J/ApJ/874/8 Follow-up spectroscopy of SDSS changing-look QSOs (Macleod+, 2019)
Changing-look quasar candidates: first results from follow-up spectroscopy of
highly optically variable quasars.
MacLeod C.L., Green P.J., Anderson S.F., Bruce A., Eracleous M., Graham M.,
Homan D., Lawrence A., Lebleu A., Ross N.P., Ruan J.J., Runnoe J.,
Stern D., Burgett W., Chambers K.C., Kaiser N., Magnier E., Metcalfe N.
<Astrophys. J., 874, 8-8 (2019)>
=2019ApJ...874....8M 2019ApJ...874....8M (SIMBAD/NED BibCode)
ADC_Keywords: QSOs; Spectra, optical; Redshifts; Photometry, SDSS
Keywords: accretion, accretion disks; catalogs; quasars: emission lines
Abstract:
Active galactic nuclei (AGNs) that show strong rest-frame optical/UV
variability in their blue continuum and broad line emission are
classified as changing-look AGN, or at higher luminosities,
changing-look quasars (CLQs). These surprisingly large and sometimes
rapid transitions challenge accepted models of quasar physics and duty
cycles, offer several new avenues for study of quasar host galaxies,
and open a wider interpretation of the cause of differences between
broad and narrow-line AGN. To better characterize extreme quasar
variability, we present follow-up spectroscopy as part of a
comprehensive search for CLQs across the full Sloan Digital Sky Survey
(SDSS) footprint using spectroscopically confirmed quasars from the
SDSS DR7 catalog. Our primary selection requires large-amplitude
(|Δg|>1mag, |Δr|>0.5mag) variability over any of the
available time baselines probed by the SDSS and Pan-STARRS 1 surveys.
We employ photometry from the Catalina Sky Survey to verify
variability behavior in CLQ candidates where available, and confirm
CLQs using optical spectroscopy from the William Herschel, MMT,
Magellan, and Palomar telescopes. For our adopted signal-to-noise
ratio threshold on variability of broad Hβ emission, we find 17
new CLQs, yielding a confirmation rate of ≳20%. These candidates are
at lower Eddington ratio relative to the overall quasar population,
which supports a disk-wind model for the broad line region. Based on
our sample, the CLQ fraction increases from 10% to roughly half as the
continuum flux ratio between repeat spectra at 3420Å increases from
1.5 to 6. We release a catalog of more than 200 highly variable
candidates to facilitate future CLQ searches.
Description:
We use imaging data from the SDSS and Pan-STARRS 1 (PS1) to select
strongly variable quasars for spectroscopic follow-up.
See Section 2 and 3.1.1.
Relatively bright (g<20.5) and highly variable (|Δg|>1.3mag)
changing-look quasar (CLQ) candidates were observed on the nights of
2016 February 6-8 and May 30, 31 using the 4.2m William Herschel
Telescope (WHT) in La Palma. Observations were performed using the
Intermediate dispersion Spectrograph and Imaging System (ISIS). The
setup gives a spectral resolution of R∼1500 at 5200Å in the blue
and R∼1000 at 7200Å in the red for a slit width of 1.0" and nominal
total coverage of ∼3100 to 10600Å (but effectively further limited
by the atmosphere).
Observations of a fainter set of targets were made with the Blue
Channel Spectrograph on the 6.5m MMT situated on Mount Hopkins,
Arizona. Observations were carried out over several dates in 2016 to
2018 (see Table 2). The central wavelength setting ranged from 5835 to
6335Å.
To observe most of our targets in the South Galactic Cap (SGC), and
most of the relatively high-redshift targets, we used the Magellan
Clay 6.5m telescope with the Low Dispersion Survey Spectrograph 3
(LDSS3)-C spectrograph. Observations were carried out over the nights
2016 July 26-29 (range 4250-10000Å).
We also obtained second-epoch spectra of two quasars with Double
Spectrograph (DBSP) on the Hale 200" Telescope at Palomar Observatory
on UT 2017 May 30, and second-epoch spectra of an additional six
quasars using the same instrument on UT 2017 June 26.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 111 262 Changing-look quasar (CLQ) candidates
table3.dat 65 108 Spectral decomposition
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See also:
II/294 : The SDSS Photometric Catalog, Release 7 (Adelman-McCarthy+, 2009)
VII/260 : The SDSS-DR7 quasar catalog (Schneider+, 2010)
II/349 : The Pan-STARRS release 1 (PS1) Survey - DR1 (Chambers+, 2016)
IX/55 : XMM-Newton Serendipitous Source Catalogue 3XMM-DR8 (XMM-SSC, 2018)
VII/286 : SDSS quasar catalog, fourteenth data release (Paris+, 2018)
J/AJ/126/1720 : ugriz and Hα of double-peaked AGN (Strateva+, 2003)
J/AJ/126/2209 : X-ray AGN from RASS and SDSS (Anderson+, 2003)
J/ApJ/613/682 : AGN central masses & broad-line region sizes (Peterson+, 2004)
J/ApJ/663/81 : SED of hard X-ray selected AGN in XMDS (Polletta+, 2007)
J/ApJ/696/870 : Catalina Real-time Transient Survey (CRTS) (Drake+, 2009)
J/ApJS/194/45 : QSO properties from SDSS-DR7 (Shen+, 2011)
J/ApJ/788/48 : X-ray through NIR photometry of NGC 2617 (Shappee+, 2014)
J/ApJ/818/30 : Lag measurements for 15 z<0.8 QSOs from SDSS-RM (Shen+, 2016)
J/ApJ/821/33 : Broad Hβ emission line in 102 Seyfert gal. (Runco+, 2016)
J/ApJ/821/56 : Space telescope RM project. III. NGC5548 LCs (Fausnaugh+, 2016)
J/ApJ/854/160 : SDSS & DES long-term extreme variability QSOs (Rumbaugh+, 2018)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS identifier (HHMMSS.ss+DDMMSS.s; J2000)
20- 24 F5.3 --- z [0.15/0.82] Spectroscopic redshift
26 I1 --- Ex? [0/1] Morphology flag (1=extended or
0=point source)
28- 32 I5 d MJD1p [51075/56031] Modified Julian Date of first
photometric observation
34- 39 F6.3 mag gmag1 [16.2/22] First SDSS g band (AB) magnitude
41- 45 F5.3 mag e_gmag1 [0.005/0.1] Uncertainty in g1mag
47- 51 I5 d MJD2p [53655/56588] Modified Julian Date of second
photometric observation
53- 58 F6.3 mag gmag2 [15.9/22.8] Second SDSS g band (AB) magnitude
60- 64 F5.3 mag e_gmag2 [0.002/0.15] Uncertainty in g2mag
66- 70 I5 d MJD1s [51630/55500] Modified Julian Date of first
spectroscopic observation
72- 76 I5 d MJDPS1 [55242/56625] Modified Julian Date of
PS1 observation
78- 83 F6.3 mag gmagPS1 [15.9/22.7] Pan-STARRS 1 g band (AB) magnitude
85- 89 F5.3 mag e_gmagPS1 [0.005/0.2] Uncertainty in gPS1mag
91- 95 I5 d MJD2s [0/58367] Modified Julian Date of second
spectroscopic observation (1)
97-104 A8 --- Tel Facility identifier
106 I1 --- CLQ? [0/1] Changing-look quasar by VI?
108-111 F4.1 --- Nsigma [-1/11.9]?=-1 Flux deviation at Hβ
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Note (1): A zero indicates the repeat spectrum was not analyzed
(due to either data source or S/N) or not available.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 18 A18 --- SDSS SDSS identifier (HHMMSS.ss+DDMMSS.s; J2000)
20- 25 F6.1 10+35W L3240-1 [27/1595] Luminosity at 3240Å
in first SDSS spectrum
27- 30 F4.1 10+35W e_L3240-1 [0.5/47] Uncertainty in L3240-1
32 A1 --- l_LHb-1 Limit fl on l_LHb-1
33- 37 F5.2 10+35W LHb-1 [0.14/20] Luminosity at Hβ
in first SDSS spectrum
39- 42 F4.2 10+35W e_LHb-1 [0.03/2] Uncertainty in LHb-1
44- 48 F5.1 10+35W L3240-2 [0/944] Luminosity at 3240Å
in follow-up spectrum
50- 53 F4.1 10+35W e_L3240-2 [0/38] Uncertainty in L3240-2
55 A1 --- l_LHb-2 Limit flag on LHb-2
56- 60 F5.2 10+35W LHb-2 [0.05/15] Luminosity at Hβ
in follow-up spectrum
62- 65 F4.2 10+35W e_LHb-2 [0/4] Uncertainty in LHb-2
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 26-Aug-2020