Discovery of late-time X-ray flare and anomalous emission line enhancement after the nuclear optical outburst in a narrow-line Seyfert 1 Galaxy

Abstract

CSS J102913+404220 is an atypical narrow-line Seyfert 1 galaxy with an energetic optical outburst occurring co-spatially with its nucleus. We present a detailed analysis of multi-wavelength photometric and spectroscopic observations of this object covering a period of a decade since outburst. We detect mid-infrared (MIR) flares delayed by about two months relative to the optical outburst and with an extremely high peak luminosity of L4.6 μm > 1044 erg s−1. The MIR peak luminosity is at least an order of magnitude higher than any known supernovae explosions, suggesting the optical outburst might be due to a stellar tidal disruption event (TDE). We find late-time X-ray brightening by a factor of ≳30 with respect to what is observed about 100 days after the optical outburst peak, followed by a flux fading by a factor of ∼4 within two weeks, making it one of the active galactic nuclei (AGNs) with extreme variability. Despite the dramatic X-ray variability, there are no coincident strong flux variations in optical, UV, and MIR bands. This unusual variability behavior has been seen in other highly accreting AGNs and could be attributed to absorption variability. In this scenario, the decrease in the covering factor of the absorber with accretion rate could cause the X-ray brightening, possibly induced by the TDE. Most strikingly, while the UV/optical continuum remains almost unchanged with time, an evident enhancement in the flux of the Hα broad emission line is observed about a decade after the nuclear optical outburst, which is an anomalous behavior never seen in any other AGN. Such an Hα anomaly could be explained by the replenishment of gas clouds and excitation within the broad line region (BLR) that perhaps originates from its interaction with outflowing stellar debris. Our results highlight the importance of the late-time evolution of a TDE, which can affect the accreting properties of the AGN, as suggested by recent simulations.

Central BH Mass of Tidal Disruption Event Candidate SDSS J0159 through Long-term Optical Variabilities

“…Frederick et al (2021) reported TDE candidates in narrow-line Seyfert 1 galaxies. More recently, Zhang et al (2022) showed the TDE expected variability patterns in a narrow line Seyfert 1 galaxy. Meanwhile, Chan et al (2019Chan et al ( , 2020 modeled TDE variabilities in AGN with a preexisting accretion disk, and showed that about a 20 day long plateau could be expected around the central BH with masses of around 10 6−7 M e .…”

Delayed and Fast-rising Radio Flares from an Optical and X-Ray-detected Tidal Disruption Event in the Center of a Dwarf Galaxy

“…However, this possibility seems unlikely based on the X-ray properties (Bykov et al 2024). In addition, the rapid (duration of <1 yr) optical flare can also rule out the normal AGN variability, as it is rarely seen in the light curves of AGNs (Drake et al 2011;Kankare et al 2017;Zhang et al 2022). The lack of detectable broad permitted lines prevents us from estimating the central BH mass of AT 2018cqh using the conventional linewidthluminosity-mass scaling relation.…”

Changing-Look Narrow-Line Seyfert 1 Galaxies, their Detection with SVOM, and the Case of NGC 1566

“…However, the situation is more complicated when the galaxy is an AGN, such that it is not immediately clear whether the change comes from an accretion disk (instability) or, less likely, from a (very rare) TDE or an SN IIn. The situation is also challenging when there is no pre-outburst spectrum available, or when there are only a few post-outburst spectra, so that long-lived AGN, rare TDE, or SN IIn scenarios cannot immediately be distinguished based on optical spectra alone (see the detailed discussion of Komossa et al [43] and Komossa et al [48]; and see Drake et al [49], Blanchard et al [50] and Zhang et al [51] for examples in NLS1-like galaxies).…”