• Einstein
• Fellowship

A Closer Look at Black Holes

Several new techniques are currently being employed to probe the strong gravitational field in the vicinity of supermassive black holes. Long baseline interferometry at sub-millimeter wavelengths constrains the silhouette of the black holes in the Galactic center (SgrA*) and M87. Stars which get tidally disrupted as they orbit too close to a single black hole are being discovered at cosmological distances. Electromagnetic counterparts of black hole binaries in galaxy mergers are being identified, and can be used to calibrate the rate of gravitational wave sources for eLISA/NGO. Most interestingly, the recoil induced by the anisotropic emission of gravitational waves in the final plunge of binaries leaves unusual imprints on their host galaxies.

Disentangling the Signatures of Supermassive Black Hole Inspiral and Recoil: A Case Study

Many supermassive black hole (SMBH) pair candidates have recently been discovered, along with a handful of gravitational-wave (GW) recoiling SMBH candidates. Among these, the galaxy CXOC J100043.1+020637 (also known as CID-42) is particularly intriguing. An apparent galaxy merger remnant, it displays signatures of both an inspiraling, kiloparsec-scale active galactic nucleus (AGN) pair and of a recoiling AGN with a kick velocity of at least 1300 km s^-1. CID-42 is the only recoiling AGN candidate with both spatial offsets (in optical and X-ray bands) and spectroscopic offsets. Accordingly, this object presents an opportunity to test our notions about the characteristic signatures of SMBH insprial and recoil. I will describe the results of numerical modeling of CID-42 as a galaxy merger remnant and the constraints that these models place on the recoiling and dual SMBH scenarios. I will also discuss the most promising prospects for distinguishing these possibilities with future observations and, more generally, the importance of multi-wavelength follow-up confirmation of any such candidate.

Testing Galaxy Formation Models: Characterizing Extended Hot Coronae Around Massive Spiral Galaxies

The presence of hot gaseous coronae in the dark matter halos of massive galaxies is a basic prediction of galaxy formation models. Theoretical models predict copious X-ray emission at large radii around massive spiral galaxies. We have studied two galaxies, NGC1961 and NGC6753, that are optically luminous and massive, with moderate star formation rates, and that can be probed to sufficiently large radii. For these two galaxies we detect emission with sufficient counts to measure X-ray gas temperatures and gas abundances. Hence, for the first time, we are able to characterize the properties - X-ray luminosity, gas temperature, elemental abundance, gas density, and gas mass - of hot coronae in normal spiral galaxies.

New Radio Minihalos Discovered in Cluster Cool Cores

Our recent systematic search for new radio minihalos has almost doubled the number of minihalo clusters known, suggesting that these radio sources are more common than previously thought. The new discoveries allow a statistical study of the minihalo properties and their correlation with the X-ray properties of their cluster hosts. In particular, minihalos are found in massive clusters with dense, sloshing cool cores, supporting the earlier hypothesis that the radio-emitting electrons are produced by gas sloshing.

Why Do Spinning Black Hole Binaries "bob" and "kick"?

Numerical simulations of binary black holes with spin have revealed some surprising behavior: for antialigned spins in the orbital plane, 1) one sees an up-and-down "bobbing" of the entire orbital plane at the orbital frequency and 2) the merged black hole receives an enormous kick that depends on the phase at merger. Natural questions are: What causes the bobbing? Can the kick be viewed as a post-merger continuation of the bobbing? We study the black hole binary in a slow-motion limit and compare to mechanical and electromagnetic analogs. We find that bobbing occurs in all systems and shares a simple kinematic origin. For the correlation to kicks, a momentum-flow analysis suggests a key role played by the curious phenomenon of "hidden mechanical momentum". We investigate, in general, the efficiency of general relativity in releasing hidden momentum during black hole formation.

The Extreme Side of AGN Feedback

AGN feedback plays a fundamental role in shaping the properties of the host galaxy and the surrounding medium. Yet, the details of how AGN feedback operates remain poorly understood and form an important part of research in modern astrophysics. Some of the most powerful and extreme examples of AGN feedback are found in clusters of galaxies, where the central black hole lying in the Brightest Cluster Galaxy is capable of driving large jetted outflows filled with radio-emitting particles. In this talk, I will review our current understanding of this field, concentrating on the most energetic AGN-driven outflows known in clusters of galaxies. I will also present recent results regarding the masses of these monster black holes, and how the outflow and radiative properties of the AGN can help constrain the black hole masses, ultimately providing evidence for the existence of ultramassive black holes (i.e. black holes with >10^10 Msun masses).

The Co-evolution of Galaxies and Supermassive Black Holes

Abstract Unavailable

Radio Polarimetry and The Pulsar Machine

Pulsar magnetospheres are geometrically simple, and observables are largely determined by two unknown parameters: ζ (α), the observer's (magnetic dipole axis') inclination to the pulsar's spin axis. By exploiting the tendency for radiation to be beamed along magnetic field lines, gamma-ray light curves can be used to infer the position and extent of the emitting volume. However, degeneracy with ζ and α prevents a unique determination of the volume. This degeneracy can be broken - or at least ameliorated - by applying constraints on ζ and α obtained from radio polarimetry. To support the search for gamma-ray pulsars, the Parkes radio telescope has performed monthly timing observations of 160 energetic radio pulsars for nearly 5 years. Co-added, these data yield some of the highest signal-to-noise polarimetry in existence. We present the constraints obtained from a uniform analysis of the polarimetry and discuss the implications for the population and for particularly well-constrained pulsars.

The Origins and Evolution of Ultra-Relativistic Electrons in the Milky Way

Abstract Unavailable

Multiscale Plasma Dynamics and Anisotropic Transport in the Intracluster Medium

The intracluster medium (ICM) of galaxy clusters is a weakly collisional, high-beta plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Assessing the efficacy of this transport is a vital step in understanding how the ICM avoids catastrophic cooling, why cool-core clusters exhibit remarkably similar temperature profiles, and what generates and sustains the observationally inferred turbulent velocity and magnetic fields. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign, the magnetothermal instability (MTI) in the outskirts of non-isothermal clusters and the heat-flux-driven buoyancy instability (HBI) in their cooling cores. The local changes in magnetic field strength that attend these instabilities cause pressure anisotropies that viscously damp motions parallel to the magnetic field. In this talk, I will discuss two important effects of pressure anisotropy on the dynamical and thermal stability of the ICM. First, by stifling the convergence/divergence of magnetic field lines, pressure anisotropy significantly affects how the ICM interacts with the temperature gradient. Instabilities which depend upon convergence/divergence of magnetic field lines to generate unstable buoyant motions (the HBI) are suppressed, whereas those which are otherwise impeded by field-line convergence/divergence (the MTI) are strengthened. Second, because the viscous heating of the ICM is regulated by the pressure anisotropy - which itself is nonlinearly regulated by the plasma beta parameter via rapidly-growing microscale instabilities - pressure anisotropy may play a crucial role in mitigating cooling flows and preventing cluster core collapse. I will discuss the physical interpretation of these effects in detail, placing them within the larger context of formulating a pragmatic analytical and numerical framework for modeling astrophysical multi-scale plasma dynamics.

Stellar Rotation and its Impact on Ionizing Spectra

Models of stellar evolution play a key role in a wide variety of research areas within astrophysics, including galaxy evolution, the properties of the first stars, supernova and GRB progenitor studies, and the hosts of extrasolar planets. Among their many applications, stellar evolutionary models are a key ingredient in stellar population synthesis codes, providing the key framework that is used to assemble a complete stellar population and track its temporal evolution. Ultimately, such codes are used to generate synthetic ionizing spectra, which can be directly compared to existing observations or used as inputs in photoionization codes to model the emission line spectra of HII regions and star-forming galaxies. Recently, a new grid of stellar models has been released that includes the first detailed implementation of stellar rotation effects. We have found that the inclusion of rotation has a substantial impact on the main-sequence lifetimes, luminosities, abundances, and effective temperatures of stars, producing a larger and hotter population of massive stars and strongly affecting the ionizing spectrum produced by such a population. By examining ionizing spectra generated with these rotating stellar evolutionary tracks and the Starburst99 stellar population synthesis code, we can evaluate the new stellar models and consider the strong influence that individual components of stellar physics can have on synthetic ionizing spectra and their applications.

Constraining the Abundance of Massive Black Hole Binaries with Quasar Spectroscopic Monitoring

A fraction of quasars have long been known to show significant bulk velocity offsets (of a few hundred to thousands of km/s) in the broad permitted emission lines with respect to host galaxy systemic redshift. Various scenarios may explain these features such as massive black hole binaries or broad line region gas kinematics. Long-term spectroscopic monitoring has been previously suggested as a promising test to discriminate between alternative scenarios. We have homogeneously selected a sample of ~300 shifted-line quasars from the SDSS DR7. For ~60 of them, we have conducted second-epoch optical spectra using MMT/BCS, ARC 3.5m/DIS, and/or FLWO 1.5m/FAST. These new observations, combined with existing SDSS multi-epoch spectra of a control quasar sample we constructed, enable us to constrain the velocity drifts of quasar broad lines with time baselines of a few years up to a decade. Previous work has been focusing on objects with extreme velocity offsets, e.g., > 1000 km/s. Our work extends to the parameter space of smaller velocity offsets, where larger velocity drifts would be expected in the binary scenario. Our results may be used to identify strong candidates for and to constrain the abundance of massive black hole binaries, which are expected to be common in hierarchical cosmologies, but have so far been elusive.

Dark Matter in a Galaxy Cluster Merger Associated with a Short Gamma-ray Burst

One of the most direct pieces of evidence to date for the existence of dark matter is in merging galaxy clusters, where the hot gas is decoupled from the peaks in mass distribution. In this talk, I report the discovery of one such merging galaxy cluster, identified through its association with a short-duration gamma-ray burst (GRB). Chandra follow-up of the burst cluster revealed the hot gas is in a highly disturbed state, and weak gravitational lensing analyses of Hubble and VLT data showed the cluster light and mass are spatially offset from the hot gas. Combined with the other recently discovered dissociative systems, our burst cluster results support the collisionless nature of dark matter.

Secular Dynamical Anti-Friction in Galactic Nuclei

I will discuss a new gravitational-dynamical process which increases the orbital eccentricity of hypothesized intermediate mass black holes as they spiral into supermassive black holes. This process is a result of a strong systematic torque caused by secular (i.e., orbit-averaged) interactions with the cluster's stars. The force which results in this torque acts, counter-intuitively, in the same direction as the intermediate mass black hole's precession and hence we refer to its action as "dynamical anti-friction."

A Resolved Measurement of the Kinetic Sunyaev-Zel'dovich Effect in MACS J0717.5+3745

I present 140 and 268 GHz Bolocam imaging of the Sunyaev-Zel'dovich effect (SZE) in the disturbed, intermediate redshift (z=0.5458) galaxy cluster MACS J0717.5+3745, a triple-merger system comprising four distinct, optically-detected subclusters. Comparing these maps to a 2-dimensional pressure map derived from Chandra X-ray observations, which fails to describe the Bolocam data at the location of the subcluster known to have a high line of sight optical velocity of ~3200 km/s. The data are adequately described when we add an additional component to the pseudo-pressure template at the location of the high velocity subcluster. The additional component cannot be described by a thermal SZE spectrum. Using flux densities extracted from our model fits, and marginalizing over the Chandra X-ray spectroscopic temperature constraints for the region, we fit a (relativistically corrected) thermal + kinetic SZE spectrum to our data and find the subcluster has a best-fit line of sight proper velocity v_z = 3600 (+3440/-2160) km/s, in agreement with the optical velocity estimates for the subcluster. The probability v_z ≤ 0 given our measurements is 2.1%. Repeating this analysis using flux densities measured directly from our maps results in a 3.4% probability v_z ≤ 0. We note that this constraint on the kinetic SZE is first of its kind on resolved, subcluster scales.

Exciting the Eccentricity: General Relativity in Hierarchical Three-body Systems

Abstract Unavailable

Winds of Change: High-Resolution X-ray Spectroscopy of Black Hole Outflows

Although accreting black holes have long been renowned for their relativistic jets, the last decade of high-resolution X-ray spectroscopy has begun to revolutionize our understanding of the central engines. We now know that in addition to relativistic jets, black holes also launch winds that carry away more than 95% of the infalling gas and exert a powerful influence on their accretion flows. In this talk, I will present some new results on outflows from Chandra observations of black holes, and I will show how these observations highlight the intricate links between the inner accretion flow, relativistic jets, and accretion disk winds.

How Black Holes Accrete

Supermassive black holes (SMBH) reside at the centre of almost all galaxies. There is a vast difference in scale-length between an SMBH and its host galaxy - about 10 orders of magnitude. Galaxies are naturally chaotic places where for example star formation, supernovae and AGN provide random energy and momentum feedback into the surrounding gas. This suggests that accretion on to an SMBH is chaotic, occurring through depositions of gas with random orientations. Therefore retrograde accretion discs are possible, indeed likely. Retrograde accretion is a natural method for keeping the spin of the SMBH low, allowing efficient growth. Accretion discs inclined to the SMBH spin are torn up by the Lense-Thirring effect, significantly enhancing accretion rates from the standard picture of slow viscous accretion. Retrograde accretion can also help to merge an SMBH binary and therefore provide a possible solution to the last parsec problem.

The Shocking History of the Early Universe and the Formation of the First Cosmic Structures.

Recently, Tseliakhovich and Hirata (2010) showed that during the cosmic Dark Ages the baryons were typically moving supersonically with respect to the dark matter. I will describe how such supersonic motion sources bow shocks and Mach cones around the first halos, which act to erase the velocity difference in overdense gas. I will show the results of numerical simulations that are the first to self consistently incorporate the dark matter-baryon velocity offset, and how it impacts the formation of the first stars and mini-halos. Finally, I will disucss the impact of the velocity differential on redshifted 21cm radiation from the epoch of first light.

Massive Black Holes in Dwarf Galaxies

Supermassive black holes (BHs) are thought to reside in the nuclei of essentially all massive galaxies with bulges, however the origin of these BHs is largely unknown. Dwarf galaxies with low masses and relatively quiet merger histories are potential hosts of the least-massive BHs, and can provide valuable constraints on the properties of the first primordial "seed" BHs as well as their formation mechanism. Observationally, however, few dwarf galaxies are known to host massive BHs. I will present new results from an on-going project to search for accreting massive BHs in dwarf galaxies using various techniques and observations spanning the electromagnetic spectrum. Furthermore, I will present recent follow-up observations of Henize 2-10 that support the case for an accreting massive BH in this bulgeless dwarf starburst galaxy.

On the Role of the X-ray Corona in Black Hole State Transitions.

It has long been speculated that the nature of the hard X-ray corona may be an important second driver of black hole state transitions, in addition to the mass accretion rate through the disk. However, a clear physical picture of coronal changes has not yet emerged. In this talk, I present evidence that both the spectral and timing properties of black hole states may be partially driven by the height of the X-ray corona above the disk, and related changes in how gravitational light bending affects the corona-disk interaction.

Three-Dimensional Simulations of Stellar Core Collapse and Collapsar Formation

Simulations of stellar core collapse are inherently difficult. They require an accurate treatment of microphyics (especially neutrinos), require high resolution due to turbulence, and do generally not contain any symmetries. In addition, they must ultimately be modeled in three spatial dimensions since effects like the standing accretion shock instability, convection, and protoneutron star pulsation are qualitatively different in two and three spatial dimensions. Previous studies were either limited to lower dimensions, or sacrifice accuracy and length of evolution. I present a new parallel code which is capable of efficiently evolving general relativistic fluids in full three spatial dimensions without any symmetry assumptions on thousands of processors. The code is based on multi blocks which allow to cover the simulation domain with multiple topologically adapted curvi-linear grid patches. Cell-centered and flux-conservative adaptive mesh refinement is used to resolve steep shocks and small scale fluid motion. Starting from a previous study in octant symmetry, I discuss the code's application to the problem of stellar core collapse and collapsar formation.

Large-Scale Structure and Gravitational Waves

Detecting a primordial gravitational wave (tensor mode) background from inflation is one of the premier observational goals in cosmology. I will describe the leading effects of tensor modes on large-scale structure, and how we can observe them. For this, I will introduce a formalism which allows for a unified derivation of a wide range of cosmological observables in the relativistic setting, in a general gauge, and with clear physical interpretation of the various contributions. Surprisingly, we find that the intrinsic alignment of galaxies by gravitational waves is one of the most promising probes of tensor modes with large-scale structure.

Double Detonations and Type Ia Supernovae

In recent years, evidence has been mounting against the textbook theoretical progenitor scenario that produces the bulk of Type Ia supernovae (SNe Ia). In an alternative scenario, a detonation in a He shell sends shock waves into the underlying C/O white dwarf. When these shock waves converge, they ignite a second, C-powered, detonation that yields a SN Ia. I will describe recent advances in this double detonation scenario, in particular, our inclusion of multiple dimensions for the He shell detonation, and spatial resolution of the shock convergence and ignition of the C detonation.

A Model-Independent Method to Identify and Constrain Source Populations Using Anisotropy Analysis

The contribution of unresolved source populations to diffuse emission at all wavelengths can induce anisotropies on small angular scales. Several recent studies have focused on the potential for anisotropy analysis of the large-scale isotropic gamma-ray background (IGRB) to identify the origin of this emission and to constrain the properties of known and proposed gamma-ray source classes. I will present a novel approach to identifying the source classes contributing to diffuse emission by combining the energy dependence of the total anisotropy and the total intensity, and demonstrate how this technique can be used in a model-independent way to extract the energy spectra of the contributing populations directly from the data, without making a priori assumptions about the spectra of any contributors. While this approach is applicable for any diffuse background at any wavelength, I will focus on its potential for understanding the IGRB. I will also discuss the implications for various source populations of the recent Fermi LAT measurement of the angular power spectrum of the IGRB.

Particle Acceleration by Magnetic Reconnection in Striped Pulsar Winds and Relativistic Jets

The relativistic wind of pulsars consists of toroidal stripes of opposite magnetic field polarity, separated by current sheets of hot plasma. By means of 2D and 3D particle-in-cell simulations, we investigate particle acceleration and magnetic field dissipation at the termination shock of a striped pulsar wind. At the shock, the flow compresses and the alternating fields annihilate by driven magnetic reconnection. Irrespective of the stripe wavelength "lambda" or the wind magnetization "sigma" (in the regime sigma>>1 of magnetically-dominated flows), shock-driven reconnection transfers all the magnetic energy of alternating fields to the particles. As the value of lambda/(r_L*sigma) increases (here, r_L is the relativistic Larmor radius in the wind), the post-shock spectrum passes from a thermal Maxwellian to a flat power-law tail with slope around -1.5, populated by particles accelerated by the reconnection electric field. The limit lambda/(r_L*sigma)>>1 (namely, of a single current sheet) is realized in relativistic jets, where kink instabilities may seed the conditions for magnetic reconnection. For a single current sheet, we find that the particle spectrum in the sheet approaches a flat power-law tail with slope between -1.5 and -2, regardless of the conditions in the jet. The spectrum extends to higher energies for larger magnetizations or colder plasma temperatures, everything else being fixed. Our results place important constraints on the emission models of Pulsar Wind Nebulae and magnetically-dominated astrophysical jets.

Conditions for Preheating

Preheating is one of the most uncertain times in the history of the universe. Beforehand is understood through the effective theory of inflation, and afterwards is understood through the standard model. Between these two times lies preheating, which must take the universe from the cold, empty state after inflation to a thermal one. Parametric resonance is one channel for preheating to happen sufficiently quickly and efficiently to be viable. I will discuss some of the conditions on model space that limit which models admit parametric resonance.

Strong Evidence for Gamma-ray Line Emission from Fermi-LAT

Using 3.7 years of Fermi-LAT data, we examine the diffuse 80-200 GeV emission in the inner Galaxy and find a resolved gamma-ray feature at 110-140 GeV. We model the spatial distribution of this emission with a ~3 degree FWHM Gaussian, finding a best fit position 1.5 degree West of the Galactic Center. Even better fits are obtained for off-center Einasto and power-law profiles, which are preferred over the null (no line) hypothesis by 6.5 sigma (5.0 sigma/5.4 sigma after trials factor correction for one/two line case) assuming an NFW density profile centered at (l, b)=(-1.5 degree, 0 degree) with a power index alpha=1.2. The energy spectrum of this structure is consistent with a single spectral line (at energy 127.0 +- 2.0 GeV with chi^2=4.48 for 4 d.o.f.). A pair of lines at 110.8 =- 0.4 GeV and 128.8 +- 2.7 GeV provides a marginally better fit (with chi^2=1.25 for 2 d.o.f.). The total luminosity of the structure is (3.2 +- 0.6) x 10^{35} reg/s, or 1.7 +- 0.4 x 10^{36} photons/sec. The energies in the two-line case are compatible with a 127.3 +- 2.7 GeV WIMP annihilating through gamma-gamma and gamma Z (with chi^2=1.67 for 3 d.o.f.). We describe a possible change to the Fermi scan strategy that would accumulate S/N on spectral lines in the Galactic center 4 times as fast as the current survey strategy. Furthermore, we examine the co-added gamma-ray spectrum of unassociated point sources in the Second Fermi-LAT catalog (2FGL) using 3.9 years of LAT data. Using the SOURCE event class, we find evidence for lines at 111 GeV and 129 GeV with a local significance of 3.3 sigma based on a conservative estimate of the background at E>135 GeV. Other 2FGL sources analyzed in the same way do not show line emission at 111 GeV and 129 GeV. The line-emitting sources are mostly within 30 degrees of the Galactic plane, although this anisotropy may be a selection effect. If the double-line emission from these object is confirmed with future data, it will provide compelling support for the hypothesis that the Galactic center line signal is indeed from dark matter annihilation.

Radio Observations of Merging Galaxy Clusters: Characterizing Shocks and Particle Acceleration.

Radio halos and relics are Mpc-size synchrotron emitting sources found in merging galaxy clusters. It has been argued that these sources trace cluster merger shock waves and turbulence. In this talk I will present the latest observational results on several unique clusters. Together with simulations, these observations can be used to probe the intracluster medium at large distances from the cluster center. Radio relics can also be used to reconstruct the mass ratio, impact parameter, and orientation of cluster merger events. In addition, I will present the results of ultra-low frequency observations of galaxy clusters with the new LOFAR telescope.

Measurement of Cosmic-Ray Positrons with the Fermi Gamma-ray Space Telescope

The Fermi Gamma-ray Space Telescope is not only an excellent gamma-ray detector but is also a very good electron and positron detector. Although the spacecraft does not have a magnetic field onboard, we used the Earth's magnetic field to distinguish electrons and positrons and measure the spectrum of each in the 20 to 200 GeV range. We confirm the surprising measurement by the PAMELA spacecraft that the positron fraction increases with energy in this range, and we extend the measurement above 100 GeV for the first time. Possible explanations for this anomaly include modified secondary production, dark matter, and primary sources such as pulsars.