We use the publicly available XMM-Newton archive to select a sample of 20 active galactic nuclei (AGN) known to exhibit reverberation signatures caused by the reflection of X-rays from the corona off the accretion disc that feeds the central black hole engine. Inverse Compton scattering by energetic electrons, coupled with accretion disc fluctuations give rise to the highly variable observed X-ray spectrum, the behaviour of which is still not fully understood. We use 121 observations in 3 - 4 distinct spectral states for each source and calculate the time lags as a function of frequency. We fit the relativistic reflection model RELXILL and explore parameter correlations. The known scaling relationship between the black hole mass and time lag is well recovered and the continuum flux is coupled strongly to the disc reflection flux. We also find that 1H 0707-495 and IRAS 13224-3809 are well described using reflection and absorption modelling in a variety of flux states. The reflection fraction is strongly coupled to the power law photon index and may be linked to dynamics of the emitting region. The data reveals hints of the power law evolutionary turnover when the 2 - 10 keV Eddington fraction is $\sim0.02$, the origin of which is not fully understood. Finally, we report the covering fraction is inversely correlated with the flux and power law photon index in IRAS 13224-3809. These findings support recent studies of 1H 0707-495 where the covering fraction may contribute to the observed variability via flux modulations from non-uniform orbiting clouds.
Using long-term monitoring data from the Rossi X-ray Timing Explorer (RXTE), we have selected 23 active galactic nuclei (AGN) with sufficient brightness and overall observation time to derive broadband X-ray spectra from 3 to >100 keV. Our sample includes mainly radio-quiet Seyferts, as well as seven radio-loud sources. Given the longevity of the RXTE mission, the greater part of our data is spread out over more than a decade, providing truly long-term average spectra and eliminating inconsistencies arising from variability. We present long-term average values of absorption, Fe line parameters…
We have analyzed spectra for all active galactic nuclei in the Rossi X-ray Timing Explorer (RXTE) archive. We present long-term average values of absorption, Fe line equivalent width, Compton reflection and photon index, as well as calculating fluxes and luminosities in the 2-10 keV band for 100 AGN with sufficient brightness and overall observation time to yield high quality spectral results. We compare these parameters across the different classifications of Seyferts and blazars. Our distributions of photon indices for Seyfert 1's and 2's are consistent with the idea that Seyferts share a co…
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of radio-loud Active Galactic Nuclei. For a summary, we refer to the paper.
A Whisper from Within: Response of a Pulsar Timing Array to an Internal Gravitational-wave Source
Peking University（北京大学）；Kavli Institute for Astronomy and Astrophysics at Peking University（北京大学科维理天文与天体物理研究所）；Shanghai Astronomical Observatory, CAS（中国科学院上海天文台）
Massachusetts Institute of Technology（麻省理工学院）；Institute for Advanced Study（高等研究院）；Princeton University（普林斯顿大学）；Harvard University（哈佛大学）；New York University（纽约大学）
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…
Pulsar timing arrays (PTAs) are Galactic-scale gravitational wave (GW) detectors consisting of precisely-timed pulsars distributed across the sky. Within the decade, PTAs are expected to detect the nanohertz GWs emitted by close-separation supermassive black hole binaries (SMBHBs), thereby opening up the low frequency end of the GW spectrum for science. Individual SMBHBs which power active galactic nuclei are also promising multi-messenger sources; they may be identified via theoretically predicted electromagnetic (EM) signatures and be followed up by PTAs for GW observations. In this work, we…
The origin of rapid neutron capture (r-process) nuclei remains one of the longest standing mysteries in nuclear astrophysics. Core collapse supernovae (SNe) and neutron star binary mergers are likely r-process sites, but little evidence yet exists for their in situ formation in such environments. Motivated by the advent of sensitive new or planned X-ray telescopes such as the Nuclear Spectroscopic Telescope Array (NuSTAR) and the Large Observatory for X-ray Timing (LOFT), we revisit the prospects for the detection of X-ray decay lines from r-process nuclei in young or nearby supernova remnants…
X-ray spectra of active galactic nuclei (AGN) are usually described as power law spectra, characterized by the spectral slope $α$ or photon index $Γ$. Here we discuss the X-ray spectral properties within the framework of clumpy accretion flows, and estimate the power law slope as a function of the source parameters. We expect harder spectra in massive objects than in less massive sources, and steeper spectra in higher accretion rate systems. The predicted values of the photon index cover the range of spectral slopes typically observed in Seyfert galaxies and quasars. The overall trends are con…
We study the origin of the soft X-ray excess seen in the 'simple' Narrow Line Seyfert 1 galaxy PG1244+026 using all available spectral-timing information. This object shows the standard switch between soft leading the hard band on long timescales, to the opposite behaviour on short timescales. This is interpreted as a combination of intrinsic fluctuations propagating down through the accretion flow giving the soft lead, together with reflection of the hard X-rays giving the soft lag. We build a full model of the spectral and time variability including both propagation and reflection, and compa…
We present low- and high-resolution mid-infrared (mid-IR) spectra and photometry for eight compact symmetric objects (CSOs) taken with the Infrared Spectrograph on the Spitzer Space Telescope. The hosts of these young, powerful radio galaxies show significant diversity in their mid-IR spectra. This includes multiple atomic fine-structure lines, H2 gas, polycyclic aromatic hydrocarbon (PAH) emission, warm dust from T = 50 to 150 K, and silicate features in both emission and absorption. There is no evidence in the mid-IR of a single template for CSO hosts, but 5/8 galaxies show similar moderate …
We present the timing and spectral analysis of the Compton Thick Seyfert 2 active galactic nuclei NGC 1068 observed using {\it NuSTAR} and {\it XMM-Newton}. In this work for the first time we calculated the coronal temperature ($\rm{kT_{e}}$) of the source and checked for its variation between the epochs if any. The data analysed in this work comprised of (a) eight epochs of observations with {\it NuSTAR} carried out during the period December 2012 to November 2017, and, (b) six epochs of observations with {\it XMM-Newton} carried out during July 2000 to February 2015. From timing analysis of …
Constraining the Binarity of Massive Black Holes in the Galactic Center and Some Nearby Galaxies via Pulsar Timing Array Observations of Gravitational Waves
We calculate the radial profiles of galaxies where the nuclear region is self-gravitating, consisting of self-interacting dark matter (SIDM) with $F$ degrees of freedom. For sufficiently high density this dark matter becomes collisional, regardless of its behaviour on galaxy scales. Our calculations show a spike in the central density profile, with properties determined by the dark matter microphysics, and the densities can reach the `mean density' of a black hole (from dividing the black-hole mass by the volume enclosed by the Schwarzschild radius). For a galaxy halo of given compactness ($χ=…
High-time-resolution X-ray observations of compact objects provide direct access to strong-field gravity, to the equation of state of ultra-dense matter and to black hole masses and spins. A 10 m^2-class instrument in combination with good spectral resolution is required to exploit the relevant diagnostics and answer two of the fundamental questions of the European Space Agency (ESA) Cosmic Vision Theme "Matter under extreme conditions", namely: does matter orbiting close to the event horizon follow the predictions of general relativity? What is the equation of state of matter in neutron stars…
X-ray Timing and Spectral studies of the bare AGN Mrk 110
Department of Astronomy, University College of Science, Osmania University（奥斯曼尼亚大学科学学院天文学系）；Space Astronomy Group, ISITE Campus, U. R. Rao Satellite Centre（U.R. Rao卫星中心ISITE校区空间天文学组）
The inner 20 gravitational radii around the black hole at the centre of luminous Active Galactic Nuclei and stellar mass Black Hole Binaries are now being routinely mapped by X-ray spectral-timing techniques. Spectral blurring and reverberation of the reflection spectrum are key tools in this work. In the most extreme AGN cases with high black hole spin, when the source appears in a low state, observations probe the region within 1 gravitational radius of the event horizon. The location, size and operation of the corona, which generates the power-law X-ray continuum, are also being revealed.
Newton's gravitational constant $G$ may vary with time at an extremely low level. The time variability of $G$ will affect the orbital motion of a millisecond pulsar in a binary system and cause a tiny difference between the orbital period-dependent measurement of the kinematic distance and the direct measurement of the annual parallax distance. PSR J0437$-$4715 is the nearest millisecond pulsar and the brightest at radio. To explore the feasibility of achieving a parallax distance accuracy of one light-year, comparable to the recent timing result, with the technique of differential astrometry,…
The All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE) has continuously monitored a number of Active galactic nuclei (AGNs) with similar sampling rates for 14 years from 1996 January to 2009 December. Utilizing the archival ASM data of 27 AGNs, we calculate the normalized excess variances of the 300-day binned X-ray light curves on the longest timescale (between 300 days and 14 years) explored so far. The observed variance appears to be independent of AGN black hole mass and bolometric luminosity, respectively. According to the scaling relation with black hole mass (and bolom…
Gravitational-Wave Signatures of Highly Eccentric Stellar-Mass Binary Black Holes in Galactic Nuclei
Monash University（莫纳什大学）；OzGrav: Australian Research Council Centre of Excellence for Gravitational Wave Discovery（澳大利亚研究委员会引力波发现卓越中心）；Department of Applied Mathematics and Theoretical Physics, Cambridge（剑桥大学应用数学与理论物理系）；Kavli Institute for Cosmology Cambridge（卡弗里宇宙学剑桥研究所）；H. H. Wills Physics Laboratory, University of Bristol（布里斯托大学H.H.威尔斯物理实验室）；Gravity Exploration Institute, School of Physics and Astronomy, Cardiff University（卡迪夫大学物理与天文学重力探索研究所）；Departamento de Astronomía, Facultad Ciencias Físicas y Matemáticas, Universidad de Concepción（康塞普西翁大学物理与数学科学学院天文学系）；National Institute for Nuclear Physics – INFN, Sezione di Trieste（意大利国家核物理研究所的里雅斯特分部）
This is a White Paper in support of the mission concept of the Large Observatory for X-ray Timing (LOFT), proposed as a medium-sized ESA mission. We discuss the potential of LOFT for the study of active galactic nuclei. For a summary, we refer to the paper.
GRAVITAS is an X-ray observatory, designed and optimised to address the ESA Cosmic Vision theme of "Matter under extreme conditions". It was submitted as a response to the call for M3 mission proposals. The concept centres around an X-ray telescope of unprecedented effective area, which will focus radiation emitted from close to the event horizon of black holes or the surface of neutron stars. To reveal the nature and behaviour of matter in the most extreme astrophysical environments, GRAVITAS targets a key feature in the X-ray spectra of compact objects: the iron Kalpha line at ~6.5 keV. The …
A Population Study for Searching Supermassive Binary Black Holes in Active Galactic Nuclei: Continuum Spectral Features and Periodic Variabilities
National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；School of Astronomy and Space Sciences, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）
A fraction of active galactic nuclei (AGN) exhibit dramatic variability, which is observed on timescales down to minutes in the X-ray band. We introduce the case study of 1H 1934-063 (z = 0.0102), a Narrow-line Seyfert 1 (NLS1) among the brightest and most variable AGN ever observed with XMM-Newton. This work includes spectral and temporal analyses of a concurrent XMM-Newton and NuSTAR 2015 observation lasting 130 kiloseconds, during which the X-ray source exhibited a steep (factor of 6) plummet and subsequent full recovery of flux level, accompanied by deviation from a single log-normal flux …
AGN, powered by accretion onto SMBHs, are thought to be scaled up versions of Galactic black hole X-ray binaries (BH-XRBs). In the past few years evidence of such correspondence include similarities in the broadband shape of the X-ray variability power spectra, with characteristic bend times-scales scaling with mass. We have performed a uniform analysis of the power spectrum densities (PSDs) of 104 nearby (z<0.4) AGN using 209 XMM-Newton/pn observations. The PSDs have been estimated in three energy bands: 0.2-10, 0.2-2, and 2-10 keV. The sample comprises 61 Type-1 AGN, 21 Type-2 AGN, 15 NLSy1,…
The focussing optics of NuSTAR have enabled high signal-to-noise spectra to be obtained from many X-ray bright Active Galactic Nuclei (AGN) and Galactic Black Hole Binaries (BHB). Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects. Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size. Our results indicate that…
X-ray selected BL Lac objects dominate the population of extragalactic sources detected in the very high energy (VHE; photon energies >100GeV) gamma-ray regime with ground-based Cherenkov Telescopes. These are not the only extragalactic VHE emitters, however, since also other types of active galactic nuclei (AGN) have recently been established as VHE sources as well. They include radio galaxies, optically-selected BL Lacs, and Flat-Spectrum Radio Quasars. Here we review the current status of the ongoing AGN studies with the H.E.S.S. instrument, concentrating on the following main three topics:…
The anti- and positive correlations between X-ray photon index and Eddington-scaled X-ray luminosity were found in decay phase of X-ray binary outbursts and a sample of active galactic nuclei in former works. We further systematically investigate the evolution of X-ray spectral index, along the X-ray flux and Eddington ratio in eight outbursts of four black-hole X-ray binaries, where all selected outbursts have observational data from Rossi X-ray Timing Explorer in both rise and decay phases. In the initial rise phase, the X-ray spectral index is anti-correlated with the flux, and the X-ray sp…
The thermal dominant state in black hole binaries (BHBs) is well understood but rarely seen in ultraluminous X-ray sources (ULXs). Using simultaneous observations of M82 with Chandra and XMM-Newton, we report the first likely identification of the thermal dominant state in a ULX based on the disappearance of X-ray oscillations, low timing noise, and a spectrum dominated by multicolor disk emission with luminosity varying to the 4th power of the disk temperature. This indicates that ULXs are similar to Galactic BHBs. The brightest X-ray spectrum can be fitted with a relativistic disk model with…
We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, mult…
To gain a better understanding of the inner disc region that comprises active galactic nuclei it is necessary to understand the pattern in which the disc is illuminated (the emissivity profile) by X-rays emitted from the continuum source above the black hole (corona). The differences in the emissivity profiles produced by various corona geometries are explored via general relativistic ray tracing simulations. Through the analysis of various parameters of the geometries simulated it is found that emissivity profiles produced by point source and extended geometries such as cylindrical slabs and …
We present an extended corona model based on ray-tracing simulations to investigate X-ray time lags in Active Galactic Nuclei (AGN). This model consists of two axial point sources illuminating an accretion disc that produce the reverberation lags. These lags are due to the time delays between the directly observed and reflection photons and are associated with the light-travel time between the source and the disc, so they allow us to probe the disc-corona geometry. We assume the variations of two X-ray sources are triggered by the same primary variations, but allow the two sources to respond i…
"Changing look" Active Galactic Nuclei (AGN) are a subset of Seyfert galaxies characterized by rapid transitions between Compton-thin and Compton-thick regimes. In their Compton-thin state, the central engine is less obscured, hence spectroscopy or timing observations can probe their innermost structures. However, it is not clear if the observed emission features and the Compton hump are associated with relativistic reflection onto the accretion disc, or complex absorption by distant, absorbing gas clouds passing by the observer's line-of-sight. Here, we investigate these two scenarios under t…
