Discovery of a fast, broad, transient outflow in NGC 985

TL;DR

This study reveals a transient, high-velocity outflow in NGC 985, characterized by variable X-ray and UV absorption features, indicating an accretion disk wind with significant changes over years.

Contribution

First simultaneous X-ray and UV observations showing the evolution of a fast, broad outflow in NGC 985, linking UV and X-ray absorbers to an accretion disk wind.

Findings

X-ray obscuration decreased by a factor of ~3 between 2013 and 2015.

Broad UV absorption lines with velocities up to -5970 km/s detected during obscuration.

UV and X-ray absorbers are consistent with originating from the same high-velocity wind.

Abstract

We observed the Seyfert 1 galaxy NGC 985 on multiple occasions to search for variability in its UV and X-ray absorption features in order to establish their location and physical properties. We use XMM-Newton to obtain X-ray spectra using the EPIC-pn camera, and the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) to obtain UV spectra. Our observations are simultaneous and span timescales of days to years. We find that the soft X-ray obscuration that absorbed the low energy continuum of NGC 985 in August 2013 diminished greatly by January 2015. The total X-ray column density decreased from 2.1 x 10^22 cm^-2 to ~6 x 10^21 cm^-2. We also detect broad, fast UV absorption lines in COS spectra obtained during the 2013 obscuration event. Lines of C III*, Ly alpha, Si IV and C IV with outflow velocities of -5970 km/s and a full-width at half-maximum of 1420 km/s are prominent in the 2013 spectrum, but have disappeared in all but Ly alpha in the 2015 spectra. The ionization state and the column density of the UV absorbing gas is compatible with arising in the same gas as that causing the X-ray obscuration. The high velocity of the UV-absorbing gas suggests that the X-ray obscurer and the associated UV outflow are manifestations of an accretion disk wind.