J/ApJ/951/134 X-ray/UV & optical evolution of ASASSN -14ko (Payne+, 2023)
Chandra, HST/STIS, NICER, Swift, and TESS detail the flare evolution of the
repeating nuclear transient ASASSN -14ko.
Payne A.V., Auchettl K., Shappee B.J., Kochanek C.S., Boyd P.T.,
Holoien T.W.-S., Fausnaugh M.M., Ashall C., Hinkle J.T., Vallely P.J.,
Stanek K.Z., Thompson T.A.
<Astrophys. J., 951, 134 (2023)>
=2023ApJ...951..134P 2023ApJ...951..134P
ADC_Keywords: Transient; Spectra, ultraviolet; Photometry; Optical;
X-ray sources
Keywords: Black holes ; Active galactic nuclei ; High energy astrophysics
Abstract:
ASASSN-14ko is a nuclear transient at the center of the AGN
ESO253-G003 that undergoes periodic flares. Optical flares were first
observed in 2014 by the All-Sky Automated Survey for Supernovae
(ASAS-SN) and their peak times are well-modeled with a period of
115.2-1.2+1.3 days and period derivative of -0.0026±0.0006. Here
we present ASAS-SN, Chandra, HST/STIS, NICER, Swift, and TESS data for
the flares that occurred on 2020 December, 2021 April, 2021 July, and
2021 November. These four flares represent flares 18-21 of the total
number of flares observed by ASAS-SN so far since 2014. The HST/STIS
UV spectra evolve from blueshifted broad absorption features to
redshifted broad emission features over ∼10days. The Swift UV/optical
light curves peaked as predicted by the timing model, but the peak UV
luminosities that varied between flares and the UV flux in Flare 20
were roughly half the brightness of the other peaks. The X-ray
luminosities consistently decreased and the spectra became harder
during the UV/optical rise, but apparently without changes in
absorption. Finally, two high-cadence TESS light curves from Flare 18
and Flare 12 showed that the slopes during the rising and declining
phases changed over time, which indicates some stochasticity in the
flare's driving mechanism. Although ASASSN-14ko remains
observationally consistent with a repeating partial tidal disruption
event, these rich multi-wavelength data are in need of a detailed
theoretical model.
Description:
ASAS-SN is an ongoing all-sky survey that uses four 14cm telescopes on
a common mount to monitor the sky to find bright and nearby
transients. There are currently five units, which are located in
Hawaii, Chile, Texas, and South Africa, and are hosted by the Las
Cumbres Observatory global telescope (LCOGT) network.
We observed ASASSN-14ko using STIS and the FUV/NUV MAMA detectors (HST
IDs: 16451, 16498; PI: Shappee). We used the 52.0"x0.2" slit and the
G140L (1425Å, FUV-MAMA) and G230L (2376Å, NUV-MAMA) gratings.
FUV and NUV spectra were obtained on 2020 December 18 (59201.6MJD),
2020 Dec 22 (59205.6MJD), 2020 Dec 29 (59212.3MJD), 2021 March 6
(59279.1MJD), and FUV-only spectra were obtained on 2021 March 29
(59302.6MJD) and 2021 April 1 (59305.3MJD).
All of the HST/STIS data that are used in this paper can be found in
MAST: DOI: 10.17909/hk87-6g84
See Section 2.2.
We requested Swift ToO observations (ToO ID: 14775, 14971, 15393,
15636, 16037, 16546; PI: Payne) using the UltraViolet/Optical
Telescope (UVOT) and the X-Ray Telescope (XRT) to coincide with the
predicted Flares 18-21. The UVOT data were obtained in six filters: V
(5425.3Å), B (4349.6Å), U (3467.1Å), UVW1 (2580.8Å), UVM2
(2246.4Å), and UVW2 (2054.6Å).
See Section 2.3.
The Transiting Exoplanet Survey Satellite (TESS) observed ASASSN-14ko
during Sectors 31-33, which occurred between 2020-10-21 and 2021-01-13,
with the flare occurring during Sectors 32-33. See Section 2.4.
ASASSN-14ko was also observed using the Neutron star Interior
Composition ExploreR (NICER) and its X-ray timing instrument (XTI).
ASASSN-14ko was observed for a total of 115 times between 2020-Dec-10
and 2021-Dec-01 (ObsIDs: 3201740101-3201740118, 4662010101-4662022801,
PI: Payne), for a total cumulative exposure of 277.7ks.
See Section 2.5.
Finally, we acquired two Chandra ACIS-S DDT observations (observation
IDs: 24875, 24876; PI: Payne). The first observation occurred on
2020-Dec-26 with an exposure time of 27.69ks, and the second
observation occurred on 2021-Jan-21 with an exposure time of 28.68ks.
See Section 2.6.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
05 25 18.13 -46 00 20.3 ASASSN-14ko = ASASSN -14ko
----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 52 15058 Photometry of ASASSN-14ko used in this analysis
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See also:
B/chandra : The Chandra Archive Log (CXC, 1999-2014)
J/ApJ/613/682 : AGN central masses & broad-line region sizes (Peterson+, 2004)
J/ApJ/687/1201 : SN and LGRB locations in their host galaxies (Kelly+, 2008)
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/A+A/594/A116 : HI4PI spectra and column density maps (HI4PI team+, 2016)
J/MNRAS/455/2918 : Phot. & line luminosities for ASASSN-14li (Holoien+, 2016)
J/MNRAS/463/3813 : ASASSN-15oi UBVI M2W1W2 light curves (Holoien+, 2016)
J/ApJ/873/92 : Opt. & UV evolution of the TDE iPTF15af (Blagorodnova+, 2019)
J/ApJ/883/111 : UV-Opt LCs of ASASSN-19bt detected by TESS (Holoien+, 2019)
J/ApJ/883/94 : Optical and UV follow-up of 1ES 1927+654 (Trakhtenbrot+, 2019)
J/ApJ/898/161 : 500days of ASASSN-18pg multiwavelength obs. (Holoien+, 2020)
J/ApJ/910/83 : Swift phot. recomputed for 38 transients (Hinkle+, 2021)
J/ApJ/910/125 : Follow-up photometry of ASASSN-14ko (Payne+, 2021)
J/ApJS/255/7 : X-ray monitoring of 1ES 1927+654 (Ricci+, 2021)
J/ApJ/930/12 : Phot. & spectroscopy follow-up of ASASSN-20hx (Hinkle+, 2022)
J/ApJ/933/196 : Photometry and spectroscopy of ASASSN-17jz (Holoien+, 2022)
J/ApJ/926/142 : UV, optical and X-ray evolution of ASASSN-14ko (Payne+, 2022)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 8 F8.2 d MJD [58410.42/59563.8] Modified Julian Date
10- 16 A7 --- Band Observational band (1)
18- 24 F7.4 10+37W Lum [-0.04/11.57]? Luminosity in Band, 1043erg/s
26- 31 F6.4 10+37W e_Lum [0.002/1.42]? Uncertainty in Lum
33- 34 A2 --- --- [HR]
41- 45 F5.2 --- HR [-0.68/0.5]? X-ray hardness ratio,
HR=(H-S)/(H+S) where H=2-10keV and S=0.3-2keV
49- 52 F4.2 --- e_HR [0/0.5]? HR uncertainty
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Note (1): Band as follows:
ITESS = TESS (13505 occurrences)
W1 = Swift/UVOT UVW1 band (104 occurrences)
M2 = Swift/UVOT UVM2 band (97 occurrences)
W2 = Swift/UVOT UVW2 band (103 occurrences)
U = Swift/UVOT U band (101 occurrences)
B = Swift/UVOT B band (99 occurrences)
V = Swift/UVOT V band (94 occurrences)
g = ASAS-SN g band (547 occurrences)
X-ray = Swift/XRT, NICER, or Chandra 0.3-10keV range (209 occurrences)
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History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 22-Aug-2025