J/ApJ/930/12 Photometry & spectroscopy follow-up of ASASSN-20hx (Hinkle+, 2022)
The curious case of ASASSN-20hx: a slowly evolving, UV- and X-ray-luminous,
ambiguous nuclear transient.
Hinkle J.T., Holoien T.W.-S., Shappee B.J., Neustadt J.M.M., Auchettl K.,
Vallely P.J., Shahbandeh M., Kluge M., Kochanek C.S., Stanek K.Z.,
Huber M.E., Post R.S., Bersier D., Ashall C., Tucker M.A., Williams J.P.,
de Jaeger T., Do A., Fausnaugh M., Gruen D., Hopp U., Myles J.,
Obermeier C., Payne A.V., Thompson T.A.
<Astrophys. J., 930, 12 (2022)>
=2022ApJ...930...12H 2022ApJ...930...12H
ADC_Keywords: Transient; Photometry, ugriz; Photometry, ultraviolet;
X-ray sources; Spectra, optical
Keywords: Accretion ; Active galactic nuclei ; Black hole physics ;
Supermassive black holes ; Tidal disruption
Abstract:
We present observations of ASASSN-20hx, a nearby ambiguous nuclear
transient (ANT) discovered in NGC 6297 by the All-Sky Automated Survey
for Supernovae (ASAS-SN). We observed ASASSN-20hx from -30 to 275 days
relative to the peak UV/optical emission using high-cadence,
multiwavelength spectroscopy and photometry. From Transiting Exoplanet
Survey Satellite data, we determine that the ANT began to brighten on
2020 June 22.8 with a linear rise in flux for at least the first week.
ASASSN-20hx peaked in the UV/optical 30 days later on 2020 July 22.8
(MJD=59052.8) at a bolometric luminosity of L=(3.15±0.04)x1043erg/s.
The subsequent decline is slower than any TDE observed to date and
consistent with many other ANTs. Compared to an archival X-ray
detection, the X-ray luminosity of ASASSN-20hx increased by an order
of magnitude to Lx∼1.5x1042erg/s and then slowly declined over time.
The X-ray emission is well fit by a power law with a photon index of
Γ∼2.3-2.6. Both the optical and near-infrared spectra of
ASASSN-20hx lack emission lines, unusual for any known class of
nuclear transient. While ASASSN-20hx has some characteristics seen in
both tidal disruption events and active galactic nuclei, it cannot be
definitively classified with current data.
Description:
ASASSN-20hx was discovered in the g-band data from the ASAS-SN
"Brutus" unit on Haleakala Hawai'i on 2020 July 10.3 UTC.
Shortly after discovery, we obtained several spectroscopic
observations. Spectra from both the SPectrograph for the Rapid
Acquisition of Transients (SPRAT) on the 2m Liverpool Telescope and
the Low-Resolution Imaging Spectrometer (LRIS) on the 10m Keck I
telescope.
We triggered spectroscopic and ground-based photometric follow-up of
ASASSN-20hx.
We reduced images from ASAS-SN, a fully automated transient survey
consisting of 20 individual telescopes on 5 robotic mounts. The five
ASAS-SN units are located at Haleakala Observatory, McDonald
Observatory, the South African Astrophysical Observatory, and two at
Cerro Tololo Inter-American Observatory. See section 2.2.
The host galaxy of ASASSN-20hx, NGC 6297, lies in the TESS northern
continuous viewing zone near the North Ecliptic Pole, so it was
observed continuously between 2019 July 18 and 2020 July 04.
See Section 2.3.
In addition to the ASAS-SN and TESS survey photometry, we obtained
photometric follow-up observations from several ground-based
observatories. We used the Las Cumbres Observatory Global Telescope
(LCOGT) network 1m and 2m telescopes located at Haleakala and McDonald
Observatory for BVgri observations, the Liverpool 2m telescope at
Observatorio del Roque de los Muchachos for ugriz observations, the
Wendelstein Observatory 2m Fraunhofer telescope for ugizJH
observations, and images from amateur astronomer Richard Post (RP) for
BVgri observations taken with a 24-in and 32-in telescope.
See Section 2.4.
Sixty-two total Neil Gehrels Swift Gamma-Ray Burst Mission (Swift)
target-of-opportunity observations were carried out between
2020-Jul-19 and 2021-Apr-15 (Swift target ID 13617, PIs: Holoien,
Hinkle). Some of these epochs were obtained from Swift guest
investigator program 1619122 (PI: Holoien). These observations used
the UVOT and the XRT to study the multiwavelength properties of the
ambiguous nuclear transient (ANT). See Section 2.5.
ASASSN-20hx was also observed using the X-ray timing instrument on
board the Neutron star Interior Composition ExploreR (NICER), which is
an external payload on the International Space Station. NICER offers
high spectral (∼85eV at 1keV) and time resolution (∼100ns)
observations in the 0.2-12keV energy range. ASASSN-20hx was observed a
total of 41 times between 2020 July 25 and 2020 November 01 (ObsIDs:
3573010101-3573014102, PI: Auchettl), for a total cumulative exposure
of 67.6ks. See Section 2.6.
In addition to our classification spectra obtained from SPRAT and
LRIS, we obtained follow-up spectra of ASASSN-20hx with the SuperNova
Integral Field Spectrograph (SNIFS) on the 88-inch University of
Hawaii telescope (UH88), LRIS on the 10m Keck I telescope, SPRAT on
the 2m Liverpool Telescope, and the Multi-Object Double Spectrographs
(MODS) on the Large Binocular Telescope (LBT). See Section 2.7.
We obtained submillimeter observations in the 850um band (353GHz)
using the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) on the
James Clerk Maxwell Telescope (JCMT). See Section 2.8.
Objects:
----------------------------------------------------------
RA (ICRS) DE Designation(s)
----------------------------------------------------------
17 03 36.49 +62 01 32.3 ASASSN-20hx = AT 2020ohl
----------------------------------------------------------
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table3.dat 40 606 Host-subtracted photometry of ASASSN-20hx
table4.dat 36 105 X-ray luminosities and hardness ratios of ASASSN-20hx
table6.dat 30 53 X-ray photon indices
fig6.dat 48 27 *List of spectra included in subdirectory "sp";
table added by CDS (Data behind Figure 6)
sp/* . 27 *Individual spectrum in ASCII format
--------------------------------------------------------------------------------
Note on fig6.dat and sp/*: Optical spectroscopic evolution of ASASSN-20hx
spanning from 10 days prior to peak UV/optical emission until 259 days
after peak (data behind Figure 6).
--------------------------------------------------------------------------------
Description of file:
apjac5f54f6.tar.gz is the Data behind Figure 6 tar file as retrieved
in the paper with the spectra in MRT format.
See also:
II/267 : The SDSS Photometric Catalog, Release 4 (Adelman-McCarthy+, 2006)
VII/233 : The 2MASS Extended sources (IPAC/UMass, 2003-2006)
J/ApJ/613/682 : AGN masses & broad-line region sizes (Peterson+, 2004)
J/A+A/451/457 : X-ray properties of AGN in CDFS (Tozzi+, 2006)
J/ApJ/696/870 : Catalina Real-time Transient Survey (CRTS) (Drake+, 2009)
J/ApJ/713/970 : Low-resolution SED templates for AGNs & gal. (Assef+, 2010)
J/ApJ/716/993 : LAMP: reverberation mapping of H and He lines (Bentz+, 2010)
J/ApJ/753/106 : Quasar variability with SDSS & POSS imaging (MacLeod+, 2012)
J/ApJ/772/26 : AGN with WISE. II. The NDWFS Bootes field (Assef+, 2013)
J/ApJ/793/38 : Palomar Transient Factory photometric obs. (Arcavi+, 2014)
J/ApJS/210/3 : SDSS bulge, disk and stellar mass estimates (Mendel+, 2014)
J/ApJ/788/48 : X-ray through NIR photometry of NGC 2617 (Shappee+, 2014)
J/ApJ/799/208 : Type IIP supernovae from Pan-STARRS1 (Sanders+, 2015)
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/A+A/596/A14 : Group catalogues of the local universe (Saulder+, 2016)
J/ApJ/844/46 : Phot. of the transient event iPTF16fnl (Blagorodnova+, 2017)
J/MNRAS/467/540 : BASS. IV. NIR lines & X-ray correlation (Lamperti+, 2017)
J/ApJS/232/8 : 16yrs of AGNs X-ray sp. analyses from 7Ms CDF-S (Liu+, 2017)
J/ApJS/233/17 : Swift/BAT AGN spectroscopic survey. V. X-ray (Ricci+, 2017)
J/ApJ/852/72 : LFs of tidal disruption flares (van Velzen, 2018)
J/ApJ/880/120 : Phot. of the tidal disruption event PS18kh (Holoien+, 2019)
J/MNRAS/484/1899 : The ASAS-SN bright supernova catalogue - IV (Holoien+, 2019)
J/ApJ/883/111 : UV-Opt LCs of ASASSN-19bt detected by TESS (Holoien+, 2019)
J/ApJ/883/94 : Optical & UV follow-up of 1ES 1927+654 (Trakhtenbrot+, 2019)
J/ApJ/898/161 : 500days of ASASSN-18pg multiwavelength obs. (Holoien+, 2020)
J/ApJ/903/31 : AT 2018hyz with Swift XRT, UVOT and Swope (Hung+, 2020)
J/MNRAS/492/3073 : CALIFA galaxies hosting an AGN (Lacerda+, 2020)
J/ApJ/901/61 : LCs of 4 superluminous SNe from the ZTF (Lunnan+, 2020)
J/MNRAS/493/1044 : Nebular spectra of 111 Type Ia supernovae (Tucker+, 2020)
J/ApJ/908/51 : TESS early LCs of type Ia Supernovae (Fausnaugh+, 2021)
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/926/142 : UV, optical & X-ray evolution of ASASSN-14ko (Payne+, 2022)
Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 10 F10.4 d MJD [59003/59327.4] Modified Julian Date of
observation (JD-2400000.5)
12- 17 F6.4 d dMJDhi [0/1.7] Range in MJD over which the data were
stacked
19- 24 F6.4 d dMJDlo [0/2] Range in MJD over which the data were
stacked
26- 29 A4 --- Filt Filter (UVM2, UVW1, UVW2, UBV, ugriz, TESS)
31- 35 F5.2 mag ohsmag [15.69/21.3] Extinction-corrected,
host-subtracted, AB-system magnitude in Filt (1)
37- 40 F4.2 mag e_ohsmag [0.02/0.4]? Uncertainty in ohsmag (1)
--------------------------------------------------------------------------------
Note (1): Values of ohsmag without uncertainties are 3σ upper limits.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table4.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 F8.2 d MJD [58326.4/59346.3] Modified Julian Date of
observation (JD-2400000.5)
10- 14 F5.2 [10-7W] logLx [40.5/42.5] log, X-ray luminosity, 0.3-10keV
16- 19 F4.2 [10-7W] e_logLx [0/0.3] Uncertainty in logLx
21- 25 F5.2 --- HR [-0.9/1] Hardness ratio (1)
27- 30 F4.2 --- e_HR [0/0.5] Uncertainty in HR
32- 36 A5 --- Sat Satellite ("Swift" or "NICER")
--------------------------------------------------------------------------------
Note (1): The hardness ratio is defined as (H-S)/(H+S), where we define
hard counts, H, as the number of counts in the 2-10keV range and
soft counts, S, as the number of counts in the 0.3-2keV.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 F8.2 d MJD [59051.3/59304] Modified Julian Date of observation
10- 14 F5.2 d dMJD [2.2/35.2]? Range in MJD over which the Swift data
were stacked
16- 19 F4.2 --- Gamma [2/3.2] Photon index (1)
21- 24 F4.2 --- e_Gamma [0.1/0.5] Uncertainty in Gamma
26- 30 A5 --- Sat Satellitee ("Swift" or "NICER")
--------------------------------------------------------------------------------
Note (1): Photon index fit to the individual NICER spectra or the
stacked Swift spectra.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: fig6.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 33 A33 --- File Spectrum file name in subdirectory "sp"
36- 40 A5 --- Inst Instrument ("SPRAT", "LRIS", "MODS" or "SNIFS")
42- 48 F7.1 d MJD [59043/59312.6] Modified Julian Date of observation
--------------------------------------------------------------------------------
Byte-by-byte Description of file: sp/*
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 8 F8.3 0.1nm lambda [3006.2/9900.9] Wavelength, redshift-corrected
in Angstroms
10- 18 E9.3 10-7W/m2/nm Flux [2e-17/1.3e-14] Flux, erg/s/cm2/Angstrom
--------------------------------------------------------------------------------
History:
From electronic version of the journal
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 13-Mar-2024