Discovery and follow-up of ASASSN-19dj: an X-ray and UV luminous TDE in an extreme post-starburst galaxy

Abstract

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 ≃ 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 ∝t2 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 ± 0.2) × 1044ergs−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 ∼225 days after peak, resulting from the expansion of the X-ray emitting region. The late-time X-ray emission is well-fit by a blackbody with an effective radius of ∼1 × 1012cm and a temperature of ∼6 × 105K. The X-ray hardness ratio becomes softer after brightening and then returns to a harder state as the X-rays fade. 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 LV ≥ 1.4 × 1043 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 HδA = 7.67 ± 0.17 Å.

Tidal Disruption Event Hosts Are Green and Centrally Concentrated: Signatures of a Post-merger System

“…Figure 4 reveals that two galaxies in this sample can be classified as E + A (AT2018hyz and AT2019azh, IDs 5 and 10, respectively) using the guidelines in Figure 4. AT2019azh is an extreme post-starburst galaxy, as noted by Hinkle et al (2021). While 2 of 19 hosts fall within the E + A region, implying an E + A fraction of 10% with a binomial confidence interval of 1%-33%, this region makes up just 0.49% of the SDSS comparison sample, implying that E + A galaxies are overrepresented in the ZTF sample by a factor of ≈22.…”

AT2019azh: an unusually long-lived, radio-bright thermal tidal disruption event

“…Miller et al 2015), and others showing delayed X-ray flares (e.g. Hinkle et al 2021). In some cases radio emission is also observed from outflowing material.…”

“…In this work we present an earlier radio detection, on 2019 March 9. Hinkle et al (2021) analysed the optical, UV, and X-ray observations of AT2019azh from 30 d before to ∼300 d after the optical peak. During the first 200 d there was very low-level X-ray emission with a harder spectral index, and strong optical/UV emission that evolved as expected for a thermal TDE.…”

“…The optical/UV flare was observed to begin on MJD 58528 (2019 Feb 14) and peaked on approximately MJD 58560 Mar 18, Hinkle et al 2021. From a powerlaw fit of the optical rise observed by ASASSN, Hinkle et al (2021) inferred that the time of first light for the event was MJD 58522 (2019 Feb 8). Liu et al (2019) found X-ray flaring episodes during the early times, which were temporally uncorrelated with the optical/UV emission.…”

“…Liu et al (2019) explain the late X-ray brightening as due to the major body of the disk forming after circularisation of the bound stellar debris. However, Hinkle et al (2021) concluded that the late-time X-ray brightening was a consequence of an increase in the area of the X-ray emitting region via the blackbody radius, while the short term X-ray variability was due to changes in the X-ray temperature. In this scenario, the late-time X-ray brightening is not due to delayed accretion disk formation, but rather an expansion of the X-ray emitting region.…”