham DH1 3LE, United Kingdom School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
Accepted XXX. Received YYY; in original form ZZZ
ABSTRACT
We have conducted a systematic survey for z < 𝜋 steradians (sky north of Declination − ◦ ) and SkyMapper has coverage of ∼ (skysouth of Declination 0 ◦ ). We use small aperture photometry to measure how colour and fluxhave changed over time, where a change may indicate a change in spectral type. Optical colourand flux are used as a proxy for changing H 𝛼 equivalent width, while WISE 𝜇 m flux is usedto look for changes in the hot dust component. We have identified four AGNs with varyingspectra selected using our optical colour selection method. Three AGNs were confirmed fromrecent observations with WiFeS on the 2.3 m telescope at Siding Spring and the other wasidentified from archival spectra alone. From this, we identify two new changing look AGNs;NGC 1346 and 2MASX J20075129-1108346. We also recover Mrk 915 and Mrk 609, whichare known to have varying spectra in the literature, but they do not meet our specific criteriafor changing look AGNs. Key words: galaxies: active – galaxies: Seyfert – methods: Observational
The classic dichotomy of Active Galactic Nuclei (AGNs) classifiestheir optical spectra as having either broad or narrow emissionlines, type 1 and type 2, respectively, with some intermediate classescontaining both emission line components (Seyfert 1943; Weedman1976; Osterbrock 1977, 1981). The widely used unified model ofAGNs proposes that observed AGN type/classes are a single typeof object, observed at different orientations along the line of sight(Osterbrock 1989; Antonucci 1993). We can directly observe boththe broad line region (BLR) and narrow line region (NLR) in type1 Seyferts. Whereas, in type 2 Seyferts, the light from the broadline region is absorbed by the dusty torus and is not visible in theoptical (although it is observable in the IR), while the light from ★ [email protected] the narrow line region is scattered. Intermediate type 1 Seyferts canhave both narrow and broad emission lines, type 1.5 Seyferts havenarrow lines with obvious broad H 𝛼 and H 𝛽 components, type 1.8shave narrow lines with a broad H 𝛼 component and a recognisablebroad H 𝛽 component and type 1.9s contain narrow lines with onlyH 𝛼 line being broad (Osterbrock 1977, 1981).Different wavelengths probe different regions of an AGN. Theinfrared (IR) wavelength range is sensitive to thermal emissionfrom warm dust, which is often attributed to the torus that canobscure the ultraviolet and optical emission (e.g., Padovani et al.2017). The optical and ultraviolet (UV) bands probe emission fromthe accretion disk and fast moving gas (1000 - 10,000 km sec − )in the BLR, but the UV and optical emission from these regionscan be obscured by dust. The X-ray band traces the emission ofthe hot corona and the ionized reflection of the X-ray continuumfrom distant neutral material like the molecular torus, the BLR and © M. R. Senarath et al.
NLR or the accretion disk (Antonucci 1993; George & Fabian 1991;Jaffe et al. 2004; Meisenheimer et al. 2007; Bianchi et al. 2008). X-rays from AGNs are believed to be a result of inverse Comptonscattering of the photons in the accretion disk by the hot corona.Changing-look AGNs (CLAGNs) are Seyferts and quasarswhere the spectral type changes from broadline to narrow line andvice versa. Given that the size of the torus is on the order of 1 pc andthe relevant velocities are < km s − , one might expect CLAGNsto take ∼ years to change spectral class in the optical. We mayexpect variability on the viscous timescale, which for AGNs is on theorder of ∼ − years (Siemiginowska et al. 1996). HoweverTohline & Osterbrock (1976), Penston & Perez (1984), Tran et al.(1992a), Storchi-Bergmann et al. (1993), Eracleous & Halpern(2001), Marchese et al. (2012), Marin et al. (2013), Denney et al.(2014), Shappee et al. (2014), Guo et al. (2016), Oknyansky et al.(2017), Ross et al. (2018), Noda & Done (2018) and Hon et al.(2020) for example, have identified CLAGNs that change spectraltype in only a few years. CLAGNs may therefore be more commonthan previously believed.Examples of low redshift CLAGNs from the literature includeMrk 833 (Canelo et al. 2018), which changed from type 1.9 totype 1.8, NGC 7603 (Tohline & Osterbrock 1976), which changedfrom type 1 to type 1.5, Mrk 372 (Penston & Perez 1984), whichchanged from type 1.5 to type 1.9 and NGC 1566 (Oknyansky et al.2018), which changed from type 1.9 to type 1.2. While there isn’t astrict definition in the literature, for consistency with the literaturewe classify objects as CLAGN if the broadline components com-pletely disappears, a new broad line component appears and/or ifthe Osterbrock (1977) and Osterbrock (1981) spectral type changesby more than 0.1 (that is a change from type 1.8 to 1.9 and 1.9to 2.0 and vice versa is not significant enough to be classified asa CLAGN). Some objects in our sample do show interesting spec-tral variability while falling below our CLAGN thresholds, and weretain them in this work while not classifying them as CLAGNs.AGNs such as Mrk 883, which change type by only 0.1 do notmeet our CLAGN criteria, however it is considered an CLAGN byCanelo et al. (2018). There are two main reasons why AGNs change their spectral type.One scenario is an obscuring cloud crosses the line of sight, causingchanges in observed light curves. Goodrich (1989) and Guo et al.(2016) have found AGNs which vary due to obscuring clouds. Inthis case, light from the inner disk and BLR is obscured by thedusty cloud, causing broad lines to disappear from the spectra. IRemission is sensitive to thermal emission from warm dust, which isoften attributed to the torus, as the change in spectral type in thisscenario is caused by obscuration of the BLR, we do not expect tomeasure a change in IR. Thus, the spectra of the AGNs should returnto their original state after a period of time. A likely example of sucha CLAGN is quasar SDSS J231742.60+000535.1 (Guo et al. 2016),where the change in its spectral type was caused by rapid outflowor inflow with an obscuring cloud passing along the line of sight.This scenario is in agreement with the unified model as changes inthe spectral type are due to changes along the line of sight.The second and more complex cause for change in spec-tral type is due to changes in accretion rate of the centralblack hole, changes in accretion disk structure, or tidal disrup-tions (Dexter & Agol 2011; Kelly et al. 2011; Merloni et al. 2015;Kokubo 2015; MacLeod et al. 2016). Ross et al. (2018) use mod-els of the innermost stable circular orbit around a black hole to determine if this is a possible driver for changes in the spectra ofSDSS J1100-0053, where the different models have combinationsof zero torque, non-zero torque, spectral hardening factor and radii.Ross et al. (2018) attributed the change in spectral type to mass flowrate switching from cold, high mass flow rate to hot, low mass flowrate. Unlike the previous scenario, the change in spectral type iscaused by changes in accretion. Thus, it does not agree with theunified model as the spectral type change is not caused by changesalong the line of sight. Until recently, the most common method by which studies haveidentified CLAGNs is by serendipity. For example, NGC 2617 is aCLAGN that was identified by Shappee et al. (2014) after an out-burst triggered a transient source alert, and the corresponding chang-ing optical spectra are displayed in Figure 1. NGC 2992 was iden-tified by Gilli et al. (2000) using
Beppo
SAX observations (Scarsi1997) which caught a rise in nuclear emission from the AGN, andthere was a corresponding change in the optical classification fromtype 1.9 to type 2.It is only recently that targeted searches for CLAGNs such asLaMassa et al. (2015), MacLeod et al. (2016), Ruan et al. (2016),Runnoe et al. (2016), Gezari et al. (2017), Yang et al. (2018),Stern et al. (2018) and MacLeod et al. (2019) have been con-ducted. This is because there is now more readily availablearchival data and multi-epoch photometry such as NEOWISE(Mainzer et al. 2014), Pan-STARRS (Chambers et al. 2019a),SDSS (Eisenstein et al. 2011), SkyMapper (Wolf et al. 2018) andGAIA (Gaia Collaboration et al. 2016). These targeted searches arefocused mainly on detecting changing-look quasars that are wellbeyond z ∼
0. That said, a number of changing-look Seyferts havebeen identified in the z < z < 𝛼 within the 𝑟 band and increases availability of archival photometry and spec-troscopy. We use the comprehensive Véron-Cetty & Véron (2010)catalogue to identify z < WISE , Wright et al. 2010)and Sloan Digital Sky Survey (
SDSS , Abazajian et al. 2004). Weselect CLAGN candidates meeting our colour and flux criteria, andthen follow-up these candidates with archival and new spectroscopy.In Section 2 we discuss the methods by which we selected CLAGNcandidates and the effectiveness of each method. In Section 3 wediscuss what objects were observed. In Section 4 we discuss the newCLAGNs we identified, including NGC 1346 which we also discussin Senarath et al. (2019). We also discuss the possible reasons whythese AGNs changed spectral type in Section 4. The spectral hardening factor, also referred to as the color correc-tion, is used to interpret multi-temperature black body fitting results(Davis & El-Abd 2019). Where for a canonical blackbody spectral, the spec-tral hardening factor is 1. MNRAS , 1–15 (2020)
Systematic Survey for z < λ rest (˚ A ) N o r m a li z e d F l u x WiFeS MJD:(58491)6dFGS MJD:(53003)
Figure 1.
Spectra of NGC 2617, a previously identified CLAGN with still evolving spectra from type 1.8 to type 1 Seyfert (Oknyansky et al. 2017).Véron-Cetty & Véron (2010) identify the 6dFGS spectrum as type 1.8, while our WiFeS 2019 January shows NGC 2617 is currently a type 1, which agreeswith observations from Oknyansky et al. (2017).
Table 1.
Known z < ID Ra(J2000) Dec(J2000) redshift Previous type(s) Current type ReferenceNGC 7603 23h18m56.65s +00d14m37.9s 0.030 1 1.5 Tohline & Osterbrock (1976)NGC 4151 12h10m32.65s +39d24m20.7s 0.003 1, 2 1.5 Penston & Perez (1984)NGC 2622 08h38m10.943s +24d53m43.02s 0.029 1.8 1 Goodrich (1989)Mrk 372 03h02m13.18s -23d35m19.8s 0.035 1.5 1.9 Gregory et al. (1991)Mrk 993 01h25m31.47s +32d08m10.5s 0.016 1 1.9 Tran et al. (1992a)NGC 1097 02h46m19.05s -30d16m29.6s 0.004 2 1 Storchi-Bergmann et al. (1993)NGC 3065 10h01m55.30s +72d10m13.0s 0.007 2 1.2 𝑎 Eracleous & Halpern (2001)NGC 2992 09h45m42.04s -14d19m34.8s 0.008 1.9, 1.5 2 Gilli et al. (2000) & Trippe et al. (2008)NGC 454E 01h14m22.50s -55d23m55.0s 0.012 2 𝑏 Marchese et al. (2012)NGC 1365 03h33m36.45s -36d08m26.3s 0.005 1.8 𝑏 Marin et al. (2013)Mrk 590 02h14m33.57s -00d46m00.2s 0.026 1 1.9-2 Denney et al. (2014)Mrk 6 06h52m12.251s +74d25m37.46s 0.019 2 1.5 Khachikian & Weedman (1971), Khachikian et al. (2011) & Afanasiev et al. (2014)ESO 362-G018 05h19m35.80s -32d39m27.0s 0.012 1.5 2 Agís-González et al. (2017)NGC 7582 23h18m23.62s -42d22m14.0s 0.005 1 2 Braito et al. (2017)NGC 2617 08h35m38.77s -04d05m17.2s 0.014 1.8 1 Oknyansky et al. (2017)NGC 1566 04h20m00.41s -54d56m16.1s 0.005 1.9 1.2 Oknyansky et al. (2018)Mrk 883 16h29m52.84s +24d26m37.4s 0.037 1.9 1.8 Canelo et al. (2018)HE 1136-2304 11h38m51.00s -23d21m32.0d 0.027 2 1.5 Zetzl et al. (2018)NGC 3516 11h06m47.490s +72d34m06.88s 0.009 1 2 Shapovalova et al. (2019)1ES 1927+654 19h27m19.54s +65d33m54.2s 0.017 2 1 Trakhtenbrot et al. (2019)NGC 1346 03h30m13.27s -05d32m36.3s 0.014 1.8 2 Senarath et al. (2019) & This work2MASX J20075129-1108346 20h07m51.29s -11d08m34.6s 0.030 2 1.8 This work 𝑎 While Eracleous & Halpern (2001) don’t explicitly state the spectral types of the changes, they do state that NGC 3065 went from lacking broad Balmer lines to containing broad Balmer lines. 𝑏 NGC 1365 is an X-ray CLAGN. X-ray CLAGNs are characterised by rapid transitions between Compton-thick to Compton-thin, where this transition can be due to absorption by gas clouds passing alongthe line of sight or relativistic reflection on to the accretion disk (Marin et al. 2013). In the case of NGC 1365, the CLAGN classification is due to the reflection-dominated scenario.
To select CLAGN candidates we use photometry from PanSTARRS,SkyMapper and the Wide-field Infrared Survey Explorer (
WISE ,Wright et al. 2010). Our methods use optical and MIR fluxes andcolours to search for variability in the optical continuum. Our firstapproach uses optical colours as a proxy of H 𝛼 equivalent widths.Our second and third approaches use variability of optical and MIRfluxes to search for changes in accretion disk and hot dust componentrespectively.We use the MIR to identify changes in the dust near the accre-tion disk of the AGNs (presumably not associated with the largertorus). The optical and UV continuum probes emission from the diskwhile the optical and UV spectral lines probe ionised gas above the disk, and changes in the optical flux and color may indicate changesin spectral type resulting from changes in accretion or changingobscuration along the line of sight. We use optical colour and in-frared fluxes and colours as proxies for H 𝛼 emission and hot diskemission respectively, where variations would indicate the presenceof a CLAGN. Therefore, we require photometry of known AGNs(the specifics of this are explained in Section 2.1). Following thephotometric selection of CLAGN candidates we obtained follow-upspectroscopy. MNRAS000
WISE ,Wright et al. 2010). Our methods use optical and MIR fluxes andcolours to search for variability in the optical continuum. Our firstapproach uses optical colours as a proxy of H 𝛼 equivalent widths.Our second and third approaches use variability of optical and MIRfluxes to search for changes in accretion disk and hot dust componentrespectively.We use the MIR to identify changes in the dust near the accre-tion disk of the AGNs (presumably not associated with the largertorus). The optical and UV continuum probes emission from the diskwhile the optical and UV spectral lines probe ionised gas above the disk, and changes in the optical flux and color may indicate changesin spectral type resulting from changes in accretion or changingobscuration along the line of sight. We use optical colour and in-frared fluxes and colours as proxies for H 𝛼 emission and hot diskemission respectively, where variations would indicate the presenceof a CLAGN. Therefore, we require photometry of known AGNs(the specifics of this are explained in Section 2.1). Following thephotometric selection of CLAGN candidates we obtained follow-upspectroscopy. MNRAS000 , 1–15 (2020)
M. R. Senarath et al.
We select known z < 𝑢𝑣𝑔𝑟𝑖𝑧 ) contains ≈
280 million objects and has a coverage area of almost the entireSouthern sky. Pan-STARRS on the other hand, surveys the sky northof Declination − ◦ (passbands 𝑔𝑟𝑖𝑧𝑦 ). Together the two surveysprovide data for the entire sky.The depth of the optical catalogues are 𝑟 ∼ . 𝑟 ∼ . < ′′ diameter apertureson stack images. For SkyMapper we use the DR1 5 ′′ diameteraperture photometry. For SDSS we use the fibre magnitudes (3 ′′ in diameter). To photometrically identify AGNs that may have avarying hot dust component, we use photometry drawn from the WISE and NEOWISE surveys (Mainzer et al. 2014). NEOWISEmeasures photometry in the W1 and W2 bands and surveys theentire sky at a cadence of 6 months, and has been doing so since
WISE was brought out of hibernation in late 2013. We present asubset of our CLAGN candidate catalogues in Tables 2 and 3 forSkyMapper and Pan-STARRS, respectively, which contain all the z < CLAGNs should change colour due to varying H 𝛼 strength, thuswe use 𝑟 - 𝑖 as a proxy for H 𝛼 equivalent width. Our optical colourselection assumes that type 1 and type 2 AGNs have relativelyblue and red 𝑟 - 𝑖 colours, respectively, resulting from the equivalentwidth of the H 𝛼 emission line. As the Pan-STARRS and SkyMapperbands differ from each other, the 𝑟 - 𝑖 colours they measure forindividual AGNs will differ. As a consequence we compare z < 𝑟 - 𝑖 colour. We apply this relation when determining our 𝑟 - 𝑖 colour criteria for our Pan-STARRS and SkyMapper catalogues.For our SkyMapper catalogue we select blue Seyfert type 2swith 𝑟 - 𝑖 < 𝑟 - 𝑖 > 𝑟 - 𝑖 < 𝑟 - 𝑖 > 𝛼 and H 𝛽 emission lines. We select candidates to observe on the basis that theyhave more than one archival spectra spectra and there is variation inemission line widths that appear to be changing in the last 10 years(after 2008), these archival spectra are referenced in Tables 2 and3. It should be noted that 46% of the candidates selected using thementioned colour criteria have no readily available archival spectraor have just one readily available spectrum. Also 26% of candidates . . . . . . . . mag i - mag z . . . . . . . m a g r - m a g i Mrk 915 Mrk 6092MASX J20075129-1108346 NGC 1346Selection criteriaInput catalogueColour selected CLAGN candidates CLAGNCLAGN identified in this workAGN with varying spectra
Figure 2.
Skymapper 𝑟𝑖𝑧 colours for our CLAGN candidates withthe other z < 𝑟 − 𝑖 > 𝑟 - 𝑖 < did not have archival spectra taken in the last 10 years. AGNs thatfall into these categories are not selected for observations.Of the 22 known z < As with 𝑟 - 𝑖 colour selection, we utilise the r-band flux variabilityto detect the changing H 𝛼 emission of CLAGNs. As, by definitionthis requires multiple 𝑟 band epochs, we have measured the vari-ability of the Véron-Cetty & Véron (2010) z < ≈ Δ m 𝑟 < 𝑟 band flux variabil-ity alone. Of the 335 type 1.8s, 1.9s and type 2s with both SDSSand Pan-STARRS photometry, we identified 22 potential CLAGNsusing Δ m 𝑟 > MNRAS , 1–15 (2020)
Systematic Survey for z < − . − . . . . . . . . . mag i - mag z − . − . . . . . . m a g r - m a g i Mrk 609NGC 1346Mrk 915Selection criteriaInput catalogueColour selected CLAGN candidates CLAGNCLAGN identified in this workAGN with varying spectra
Figure 3.
Pan-STARRS catalogue 𝑟𝑖𝑧 colours for our CLAGN candi-dates with the other z < 𝑟 - 𝑖 > 𝑟 - 𝑖 < the scatter is dominated by PSF differences, zeropoint errors, fil-ter curve differences and AGN variability. The dominant source oferror is systematic errors rather than easily quantifiable errors, andthus we have not included individual error bars into Figure 4.After further investigation of the 22 potential CLAGNs, fourCLAGN candidates were removed as their measurement of vari-ability resulted from centroid errors, where two of these AGNswere in galaxy pairs. We also recover the known CLAGNsNGC 2617 (Oknyansky et al. 2017) and Mrk 883. Further in-spection into archival spectra of the AGNs with Δ m 𝑟 > We use NEOWISE variability to search for z <
13 14 15 16 17 18
PS1 − . . . . . . . ∆ m r Selection criteriaSDSS m r - PS1 m r NGC 2617 (SDSS m r - PS1 m r ) Mrk 883 (SDSS m r - PS1 m r ) Figure 4.
The 𝑟 -band magnitude as a function of apparent magnitude forVéron-Cetty & Véron (2010) type 1.8s, type 1.9s and type 2s, measuredwith SDSS and Pan-STARRS 3 ′′ aperture photometry. Known CLAGNsNGC 2617 and Mrk 883 are highlighted, NGC 2617 became 1 magnitudebrighter in 𝑟 -band between the SDSS and Pan-STARRS imaging surveys. W1mpro (mag) . . . . . . . ∆ W ( m a g ) Selection criteria ∆ W1 S1.8 ∆ W1 S1.9 ∆ W1 S2NGC2617
Figure 5.
Change in NEOWISE W1 photometry as a function of W1 pho-tometry. As type 1s vary at ∼ . 𝜇 m without changing spectral type, wecan only plot and draw CLAGN candidates from AGNs classes type 1.8s,1.9s and type 2.0s. The purple star is NGC 2617, a known CLAGN. and the varying MIR emission is attributed to a dusty wind inthe AGNs polar region (Hönig et al. 2013). As with the previousCLAGN candidate selection methods, we inspect the archival spec-tra of AGNs where 1.8s, 1.9s and type 2s had Δ W1 > WISE variability andspectra more recent than 2017, we found that the recent spectra stillexhibited narrow lines (i.e., NGC 4135, NGC 6230, IC 1495 andMrk 670). Thus, we did not undertake any spectroscopic follow upof AGNs on the basis of infrared variability alone.
MNRAS , 1–15 (2020) M . R . S e na r a t h e t a l . Table 2.
Sample taken from our SkyMapper catalogue used to select CLAGN candidates. Note that the full table is available online.
SkyMapper SDSS Pan-STARRS 𝑎 RA Dec Name z VCV r i z MJD r i z MJD r i z Flag 𝑏 Spectra Spec.(deg) (deg) Spec. Type (mag) (mag) (mag) (mag) (mag) (mag) (deg) (deg) (deg) Notes 𝑐 Sources 𝑑 𝑎 NOTE: The Pan-STARRS photometry in this table has been measured by us using the Pan-STARRS cutouts (where available) with a 3 ′′ diameter aperture for the AGNs in our Skymapper catalogue. 𝑏 Optical colour selection flag where 1 indicates possible narrowing spectra and 2 indicates possible broadening spectra. 𝑐 CLAGN spectra flag where: 0 No archival spectra online, 1 Varying in archival and/or WiFeS spectra, 2 Only one archival spectrum found and no WiFeS, 3 Two or more archival spectra, not varying in WiFeS and archival spectra, 4 No archival spectra found from the last 10 years,but shows signs of varying before then and 5 No archival spectra found from the past 10 years, does not show signs of varying before then. 𝑑 If Spectra Notes is 0, then this is also 0. 1 WiFeS 2 6dFGS 3 SDSS 4 S7 5 BASS 6 2dFGRS 7 Ho et al. (1995) 8 MaNGA 9 Fosbury et al. (1982) 10 Phillips et al. (1983) 11 Kennicutt & Keel (1984) 12 Bergvall et al. (1986) 13 Veron-Cetty & Veron (1986a) 14 Veron-Cetty & Veron(1986b) 15 Kollatschny & Fricke (1987) 16 Maia et al. (1987) 17 Rudy et al. (1988) 18 Morris & Ward (1988) 19 Storchi Bergmann et al. (1990) 20 Winkler (1992) 21 de Grijp et al. (1992) 22 Moran et al. (1994) 23 Cruz-Gonzalez et al. (1994) 24 Goodrich (1995) 25 Maia et al.(1996) 26 Moran et al. (1996) 27 Reimers et al. (1996) 28 Scarpa et al. (1996) 29 Coziol et al. (1997) 30 Pietsch et al. (1998) 31 Fraquelli et al. (2000) 32 Jansen et al. (2000) 33 Kewley et al. (2001) 34 Reunanen et al. (2003) 35 Márquez et al. (2004) 36 Georgantopoulos et al.(2004) 37 Masetti et al. (2006a) 38 Masetti et al. (2006b) 39 Moustakas & Kennicutt (2006) 40 Ho & Kim (2009) 41 Trippe et al. (2010) 42 Dopita et al. (2014) 43 Schmidt et al. (2016) 44 Ramos Almeida et al. (2016) 45 Thomas et al. (2017) M N R A S , ( ) S y s t e m a ti c Su r vey f o rz < . C hang i ng - L oo k A G N s Table 3.
Sample taken from our Pan-STARRS (PS1) catalogue used to select CLAGN candidates. Note that the full table is available online.
Pan-STARRS (PS1) SDSSRA Dec Name z VCV r i z MJD r i z Flag 𝑎 Spectra Spec.(deg) (deg) Spec. Type (mag) (mag) (mag) (mag) (mag) (mag) Notes 𝑏 Sources 𝑐 𝑎 Optical colour selection flag where 1 indicates possible narrowing spectra and 2 indicates possible broadening spectra. 𝑏 CLAGN spectra flag where: 0 No archival spectra online, 1 Varying in archival and/or WiFeS spectra, 2 Only one archival spectrum found and no WiFeS, 3 Two or more archival spectra, not varying in WiFeS and archival spectra, 4 No archival spectra found from the last 10 years,but shows signs of varying before then and 5 No archival spectra found from the past 10 years, does not show signs of varying before then. 𝑐 If Spectra Notes is 0, then this is also 0. 1 WiFeS 2 6dFGS 3 SDSS 4 S7 5 BASS 6 2dFGRS 7 Ho et al. (1995) 8 MaNGA 9 MUSE 10 Phillips et al. (1983) 11 Goodrich & Osterbrock (1983) 12 Penston & Perez (1984) 13 Osterbrock (1985) 14 Bergvall et al. (1986) 15Veron-Cetty & Veron (1986a) 16 Rudy et al. (1988) 17 Morris & Ward (1988) 18 Sabbadin et al. (1989) 19 Gregory et al. (1991) 20 Kennicutt (1992) 21 Tran et al. (1992b) 22 de Grijp et al. (1992) 23 Durret (1994) 24 Kim et al. (1995) 25 Goodrich (1995) 26 Moran et al. (1996) 27Owen et al. (1996) 28 Scarpa et al. (1996) 29 Coziol et al. (1997) 30 Pietsch et al. (1998) 31 Wei et al. (1999) 32 Gonçalves et al. (1999) 33 White et al. (2000) 34 2000UZC...C......0F 35 Reichardt et al. (2001) 36 Stepanian et al. (2002) 37 Rossa et al. (2006) 38 Moustakas & Kennicutt(2006) 39 Lira et al. (2007) 40 Buttiglione et al. (2009) 41 Stoklasová et al. (2009) 42 Tsalmantza et al. (2009) 43 Trippe et al. (2010) 44 Gavazzi et al. (2013) 45 Barth et al. (2015) 46 Dopita et al. (2015) 47 Schmidt et al. (2016) 48 Ramos Almeida et al. (2016) 49 Thomas et al.(2017) 50 Greenawalt et al. (1997) M N R A S , ( ) M. R. Senarath et al.
To measure the completeness of our colour and flux selection criteriawe inspected AGNs that have 2 or more archival spectra. Our mainsources of spectra are WiFeS Siding Spring Southern Seyfert Spec-troscopic Snapshot Survey (S7; Dopita et al. 2015), SDSS, 6dFGSand Ho et al. (1995). The date ranges for these spectra sources areas follows: S7 spectra were observed from 2013-2016, SDSS spec-tra were observed between 2000-2019 (where DR16 spectra wereobserved through 2019), 6dFGS spectra were observed from 2001-2009 and Ho et al. (1995) spectra were observed between 1984-1990. Of all the 𝑧 <
Once candidates were identified using the colour and flux crite-ria discussed in Sections 2.2, 2.3 and 2.4, we inspected the archivalspectra of these objects in order to identify potential CLAGNs with-out obtaining new spectra. It should be noted that we did not followup candidates where archival spectra from the past two decadesshowed no variability (irrespective of the selection criteria).We used the Wide Field Spectrograph (WiFeS, Dopita et al.2010) Integral Field Unit (IFU) on the Australian National Univer-sity’s 2.3 m telescope at Siding Spring to obtain new spectra of ourcandidates to confirm that they are indeed CLAGN. WiFeS has afield of view of 25 ′′ × ′′ , divided into 950 spaxels. The advantageof using an IFU for follow-up observations of candidates is that ex-traction aperture size can be matched to previous observations (e.g.the 7 ′′ fibre of 6dF, Jones et al. 2009). The wavelength coverageis 3800 − 𝛼 , H 𝛽 and OIII lines at z < 𝛼 to show the most clear signs of change inCLAGNs.We observed in nod-and-shuffle mode taking at least threeframes with 60s on object, 60s on sky and 10 cycles per frame. Thisresults in 40 min on object, 40 min on sky, and ∼
15 min on overheadsincluding telescope nod time, guide star re-acquisition and CCDreadout. In total we allow ∼
100 mins per galaxy. Our observationswere taken between July 2018 and 2019 March. We reduced our datawith PyWiFeS, Python-based pipeline (Childress et al. 2014). Weobserved 15 CLAGN candidates over multiple observing sessions,the AGNs are presented in Table 4.
We next present the spectra of the new CLAGNs that we have iden-tified using the selection criteria mentioned in Sections 2.2, 2.3 and2.4, where we compare archival spectra of the galaxies with spectra taken using WiFeS. We plot archival spectra, where available, withthe spectra taken using WiFeS to display the change. Multiplica-tive scaling has been applied so that the continuum spectra agree,highlighting changes in the emission lines. We match apertures ofthe multiple spectra of the new CLAGNs and AGNs with varyingspectra below (where possible). It should be noted however, that thefeatures in our observed WiFeS spectra displayed their respectivebroad line and narrow line features irrespective of the extractionaperture used. 𝑟 - 𝑖 < Mrk 609 is classified by Véron-Cetty & Véron (2010) as a type 1.8using the spectra from Rudy et al. (1988), but has 𝑟 - 𝑖 < 𝛼 components. As the Rudy et al. (1988) spectral classification isinconclusive, we use the SDSS and 6dFGS spectra in Figure 7 as thebaseline for determining whether Mrk 609 is changing spectral type.We classify the SDSS and 6dFGS spectra as type 1.9 and type 2,respectively, in accordance with the criteria outlined by Osterbrock(1981). Our WiFeS spectra is consistent with that of a type 1.9 andis similar to that of SDSS indicating that Mrk 609 changed spectraltype between 2001 and 2018 to a type 2, and has returned to beinga type 1.9. This small variation in spectra is not consistent with ourCLAGN criterion, but additional spectroscopy may reveal furtherchanges in the spectral class of Mrk 609. Mrk 915 was classified as a type 1.8 by Véron-Cetty & Véron(2010) using the Dahari & De Robertis (1988) spectrum, but hasPan-STARRS 𝑟 - 𝑖 < 𝛼 line between 1993 and 1994. While the 1993 spectrum in Figure 8 isof relatively poor quality, it does not show the broadline componentof subsequent spectra, and we conclude Mrk 915 as a type 2 Seyfert MNRAS , 1–15 (2020)
Systematic Survey for z < Table 4.
CLAGN candidates that were observed with WiFeS between 2018 July and 2019 March. 𝑎 NGC 1346 0.0135 1.8 2 58457J05521140-0727222 NGC 2110 0.0078 1 1 58491J08044636+1046363 UGC 04211 0.0344 1 1 58548J08353877-0405172 𝑏 NGC 2617 0.0142 1.8 1 58491J10445172+0635488 NGC 3362 0.0277 2 2 58491J13254405-2950012 NGC 5135 0.0137 2 2 58549J13311382-2524096 ESO 509- G 038 0.0260 1 1 58548J13352457+0124376 NGC 5227 0.0175 2 2 58548J15461637+0224558 NGC 5990 0.0128 2 2 58549J20075129-1108346 𝑎 𝑎 New CLAGN that we have identified 𝑏 Known CLAGN at the time. We classify the
BAT AGN Spectroscopic Survey (BASS:Koss et al. 2017) spectrum as type 1.9; the 2019 WiFeS spectrumin Figure 8 is also consistent with a type 1.9. The H 𝛼 line begins tobroaden in the 2008 and 2010 spectrum (note: both spectra are fromthe same survey) and is broader still in the WiFeS 2019 spectrum.This variation in spectra from type 2 to type 1.9 does not meet ourcriteria for CLAGN. Although this may be the case, it is a goodcandidate CLAGN and further observations are needed. NGC 1346 is a newly discovered CLAGN. We identified NGC 1346as a broad-line AGN with unusually red colours with the SDSS andPan-STARRS photometry and we designated it as a CLAGN viavisual inspection of spectra from SDSS, 6dFGS and S7. NGC 1346was classified as a Seyfert 1 galaxy by Véron-Cetty & Véron (2003)using the SDSS spectra in Figure 9. We classify this spectrum as atype 1.8 according to the definitions outlined by Osterbrock (1981).The spectrum from SDSS (taken in 2001) showed a significant broadline component, however the 2004 December 6dFGS (Jones et al.2009) spectrum contains only narrow emission lines. The S7 spec-trum of NGC 1346 and the WiFeS 2018 spectrum showed onlynarrow lines. Therefore, NGC 1346 was a type 2 prior to 2004 andit changed spectral type between 2001 and 2004. We use infraredphotometry to investigate why this AGN is changing spectral type.To determine if a varying hot dust component of NGC 1346could be responsible for the change in spectral type we use NEO-WISE photometry and compared 2MASS photometry with recenttargeted InfraRed Survey Facility (IRSF; Nagayama 2012) photom-etry. The NEOWISE photometry was taken between 2014 and 2018with a cadence of 6 months, and was thus taken after the change inspectral type. We measure a change in NEOWISE photometry of0.11 mag; this is not a significant change and is not considered highenough to suggest a change in spectral type. This is because theNEOWISE survey has data from 2014 onward, and as suggested bythe spectra, NGC 1346 had already changed spectral types by then.We find that NGC 1346 has faded by 0.82 mag in the 𝐾 𝑠 -band between 1998 and 2018, where we measure photometry from2MASS and IRSF respectively. IR wavelength is sensitive to emis-sion from warm dust attributed to the torus, as we measure a changein the IR photometry, this indicates that the change in spectral typewe measure is a result of changes in the torus and not due to a simpleobscuring cloud crossing the line of sight. We have conducted a systematic survey for CLAGNs by identifyingcandidates using optical and infrared photometry from SkyMapper,Pan-STARRS, SDSS and NEOWISE. Using SkyMapper, we selecttype 1s with 𝑟 − 𝑖 > 𝑟 - 𝑖 < 𝑟 − 𝑖 > 𝑟 - 𝑖 < 𝑟 -band flux where type 1.8s, 1.9s and type 2s had Δ m 𝑟 > WISE
W1(3.4 𝜇 m) (type 1.8s,1.9s and type 2s where Δ W1 > 𝜇 m) photometry we find majorityof type 1s and type 2s have exhibited > < > WISE
W1 variability didn’t prove useful for identifying changing-look Seyferts, but it could work with cleaner input catalogues andit has been used to identify changing-look quasars (e.g., Guo et al.2016; Ross et al. 2018; Stern et al. 2018).Using our optical colour selection method we were able to iden-tify four AGNs with varying spectra. 2MASX J20075129-110834and NGC 1346 are new CLAGNs that were identified in this workusing optical colour selection. Mrk 915 and Mrk 609 have varyingspectra which do not meet our criteria for CLAGN and only havea small change from type 2 to type 1.9 and type 1.9 to type 2, re-spectively. These AGNs remain CLAGN candidates and additionalfollowup spectroscopy may reveal further changes in their spectraltypes. 46% of candidates selected using this method either didn’thave archival spectra at all, didn’t have archival spectra from the last10 years or only had one archival spectrum. Extrapolating this, wecan estimate that we have only identified 54% of possible CLAGNsin this sample due to lack of spectra. The optical colour selectionmethod also only identifies ≈
50 % of known CLAGN.We note that as we refined our CLAGN candidates by select-ing candidates where the archival spectra already showed signs ofchange, we may have missed CLAGNs that may have changed after
MNRAS , 1–15 (2020) M. R. Senarath et al. N o r m a li z e d F l u x + o ff s e t WiFeS (2018) Sy1.8 MJD: 583086dFGS (2003) Sy2 MJD: 52902 λ rest (˚ A ) − N o r m a li z e d F l u x + o ff s e t WiFeS (7”)/6dFGS (6.7”) λ rest (˚ A ) Figure 6. the last archival spectra was taken and/or changed type relativelybriefly (i.e. < = ∼ ≈
18 CLAGN as z < ACKNOWLEDGEMENTS
The authors would like to thank the anonymous referee for theirhelpful comments which improved the manuscript overall.The authors wish to thank the staff at ANU 2.3m telescopeand the WiFeS instrument for their technical support. We wouldalso like to thank the ANU telescope time allocation committee forsupporting this project and the observations in this paper.Michelle E. Cluver is a recipient of an Australian ResearchCouncil Future Fellowship (project number FT170100273) fundedby the Australian Government.John R. Lucey was supported by the Science and TechnologyFacilities Council through the Durham Astronomy ConsolidatedGrants ST/P000541/1 and ST/T000244/1.The national facility capability for SkyMapper has been funded
MNRAS , 1–15 (2020)
Systematic Survey for z < N o r m a li z e d F l u x + o ff s e t WiFeS (2018) Sy1.9 MJD: 583756dFGS (2004) Sy2 MJD: 53347SDSS (2001) Sy1.9 MJD: 51924 λ rest (˚ A ) N o r m a li z e d F l u x + o ff s e t WiFeS (7”)/6dFGS (6.7”) λ rest (˚ A ) Figure 7.
Mrk 609 has varying spectra and was identified using optical colour selection. The SDSS spectrum is consistent with a type 1.9 where the 6dFGSspectrum is completely narrow indicating it is a type 2. We classify Mrk 609 as type 1.9 using our WiFeS spectrum, and thus the changes in spectral class areinsufficient to meet our CLAGN criterion. through ARC LIEF grant LE130100104 from the Australian Re-search Council, awarded to the University of Sydney, the AustralianNational University, Swinburne University of Technology, the Uni-versity of Queensland, the University of Western Australia, the Uni-versity of Melbourne, Curtin University of Technology, MonashUniversity and the Australian Astronomical Observatory. SkyMap-per is owned and operated by The Australian National University’sResearch School of Astronomy and Astrophysics. The survey datawere processed and provided by the SkyMapper Team at ANU. TheSkyMapper node of the All-Sky Virtual Observatory (ASVO) ishosted at the National Computational Infrastructure (NCI). Devel-opment and support the SkyMapper node of the ASVO has beenfunded in part by Astronomy Australia Limited (AAL) and the Aus-tralian Government through the Commonwealth’s Education Invest-ment Fund (EIF) and National Collaborative Research InfrastructureStrategy (NCRIS), particularly the National eResearch Collabora-tion Tools and Resources (NeCTAR) and the Australian NationalData Service Projects (ANDS).The Pan-STARRS1 Surveys (PS1) and the PS1 public science archive have been made possible through contributions by the In-stitute for Astronomy, the University of Hawaii, the Pan-STARRSProject Office, the Max-Planck Society and its participating in-stitutes, the Max Planck Institute for Astronomy, Heidelberg andthe Max Planck Institute for Extraterrestrial Physics, Garching, TheJohns Hopkins University, Durham University, the University of Ed-inburgh, the Queen’s University Belfast, the Harvard-SmithsonianCenter for Astrophysics, the Las Cumbres Observatory Global Tele-scope Network Incorporated, the National Central University ofTaiwan, the Space Telescope Science Institute, the National Aero-nautics and Space Administration under Grant No. NNX08AR22Gissued through the Planetary Science Division of the NASA ScienceMission Directorate, the National Science Foundation Grant No.AST-1238877, the University of Maryland, Eotvos Lorand Univer-sity (ELTE), the Los Alamos National Laboratory, and the Gordonand Betty Moore Foundation.SDSS-IV is managed by the Astrophysical Research Consor-tium for the Participating Institutions of the SDSS Collaborationincluding the Brazilian Participation Group, the Carnegie Institu-
MNRAS , 1–15 (2020) M. R. Senarath et al. N o r m a li z e d F l u x + o ff s e t WiFeS (2019) Sy1.9 MJD: 58724Trippe et al (2010) Sy1.9 MJD:54646BASS (2008) Sy 1.9 MJD:54661Goodrich (1993) Sy2 MJD:49270 λ rest (˚ A ) N o r m a li z e d F l u x + o ff s e t WiFeS/BASS λ rest (˚ A ) Figure 8.
Mrk 915 is a varying AGN that was identified using optical colour selection. We classify the 1993 (Goodrich 1995) spectrum as a type 2 as it containsonly narrow lines and the BASS 2008 spectrum is consistent with a type 1.9. The WiFeS 2019 spectra is that of an type 1.9 . However, this change from type 2to type 1.9 is not significant enough to meet out CLAGN criteria. Although this is the case, it is a good CLAGN candidate that will require further investigation.
MNRAS , 1–15 (2020)
Systematic Survey for z < N o r m a li z e d F l u x + o ff s e t WiFeS (2018) Sy 2 MJD: 58457WiFeS (S7) (2013) Sy 2 MJD: 565986dF (2004) Sy 2 MJD: 53347SDSS (2001) Sy 1.8 MJD: 51910 λ rest (˚ A ) . . . . N o r m a li z e d F l u x + o ff s e t SDSS/Wifes (2018) λ rest (˚ A ) Figure 9.
Spectra of CLAGN NGC 1346, with the SDSS spectrum revealing broad H 𝛼 consistent with a type 1.8, while subsequent archival 6dFGS and WiFeSspectra, and our new WiFeS spectra, indicate a Seyfert 2 (Senarath et al. 2019). DATA AVAILABILITY
The data underlying this article are available in the article and in itsonline supplementary material.
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