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

Kavli Affiliate: Michael M. Fausnaugh

| First 5 Authors: Anna V. Payne, Katie Auchettl, Benjamin J. Shappee, Christopher S. Kochanek, Patricia T. Boyd

| Summary:

ASASSN-14ko is a nuclear transient at the center of the AGN ESO 253-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 $115.2^{+1.3}_{-1.2}$ days and period derivative
of $-0.0026 pm 0.0006$. Here we present ASAS-SN, Chandra, HST/STIS, NICER,
Swift, and TESS data for the flares that occurred in December 2020, April 2021,
July 2021, and November 2021. The HST/STIS UV spectra evolve from blue shifted
broad absorption features to red shifted broad emission features over $sim$10
days. The Swift UV/optical light curves peaked as predicted by the timing
model, but the peak UV luminosities varied between flares and the UV flux in
July 2021 was roughly half the brightness of all 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 December 2020 and November 2018 showed that
the slopes during the rising and declining phases changed over time, which
indicates some stochasticity in the flare’s driving mechanism. ASASSN-14ko
remains observationally consistent with a repeating partial tidal disruption
event, but, these rich multi-wavelength data are in need of a detailed
theoretical model.

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