The role of failed accretion disk winds in active galactic nuclei

Both observational and theoretical evidence point at outflows originating from accretion disks as fundamental ingredients of active galactic nuclei (AGN). These outflows can have more than one component, for example an unbound supersonic wind and a failed wind (FW). The latter is a prediction of the simulations of radiation-driven disk outflows which show that the former is accompanied by an inner failed component, where the flow struggles to escape from the strong gravitational pull of the supermassive black hole. This FW component could provide a physical framework to interpret various phenomenological components of AGN. Here we briefly discuss a few of them: the broad line region, the X-ray obscurer, and the X-ray corona.Comment: 5 pages no figures. Proceedings of the IAU Symposium No. 356, "Nuclear Activity in Galaxies Across Cosmic Time" (Addis Ababa, 7-11 Oct 2019

Rapid late-time X-ray brightening of the tidal disruption event OGLE16aaa

Stars that pass too close to a super-massive black hole may be disrupted by strong tidal forces. OGLE16aaa is one such tidal disruption event (TDE) which rapidly brightened and peaked in the optical/UV bands in early 2016 and subsequently decayed over the rest of the year. OGLE16aaa was detected in an XMM-Newton X-ray observation on June 9, 2016 with a flux slightly below the Swift/XRT upper limits obtained during the optical light curve peak. Between June 16-21, 2016, Swift/XRT also detected OGLE16aaa and based on the stacked spectrum, we could infer that the X-ray luminosity had jumped up by more than a factor of ten in just one week. No brightening signal was seen in the simultaneous optical/UV data to cause the X-ray luminosity to exceed the optical/UV one. A further XMM-Newton observation on November 30, 2016 showed that almost a year after the optical/UV peak, the X-ray emission was still at an elevated level, while the optical/UV flux decay had already leveled off to values comparable to those of the host galaxy. In all X-ray observations, the spectra were nicely modeled with a 50-70 eV thermal component with no intrinsic absorption, with a weak X-ray tail seen only in the November 30 XMM-Newton observation. The late-time X-ray behavior of OGLE16aaa strongly resembles the tidal disruption events ASASSN-15oi and AT2019azh. We were able to pinpoint the time delay between the initial optical TDE onset and the X-ray brightening to $182 \pm 5$ days, which may possibly represent the timescale between the initial circularization of the disrupted star around the super-massive black hole and the subsequent delayed accretion. Alternatively, the delayed X-ray brightening could be related to a rapid clearing of a thick envelope that covers the central X-ray engine during the first six months.Comment: 8 pages, 2 figures, Accepted for publication in A&

Variable X-ray absorption in the mini-BAL QSO PG 1126-041

X-ray studies of active galactic nuclei (AGN) with powerful nuclear winds are important for constraining the physics of the inner accretion/ejection flow around supermassive black holes (SMBHs) and for understanding the impact of such winds on the AGN environment. Our main scientific goal is to constrain the properties of the circum-nuclear matter close to the SMBH in the mini-broad absorption line quasar (mini-BAL QSO) PG 1126-041 using a multi-epoch observational campaign with XMM-Newton. We performed temporally resolved X-ray spectroscopy and simultaneous UV and X-ray photometry on the most complete set of observations and on the deepest X-ray exposure of a mini-BAL QSO ever. We found complex X-ray spectral variability on time scales of both months and hours, which is best reproduced by means of variable massive ionized absorbers along the line of sight. As a consequence, the observed optical-to-X-ray spectral index is found to be variable with time. In the highest signal-to-noise observation we detected highly ionized X-ray absorbing material outflowing much faster (v ~ 16500 km/s) than the UV absorbing one (v ~ 5000 km/s). This highly ionized absorber is found to be variable on very short (a few kiloseconds) time scales. Our findings are qualitatively consistent with line-driven accretion disk winds scenarios. Our observations have opened the time-resolved X-ray spectral analysis field for mini-BAL QSOs. Only with future deep studies will we be able to map the dynamics of the inner flow and understand the physics of AGN winds and their impact on the environment.Comment: Replaced to match the published versio

On the diversity and complexity of absorption line profiles produced by outflows in Active Galactic Nuclei

Understanding the origin of AGN absorption line profiles and their diversity could help to explain the physical structure of the accretion flow, and also to assess the impact of accretion on the evolution of the AGN host galaxies. Here we present our first attempt to systematically address the issue of the origin of the complexities observed in absorption profiles. Using a simple method, we compute absorption line profiles against a continuum point source for several simulations of accretion disk winds. We investigate the geometrical, ionization, and dynamical effects on the absorption line shapes. We find that significant complexity and diversity of the absorption line profile shapes can be produced by the non-monotonic distribution of the wind velocity, density, and ionization state. Non-monotonic distributions of such quantities are present even in steady-state, smooth disk winds, and naturally lead to the formation of multiple and detached absorption troughs. These results demonstrate that the part of a wind where an absorption line is formed is not representative of the entire wind. Thus, the information contained in the absorption line is incomplete if not even insufficient to well estimate gross properties of the wind such as the total mass and energy fluxes. In addition, the highly dynamical nature of certain portions of disk winds can have important effects on the estimates of the wind properties. For example, the mass outflow rates can be off up to two orders of magnitude with respect to estimates based on a spherically symmetric, homogeneous, constant velocity wind.Comment: 10 pages, 10 figures, to appear in Ap

The WISSH quasars project. VII. The impact of extreme radiative field in the accretion disc and X-ray corona interplay

Hyper-luminous quasars (Lbol >10^47 erg/s) are ideal laboratories to study the interaction and impact of the extreme radiative field and the most powerful winds in the active galactic nuclei (AGN) nuclear regions. They typically exhibit low coronal X-ray luminosity (LX) compared to the ultraviolet (UV) and mid-infrared (MIR) radiative outputs (LUV and LUV); a non-negligible fraction of them report even ~1 dex weaker LX compared to the prediction of the well established LX-LUV and LX-LUV relations followed by the bulk of the AGN population. In our WISE/SDSS-selected Hyper-luminous (WISSH) z = 2 - 4 broad-line quasar sample, we report on the discovery of a dependence between the intrinsic 2-10 keV luminosity (L2-10) and the blueshifted velocity of the CIV emission line (vCIV) that is indicative of accretion disc winds. In particular, sources with the fastest winds (vCIV > 3000 km/s) possess ~0.5-1 dex lower L2-10 than sources with negligible vCIV. No similar dependence is found on LUV, LUV, Lbol, the photon index, or the absorption column density. We interpret these findings in the context of accretion disc wind models. Both magnetohydrodynamic and line-driven models can qualitatively explain the reported relations as a consequence of X-ray shielding from the inner wind regions. In case of line-driven winds, the launch of fast winds is favoured by a reduced X-ray emission, and we speculate that these winds may play a role in directly limiting the coronal hard X-ray production.With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737

The Quest for Dual and Binary Supermassive Black Holes: A Multi-Messenger View

The quest for binary and dual supermassive black holes (SMBHs) at the dawn of the multi-messenger era is compelling. Detecting dual active galactic nuclei (AGN) – active SMBHs at projected separations larger than several parsecs – and binary AGN – probing the scale where SMBHs are bound in a Keplerian binary – is an observational challenge. The study of AGN pairs (either dual or binary) also represents an overarching theoretical problem in cosmology and astrophysics. The AGN triggering calls for detailed knowledge of the hydrodynamical conditions of gas in the imminent surroundings of the SMBHs and, at the same time, their duality calls for detailed knowledge on how galaxies assemble through major and minor mergers and grow fed by matter along the filaments of the cosmic web. This review describes the techniques used across the electromagnetic spectrum to detect dual and binary AGN candidates and proposes new avenues for their search. The current observational status is compared with the state-of-the-art numerical simulations and models for formation of dual and binary AGN. Binary SMBHs are among the loudest sources of gravitational waves (GWs) in the Universe. The search for a background of GWs at nHz frequencies from inspiralling SMBHs at low redshifts, and the direct detection of signals from their coalescence by the Laser Interferometer Space Antenna in the next decade, make this a theme of major interest for multi-messenger astrophysics. This review discusses the future facilities and observational strategies that are likely to significantly advance this fascinating field

The Quiescent Intracluster Medium in the Core of the Perseus Cluster

Clusters of galaxies are the most massive gravitationally-bound objects in the Universe and are still forming. They are thus important probes of cosmological parameters and a host of astrophysical processes. Knowledge of the dynamics of the pervasive hot gas, which dominates in mass over stars in a cluster, is a crucial missing ingredient. It can enable new insights into mechanical energy injection by the central supermassive black hole and the use of hydrostatic equilibrium for the determination of cluster masses. X-rays from the core of the Perseus cluster are emitted by the 50 million K diffuse hot plasma filling its gravitational potential well. The Active Galactic Nucleus of the central galaxy NGC1275 is pumping jetted energy into the surrounding intracluster medium, creating buoyant bubbles filled with relativistic plasma. These likely induce motions in the intracluster medium and heat the inner gas preventing runaway radiative cooling; a process known as Active Galactic Nucleus Feedback. Here we report on Hitomi X-ray observations of the Perseus cluster core, which reveal a remarkably quiescent atmosphere where the gas has a line-of-sight velocity dispersion of 164+/-10 km/s in a region 30-60 kpc from the central nucleus. A gradient in the line-of-sight velocity of 150+/-70 km/s is found across the 60 kpc image of the cluster core. Turbulent pressure support in the gas is 4% or less of the thermodynamic pressure, with large scale shear at most doubling that estimate. We infer that total cluster masses determined from hydrostatic equilibrium in the central regions need little correction for turbulent pressure.Comment: 31 pages, 11 Figs, published in Nature July