Chandra Observations of Candidate Sub-Parsec Binary Supermassive Black Holes
M. Lynne Saade, Daniel Stern, Murray Brightman, Zoltán Haiman, S. G. Djorgovski, Daniel D'Orazio, K. E. S. Ford, Matthew J. Graham, Hyunsung D. Jun, Ralph P. Kr… 展开作者
M. Lynne Saade, Daniel Stern, Murray Brightman, Zoltán Haiman, S. G. Djorgovski, Daniel D'Orazio, K. E. S. Ford, Matthew J. Graham, Hyunsung D. Jun, Ralph P. Kraft, Barry McKernan, Alexei Vikhlinin, Dominic J. Walton
发表信息：
2020, The Astrophysical Journal, Volume 900, Page 148
We present analysis of Chandra X-ray observations of seven quasars that were identified as candidate sub-parsec binary supermassive black hole (SMBH) systems in the Catalina Real-Time Transient Survey (CRTS) based on apparent periodicity in their optical light curves. Simulations predict close-separation accreting SMBH binaries will have different X-ray spectra than single accreting SMBHs, including harder or softer X-ray spectra, ripple-like profiles in the Fe K-$α$ line, and distinct peaks in the spectrum due to the separation of the accretion disk into a circumbinary disk and mini-disks around each SMBH. We obtained Chandra observations to test these models and assess whether these quasars could contain binary SMBHs. We instead find that the quasar spectra are all well fit by simple absorbed power law models, with the rest frame 2-10 keV photon indices, $Γ$, and the X-ray-to-optical power slopes, $α_{\rm OX}$, indistinguishable from the larger quasar population. This may indicate that these seven quasars are not truly sub-parsec binary SMBH systems, or it may simply reflect that our sample size was too small to robustly detect any differences. Alternatively, the X-ray spectral changes might only be evident at higher energies than probed by Chandra. Given the available models and current data, no firm conclusions are drawn. These observations will help motivate and direct further work on theoretical models of binary SMBH systems, such as modeling systems with thinner accretion disks and larger binary separations.
As a natural consequence of cosmological hierarchical structure formation, sub-parsec supermassive black hole binaries (SMBHBs) should be common in galaxies but thus far have eluded spectroscopic identification. Based on four decades of optical spectroscopic monitoring, we report that the nucleus of NGC 5548, a nearby Seyfert galaxy long suspected to have experienced a major merger about one billion years ago, exhibits long-term variability with a period of 14 years in the optical continuum and broad Hbeta emission line. Remarkably, the double-peaked profile of Hbeta shows systematic velocity …
The jet in active galactic nuclei (AGN) is a key ingredient in understanding the co-evolution of galaxies and their central supermassive black holes (SMBHs). Unfortunately, the mechanism of jet launching and collimation is still elusive. The observational evidence of decreasing radio loudness with increasing Eddington ratio implies that the jet should be coupled with the accretion process. To further explore the relationship between the jet and accretion, it is necessary to extend our knowledge of the jet to an extreme end of the Eddington ratio distribution of AGN. Using Very Long Baseline Ar…
The formation of massive black hole binaries (MBHBs) is an unavoidable outcome of galaxy evolution via successive mergers. However, the mechanism that drives their orbital evolution from parsec separations down to the gravitational wave (GW) dominated regime is poorly understood, and their final fate is still unclear. If such binaries are embedded in gas-rich and turbulent environments, as observed in remnants of galaxy mergers, the interaction with gas clumps (such as molecular clouds) may efficiently drive their orbital evolution. Using numerical simulations, we test this hypothesis by study…
One of the most intriguing scenarios proposed to explain how active galactic nuclei are triggered involves the existence of a supermassive binary black hole system in their cores. Here we present an observational evidence for the first spectroscopically resolved sub-parsec orbit of a such system in the core of Seyfert galaxy NGC 4151. Using a method similar to those typically applied for spectroscopic binary stars we obtained radial velocity curves of the supermassive binary system, from which we calculated orbital elements and made estimates about the masses of components. Our analysis shows …
Supermassive binary black holes (BBHs) on sub-parsec scales are prime targets for gravitational wave experiments. They also provide insights on close binary evolution and hierarchical structure formation. Sub-parsec BBHs cannot be spatially resolved but indirect methods can identify candidates. In 2015 Liu et al. reported an optical-continuum periodicity in the quasar PSO J334.2028+01.4075, with the estimated mass and rest-frame period suggesting an orbital separation of about 0.006 pc (0.7 micro-arcsec). The persistence of the quasar's optical periodicity has recently been disfavored over an …
Motivated by advances in observational searches for sub-parsec supermassive black hole binaries (SBHBs) made in the past few years we develop a semi-analytic model to describe spectral emission line signatures of these systems. The goal of this study is to aid the interpretation of spectroscopic searches for binaries and help test one of the leading models of binary accretion flows in the literature: SBHB in a circumbinary disk. In this work we present the methodology and a comparison of the preliminary model with the data. We model SBHB accretion flows as a set of three accretion disks: two m…
Supermassive binary black holes (BBHs) are unavoidable products of galaxy mergers and are expected to exist in the cores of many quasars. Great effort has been made during the past several decades to search for BBHs among quasars; however, observational evidence for BBHs remains elusive and ambiguous, which is difficult to reconcile with theoretical expectations. In this paper, we show that the distinct optical-to-UV spectrum of Mrk 231 can be well interpreted as emission from accretion flows onto a BBH, with a semimajor axis of ~590AU and an orbital period of ~1.2 year. The flat optical and U…
We present unambiguous evidence for a parsec scale wind in the Broad-Line Radio Galaxy (BLRG) 3C 382, the first radio-loud AGN, with $R_{\rm L} = \log_{10}(f_{\rm 5GHz}/f_{4400})>1$, whereby an outflow has been measured with X-ray grating spectroscopy. A 118 ks Chandra grating (HETG) observation of 3C 382 has revealed the presence of several high ionization absorption lines in the soft X-ray band, from Fe, Ne, Mg and Si. The absorption lines are blue-shifted with respect to the systemic velocity of 3C 382 by -840\pm60 km/s and are resolved by Chandra with a velocity width of 340\pm70 km/s. The…
We present an improved semi-analytic model for calculation of the broad optical emission-line signatures from sub-parsec supermassive black hole binaries (SBHBs) in circumbinary disks. The second-generation model improves upon the treatment of radiative transfer by taking into account the effect of the radiation driven accretion disk wind on the properties of the emission-line profiles. Analysis of 42.5 million modeled emission-line profiles shows that correlations between the profile properties and SBHB parameters identified in the first-generation model are preserved, indicating that their d…
Elusive supermassive black hole binaries (SMBHBs) are thought to be the penultimate stage of galaxy mergers, preceding a final coalescence phase. SMBHBs are sources of continuous gravitational waves, possibly detectable by pulsar timing arrays; the identification of candidates could help in performing targeted gravitational wave searches. Due to their origin in the innermost parts of active galactic nuclei (AGN), X-rays are a promising tool to unveil the presence of SMBHBs, by means of either double Fe K$α$ emission lines or periodicity in their light curve. Here we report on a new method to s…
Quasars whose broad emission lines show temporal, bulk radial velocity (RV) shifts have been proposed as candidate sub-parsec (sub-pc), binary supermassive black holes (BSBHs). We identified a sample of 16 BSBH candidates based on two-epoch spectroscopy among 52 quasars with significant RV shifts over a few rest-frame years. The candidates showed consistent velocity shifts independently measured from two broad lines (H$β$ and H$α$ or Mg${\rm \,II}$) without significant changes in the broad-line profiles. Here in the third paper of the series, we present further third- and fourth-epoch spectros…
We present a method for comparing the H$β$ emission-line profiles of observed supermassive black hole (SBHB) candidates and models of sub-parsec SBHBs in circumbinary disks. Using the approach based on principal component analysis we infer the values of the binary parameters for the spectroscopic SBHB candidates and evaluate the parameter degeneracies, representative of the uncertainties intrinsic to such measurements. We find that as a population, the SBHB candidates favor the average value of the semimajor axis corresponding to $\log(a/M) \approx 4.20\pm 0.42$ and comparable mass ratios, $q>…
We consider the interaction between a binary system (e.g. two supermassive black holes or two stars) and an external accretion disc with misaligned angular momentum. This situation occurs in galaxy merger events involving supermassive black holes, and in the formation of stellar--mass binaries in star clusters. We work out the gravitational torque between the binary and disc, and show that their angular momenta J_b, J_d stably counteralign if their initial orientation is sufficiently retrograde, specifically if the angle theta between them obeys cos(theta) < -J_d/2J_b, on a time short compared…
Motivated by observational searches for sub-parsec supermassive black hole binaries (SBHBs) we develop a modular analytic model to determine the likelihood for detection of SBHBs by ongoing spectroscopic surveys. The model combines the parametrized rate of orbital evolution of SBHBs in circumbinary disks with the selection effects of spectroscopic surveys and returns a multivariate likelihood for SBHB detection. Based on this model we find that in order to evolve into the detection window of the spectroscopic searches from larger separations in less than a Hubble time, $10^8M_\odot$ SBHBs must…
The capture and disruption of stars by supermassive black holes (SMBHs), and the formation and coalescence of binaries, are inevitable consequences of the presence of SMBHs at the cores of galaxies. Pairs of active galactic nuclei (AGN) and binary SMBHs are important stages in the evolution of galaxy mergers, and an intense search for these systems is currently ongoing. In the early and advanced stages of galaxy merging, observations of the triggering of accretion onto one or both BHs inform us about feedback processes and BH growth. Identification of the compact binary SMBHs at parsec and sub…
The main idea of our research is to estimate the physical coalescence time of the double supermassive black hole (SMBH) system in the centre of NGC 6240 based on the X-ray observations from the Chandra space observatory. The spectra of the Northern and Southern nuclei were fitted by spectral models from Sherpa and both presented the narrow component of the Fe K$α$ emission line. It enabled us to apply the spectral model to these lines and to find relative offset $\approx0.02$ keV. The enclosed dynamical mass of the central region of NGC 6240 with radius 1 kpc was estimated $\approx 2.04\times …
We have investigated the linear X-ray filament, G359.944-0.052, previously identified as a likely X-ray counterpart of a parsec-scale jet from the Galactic Center super-massive black hole (SMBH), Sagittarius A* (Sgr A*), using a total of ~5.6 Ms ultra-deep Chandra observations taken from September 1999 to July 2017. This unprecedented dataset enables us to examine flux and spectral variations that might be related to intrinsic properties of the weakly accreting SMBH. We find no flux or spectral variation in G359.944-0.052 after the G2 periapsis passage around early 2014, however, a moderate fl…
Spin measurements of supermassive black holes (SMBHs) provide crucial constraints on the accretion processes that power active galactic nuclei (AGN), fuel outflows, and trigger black hole growth. However, spin measurements are mainly limited to a few dozen nearby sources for which high quality, high S/N spectra (e.g., from Chandra, XMM-Newton, Suzaku, NuSTAR) are available. Here we measure the average SMBH spin of $\sim$1900 AGN in the Chandra COSMOS-Legacy survey using spectral stacking analysis. We find broad Fe K$α$ line emission in the average COSMOS spectrum (Gaussian width $σ=0.27\pm0.05…
We present Chandra observations of 12 galaxies that contain supermassive black holes with dynamical mass measurements. Each galaxy was observed for 30 ksec and resulted in a total of 68 point source detections in the target galaxies including supermassive black hole sources, ultraluminous X-ray sources, and extragalactic X-ray binaries. Based on our fits of the X-ray spectra, we report fluxes, luminosities, Eddington ratios, and slope of the power-law spectrum. Normalized to the Eddington luminosity, the 2--10 keV band X-ray luminosities of the SMBH sources range from $10^{-8}$ to $10^{-6}$, a…
We show that a 10 year Gaia mission could astrometrically detect the orbital motion of ~1 sub-parsec separation supermassive black hole binary in the heart of nearby, bright active galactic nuclei (AGN). Candidate AGN lie out to a redshift of z=0.02 and in the V-band magnitude range $10 \lesssim m_V \lesssim 13$. The distribution of detectable binary masses peaks at a few times $10^7$ solar masses and is truncated above a few times $10^8$ solar masses.
Galaxies are believed to evolve through merging, which should lead to multiple supermassive black holes in some. There are four known triple black hole systems, with the closest pair being 2.4 kiloparsecs apart (the third component is more distant at 3 kiloparsecs), which is far from the gravitational sphere of influence of a black hole with mass $\sim$10$^9$ M$_\odot$ (about 100 parsecs). Previous searches for compact black hole systems concluded that they were rare, with the tightest binary system having a separation of 7 parsecs. Here we report observations of a triple black hole system at …
Extragalactic X-ray surveys over the past decade have dramatically improved understanding of the majority populations of active galactic nuclei (AGNs) over most of the history of the Universe. Here we briefly highlight some of the exciting discoveries about AGN demography, physics, and ecology with a focus on results from Chandra. We also discuss some key unresolved questions and future prospects.
The dynamical evolution of supermassive black-hole binary systems is tethered on large scales to the merger rate of massive galaxies, and on small scales to the stellar and gaseous environments of galactic cores. The population of these systems will create an ensemble gravitational-wave signal at nanohertz frequencies that is detectable by Pulsar Timing Arrays (PTA), such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). By measuring nanohertz gravitational waves, we will be able to definitively demonstrate that supermassive black-hole binaries reach milliparsec s…
Jet precession can reveal the presence of binary systems of supermassive black holes. The ability to accurately measure the parameters of jet precession from radio-loud AGN is important for constraining the binary supermassive black hole population, which are expected as a result of hierarchical galaxy evolution. The age, morphology, and orientation along the line of sight of a given source often result in uncertainties regarding jet path. This paper presents a new approach for efficient determination of precession parameters using a 2D MCMC curve-fitting algorithm which provides us a full pos…
We report a candidate centi-parsec supermassive black hole binary (SMBHB) in the radio-quiet quasar SDSS J0159+0105 at z=0.217. With a modified lomb-scargle code GLSdeDRW and the auto-correlation analysis ACF, we detect two significant (at P>99%) periodic signals at ~741 day and ~1500 day from the 8.1-year Catalina V-band light curve of this quasar. The period ratio, which is close to 1:2, is typical of a black-hole binary system with a mass ratio of 0.05<q<0.8 according to recent numerical simulations. SDSS J0159+0105 has two SDSS spectroscopic observations separated by ~10 years. There is a …
X-ray emission from quasars has been detected up to redshift $z=7.5$, although only limited to a few objects at $z>6.5$. In this work, we present new Chandra observations of five $z>6.5$ quasars. By combining with archival Chandra observations of six additional $z>6.5$ quasars, we perform a systematic analysis on the X-ray properties of these earliest accreting supermassive black holes (SMBHs). We measure the black hole masses, bolometric luminosities ($L_{\rm bol}$), Eddington ratios ($λ_{\rm Edd}$), emission line properties, and infrared luminosities ($L_{\rm IR}$) of these quasars using inf…
Effects of dark dipole radiation on eccentric supermassive black hole binary inspirals
School of Astronomy and Space Science, University of Chinese Academy of Sciences (UCAS)（中国科学院大学天文与空间科学学院）；School of Physics, Nanjing University（南京大学物理学院）
The gravitational interaction at parsec to sub-parsec scales between a circumbinary gas disc and a super massive black hole binary (SMBHB) is a promising mechanism to drive the migration of SMBHBs toward coalescence. The typical dynamical evolution can be separated in two main regimes: I) Slow migration ($T_{\rm mig}$ $\sim$ $10^{3-4}\times T_{\rm orb}$), where viscous torques are not efficient enough to redistribute the extracted angular momentum from the binary, leading to the formation of a low density cavity around the binary. II) Fast migration ($T_{\rm mig}$ $\sim$ $10^{1-2} \times T_{\r…
We report NuSTAR and Chandra observations of two X-ray transients, SWIFT J174540.7$-$290015 (T15) and SWIFT J174540.2$-$290037 (T37), which were discovered by the Neil Gehrels Swift Observatory in 2016 within $r\sim1$ pc of Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the soft and hard states, with 3-79~keV luminosities of $8\times10^{36}$ and $3\times10^{37}$ erg/s, respectively. No X-ray outbursts have previously been detected from the two transients and our Chandra ACIS analysis puts an upper limit of $L_X \lesssim 2 \times10^{31}$ erg/s on their quiescent 2-8 …
Gas undergoing Bondi accretion onto a supermassive black hole (SMBH) becomes hotter toward smaller radii. We searched for this signature with a Chandra observation of the hot gas in NGC 3115, which optical observations show has a very massive SMBH. Our analysis suggests that we are resolving, for the first time, the accretion flow within the Bondi radius of an SMBH. We show that the temperature is rising toward the galaxy center as expected in all accretion models in which the black hole is gravitationally capturing the ambient gas. There is no hard central point source that could cause such a…
In this paper, we study the interaction between the supermassive binary black holes in elliptical orbit and their surrounding disk with a gap. The gap in the disk is a low density region formed due to the tidal effects of the less massive black hole. The binary we have investigated has a sub-parsec separation and is coplanar with the disk. We find that the maximum variation of the surface density in the gap reaches 50% during an orbital period. However, in other regions of the disk, the density variation is much less than 1%. Furthermore, we calculate the corresponding variation of spectral en…
We present continued radio observations of the tidal disruption event SwiftJ164449.3+573451 extending to \sim216 days after discovery. The data are part of a long-term program to monitor the expansion and energy scale of the relativistic outflow, and to trace the parsec-scale environment around a previously-dormant supermassive black hole (SMBH). The new observations reveal a significant change in the radio evolution starting at \sim1 month, with a brightening at all frequencies that requires an increase in the energy by about an order of magnitude, and an overall density profile around the SM…
Over the last decade, X-ray observations of Sgr A* have revealed a black hole in a deep sleep, punctuated roughly once per day by brief flares. The extreme X-ray faintness of this supermassive black hole has been a long-standing puzzle in black hole accretion. To study the accretion processes in the Galactic Center, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012. With its excellent observing cadence, sensitivity, and spectral resolution, this Chandra X-ray Visionary Project (XVP) provides an unprecedented opportunity to stud…
The X-ray emission from the jets in Active Galactic Nuclei (AGN) carries important information on the distributions of relativistic electrons and magnetic fields on large scales. We reanalyze archival Chandra observations on the jet of M87 from 2000 to 2016 with a total exposure of 1460 kiloseconds to explore the X-ray emission characteristics along the jet. We investigate the variability behaviours of the nucleus and the inner jet component HST-1, and confirm indications for day-scale X-ray variability in the nucleus contemporaneous to the 2010 high TeV gamma-ray state. HST-1 shows a general …
We discuss the formation and evolution of systems comprised of a low-mass ($M_\star \lesssim 4 \, \rm M_\odot$) main sequence star, orbiting a $10^5-10^7 \, \rm M_\odot$ supermassive black hole with an orbital period of order $\sim$hours, and a mild eccentricity ($e\approx0.1-0.2$), episodically shedding mass at each pericenter passage. We argue that the resulting mass transfer is likely unstable, with Roche lobe overflow initially driven by gravitational wave emission, but then being accelerated by the star's expansion in response to its mass loss, undergoing a runaway process. We show that s…
Extragalactic X-ray surveys are exceptionally powerful tools for studying the evolution of supermassive black holes and their host galaxies, by detecting large numbers of active galactic nuclei (AGN) and star-forming galaxies over a wide range of redshifts and cosmic environments. With its sensitivity and superb angular resolution, Chandra has been at the forefront of recent extragalactic surveys. This article provides a brief overview of Chandra surveys, and highlights a few recent results on the composition of the cosmic X-ray background, insights on X-ray source populations, and links betwe…
(ABRIDGED) We present here the results from new Very Long Baseline Array observations at 1.6 and 5 GHz of 19 galaxies of a complete sample of 21 UGC FRI radio galaxies. New Chandra data of two sources, viz., UGC00408 and UGC08433, are combined with the Chandra archival data of 13 sources. The 5 GHz observations of ten "core-jet" sources are polarization-sensitive, while the 1.6 GHz observations constitute second epoch total intensity observations of nine "core-only" sources. Polarized emission is detected in the jets of seven sources at 5 GHz, but the cores are essentially unpolarized, except …
We investigate the evolution of low mass (Md /Mb = 0.005) misaligned gaseous discs around eccentric supermassive black hole (SMBH) binaries. These are expected to form from randomly oriented accretion events onto a SMBH binary formed in a galaxy merger. When expanding the interaction terms between the binary and a circular ring to quadrupole order and averaging over the binary orbit, we expect four non-precessing disc orientations: aligned or counter-aligned with the binary, or polar orbits around the binary eccentricity vector with either sense of rotation. All other orientations precess arou…
Supermassive black hole binaries (SMBHBs) are a natural outcome of galaxy mergers and should form frequently in galactic nuclei. Sub-parsec binaries can be identified from their bright electromagnetic emission, e.g., Active Galactic Nuclei (AGN) with Doppler shifted broad emission lines or AGN with periodic variability, as well as from the emission of strong gravitational radiation. The most massive binaries (with total mass >10^8 M_sol) emit in the nanohertz band and are targeted by Pulsar Timing Arrays (PTAs). Here we examine the synergy between electromagnetic and gravitational wave signatu…
The Large Area Telescope (LAT) onboard the Fermi satellite opened a new era for pulsar astronomy, detecting gamma-ray pulsations from more than 60 pulsars, ~40% of which are not seen at radio wavelengths. One of the most interesting sources discovered by LAT is PSR J0357+3205, a radio-quiet, middle-aged (tau_C ~0.5 Myr) pulsar standing out for its very low spin-down luminosity (Erot ~6x10^33 erg/s), indeed the lowest among non-recycled gamma-ray pulsars. A deep X-ray observation with Chandra (0.5-10 keV), coupled with sensitive optical/infrared ground-based images of the field, allowed us to i…
Resolving the sub-parsec circumnuclear density profiles of quiescent galaxies: Evidence for Bondi accretion flows in tidal disruption event hosts
International Centre for Radio Astronomy Research – Curtin University（国际射电天文中心 - 科廷大学）；School of Natural Sciences, Institute for Advanced Study（自然科学与高级研究院）
The detection of a stochastic gravitational-wave signal from the superposition of many inspiraling supermassive black holes with pulsar timing arrays (PTAs) is likely to occur within the next decade. With this detection will come the opportunity to learn about the processes that drive black-hole-binary systems toward merger through their effects on the gravitational-wave spectrum. We use Bayesian methods to investigate the extent to which effects other than gravitational-wave emission can be distinguished using PTA observations. We show that, even in the absence of a detection, it is possible …
We study the X-ray variability properties of distant AGNs in the Chandra Deep Field-South region over 17 years, up to $z\sim 4$, and compare them with those predicted by models based on local samples. We use the results of Monte Carlo simulations to account for the biases introduced by the discontinuous sampling and the low-count regime. We confirm that variability is an ubiquitous property of AGNs, with no clear dependence on the density of the environment. The variability properties of high-z AGNs, over different temporal timescales, are most consistent with a Power Spectral Density (PSD) de…
Nuclear star clusters that surround supermassive black holes (SMBHs) in galactic nuclei are among the densest systems in the Universe, harbouring millions of stars and compact objects (COs). Within a few parsecs from the SMBH, stars can form binaries. In this paper, we model the supernova (SN) process of massive binaries that are born in proximity of the SMBH and that produce CO binaries. These binaries can later merge via emission of gravitational waves as a consequence of the Lidov-Kozai mechanism. We study the dynamical evolution of these systems by means of high-precision $N$-body simulati…
Supermassive black hole binaries may be detectable by an upcoming suite of gravitational wave experiments. Their binary nature can also be revealed by radio jets via a short-period precession driven by the orbital motion as well as the geodetic precession at typically longer periods. We have investigated Karl G. Jansky Very Large Array (VLA) and MERLIN radio maps of powerful jet sources for morphological evidence of geodetic precession. For perhaps the best studied source, Cygnus A, we find strong evidence for geodetic precession. Projection effects can enhance precession features, for which w…
We present evidence for X-ray line emitting and absorbing gas in the nucleus of the Broad-Line Radio Galaxy (BLRG), 3C 445. A 200ks Chandra LETG observation of 3C 445 reveals the presence of several highly ionized emission lines in the soft X-ray spectrum, primarily from the He and H-like ions of O, Ne, Mg and Si. Radiative recombination emission is detected from O VII and O VIII, indicating that the emitting gas is photoionized. The He-like emission appears to be resolved into forbidden and intercombination line components, which implies a high density of >10^{10} cm^{-3}, while the Oxygen li…
From a Chandra sample of active galactic nuclei (AGNs) in nearby galaxies, we find that for low-luminosity AGNs (LLAGNs), either the intrinsic absorption column density, or the fraction of absorbed AGNs, positively scales with the Eddington ratio for $L_{\rm bol}/L_{\rm Edd} \lesssim 10^{-2}$. Such a behavior, along with the softness of the X-ray spectrum at low luminosities, is in good agreement with the picture that they are powered by hot accretion flows surrounding supermassive black holes. Numerical simulations find that outflows are inevitable with hot accretion flows, and the outflow ra…
Extragalactic jets originating from the central supermassive black holes of active galaxies are powerful, highly relativistic plasma outflows, emitting light from the radio up to the gamma-ray regime. The details of their formation, composition and emission mechanisms are still not completely clear. The combination of high-resolution observations using very long baseline interferometry (VLBI) and multiwavelength monitoring provides the best insight into these objects. Here, such a combined study of sources of the TANAMI sample is presented, investigating the parsec-scale and high-energy proper…
Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be common in galactic nuclei, as a result of frequent galaxy mergers. Hydrodynamical simulations of circumbinary discs predict strong periodic modulation of the mass accretion rate on time-scales comparable to the orbital period of the binary. As a result, SMBHBs may be recognized by the periodic modulation of their brightness. We conducted a statistical search for periodic variability in a sample of 35,383 spectroscopically confirmed quasars in the photometric database of the Palomar Transient Factory (PTF). We analyse…
