Long-term multiwavelength monitoring and reverberation mapping of NGC 2617 during a changing-look event

TL;DR

This study presents a comprehensive multiwavelength monitoring of the changing-look AGN NGC 2617 over six years, revealing variability, reverberation mapping results, and the physical processes behind its spectral type change.

Contribution

It provides the first long-term, multiwavelength dataset of NGC 2617 during a changing-look event, including reverberation mapping and analysis of dust and accretion variability.

Findings

Measured time delays of ~6 days for H-beta and ~8 days for H-alpha lines.

Found X-ray variations correlate with UV and optical with a few days delay.

Observed K-band lag of 14 days relative to B band, shorter than previous measurements.

Abstract

We present the results of photometric and spectroscopic monitoring campaigns of the changing look AGN NGC~2617 carried out from 2016 until 2022 and covering the wavelength range from the X-ray to the near-IR. The facilities included the telescopes of the SAI MSU, MASTER Global Robotic Net, the 2.3-m WIRO telescope, Swift, and others. We found significant variability at all wavelengths and, specifically, in the intensities and profiles of the broad Balmer lines. We measured time delays of ~ 6 days (~ 8 days) in the responses of the H-beta (H-alpha) line to continuum variations. We found the X-ray variations to correlate well with the UV and optical (with a small time delay of a few days for longer wavelengths). The K-band lagged the B band by 14 +- 4 days during the last 3 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 Balmer decrement of the broad-line components is inversely correlated with the UV flux. The change of the object's type, from Sy1 to Sy1.8, was recorded over a period of ~ 8 years. We interpret these changes as a combination of two factors: changes in the accretion rate and dust recovery along the line of sight.