Einstein Fellows Symposium 2017
October 12-13, 2017
Phillips Auditorium
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
Watch the talks on our channel, Day 1 and Day 2.
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- Thursday, October 12
- 9:00 - 9:15
- Welcome
- Session 1: Supernovae & Pulsars
9:15 - 10:30 - Chair: Paul Plucinsky
- 9:15 - 9:30
The Impossible Supernova
iPTF14hls is one of the most peculiar transients we've ever seen. It displays spectra identical to those of the most common type of supernova (type IIP) but the spectra evolve ~10 times slower than normal, the light curve displays at least five peaks, and there is evidence of a strong eruption at the position of iPTF14hls in images taken in 1954. None of these properties have ever been seen in a supernova before. iPTF14hls might constitute the first case of the theoretically-predicted pulsational pair instability supernova, but it still violates some principles of that model. In fact, the properties of iPTF14hls are difficult to explain in the framework of any existing supernova model.
- 9:30 - 9:45
Mass Ejection in Failed Supernovae
During the core collapse of a massive star, the formation of the proto-neutron star is accompanied by the emission of ~ 0.1 M_{Sun} in neutrinos. This mass loss generates a time-dependent gravitational field and launches a pressure wave from the stellar core. If the supernova fails, then this pressure wave traverses the stellar envelope and steepens into a shock, erupting from the surface of the star and providing observational evidence of a failed supernova. I will argue that the characteristic energy of this shock is ~ 10^{48} erg. I will then describe a general formalism for analyzing the formation, evolution, and steepening of the pressure wave, and apply this formalism to polytropes. I will also discuss the observational implications for detecting failed supernovae.
Presentation- 9:45 - 10:00
Two for One: A Long GRB And Broad-Lined Sn Ic from a Single Central Engine
High velocities (~0.1c) and kinetic energies have been observed in a subset of Type Ic supernovae ("broad-lined SNe Ic"), prompting a search for a central engine model capable of generating such energetic explosions. A clue to the engine may lie in the fact that all SNe that accompany long-duration gamma-ray bursts belong to the Ic-BL class. A combination of 2D relativistic hydrodynamics and radiation transport calculations demonstrates that the central engine responsible for long gamma-ray bursts can also trigger a SN Ic-BL. A reasonable gamma-ray burst engine injected into a stripped Wolf-Rayet progenitor produces a relativistic jet with energy ~10^51 ergs, and a SN whose synthetic light curves and spectra are fully consistent with observed SNe Ic-BL during the photospheric phase.
Presentation- 10:00 - 10:15
Neutron Star Post-Merger Simulations: The Origin of Kilonovae and the Heavy Elements
Neutron star (NS) mergers are expected to be detected by LIGO in the very near future. Such mergers can form massive accretion disks around the remnant black hole or NS. Outflows from these disks are an important source of mass ejection and rapid neutron capture (r-process) nucleosynthesis. In this talk, I will present the first 3D general-relativistic magnetohydrodynamic simulations with neutrino cooling of such accretion disks. I will argue that mass ejection from these disks is much higher than previously thought. This material may provide the dominant reservoir to fuel a kilonova, which is among the most promising electromagnetic counterparts of the merger. Furthermore, I will present strong evidence for NS mergers being the prime production site for the heavy elements in the universe.
Presentation- 10:15 - 10:30
The Most Powerful Transients in 3D
Extreme (hyperenergetic/superluminous) core-collapse supernovae and neutron star mergers belong to the most extreme transient events in the universe and are the leading engine candidates for both long and short gamma-ray bursts. I will discuss the unique challenges in both input physics and computational modeling for these systems involving all four fundamental forces and highlight recent breakthroughs in full 3D simulations. I will pay particular attention to the nucleosynthetic signatures of the these explosions and conclude by discussing what remains to be done in order to maximize what we can learn from current and future time-domain transient surveys.
Presentation- 10:30 - 11:00 Coffee Break
- Session 2: Stars
11:00 - 12:00 - Chair: Scott Wolk
- 11:00 - 11:15
Illuminating the Night Sky with Common Envelope Events
It is becoming an observational reality to catch common envelope event--where one star in a binary evolves to engulf its companion--in action with time domain surveys. These data are beginning to constrain long-standing theoretical uncertainties surrounding mass ejection in common envelope phases and their role in transforming binary star systems. I will discuss an effort to understand the details of mass ejection in these events through hydrodynamic simulations.
Presentation- 11:15 - 11:30
An Unstable Truth: How Massive Stars get their Mass
The pressure exerted by massive stars' radiation fields is an important mechanism regulating their formation. Therefore detailed simulation of massive star formation requires an accurate treatment of radiation. For this purpose, I developed a new, highly accurate radiation algorithm that properly treats the absorption of radiation from stars and the re-emission and processing by interstellar dust. With this new tool, we performed a set of 3D radiation-hydrodynamic simulations of the collapse of massive pre-stellar cores. We find that mass is channeled to the stellar system via gravitational and Rayleigh-Taylor instabilities through non-axisymmetric disks and filaments that self-shield against radiation pressure while allowing for radiation to escape through optically thin regions.
Presentation- 11:30 - 11:45
The Role of Magneto-Turbulence in Star Formation
Fundamental physical processes involving the magnetic field and turbulence are key to understanding star formation. Here I present work published in my thesis on developing state-of-the-art moving mesh simulations of prestellar collapse in a a turbulent medium. Two regimes of collapse are found depending on whether the turbulent kinetic energy density or the mean magnetic field energy density dominates the star forming environment. I discuss the theoretical and observational properties of the two regimes, and lay out plans for future work.
Presentation- 11:45 - 12:00
Formation, Evolution, and Future of Binary Stars
Formation: close binaries cannot form in situ. While secular evolution in triples via Kozai-Lidov cycles and equilibrium tides may produce some close binaries, we find that non-secular evolution in unstable triples, dynamical tides, and energy dissipation in the primordial disk play a larger role in their migration. Evolution: hot, massive helium stars are the cores of OB stars whose hydrogen envelopes have been stripped due to binary interactions. They are the progenitors of Type Ib/c supernovae and may produce enough UV photons to account for reionization. I will discuss the properties of several massive helium stars we recently discovered in the LMC. Future: I will give a status update on VARSTAGA, the first simultaneously deep and high-cadence survey of a local group galaxy (M33).
Presentation- 12:00 - 1:30 Lunch
- 1:30 - 2:15
LYNX: Tomorrow's Super-Sensitive Eye on the X-ray Sky
- Session 3: Black Holes
2:15 - 3:15 - Chair: Francesca Civano
- 2:15 - 2:30
A NICER Look at Accreting Black Holes
NICER is a new instrument and premier X-ray timing facility installed on the ISS this June. I will present some of our first results with NICER on bright black hole systems.
Presentation- 2:30 - 2:45
Unveiling The Structure and Evolution of Black Hole Coronae
The corona is an important component of a black hole accretion flow, responsible for a significant fraction of the energy output in the form of the X-ray continuum. The precise nature of the corona, however, remains a mystery, in terms of its structure, how energy is injected into it from the accretion flow and how it's output is regulated as the system evolves.
Detailed measurements of the reverberation of the X-ray continuum off the inner regions of the accretion disc, compared with models derived from general relativistic ray tracing simulations, are revealing, for the first time, structures present within the corona. They reveal how the corona evolves giving rise to flares in the X-ray emission and even how the corona may be linked to the launching of jets in radio-loud AGN.
Presentation- 2:45 - 3:00
More Tips for Characterizing a Population of Massive Black Hole Binaries
The inspiral and merger of massive black hole binaries in galactic nuclei make up a dominant component of the low frequency gravitational sky, soon detectable by the Pulsar Timing Arrays and space based interferometer LISA. Characterization of the massive black hole binary population will shed light on the environments of galactic nuclei and also on the mutual growth of galaxies and their central black holes. This talk addresses novel predictions for electromagnetic tracers that are needed in combination with gravitational observations to characterize the poorly understood population of massive black hole binaries.
Presentation- 3:00 - 3:15
Quenching, Black Hole Feedback and Anisotropic Thermal Conduction
Feedback from central supermassive black holes is often invoked to explain the low star formation rates in the massive galaxies, however, the detailed physics of the coupling of the injected energy with the intracluster medium is still unclear. We show that stratified anisotropically conducting plasmas are formally always unstable, and thus more prone to mixing. The increased mixing efficiently isotropizes the injected feedback energy, which in turn significantly improves the coupling between the feedback energy and the ICM. This facilitates an earlier disruption of the cool-core, reduces the SFR by more than an order of magnitude, and results in earlier quenching despite an overall lower amount of feedback energy injected into the cluster core.
Presentation- 3:15 - 3:45 Coffee Break
- Session 4: Accretion
3:45 - 5:00 - Chair: Fred Baganoff
- 3:45 - 4:00
Relativistic Accretion onto Millisecond Pulsars
I will present results from the first relativistic MHD simulations of accretion onto magnetized neutron stars, performed in general relativity in the Kerr spacetime geometry. Four regimes are recovered, in order of increasing stellar magnetic field strength (or decreasing accretion rate): (a) direct equatorial accretion; (b) magnetically channeled accretion; (c) the propeller state; (d) exclusion of the accretion flow from the light cylinder by the pulsar wind. A Poynting-flux-dominated relativistic jet, powered by stellar rotation, is produced at high accretion rates. I will discuss our results in the context of the transitional millisecond pulsars and the neutron-star-powered ultra-luminous X-ray sources.
Presentation- 4:00 - 4:15
Discovery of a time lag between the soft X-ray and Radio emission of the tidal disruption flare ASASSN-14li: Evidence for linear Disk--jet coupling
The tidal disruption of stars by massive black holes can result in transient radio emission. The electrons producing these synchrotron radio flares could either be accelerated inside a relativistic jet or externally by shocks resulting from an outflow interacting with the circumnuclear medium. Until now, all tidal disruption flare (TDF) studies have adopted the external shock model to explain their radio properties. I will talk about our discovery of a correlation between changes in the X-ray and radio flux of a TDF. The radio lags the X-ray emission by about 13 days. This demonstrates that X-ray emitting accretion disk regulates the radio emission. This coupling is inconsistent with all previous external models but is explained if the radio emission originates from a freely expanding jet.
Presentation- 4:15 - 4:30
The Delay Time Distribution of Tidal Disruption Flares
Recent observations suggest that stellar tidal disruption events (TDEs) are strongly overrepresented in rare, post-starburst galaxies. I will examine three dynamical explanations for this apparent rate enhancement: unresolved supermassive black hole binaries (SMBHBs), overdense central stellar systems, and radial anisotropies. I will show that various factors disfavor the SMBHB hypothesis, while the other two remain theoretically viable. I will also compare theoretical and empirical TDE delay time distributions, and argue that this observable may become a powerful tool for understanding nuclear stellar dynamics.
Presentation- 4:30 - 4:45
Unifying the Micro and Macro Properties of AGN Feeding and Feedback
I highlight key advancements in the modeling of multiphase halos, focusing on the gas kinematics. High-resolution ab-initio HD simulations show the X-ray plasma radiatively cools along the turbulent eddies stimulating a top-down condensation cascade. As ensemble, the warm filaments and molecular clouds inherit the turbulent kinematics of the parent hot halo; in the nuclear region, they rapidly fall toward the SMBH via chaotic cold accretion. GR-MHD runs show that near the horizon the cloud gravitational energy is transformed into ultrafast outflows, which thermalize via cavities, shocks, and mixing beyond the kpc scale, thus re-heating the halo in a self-regulated cycle. The micro and macro feeding-feedback cycle is unified in a minimal model consistent with multiwavelength observations.
Presentation- 4:45 - 5:00
How do Pulsars Shine?
In this talk I will describe how plasma is produced in magnetospheres of pulsars and show that effects of general relativity are crucial for the activity of pulsars with low inclination angles. I will present modeling of high-energy lightcurves, calculated self-consistently from particle motion in the pulsar magnetosphere. I will also show evidence that observed radio emission is powered by non-stationary discharge at the polar cap. Finally, I will argue that giant radio pulses in the Crab are produced by coherent plasma currents, which appear at the interfaces of merging plasmoids in the current sheet beyond the light cylinder.
Presentation- 6:00pm Dinner for Fellows at the NuBar
- Friday, October 13
- Session 5: Active Galaxies
9:15 - 10:45 - Chair: Belinda Wilkes
- 9:15 - 9:30
Inflow And Outflow in the Broad Line Region of AGN
Understanding the detailed structure of the broad line region in active galactic nuclei (AGN) can help us measure the masses of supermassive black holes outside the local Universe. By modeling light echoes in the broad line region from reverberation mapping data, we find that the H-beta broad line emission comes from a thick disk viewed close to face-on, often with signatures of either inflowing or outflowing gas. I will discuss the emerging picture of broad line region structure now with over a dozen AGN in the sample and the high-quality reverberation mapping experiments pushing the boundaries of what we can learn about the inner regions of AGN.
Presentation- 9:30 - 9:45
Characterizing AGNs in Dwarf Galaxies
Searching for signs of black hole accretion is the best way of identifying black holes in dwarf galaxies. Recently, the number of dwarf galaxies with known active galactic nuclei (AGNs) has increased by over an order of magnitude, thanks to large scale surveys such as the Sloan Digital Sky Survey. These objects comprise a new population of AGNs. It is important to characterize this population to understand how they compare to more massive AGN, as well as to determine which factors influence the presence of AGNs in dwarf galaxies. I will discuss X-ray and ultraviolet observations of a sample of dwarf galaxies with optical signatures of AGN activity. I will also discuss new results from HST imaging of RGG 118, a dwarf galaxy with an active 50,000 solar mass black hole in its nucleus.
Presentation- 9:45 - 10:00
X-ray AGN in Galaxy Clusters
A critical prerequisite for both AGN activity, and the formation of new stars in host galaxies, is the availability of gas. The cluster environment affects gas reservoirs in galaxies through processes such as ram-pressure stripping, evaporation, starvation, and tidal effects of the cluster potential. The density of cluster members and their relative velocities also depend on the cluster mass. As such, the rates of violent processes will differ in clusters and the field. The relative importance of these processes depends on both the position within, the mass of, and the redshift of the host galaxy cluster. I'll give an overview of the Cluster AGN Topography Survey (CATS) which is mapping the AGN distributions within clusters.
Presentation- 10:00 - 10:15
Tides and Mergers Trigger Cluster Radio AGN
Radio-mode feedback from cluster AGN redistributes energy and material across the cluster environment. But what triggers such powerful events? We examine a sample of 183 clusters found in FIRST in an aim to address this question. Describing the data as a non-homogeneous spatial point process, we model the number density of radio AGN as a function of cluster radius. We find an inverse scaling relation between the radio AGN number density and cluster mass. This inverse trend is stronger at the center of the cluster than at the outskirts. We discuss these results in the context of the analytic model for mergers and tidal interactions presented in Mamon 2000.
Presentation- 10:15 - 10:30
The Kepler Light Curves of Active Galaxies: A New Regime of Optical Variability
The optical light curves of AGN provide a unique window into the conditions and behavior within the accretion disk. The development of a specialized pipeline for AGN science with the exquisite photometry of exoplanet-hunting satellites allows us to explore new optical variability phenomena. Among the insights from these new light curves are bimodal flux distributions, steep power spectral slopes, characteristic variability timescales, a candidate quasi-periodic oscillation, and more. Such data provide an opportunity for direct comparison with X-ray light curves for the first time, and promise to inform models of both accretion physics and the relationship between X-ray and optical emitting regions in the central engine.
Presentation- 10:30 - 10:45
The Merger-Free Growth of Supermassive Black Holes and their Host Galaxies
Merger-free evolution contributes roughly equally to the overall growth of black holes in the Universe and is also responsible for a significant amount of galaxy growth over cosmic time. Black holes hosted in unambiguously disk-dominated galaxies reach quasar-like luminosities and black hole masses typical of those hosted in galaxies with more merger-driven evolutionary histories. Moreover, the fitted correlation between black hole mass and total galaxy stellar mass in these merger-free systems is fully consistent with the canonical relationship based on merger-driven systems. The same correlation between black hole & galaxy in merger-free systems indicates the black hole-galaxy connection must originate with a process more fundamental than the dynamical configuration of a galaxy's stars.
Presentation- 10:45 - 11:15 Coffee Break
- Session 6: Gravity & Lensing
11:15 - 12:15 - Chair: Dan Schwartz
- 11:15 - 11:30
Formation of Eccentric Black Hole Mergers
Binary black hole gravitational wave mergers have been observed, but their astrophysical origin is still unknown. One way of determining their formation channel is to consider their relative spin orientation, which should be random in cluster mergers and less random in field mergers. In my talk I show how the binary eccentricity at merger also can give strong constraints on the formation channel. I show that about one percent of cluster mergers will be highly eccentric when observed, whereas zero percent is expected from field binaries.
Presentation- 11:30 - 11:45
Careful with the Priors: A Reanalysis of LIGO Black Hole Coalescences
In a regime where data are only mildly informative, prior choices can play a significant role in Bayesian statistical inference, potentially affecting the inferred physics. We show this is indeed the case for some of the parameters inferred from current gravitational-wave measurements of binary black hole coalescences. We reanalyze the LIGO data using alternative (and astrophysically motivated) prior assumptions, and find different prior distributions can indeed impact the physical interpretation of these systems. More on arXiv:1707.04637.
Presentation- 11:45 - 12:00
Caustic Crossing Stars in Cluster Strong-Lensing Systems and the Small-Scale Structure of Dark Matter
When a background galaxy straddles the lensing caustic of a foreground galaxy cluster, its member stars can acquire extreme magnification factors as large as hundreds to thousands, and hence the brightest ones may be individually detectable to the HST or the forthcoming JWST. One candidate event of this kind has recently been discovered in HST observations. I will explain the basic structure of such caustic-crossing phenomenon after taking into account the microlensing effects of the intracluster stars. By detecting more caustic-crossing stars in deep images, it is possible to probe small-scale structures in the cluster's dark matter halo, which include either stellar-mass compact objects, or baryon-free subhalos as predicted in the paradigm of cold dark matter.
Presentation- 12:00 - 12:15
Gravitational Lenses as High-Resolution Telescopes
Many of the most extreme and energetic sources in the universe can not be resolved with direct observation, for example, the inner regions of active galaxies that can host relativistic jets. I show that strong gravitational lensing can be used to elucidate the structure of these sources from radio frequencies up to very high energy gamma rays. Future surveys, including LSST, SKA, and Euclid, will provide observations for hundreds of thousands of gravitationally lensed sources, which will allow us to apply these methods to study the multi-wavelength structure for large ensembles of sources. This large ensemble of sources will allow us to elucidate physical origin of multi-wavelength emissions, their connection to supermassive black holes, and their cosmic evolution.
Presentation- 12:15 - 1:45 Lunch
- Session 7: Cosmology
1:45 - 3:00 - Chair: Larry David
- 1:45 - 2:00
Cosmological Constraints from Weak Lensing in the Dark Energy Survey
The Dark Energy Survey has combined analyses of galaxy clustering and weak gravitational lensing two-point correlation functions in its first year (Y1) of observations. The goal of this is to constrain cosmological parameters from structure in the evolved Universe. These measurements provide information on the amplitude of density fluctuations (S8=0.794+0.029-0.027) and the dark energy equation of state (w=-0.80+0.20-0.22) that is competitive with Planck CMB data. I will review these measurements and give an outlook on upcoming results from a lensing view of higher than second moments of the matter density field.
Presentation- 2:00 - 2:15
Measuring the Cosmological Distance Scale with Spectroscopic and Photometric Data
The baryon acoustic oscillations (BAO) peak in the clustering of galaxies provides a powerful probe of the expansion history of the Universe. Spectroscopic surveys enable the measurement of BAO using galaxy clustering primarily at low to intermediate redshifts, where these surveys can obtain redshift estimates for sufficiently dense samples of galaxies. I will show recent work in which we instead measure the BAO at z=0.64 via the cross-correlation of a sparse spectroscopic sample with a dense photometric sample and I will discuss applications to future surveys.
Presentation- 2:15 - 2:30
Detecting DLAs with Machine Learning
DLAs are strong neutral hydrogen absorbers, traditionally detected by astronomers staring at spectra. With current surveys containing half a million such objects, this does not scale. I will talk about how we taught a computer to do it instead, using a probabilistic trained Gaussian Process.
Presentation- 2:30 - 2:45
Morphological Drivers of Milky May-Mass Galaxies: Insights from the FIRE Simulations
I explore the morphologies of fifteen MW-mass galaxies from the FIRE-2 suite of simulations. I show that the disk radius does scale with the halo spin and specific angular momentum of the disk, as predicted by Mo et al. 1998, but their model predicts the wrong normalization when including adiabatic contraction. Morphology also correlates with the gaseous history of each galaxy: those that maintain a high gas fraction after z~1 develop well-ordered stellar disks. I also examine the kinematic disk fraction as a function of time and the fraction of stars that are born in a disk. By z = 0, MW-mass galaxies have formed >60% of their stars in disks, even when only ~20% of those stars are in a disk now. Nearly all stars forming at z = 0 do so out of gas co-rotating with the stellar disk.
Presentation- 2:45 - 3:00
The Missing Satellites Problem Re-examined: The Baryon-Dark Matter Relative Velocity in the Milky Way
LCDM simulations of structure formation over-predict the abundance of small satellite galaxies relative to that observed in the Milky Way, suggesting the current paradigm of cold, collision-less DM may be incorrect. But it may instead be a symptom that our picture of LCDM structure formation is missing pieces. One missing piece in most simulations is that, due to their different behaviors before decoupling (z~1020), baryons and dark matter have a large-scale coherent relative velocity that fluctuates statistically throughout the Universe. If the Milky Way is in a patch with a 2-3sigma high relative velocity, this can starve small satellites of gas, suppressing their formation. Using data from the 2MASS Redshift Survey, we can compute the relative velocity around the Milky Way to test this idea. I will show preliminary results suggesting that the relative velocity is indeed 2-3sigma high locally. This argues that incorporating a standard but up to now rarely-included piece of linear theory physics is enough to relieve the tension between simulation and observation without need for exotic DM models.
Presentation- 3:00 - 3:30 Coffee Break
- Session 8: Milky Way
3:30 - 4:15 - Chair: Rudy Montez
- 3:30 - 3:45
What is the Origin of the Fermi Bubbles?
The Fermi bubbles are two giant gamma-ray bubbles above and below the Galactic center (GC), revealed by the Fermi Gamma-ray Space Telescope. Because of their proximity, the spatially resolved, multi-wavelength observations offer unprecedented opportunities for constraining the origin of the bubbles, which is still elusive despite the many theories proposed. In this talk, I will briefly review the status of theoretical modeling of the bubble formation. I will also introduce CRSPEC, a new module in the FLASH code that could track the evolution of CR spectrum, and how we used simulations with CRSPEC to understand the high-energy cutoff at ~100 GeV and the spatial uniformity of the observed gamma-ray spectrum in the leptonic AGN jet scenario.
Presentation- 3:45 - 4:00
Things that Go Bang in the Night: Using X-ray Echolocation to Study the Milky Way
X-ray outbursts propagating through the interstellar medium (ISM) scatter off dust, producing scattering halos that appear as rings that grow with time. Like echolocation, these light rings can be used to triangulate the distances to ISM structures with more precision than any other method. I will review nature's recipe for dust echoes, the results from the three brightest ring echo events to date, and science prospects for the future--from the local to the intergalactic medium.
Presentation- 4:00 - 4:15
Data-Driven Models of the Milky Way in the Gaia Era
The Gaia satellite will soon deliver magnitudes, parallaxes, and proper motions of billions of stars, offering a unprecedentedly detailed view of the structure and dynamics of our Galaxy. Correctly exploiting Gaia is challenging due to the colossal amount of data, the complicated selection effects and noise, and the statistical inaccuracy of current stellar and 3D Galactic models. I will demonstrate how hierarchical probabilistic descriptions of the data address those issues, and how flexible data-driven models allow us to bridge the gap between the statistical power of the data and the physics of interest. I will show color-magnitude diagram and phase space reconstruction built from the first Gaia data release, and discuss upcoming data releases.
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