J/ApJ/883/94 Optical and UV follow-up of 1ES 1927+654 (Trakhtenbrot+, 2019)
1ES 1927+654: an AGN caught changing look on a timescale of months.
Trakhtenbrot B., Arcavi I., MacLeod C.L., Ricci C., Kara E., Graham M.L.,
Stern D., Harrison F.A., Burke J., Hiramatsu D., Hosseinzadeh G.,
Howell D.A., Smartt S.J., Rest A., Prieto J.L., Shappee B.J.,
Holoien T.W.-S., Bersier D., Filippenko A.V., Brink T.G., Zheng W., Li R.,
Remillard R.A., Loewenstein M.
<Astrophys. J., 883, 94 (2019)>
=2019ApJ...883...94T 2019ApJ...883...94T
ADC_Keywords: Active gal. nuclei; Spectra, optical; Photometry, ultraviolet;
X-ray sources
Keywords: galaxies: active ; galaxies: individual (1ES 1927+654) ;
galaxies: nuclei ; quasars: emission lines ; quasars: general
Abstract:
We study the sudden optical and ultraviolet (UV) brightening of
1ES1927+654, which until now was known as a narrow-line active
galactic nucleus (AGN). 1ES 1927+654 was part of the small and
peculiar class of "true Type-2" AGNs that lack broad emission lines
and line-of-sight obscuration. Our high-cadence spectroscopic
monitoring captures the appearance of a blue, featureless continuum,
followed several weeks later by the appearance of broad Balmer
emission lines. This timescale is generally consistent with the
expected light travel time between the central engine and the
broadline emission region in (persistent) broadline AGN. Hubble Space
Telescope spectroscopy reveals no evidence for broad UV emission lines
(e.g., CIVλ1549, CIII]λ1909, MgIIλ2798), probably
owing to dust in the broadline emission region. To the best of our
knowledge, this is the first case where the lag between the change in
continuum and in broadline emission of a "changing look" AGN has been
temporally resolved. The nature and timescales of the photometric and
spectral evolution disfavor both a change in line-of-sight obscuration
and a change of the overall rate of gas inflow as driving the drastic
spectral transformations seen in this AGN. Although the peak
luminosity and timescales are consistent with those of tidal
disruption events seen in inactive galaxies, the spectral properties
are not. The X-ray emission displays a markedly different behavior,
with frequent flares on timescales of hours to days, and will be
presented in a companion publication.
Description:
An increase in optical flux from 1ES 1927+654 was discovered on 2018
March 3. The transient event was reported as ASASSN-18el and given the
name AT 2018zf by the transient name server.
Pre-discovery detections from 2017 December 23 were recovered by the
Asteroid Terrestrial-impact Last Alert System (ATLAS) and designated
as ATLAS18mgv. In what follows, we adopt this earlier ATLAS
pre-discovery measurement as the detection date.
Following the sudden optical flux increase, we initiated a near-UV
(NUV) and X-ray monitoring campaign using the Neil Gehrels Swift
Observatory (PI: I. Arcavi).
The host flux in the NUV UVW1 and UVM2 bands was determined from
XMM-Newton/Optical Monitor (OM) data obtained in 2011 May 20
(Gallo+ 2013MNRAS.433..421G 2013MNRAS.433..421G AB magnitudes of 18.14 and 18.65,
respectively, using the same aperture), and subtracted from our new
UVOT measurements.
The X-ray analysis is discussed in detail in a companion paper
(C. Ricci+ 2021, arXiv:2102.05666).
We obtained optical spectra of 1ES 1927+654 with the Ohio State
Multi-Object Spectrograph (OSMOS) mounted on the Hiltner 2.4m
Telescope at the MDM Observatory; the Spectrograph for the Rapid
Acquisition of Transients (SPRAT) instrument mounted on the 2m
Liverpool Telescope at the Roque de los Muchachos Observatory; the
Kast spectrograph mounted on the 3m Shane Telescope at the Lick
Observatory; the FLOYDS spectrograph mounted on the 2m Faulkes
Telescope North at Haleakala, Hawaii (part of the Las Cumbres
Observatory network); the Dual Imaging Spectrograph mounted on the
Apache Point Observatory 3.5m telescope; the Low Resolution Imaging
Spectrograph (LRIS) mounted on the Keck I telescope at Maunakea; and
the Double Beam Spectrograph (DBSP) mounted on the Palomar Hale 5m
telescope.
We also obtained far-UV (FUV) and NUV spectra of 1ES 1927+654 using
the Cosmic Origins Spectrograph (COS) and Space Telescope Imaging
Spectrograph (STIS) instruments, respectively, on board the Hubble
Space Telescope (HST; Program ID 15604, PI: C.L. MacLeod).
A log of all our spectral observations of 1ES 1927+654 is detailed in
Table 1.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
19 27 19.54 +65 33 54.3 1ES 1927+654 = 1ES 1927+65.4
----------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 61 28 Log of spectroscopic observations
fig1.dat 56 714 Long-term optical light curve of 1ES1927+654
fig2.dat 104 2909 Light curve and equivalent-width evolution of
1ES 1927+654 during the flare
fig3/* . 26 Optical spectra of 1ES 1927+654 in ASCII format
for 26 observation dates
--------------------------------------------------------------------------------
See also:
J/ApJ/666/757 : CIV 1549Å emission line in AGNs (Sulentic+, 2007)
J/MNRAS/383/581 : Broad-line Balmer decrements in blue AGN (Dong+, 2008)
J/ApJS/194/45 : QSO properties from SDSS-DR7 (Shen+, 2011)
J/MNRAS/423/600 : Type-1 low-z AGN emission properties (Stern+, 2012)
J/MNRAS/426/2703 : Type 1 AGN at low z. II. (Stern+, 2012)
J/ApJ/793/38 : Palomar Transient Factory photometric obs. (Arcavi+, 2014)
J/ApJ/788/48 : X-ray through NIR photometry of NGC 2617 (Shappee+, 2014)
J/ApJS/219/1 : Catalog of Type-1 AGNs from SDSS-DR7 (Oh+, 2015)
J/ApJ/850/74 : Swift/BAT AGN Spectroscopic Survey. I. (Koss+, 2017)
J/ApJ/852/72 : LFs of tidal disruption flares (van Velzen, 2018)
J/ApJ/874/8 : Follow-up spec. of SDSS changing-look QSOs (Macleod+, 2019)
Byte-by-byte Description of file: table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 A10 "Y/M/D" Date Date of observation (UT) at exposure start
12 A1 --- f_Date [*c] Flag on Date (1)
14- 20 F7.1 d MJD [58183.5/58622.6] Modified Julian Date
at exposure start
22- 24 I3 d delT [72/503] Time since ATLAS first detection of
the optical flare (MJD=58110.2),
in rounded rest-frame days
26- 47 A22 --- Tel Telescope
49- 54 A6 --- Inst Instrument
56 A1 --- u_Exp [~] Uncertainty flag on Exp
57- 60 I4 s Exp [300/3600] Exposure time
61 A1 --- f_Exp [d] Flag on Exp (1)
--------------------------------------------------------------------------------
Note (1): Flag as follows:
* = spectra taken under different weather and/or instrument conditions
compared to a standard star, thus affecting the validity of their
continuum shape.
c = A low-resolution spectrum, not shown in Figure 3.
d = The Shane/Kast exposure times were 3x500s for the red arm, and
1x1560s for the blue.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: fig1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.3 d MJD [56734.5/58630.6] Modified Julian Date
of Observation (JD-2400000.5)
11- 19 F9.3 d Phase [-1349.42/510.5] Rest-frame days since
2017 December 23 (MJD=58110.2)
21 A1 --- l_mag [l] 3-sigma non-detection limit flag on mag
23- 28 F6.3 mag mag [14.8/19.5] Apparent magnitude in Filt
30- 34 F5.3 mag e_mag [0.014/0.4]?=0 Uncertainty in mag (G1)
36- 42 A7 --- Tel Telescope (ASAS-SN or ATLAS)
44- 44 A1 --- Filt Filter (g, V or c, o)
46- 56 A11 --- q_mag Quality for mag: "bad weather"
--------------------------------------------------------------------------------
Byte-by-byte Description of file: fig2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.3 d MJD [56734.5/58630.6] Modified Julian Date
of Observation (JD-2400000.5)
11- 19 F9.3 d Phase [-1349.42/510.5] Phase in days since
2017 December 23 (MJD=58110.2)
21- 27 F7.3 0.1nm EW [0.014/205]? Equivalent width in Angstrom units
29- 34 F6.3 0.1nm e_EW [0.014/27.2]? Lower uncertainty on EW
36- 41 F6.3 0.1nm E_EW [0.2/13.4]? Upper uncertainty on EW
43- 50 A8 --- Line Hα or Hβ
52- 59 F8.3 --- Fx [0.3/1298]? 0.5-10keV X-ray flux
61- 66 F6.3 --- e_Fx [0/24]? Lower uncertainty on Fx
68- 73 F6.3 --- E_Fx [0.1/16.5]? Upper uncertainty on Fx
75- 84 A10 --- Tel Telescope (ASAS-SN, ATLAS, Swift-UVOT,
Swift-XRT, NICER or XMM)
86 A1 --- l_mag [l] 3-sigma non-detection limit flag on mag
88- 93 F6.3 mag mag [13.89/19.5]? Apparent magnitude in Filt
95- 99 F5.3 mag e_mag [0.014/0.4]? Uncertainty on mag (G1)
101-104 A4 --- Filt Filter (g, V; c, o or U, UVM2, UVW1, UVW2)
--------------------------------------------------------------------------------
Byte-by-byte Description of file: fig3/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 F8.2 0.1nm lambda [3198.2/10706] Observed wavelength
10- 20 E11.4 cW/m2/nm Flux [1.5e-17/7e-14] Flux density,
in erg/cm2/s/Å units
--------------------------------------------------------------------------------
Global notes:
Note (G1): When the uncertainty is null the observation is a non-detection.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 04-Mar-2021