The Rapid X-Ray and UV Evolution of ASASSN-14ko

Abstract

ASASSN-14ko is a recently discovered periodically flaring transient at the center of the active galactic nucleus (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 2020 May and September 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, by contrast, first rapidly drops over ∼2.6 days, rises for ∼5.8 days, drops again over ∼4.3 days, and then recovers. The X-ray spectral evolution of the two flares differ, however. During the 2020 May peak the spectrum softened with increases in the X-ray luminosity, while we observed the reverse for the 2020 September peak. We found a small change in the period derivative, which seems to indicate that the system does not have a static period derivative and there is some stochasticity in its evolution.

The Curious Case of ASASSN-20hx: A Slowly Evolving, UV- and X-Ray-Luminous, Ambiguous Nuclear Transient

“…while the flux of ASASSN-20hx rises linearly with time. However, the transient ASASSN-14ko, which is presently interpreted as a repeating partial TDE (Tucker et al 2021;Payne et al 2021Payne et al , 2022) also shows a linear rise in its TESS light curve. The overall time for ASASSN-20hx to reach its peak UV/optical emission is similar to typical TDEs.…”

“…Figure 19 shows the X-ray luminosity of ASASSN-20hx compared to the well-studied TDEs ASASSN-14li (Brown et al 2017), ASASSN-15oi (Holoien et al 2018), ASASSN-18ul (AT2018fyk;Wevers et al 2019a, A. V. Payne et al 2022, ASASSN-19dj (Hinkle et al 2021b), andZTF19aapreis (AT2019dsg;van Velzen et al 2021;K. Auchettl et al 2022, in preparation) and to the nuclear transients ASASSN-17jz (Holoien et al 2021), ASASSN-18el (Trakhtenbrot et al 2019a; J. T. Hinkle et al 2022, in preparation), ASASSN-18jd (Neustadt et al 2020), andZTF19abvgxrq (AT2019pev;Frederick et al 2020;K.…”

“…ASASSN-20hx was also observed using the X-ray timing instrument on board the Neutron star Interior Composition ExploreR (NICER: Gendreau et al 2012), which is an external payload on the International Space Station. NICER offers high spectral (∼85 eV at 1 keV) and time resolution (∼100 ns) observations in the 0.2-12 keV energy range and has been used to observe a number of nuclear transients (Trakhtenbrot et al 2019a;Pasham et al 2020aPasham et al , 2020bPasham & Gendreau 2020;Payne et al 2022;Hinkle et al 2021b;Wevers et al 2021;Cannizzaro et al 2021;Ricci et al 2021). 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 67.6 ks.…”

Using the Hills Mechanism to Generate Repeating Partial Tidal Disruption Events and ASASSN-14ko

“…Repeating optical and X-ray flares have recently been detected in the nuclei of galaxies (e.g., Miniutti et al 2019;Song et al 2020;Chakraborty et al 2021;Arcodia et al 2021;Payne et al 2021;Payne et al 2022), with ASASSN-14ko in particular displaying outbursts at an extremely regular period of P = 114 days (Payne et al 2021; see Shappee et al 2014;Holoien et al 2017;Kochanek et al 2017 for specifics related to the ASAS-SN survey). The physical origin of these flares (in general) remains an open question, but if the flaring activity is related to accretion onto the supermassive black hole (SMBH) in the galactic nucleus, the high degree of regularity in the flaring rate of ASASSN-14ko (specifically) is strongly indicative of a binary origin.…”

Chandra, HST/STIS, NICER, Swift, and TESS Detail the Flare Evolution of the Repeating Nuclear Transient ASASSN-14ko

“…Here we present observations of the latest four flares of ASASSN-14ko, a nuclear transient at the center of the AGN ESO 253−G003 that undergoes periodic flares (Payne et al 2021(Payne et al , 2022. This transient was initially discovered by the All-Sky Automated Survey for Supernovae (ASAS-SN, Shappee et al 2014;Kochanek et al 2017) in 2014 but classified as a Type IIn supernova with a blue continuum projected very close to the nucleus of a Type 2 Seyfert, although AGN activity was not ruled out as a possibility (Holoien et al 2014b).…”