Effects of ultrafast outflows on X-ray time lags in active galactic nuclei

¹ INAF - IASF Palermo, Via U. La Malfa 153, I-90146 Palermo, Italy
² Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, I-90123 Palermo, Italy
³ Department of Astronomy & Physics, Saint Mary’s University, 923 Robie Street, Halifax, NS B3H 3C3, Canada
⁴ MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁵ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, I-00146 Roma, Italy
⁶ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁷ Department of Physics, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Rome, Italy
⁸ INAF - Astronomical Observatory of Rome, Via Frascati 33, 00040 Monte Porzio Catone, Italy
⁹ INFN - Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy

^⋆ Corresponding author; yerong.xu@inaf.it

Received: 29 July 2024
Accepted: 5 November 2024

Abstract

Context. The time lag between soft (e.g., 0.3–1 keV) and hard (e.g., 1–4 keV) X-ray photons has been observed in many active galactic nuclei (AGNs) and can reveal the accretion process and geometry around supermassive black holes. High-frequency Fe K and soft lags are considered to originate from the light-travel distances between the corona and the accretion disk, while the propagation of the inward mass accretion fluctuation usually explains the low-frequency hard lags. Ultrafast outflows (UFOs) with a velocity range of ∼0.03 to 0.3c have also been discovered in numerous AGNs and are believed to be launched from the inner accretion disk. However, it remains unclear whether UFOs can affect the X-ray time lags.

Aims. As a pilot work, we aim to investigate the potential influence of UFOs on X-ray time lags of AGNs in a small sample.

Methods. By performing the UFO-resolved Fourier spectral timing analysis of archival XMM-Newton observations of three AGNs with transient UFOs – PG 1448+273, IRAS 13224-3809, and PG 1211+143 – we compare the X-ray timing products, such as lag-frequency and lag-energy spectra, of observations with and without UFO obscuration.

Results. Our results find that in each AGN, low-frequency hard lags become weak or even disappear when they are accompanied by UFOs. This change is confirmed by Monte Carlo simulations at a confidence level of at least 2.7σ. In the high-frequency domain, soft lags remain unchanged, while the Fe K reverberation lags tentatively disappear. The comparison between timing products of low- and high-flux observations on another three AGNs without UFOs (Ark 564, NGC 7469, and Mrk 335) suggests that the disappearance of low-frequency hard lags is likely related to the emergence of UFOs, not necessarily related to the source flux.

Conclusions. The presence of UFOs can affect X-ray time lags of AGNs by suppressing the low-frequency hard lags, which can be explained by an additional time delay introduced by UFOs or disk accretion energy, which should transferred to heat the corona, carried away by UFOs.