Multi-wavelength observations of the obscuring wind in the radio-quiet quasar MR 2251-178

TL;DR

This study reports multi-wavelength observations of a transient obscuring wind in the radio-quiet quasar MR 2251-178, revealing unique UV to NIR features and suggesting episodic wind activity linked to X-ray obscuration events.

Contribution

First simultaneous multi-wavelength observations of the obscuring wind in MR 2251-178, highlighting its transient nature and unique spectral features across UV to NIR bands.

Findings

X-ray flux drop indicating obscuration in late 2020-2021

Emergence of blueshifted absorption features in UV Lyα

Presence of both blueshifted and redshifted He I absorption features

Abstract

Obscuring winds driven away from active supermassive black holes are rarely seen due to their transient nature. They have been observed with multi-wavelength observations in a few Seyfert 1 galaxies and one broad absorption line radio-quiet quasar so far. An X-ray obscuration event in MR 2251-178 was caught in late 2020, which triggered multi-wavelength (NIR to X-ray) observations targeting this radio-quiet quasar. In the X-ray band, the obscurer leads to a flux drop in the soft X-ray band from late 2020 to early 2021. X-ray obscuration events might have a quasi-period of two decades considering earlier events in 1980 and 1996. In the UV band, a forest of weak blueshifted absorption features emerged in the blue wing of Ly$\alpha$ $\lambda1216$ in late 2020. Our XMM-Newton, NuSTAR, and HST/COS observations are obtained simultaneously, hence, the transient X-ray obscuration event is expected to account for the UV outflow, although they are not necessarily caused by the same part of the wind. Both blueshifted and redshifted absorption features were found for He {\sc i} $\lambda10830$, but no previous NIR spectra are available for comparison. The X-ray observational features of MR 2251-178 shared similarities with some other type 1 AGNs with obscuring wind. However, observational features in the UV to NIR bands are distinctly different from those seen in other AGN with obscuring winds. A general understanding of the observational variety and the nature of obscuring wind is still lacking.