Search

We present the results of a new X-ray to near-IR photometric and spectroscopic monitoring of the changing look active galactic nucleus NGC 2617 carried out from March 2022 to March 2024. We found significant variability at all wavelengths and in the intensities and profiles of the broad Balmer lines. Reverberation mapping was carried out for three observing seasons during 2021–2024. We obtained time delays of |$\sim$| 4 d for the response of the H  |${\beta }$| line to optical continuum variations. The X-ray variations correlate well with the UV and optical, with a few days lag for longer wavelengths. The K band lagged the B band by |$\sim$| 15 d during the last three seasons, which is significantly shorter than the delays reported previously by the 2016 and 2017–2019 campaigns. Near-IR variability arises from two different emission regions: the outer part of the accretion disc and a more distant dust component. The |$HK$| -band variability is governed primarily by dust. The H  |${\beta }$| /H  |${\alpha }$| ratio variations (for broad components only) correlate with the X-ray and UV fluxes. The spectral type changed from type 1.8 to type 1.9 during 2023 October–2024 January and then a more rapid change to type 1.5 occurred in 2024 February. We interpret these changes as a combination of two factors: changes in the accretion rate as a dominant cause but also the sublimation or recovery of dust along the line of sight. [ABSTRACT FROM AUTHOR]

A star that wanders too close to a massive black hole (BH) is shredded by the BH's tidal gravity. Stellar gas falls back to the BH, releasing a flare of energy. In anticipation of upcoming transient surveys, we predict the light curves and spectra of tidal flares as a function of time, highlighting the unique signatures of tidal flares in the optical and near-IR. Some of the gas initially bound to the BH is likely blown away when the fallback rate is super-Eddington at early times. This outflow produces an optical luminosity comparable to that of a supernova (Figure 1, left panel); such events have durations of ~ 10 days and may have been missed in supernova searches that exclude the nuclear regions of galaxies. When the fallback rate subsides below Eddington, the gas accretes onto the BH via a thin disk whose emission peaks in the UV to soft X-rays. Some of this emission is reprocessed by the unbound stellar debris, producing a spectrum of very broad emission lines, with no corresponding narrow forbidden lines (center panel). These lines are strongest for BHs with MBH ~ 105–106M⊙ and thus optical surveys are particularly sensitive to the lowest mass BHs in galactic nuclei. Calibrating our models to ROSAT and GALEX observations, we predict detection rates for Pan-STARRS, Palomar Transit Factory, and LSST (right panel) and highlight observational challenges in the optical. Pan-STARRS should detect at least several events per year — many more if current theoretical models of super-Eddington outflows are correct. These surveys will significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of intermediate mass BHs, and the role of tidal disruption in the growth of massive BHs. [ABSTRACT FROM PUBLISHER]