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NASA NTRS2021

The Peculiar X-Ray Transient Swift J0840.7-3516: An Unusual Low-mass X-Ray Binary or a Tidal Disruption Event?

We report on the X-ray properties of the new transient Swift J0840.7−3516, discovered with Swift/BAT in 2020February, using extensive data from Swift, MAXI, NICER, and NuSTAR. The source flux increased for∼103safter the discovery, decayed rapidly over∼5 orders of magnitude in five days, and then remained almost constant over nine months. Large-amplitude short-term variations on timescales of 1–104s were observed throughout the decay. In the initial flux rise, the source showed a hard power-law-shaped spectrum with a photon index of∼1.0extending up to∼30 keV, above which an exponential cutoff was present. The photon index increased in the following rapid decay and became∼2 at the end of the decay. A spectral absorption feature at 3–4 keV was detected in the decay. It is not straightforward to explain all the observed properties by any known class of X-ray sources. We discuss the possible nature of the source, including a Galactic low-mass X-ray binary with multiple extreme properties and a tidal disruption event by a supermassive black hole or a Galactic neutron star.

Megumi Shidatsu↗

NASA NTRS2003

What can we learn about cosmic structure from gravitational waves?

Observations of low frequency gravitational waves by the space-based LISA mission will open a new observational window on the early universe and the emergence of structure. LISA will observe the dynamical coalescence of massive black hole binaries at high redshifts, giving an unprecedented look at the merger history of galaxies and the reionization epoch. LISA will also observe gravitational waves from the collapse of supermassive stars to form black holes, and will map the spacetime in the central regions of galaxy cusps at high precision.

Centrella, Joan M.↗

NASA NTRS2008

Neutron Stars and Black Holes Seen with the Rossi X-Ray Timing Explorer (RXTE)

Astrophysical X-rays bring information about location, energy, time, and polarization. X-rays from compact objects were seen in the first explorations to vary in time. Eclipses and pulsations have simple explanations that identified the importance of X-ray binaries and magnetic neutron stars in the first decade of X-ray astronomy. The dynamics of accretion onto stellar and supermassive black holes and onto neutron stars with relatively low magnetic fields shows up as more complex variations, quasi-periodic oscillations, noise with characteristic frequency spectra, broad-band changes in the energy spectra. To study these variations, RXTE instruments needed to have large area and operational flexibility to find transient activity and observe when it was present. Proportional counters and Phoswich scintillators provided it in a modest mission that has made textbook level contributions to understanding of compact objects. The first seen, and the brightest known, X-ray binary, Sco X-1 is one of a class of neutron stars with low mass companions. Before RXTE, none of these had been seen to show pulsations, though they were hypothesized to be the precursors of radio pulsars with millisecond periods and low magnetic fields. RXTE's large area led to identifying coherent millisecond pulsars in a subset which are relatively faint transients. It also led to identifying short episodes of pulsation during thermonuclear bursts, in sources where a steady signal is not seen. The X-ray stage verifies the evolution that produces millisecond radio pulsars.Masses and radii of neutron stars are being determined by various techniques, constraining the equation of state of matter at nuclear densities. Accretion should lead to a range of neutron star masses. An early stage of superstrong magnetic field neutron stars is now known to produce X-ray and gamma-ray bursts in crust quakes and magnetic field reconnection releases of energy. Soft Gamma Repeaters, Anomolous X-ray Pulsars, and high magnetic field rotation-powered pulsars are all now called magnetars, because they have pulse periods indicating they are slowing down as they would with magnetic dipole radiation for a surface field above 5x1013 gauss. The accretion disk has been connected to the launching of radio jets from black holes, and even from neutron stars. Estimates of the angular momenta of black holes are being made from different approaches, modelling a high frequency oscillation that may be related to how close the inner part of the accretion disk is to the black hole, modelling the continua spectra of the X-ray emission, and modeling the emission of red-shifted iron that may be emitted from the accretion disk. These investigations require early discovery of the black hole transient with the All Sky Monitor on RXTE or other monitoring information, frequent extended observations, and coordinated observations with missions that give higher energy resolution, or radio and infrared information.

Swank, Jean↗

NASA NTRS2016

eXTP: Enhanced X-Ray Timing and Polarimetry Mission

eXTP is a science mission designed to study the state of matter under extreme conditions of density, gravity and magnetism. Primary goals are the determination of the equation of state of matter at supra-nuclear density, the measurement of QED effects in highly magnetized star, and the study of accretion in the strong-field regime of gravity. Primary targets include isolated and binary neutron stars, strong magnetic field systems like magnetars, and stellar-mass and supermassive black holes. The mission carries a unique and unprecedented suite of state-of-the-art scientific instruments enabling for the first time ever the simultaneous spectral-timing-polarimetry studies of cosmic sources in the energy range from 0.5-30 keV (and beyond). Key elements of the payload are: the Spectroscopic Focusing Array (SFA) - a set of 11 X-ray optics for a total effective area of approx. 0.9 m(exp. 2) and 0.6 m(exp. 2) at 2 keV and 6 keV respectively, equipped with Silicon Drift Detectors offering less than 180 eV spectral resolution; the Large Area Detector (LAD) - a deployable set of 640 Silicon Drift Detectors, for a total effective area of approx. 3.4 m(exp. 2), between 6 and 10 keV, and spectral resolution better than 250 eV; the Polarimetry Focusing Array (PFA) - a set of 2 X-ray telescope, for a total effective area of 250 cm(exp. 2) at 2 keV, equipped with imaging gas pixel photoelectric polarimeters; the Wide Field Monitor (WFM) - a set of 3 coded mask wide field units, equipped with position-sensitive Silicon Drift Detectors, each covering a 90 degrees x 90 degrees field of view. The eXTP international consortium includes major institutions of the Chinese Academy of Sciences and Universities in China, as well as major institutions in several European countries and the United States. The predecessor of eXTP, the XTP mission concept, has been selected and funded as one of the so-called background missions in the Strategic Priority Space Science Program of the Chinese Academy of Sciences since 2011. The strong European participation has significantly enhanced the scientific capabilities of eXTP. The planned launch date of the mission is earlier than 2025.

X-ray astronomy↗

NASA NTRS2021

The high energy Universe at ultra-high resolution: the power and promise of X-ray interferometry

We propose the development of X-ray interferometry (XRI), to reveal the Universe at high energies with ultra-high spatial resolution. With baselines which can be accommodated on a single spacecraft, XRI can reach 100 μ as resolution at 10 Å (1.2 keV) and 20 μ as at 2 Å (6 keV), enabling imaging and imaging-spectroscopy of (for example) X-ray coronae of nearby accreting supermassive black holes (SMBH) and the SMBH ‘shadow’; SMBH accretion flows and outflows; X-ray binary winds and orbits; stellar coronae within ∼100 pc and many exoplanets which transit across them. For sufficiently luminous sources XRI will resolve sub-pc scales across the entire observable Universe, revealing accreting binary SMBHs and enabling trigonometric measurements of the Hubble constant with X-ray light echoes from quasars or explosive transients. A multi-spacecraft ‘constellation’ interferometer would resolve well below 1 μ as, enabling SMBH event horizons to be resolved in many active galaxies and the detailed study of the effects of strong field gravity on the dynamics and emission from accreting gas close to the black hole.

Phil Uttley↗

DOE OSTI2022

Long-term Evolution of Tightly Packed Stellar Black Holes in AGN Disks: Formation of Merging Black Hole Binaries via Close Encounters

We study the long-term evolution of two or more stellar black holes (BHs) on initially separated but unstable circular orbits around a supermassive BH (SMBH). Such a close-packed orbital configuration can naturally arise from BH migrations in the AGN disk. Dynamical instability of the orbits leads to recurring close encounters between two BHs, during which the BH separation r p becomes less than the Hill radius R H . In rare very close encounters, a tight merging BH binary can form with the help of gravitational wave emission. We use N-body simulations to study the time evolution of close encounters of various degrees of closeness. For a typical "SMBH+2BH" system, the averaged cumulative number of close encounters (with r p ≲ R H ) scales approximately as ∝ t 0.5 . The minimum encounter separation r p follows a cumulative distribution P( p ) ∝ r p for r p « R H . We obtain a semi-analytical expression for the averaged rate of binary captures that lead to BH mergers. Our results suggest that close-packed BHs in AGN disks may take a long time (≳10 7 orbits around the SMBH) to experience a sufficiently close encounter and form a bound binary. This time can be shorter if the initial BH orbits are highly aligned. The BH binary mergers produced in this scenario have high eccentricities when entering the LIGO band and broad distribution of orbital inclinations relative to the original AGN disk. We explore the effects of the gas disk and find that simple gas drags on the BHs do not necessarily lead to an enhanced BH binary capture rate.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2024

Exploring dark forces with multimessenger studies of extreme mass ratio inspirals

Abstract The exploration of dark sector interactions via gravitational waves (GWs) from binary inspirals has been a subject of recent interest. We study dark forces using extreme mass ratio inspirals (EMRIs), pointing out two issues of interest. Firstly, the innermost stable circular orbit (ISCO) of the EMRI, which sets the characteristic length scale of the system and hence the dark force range to which it exhibits enhanced sensitivity, probes force mediator masses that complement those studied with supermassive black hole (SMBH) or neutron star binaries. The LISA mission (the proposedμAres detector) will probe mediators with massesm V ∼ 10 -16 eV (m V ∼ 10 -18 eV), corresponding to ISCOs of 10 6 M ⊙ (10 8 M ⊙ ) central SMBHs. Secondly, while the sensitivity to dark couplings is typically limited by the uncertainty in the binary component masses, independent mass measurements of the central SMBH through reverberation mapping campaigns or the motion of dynamical tracers enable one to break this degeneracy. Our results therefore highlight the necessity for coordinated studies, loosely referred to as “multimessenger”, between futureμHz- mHz GW observatories and ongoing and forthcoming SMBH mass measurement campaigns, including OzDES-RM, SDSS-RM, and SDSS-V Black Hole Mapper.

Astronomy & Astrophysics↗

DOE OSTI2023

Probing the Origin of Changing-look Quasar Transitions with Chandra

Extremely variable quasars can also show strong changes in broad-line emission strength and are known as changing-look quasars (CLQs). To study the CLQ transition mechanism, we present a pilot sample of CLQs with X-ray observations in both the bright and faint states. From a sample of quasars with bright-state archival SDSS spectra and (Chandra or XMM-Newton) X-ray data, we identified five new CLQs via optical spectroscopic follow-up and then obtained new target-of-opportunity X-ray observations with Chandra. No strong absorption is detected in either the bright- or the faint-state X-ray spectra. The intrinsic X-ray flux generally changes along with the optical variability, and the X-ray power-law slope becomes harder in the faint state. Large-amplitude mid-infrared variability is detected in all five CLQs, and it echoes the variability in the optical with a time lag expected from the light-crossing time of the dusty torus for CLQs with robust lag measurements. The changing-obscuration model is not consistent with the observed X-ray spectra and spectral energy distribution changes seen in these CLQs. It is highly likely that the observed changes are due to the changing accretion rate of the supermassive black hole, so the multiwavelength emission varies accordingly, with promising analogies to the accretion states of X-ray binaries.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2011

Black Hole Coalescence: The Gravitational Wave Driven Phase

When two supermassive black holes (SMBHS) approach within 1-10 mpc, gravitational wave (GW) losses begin to dominate the evolution of the binary, pushing the system to merge in a relatively small time. During this final inspiral regime, the system will emit copious energy in GWs, which should be directly detectable by pulsar timing arrays and space-based interferometers. At the same time, any gas or stars in the immediate vicinity of the merging 5MBHs can get heated and produce bright electromagnetic (EM) counterparts to the GW signals. We present here a number of possible mechanisms by which simultaneous EM and GW signals will yield valuable new information about galaxy evolution, accretion disk dynamics, and fundamental physics in the most extreme gravitational fields.

Schnittman, Jeremy D.↗

NASA NTRS2011

The Astrophysics of Merging Black Holes

When two supermassive black holes (SMBHs) approach within 1-10 mpc, gravitational wave (GW) losses begin to dominate the evolution of the binary, pushing the system to merge in a relatively small time. During this final inspiral regime, the system will emit copious energy in GWs, which should be directly detectable by pulsar timing arrays and space-based interferometers. At the same time, any gas or stars in the immediate vicinity of the merging 5MBHs can get heated and produce bright electromagnetic (EM) counterparts to the GW signals. We present here a number of possible mechanisms by which simultaneous EM and GW signals will yield valuable new information about galaxy evolution, accretion disk dynamics, and fundamental physics in the most extreme gravitational fields.

Schnittman, Jeremy D.↗

NASA NTRS2012

Electromagnetic Signatures of SMBH Coalescence

Schnittman, Jeremy↗

NASA NTRS2003

Ring Around the Black Hole

Regardless of size, black holes easily acquire accretion disks. Supermassive black holes can feast on the bountiful interstellar gas in galactic nuclei. Small black holes formed from collapsing stars often belong to binary systems in which a bulging companion star can spill some of its gas into the black hole s reach. In the chaotic mess of the accretion disk, atoms collide with one another. Swirling plasma reaches speeds upward of 10% that of light and glows brightly in many wavebands, particularly in X-rays. Gas gets blown back by a wind of radiation from the inner disk. New material enters the disks from different directions.

Wanjek, Christopher↗

NASA NTRS2012

Variability Selected Low-Luminosity Active Galactic Nuclei in the 4 Ms Chandra Deep Field-South

The 4 Ms Chandra Deep Field-South (CDF-S) and other deep X-ray surveys have been highly effective at selecting active galactic nuclei (AGN). However, cosmologically distant low-luminosity AGN (LLAGN) have remained a challenge to identify due to significant contribution from the host galaxy. We identify long-term X ray variability (approx. month years, observed frame) in 20 of 92 CDF-S galaxies spanning redshifts approx equals 00.8 - 1.02 that do not meet other AGN selection criteria. We show that the observed variability cannot be explained by X-ray binary populations or ultraluminous X-ray sources, so the variability is most likely caused by accretion onto a supermassive black hole. The variable galaxies are not heavily obscured in general, with a stacked effective power-law photon index of Gamma(sub Stack) approx equals 1.93 +/- 0.13, and arc therefore likely LLAGN. The LLAGN tend to lie it factor of approx equal 6-89 below the extrapolated linear variability-luminosity relation measured for luminous AGN. This may he explained by their lower accretion rates. Variability-independent black-hole mass and accretion-rate estimates for variable galaxies show that they sample a significantly different black hole mass-accretion-rate space, with masses a factor of 2.4 lower and accretion rates a factor of 22.5 lower than variable luminous AGNs at the same redshift. We find that an empirical model based on a universal broken power-law power spectral density function, where the break frequency depends on SMBH mass and accretion rate, roughly reproduces the shape, but not the normalization, of the variability-luminosity trends measured for variable galaxies and more luminous AGNs.

Young, M.↗

DOE OSTI2021

Confrontation of Observation and Theory: High-frequency QPOs in X-Ray Binaries, Tidal Disruption Events, and Active Galactic Nuclei

In this paper, we compile observations of high-frequency quasi-periodic oscillations (QPOs) around black holes, both stellar and supermassive, and compare their positions in the parameter space of black hole mass, spin, and oscillation frequency. We find that supermassive black holes occupy a separate region of parameter space than stellar, and further, that QPOs seen around tidal disruption events rather than Seyfert-type active galactic nuclei (AGN) occupy an entirely different space. We then compare these results to the orbital resonance, diskoseismic, warped disk, and disk–jet coupling theoretical models for the origin of high-frequency QPOs. We find that while oscillations around stellar mass black holes are generally consistent with the above models, supermassive black holes are decidedly not. Oscillations seen in tidal disruption events are consistent with oscillations near the frequency of the innermost stable circular orbit, while QPOs in AGN are not accounted for by any of the physical models in consideration. This indicates that despite the scale invariance of accretion processes implied by a decades-wide correlation between QPO frequency and black hole mass, any theory of high-frequency QPOs must relate the frequency to more than just the mass and spin.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2025

Gravitational Waves from Primordial Black Hole Dark Matter Spikes

The origin of the binary black hole mergers observed by LIGO–Virgo–KAGRA remains an open question. We calculate the merger rate from primordial black holes (PBHs) within the density spike around supermassive black holes (SMBHs) at the centers of galaxies. We show that the merger rate within the spike is comparable to that within the wider dark matter halo. We also calculate the extreme mass ratio inspiral (EMRI) signal from PBHs hosted within the density spike spiralling into their host SMBHs due to gravitational-wave emission. We predict that LISA may detect ∼10 4 of these EMRIs with a signal-to-noise ratio threshold of 20 within a 4 yr observation run, if all dark matter is made up of ∼30 M ⊙ PBHs. Uncertainties in our rates come from the uncertain mass fraction of PBHs within the dark matter spike, relative to the host central SMBHs, which defines the parameter space LISA can constrain.

dark matter↗

DOE OSTI2021

AGNs on the Move: A Search for Off-nuclear AGNs from Recoiling Supermassive Black Holes and Ongoing Galaxy Mergers with the Zwicky Transient Facility

A supermassive black hole (SMBH) ejected from the potential well of its host galaxy via gravitational wave recoil carries important information about the mass ratio and spin alignment of the pre-merger SMBH binary. Such a recoiling SMBH may be detectable as an active galactic nucleus (AGN) broad-line region offset by up to 10 kpc from a disturbed host galaxy. We describe a novel methodology using forward modeling with The Tractor to search for such offset AGNs in a sample of 5493 optically variable AGNs detected with the Zwicky Transient Facility (ZTF). We present the discovery of nine AGNs that may be spatially offset from their host galaxies and are candidates for recoiling SMBHs. Five of these offset AGNs exhibit double-peaked broad Balmer lines, which may have arisen from unobscured accretion disk emission, and four show radio emission indicative of a relativistic jet. The fraction of double-peaked emitters in our spatially offset AGN sample is significantly larger than the 16% double-peaked emitter fraction observed for ZTF AGNs overall. In our sample of variable AGNs we also identified 52 merging galaxies, including a new spectroscopically confirmed dual AGN. Finally, we detected the dramatic rebrightening of SDSS 1133, a previously discovered variable object and recoiling SMBH candidate, in ZTF. The flare was accompanied by the reemergence of strong P Cygni line features, indicating that SDSS 1133 may be an outbursting luminous blue variable star.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2011

A Galactic Binary Detection Pipeline

The Galaxy is suspected to contain hundreds of millions of binary white dwarf systems, a large fraction of which will have sufficiently small orbital period to emit gravitational radiation in band for space-based gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA). LISA's main science goal is the detection of cosmological events (supermassive black hole mergers, etc.) however the gravitational signal from the galaxy will be the dominant contribution to the data - including instrumental noise over approximately two decades in frequency. The catalogue of detectable binary systems will serve as an unparalleled means of studying the Galaxy. Furthermore, to maximize the scientific return from the mission, the data must be "cleansed" of the galactic foreground. We will present an algorithm that can accurately resolve and subtract 2:: 10000 of these sources from simulated data supplied by the Mock LISA Data Challenge Task Force. Using the time evolution of the gravitational wave frequency, we will reconstruct the position of the recovered binaries and show how LISA will sample the entire compact binary population in the Galaxy.

Littenberg, Tyson B.↗

NASA NTRS2012

A Detection Pipeline for Galactic Binaries in LISA Data

The Galaxy is suspected to contain hundreds of millions of binary white dwarf systems, a large fraction of which will have sufficiently small orbital period to emit gravitational radiation in band for space-based gravitational wave detectors such as the Laser Interferometer Space Antenna (LISA). LISA's main science goal is the detection of cosmological events (supermassive black hole mergers) etc.) however the gravitational signal from the galaxy will be the dominant contribution to the data - including instrumental noise - over approximately two decades in frequency. The catalogue of detectable binary systems will serve as an unparalleled means of studying the Galaxy. Furthermore, to maximize the scientific return from the mission, the data must be "cleansed" of the galactic foreground. We will present an algorithm that can accurately resolve and subtract greater than or equal to 10000 of these sources from simulated data supplied by the Mock LISA Data Challenge Task Force. Using the time evolution of the gravitational wave frequency, we will reconstruct the position of the recovered binaries and show how LISA will sample the entire compact binary population in the Galaxy.

Littenberg, Tyson B.↗