Multi-epoch properties of the warm absorber in the Seyfert 1 galaxy NGC 985

¹ Telespazio UK for the European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo, s/n, 28692 Villanueva de la Cañada, Madrid, Spain
e-mail: jebrero@sciops.esa.int
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴ Department of Theoretical Physics and Astrophysics, Masaryk University, Kotlářská 2 61137, Czech Republic
⁵ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 2 December 2020
Accepted: 22 June 2021

Abstract

Context. NGC 985 was observed by XMM-Newton twice in 2015, revealing that the source was coming out from a soft X-ray obscuration event that took place in 2013. These kinds of events are possibly recurrent since a previous XMM-Newton archival observation in 2003 also showed signatures of partial obscuration.

Aims. We have analyzed the high-resolution X-ray spectra of NGC 985 obtained by the Reflection Grating Spectrometer onboard XMM-Newton in 2003, 2013, and 2015 in order to characterize the ionized absorbers superimposed to the continuum and to study their response as the ionizing flux varies.

Methods. The spectra were analyzed with the SPEX fitting package and the photoionization code CLOUDY.

Results. We found that up to four warm absorber (WA) components were present in the grating spectra of NGC 985, plus a mildy ionized (logξ ∼ 0.2−0.5) obscuring (N_H ∼ 2 × 10²² cm⁻²) wind outflowing at ∼ − 6000 km s⁻¹. The absorbers have a column density that ranges from ∼10²¹ to a few times 10²² cm⁻², and ionization parameters ranging from logξ ∼ 1.6 to ∼2.9. The most ionized component is also the fastest, moving away at ∼ − 5100 km s⁻¹, while the others outflow in two kinematic regimes, ∼ − 600 and ∼ − 350 km s⁻¹. These components showed variability at different time scales in response to changes in the ionizing continuum. Assuming that these changes are due to photoionization and recombination mechanisms, we have obtained upper and lower limits on the density of the gas. We used these limits to pinpoint the location of the warm absorbers, finding that the closest two components are at parsec-scale distances, while the rest may extend up to tens of parsecs from the central source. With these constraints on the density and location, we found that the fastest, most ionized WA component accounts for the bulk of the kinetic luminosity injected back into the interstellar medium of the host galaxy, which is on the order of 0.8% of the bolometric luminosity of NGC 985. According to the models, this amount of kinetic energy per unit time would be sufficient to account for cosmic feedback.

Conclusions. Observations of the onset and conclusion of transient obscuring events in active galactic nuclei are a key tool to understand both the dynamics and physics of the gas in their innermost regions, and also to study the response of the surrounding gas as the ionizing continuum varies.