The rise and fall of the iron-strong nuclear transient PS16dtm

Abstract

Context. Thanks to the advent of large-scale optical surveys, a diverse set of flares from the nuclear regions of galaxies has recently been discovered. These include the disruption of stars by supermassive black holes at the centers of galaxies -nuclear transients known as tidal disruption events (TDEs). Active galactic nuclei (AGN) can show extreme changes in the brightness and emission line intensities, often referred to as changing-look AGN (CLAGN). Given the physical and observational similarities, the interpretation and distinction of nuclear transients as CLAGN or TDEs remains difficult. One of the obstacles of making progress in the field is the lack of well-sampled data of long-lived nuclear outbursts in AGN.Aims. Here, we study PS16dtm, a nuclear transient in a Narrow Line Seyfert 1 (NLSy1) galaxy, which has been proposed to be a TDE candidate. Our aim is to study the spectroscopic and photometric properties of PS16dtm, in order to better understand the outbursts originating in NLSy1 galaxies. Methods. Our extensive multiwavelength follow-up that spans around 2000 days includes photometry and spectroscopy in the UV/optical, as well as mid-infrared (MIR) and X-ray observations. Furthermore, we improved an existing semiempirical model in order to reproduce the spectra and study the evolution of the spectral lines.Results. The UV/optical light curve shows a double peak at ∼ 50 and ∼ 100 days after the first detection, and it declines and flattens afterward, reaching preoutburst levels after 2000 days of monitoring. The MIR light curve rises almost simultaneously with the optical, but unlike the UV/optical which is approaching the preoutburst levels in the last epochs of our observations, the MIR emission is still rising at the time of writing. The optical spectra show broad Balmer features and the strongest broad Fe II emission ever detected in a nuclear transient. This broad Fe II emission was not present in the archival preoutburst spectrum and almost completely disappeared +1868 days after the outburst. We found that the majority of the flux of the broad Balmer and Fe II lines is produced by photoionization. We detect only weak X-ray emission in the 0.5-8 keV band at the location of PS16dtm, at +848, +1130, and +1429 days past the outburst. This means that the X-ray emission continues to be lower by at least an order of magnitude, compared to archival, preoutburst measurements. Conclusions. We confirm that the observed properties of PS16dtm are difficult to reconcile with normal AGN variability. The TDE scenario continues to be a plausible explanation for the observed properties, even though PS16dtm shows differences compared to TDE in quiescent galaxies. We suggest that this event is part of a growing sample of TDEs that show broad Balmer line profiles and Fe II complexes. We argue that the extreme variability seen in the AGN host due to PS16dtm may have easily been misclassified as a CLAGN, especially if the rising part of the light curve had been missed. This implies that some changing look episodes ...

Multiwavelength observations of the extraordinary accretion event AT2021lwx

“…Similar effects have been seen in TDEs (e.g. van Velzen et al 2016;Jiang et al 2021;Onori et al 2022;Petrushevska et al 2023) and are interpreted as the echoes of the UV emission from a dusty 'torus' surrounding the SMBH. From a pseudo-bolometric luminosity of 7 × 10 45 erg s −1 we follow Jiang et al (2021) Velocity (km/s) flected in a lag between the UV-optical and MIR emission.…”

“…Leloudas et al 2019;van Velzen et al 2021;Charalampopoulos et al 2022). Around half of TDEs appear to show He , Fe and/or Bowen features, and while it is fairly common for TDEs to exhibit only Balmer features, such features tend not to include such strong narrow (in our case unresolved) components as AT2021lwx (although see PS16dtm; Petrushevska et al 2023 andAT2019dsg;van Velzen et al 2021). If the transient is caused by the disruption of a star, then these spectral features must originate from a reprocessing region not present in typical TDEs.…”

“…Leloudas et al 2019), and the MIR-brightening PS16dtm (e.g. Petrushevska et al 2023). AT2021lwx is 100 times brighter and decays slower than these TDEs.…”

Fantastic Fits with fantasy of Active Galactic Nuclei Spectra: Exploring the Fe ii Emission near the Hα Line

“…Moreover, recent observations of the specific transient phenomena of stellar disruption in galactic nuclei, so-called tidal disruption events (TDEs; for a review see Gezari 2021) show that some of these events are strong Fe II emitters (Petrushevska et al 2023). As pointed out in Frederick et al (2021), misleading identification of emission line features can cause errors in the classification of these events.…”

“…Recently, during a transient event in an NLSy1 galaxy, the wavelength region redward from the Hα line was heavily contaminated by iron emission (for details see Petrushevska et al 2023). Most available iron templates focus on the Hβ region, since this line is most commonly observed in distant AGNs and is widely used for SMBH mass estimates from single-epoch observations (for a review see Popović 2020).…”