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Register a new userDiscovery and Follow-up of ASASSN-19dj: An X-ray and UV Luminous TDE in an Extreme Post-Starburst Galaxy
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We present observations of ASASSN-19dj, a nearby tidal disruption event (TDE) discovered in the post-starburst galaxy KUG 0810+227 by the All-Sky Automated Survey for Supernovae (ASAS-SN) at a distance of d $simeq$ 98 Mpc. We observed ASASSN-19dj from $-$21 to 392 days relative to peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From the ASAS-SN $g$-band data, we determine that the TDE began to brighten on 2019 February 6.8 and for the first 25 days the rise was consistent with a flux $propto$ $t^2$ power-law. ASASSN-19dj peaked in the UV/optical on 2019 March 6.5 (MJD = 58548.5) at a bolometric luminosity of $L = (6.2 pm 0.2) times 10^{44} text{ erg s}^{-1}$. Initially remaining roughly constant in X-rays and slowly fading in the UV/optical, the X-ray flux increased by over an order of magnitude $sim$225 days after peak, resulting from the expansion of the X-ray emitting surface. The late-time X-ray emission is well-fit by a blackbody with an effective radius of $sim 1 times 10^{12} text{ cm}$ and a temperature of $sim 6 times 10^{5} text{ K}$. Analysis of Catalina Real-Time Transient Survey images reveals a nuclear outburst roughly 14.5 years earlier with a smooth decline and a luminosity of $L_V$ $geq$ $1.4 times 10^{43}$ erg s$^{-1}$, although the nature of the flare is unknown. ASASSN-19dj occurred in the most extreme post-starburst galaxy yet to host a TDE, with Lick H$delta_{A}$ = $7.67 pm 0.17$ AA.
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We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/UV/X-ray transient located in the nucleus of the galaxy 2MASX J22434289$-$1659083 at $z=0.1192$. Swift UVOT photometry shows the UV SED of the transient to be well modeled by a slowly shrinking blackbody with temperature $T sim 2.5 times 10^{4} rm ~K$, a maximum observed luminosity of $L_text{max} = 4.5^{+0.6}_{-0.3} times 10^{44} rm ~erg ~s^{-1}$, and a total radiated energy of $E = 9.6^{+1.1}_{-0.6} times 10^{51} rm ~erg$. X-ray data from Swift XRT and XMM-Newton show a transient, variable X-ray flux with blackbody and power-law components. Optical spectra show strong, roughly constant broad Balmer emission as well as transient features attributable to He II, N III-V, O III, and coronal Fe. While ASASSN-18jd shares similarities with Tidal Disruption Events (TDEs), it is also similar to the rapid turn-on events seen in quiescent galaxies and in faint Active Galactic Nuclei (AGNs).
ASASSN-14ko is a recently discovered periodically flaring transient at the center of the AGN ESO 253-G003 with a slowly decreasing period. Here we show that the flares originate from the northern, brighter nucleus in this dual-AGN, post-merger system. The light curves for the two flares that occurred in May 2020 and September 2020 are nearly identical over all wavelengths. For both events, Swift observations showed that the UV and optical wavelengths brightened in unison. The effective temperature of the UV/optical emission rises and falls with the increase and subsequent decline in the luminosity. The X-ray flux, in contrast, first rapidly drops over $sim$2.6 days, rises for $sim$5.8 days, drops again over $sim$4.3 days and then recovers. The X-ray spectral evolution of the two flares differ, however. During the May 2020 peak the spectrum softened with increases in the X-ray luminosity, while we observed the reverse for the September 2020 peak.
We present ground-based and Swift photometric and spectroscopic observations of the candidate tidal disruption event (TDE) ASASSN-14li, found at the center of PGC 043234 ($dsimeq90$ Mpc) by the All-Sky Automated Survey for SuperNovae (ASAS-SN). The source had a peak bolometric luminosity of $Lsimeq10^{44}$ ergs s$^{-1}$ and a total integrated energy of $Esimeq7times10^{50}$ ergs radiated over the $sim6$ months of observations presented. The UV/optical emission of the source is well-fit by a blackbody with roughly constant temperature of $Tsim35,000$ K, while the luminosity declines by roughly a factor of 16 over this time. The optical/UV luminosity decline is broadly consistent with an exponential decline, $Lpropto e^{-t/t_0}$, with $t_0simeq60$ days. ASASSN-14li also exhibits soft X-ray emission comparable in luminosity to the optical and UV emission but declining at a slower rate, and the X-ray emission now dominates. Spectra of the source show broad Balmer and helium lines in emission as well as strong blue continuum emission at all epochs. We use the discoveries of ASASSN-14li and ASASSN-14ae to estimate the TDE rate implied by ASAS-SN, finding an average rate of $r simeq 4.1 times 10^{-5}~{rm yr}^{-1}$ per galaxy with a 90% confidence interval of $(2.2 - 17.0) times 10^{-5}~{rm yr}^{-1}$ per galaxy. ASAS-SN found roughly 1 TDE for every 70 Type Ia supernovae in 2014, a rate that is much higher than that of other surveys.
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 peak UV/optical emission using high-cadence, multi-wavelength spectroscopy and photometry. From Transiting Exoplanet Survey Satellite (TESS) data, we determine that the ANT began to brighten on 2020 June 23.3 with a linear rise in flux for at least the first week. ASASSN-20hx peaked in the UV/optical 29.5 days later on 2020 July 22.8 (MJD = 59052.8) at a bolometric luminosity of $L = (3.15 pm 0.04) times 10^{43}$ erg s$^{-1}$. 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 $L_{x} sim 1.5 times 10^{42}$ erg s$^{-1}$ and then slowly declined over time. The X-ray emission is well-fit by a power law with a photon index of $Gamma sim 2.3 - 2.6$. 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 (TDEs) and active galactic nuclei (AGNs), it cannot be definitively classified with current data.
In the canonical model for tidal disruption events (TDEs), the stellar debris circularizes quickly to form an accretion disk of size about twice the orbital pericenter of the star. Most TDEs and candidates discovered in the optical/UV have broad optical emission lines with complex and diverse profiles of puzzling origin. Liu et al. recently developed a relativistic elliptical disk model of constant eccentricity in radius for the broad optical emission lines of TDEs and well reproduced the double-peaked line profiles of the TDE candidate PTF09djl with a large and extremely eccentric accretion disk. In this paper, we show that the optical emission lines of the TDE ASASSN-14li with radically different profiles are well modelled with the relativistic elliptical disk model, too. The accretion disk of ASASSN-14li has an eccentricity 0.97 and semimajor axis of 847 times the Schwarzschild radius (r_S) of the black hole (BH). It forms as the consequence of tidal disruption of a star passing by a massive BH with orbital pericenter 25r_S. The optical emission lines of ASASSN-14li are powered by an extended X-ray source of flat radial distribution overlapping the bulk of the accretion disk and the single-peaked asymmetric line profiles are mainly due to the orbital motion of the emitting matter within the disk plane of inclination about 26degr and of pericenter orientation closely toward the observer. Our results suggest that modelling the complex line profiles is powerful in probing the structures of accretion disks and coronal X-ray sources in TDEs.
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