Einstein Fellows Symposium 2016
October 18-19, 2016
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA
To see abstracts and presentation slides, click on the talk titles.
- Tuesday, October 18
- Session 1: Stars, Dust
- Chair: Jeremy Drake
- 9:20 - 9:40
The Formation and Evolution of Binary StarsCollaborators and I have recently completed a meta-analysis of binary stars based on 30+ surveys. The statistical distributions of binary star properties provide powerful insight into the binary star formation process. In turn, the updated multiplicity statistics serve as more reliable initial conditions in binary population synthesis studies of Type Ia supernovae, X-ray binaries, and sources of gravitational waves. I have recently been advising undergraduate students to use large samples of eclipsing binaries to constrain tidal evolution and mass transfer in early-type binaries. Finally, I will discuss some ongoing observational work on nascent eclipsing binaries, Type Ia supernovae, and a proposed deep and high-cadence photometric variability survey of M33.
- 9:40 - 10:00
Unwrapping the Mystery of Common Envelope EpisodesA dynamic and crucial phase in the evolution of a binary star system comes when one star engulfs its companion in a shared gaseous envelope. The two stellar cores spiral tighter in response to drag forces and develop differential rotational with respect to the surrounding envelope gas. Eventually, they may deposit enough energy and momentum to expel the envelope, leaving behind a binary transformed by orbital tightening and mass accretion. In other cases, the objects merge. In this talk, I will share recent results and propose a strategy to unwrap the mystery of this critical phase of binary evolution through a combination of hydrodynamic modeling and new observational constraints.
- 10:00 - 10:20
Turbulent Central Engines of Extreme Core-Collapse SupernovaeHyperenergetic type Ic-bl and superluminous core-collapse supernovae belong to the most extreme transient events in the universe and are a key factor in the supernova gamma-ray burst connection. I will present results from high-fidelity three-dimensional magnetohydrodynamics simulations of extreme core-collapse supernovae with an emphasis on the engines driving the explosion. In particular I will discuss how magnetoturbulence can form protomagnetars and fuel an explosion in rapidly rotating stars making them plausible progenitors for hyperenergetic and superluminous supernovae.
- 10:20 - 10:40
Discovery of a Dust Scattering Halo from a Galactic Center Transient, Swift J174540.7-290015Earth's position on the outskirts of the Milky Way galaxy means that the Galactic Center is obscured by a particularly large column of dust and gas, which is optically thick to the scattering of soft X-rays (tau~5). The dust scattering halos around compact objects in the dense GC environment cause significant blurring, casting a fog over the heart of our Galaxy. By studying the foreground ISM, we can measure and correct for the effects of dust scattering. I will report the preliminary discovery of a dust scattering halo around a recently discovered X-ray transient, Swift J174540.7-290015, which underwent one of the brightest flares ever observed from a GC compact object. Residual surface brightness around the object gives evidence of foreground dust scattering.
- 10:40 - 11:00 Coffee Break
- Session 2: Tidal Disruption
- Chair: Malgosia Sobolewska
- 11:00 - 11:20
X-ray, Optical, and UV Echoes from a Tidal Disruption FlareWhen a star gets close to a super-massive black hole (SMBH), it can be disrupted in a flare that lasts anywhere between a few months to a few years. While a fraction of the stellar debris becomes unbound, the rest is accreted onto the black hole. This provides a unique opportunity to study the accretion disk formation process around a SMBH in real time. I will present results from the first multi-wavelength photometric reverberation mapping study of a tidal disruption flare. We find that the optical and the UV emission lead the X-rays by 10-40 days with the lag increasing with increasing wavelength. I will discuss how these time lags allow us to constrain and map the accretion structure following a tidal disruption flare.
- 11:20 - 11:40
Tidal Disruption Events from Binary Supermassive Black HolesThe destruction of a star by the gravitational field of a supermassive black hole (SMBH) powers a luminous accretion episode, and this process is known as a tidal disruption event (TDE). I will present the results of an ensemble of numerical simulations of TDEs generated by SMBH binaries. We show that both the pre-disruption interaction of the star with the binary, and the post-disruption evolution of the disrupted debris induce substantial variability in the SMBH accretion rates. Nevertheless, we find that many cases exhibit some degree of periodicity on timescales comparable to the binary orbital period, which may be used to indirectly determine the presence of SMBH binaries within galaxies.
- 11:40 - 12:00
Tidal Disruption Events and Their Surprising Host Galaxy PreferenceThe search for the tidal disruption of stars by supermassive black holes (tidal disruption events; TDEs) is now yielding exciting results. Recently, we tied several events together into a coherent class of outbursts. This picture offers two surprising insights: (1) TDE emission properties are different than what was expected by theory, and new models are now being developed to try and explain the observations. (2) TDEs show a strong (200x) preference for post-starburst galaxies. The reason for this is still not know. We are trying to answer this puzzle, and in parallel are also taking advantage of it by starting the first-ever post-starburst galaxy monitoring survey, with the goal of finding many new TDEs.
- 12:00 - 1:10 Lunch
- Session 3: Feedback, BH/Galaxy Interactions
- Chair: Harvey Tananbaum
- 1:10 - 1:30
Merger-Free Black Hole-Galaxy Co-EvolutionSupermassive black holes and their host galaxies appear to co-evolve over a large redshift range, but the origin of this co-evolution remains unknown. Studies are complicated by the fact that most galaxies' evolutionary histories include a mix of interactions and mergers of many strengths and merger-free "secular" evolution, making it hard to disentangle which of these processes contributes (or dominates) black hole-galaxy co-evolution. Using a sample of unambiguously disk-dominated galaxies hosting luminous, growing supermassive black holes provides a useful means of studying secular black hole growth. This sample cannot have had significant merger activity since z ~ 2, yet the black holes have grown significantly, up to ~10^9 M_sun, and may co-evolve with their host galaxies.
- 1:30 - 1:50
The Effects of AGN Feedback on the Low-z Lyman Alpha ForestIn this talk I will demonstrate that the low-redshift Lyman-alpha forest column density distribution in the Illustris simulation reproduces observations extremely well in the column density range 10^12.5-10^14.5 cm^-2, relevant for the "photon underproduction crisis." We attribute this to the inclusion of AGN feedback, which changes the gas distribution so as to mimic the effect of extra photons, as well as the use of the Faucher-Giguere (2009) ultra-violet background, which is more ionizing at z=0.1 than the Haardt & Madau (2012) background previously considered.
- 1:50 - 2:10
The Evolution of X-ray AGN in ClustersA 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, and the mass of, the host galaxy cluster. I will share new results of the AGN distribution in the highest-z clusters and detail our survey which will identify ~40,000 X-ray AGN.
- 2:10 - 2:30
The Effects of Cluster Environment on Radio AGNFeedback from AGN takes the form of either radiative or mechanical modes, dispersing energy and momentum to the surrounding environment. But does the environment have a role in determining which mode the AGN's are in? I will present our study examining radio-loud AGN in cluster environments with a direct comparison to the field populations. We compare this radio-loud sample with their X-ray detected counterparts to assess the differences in mechanical versus radiative mode feedback, respectively. Finally, we examine the radio-AGN fraction as a function of both redshift and mass of the cluster.
- 2:30 - 2:50
Black Hole Feedback is Elegant, not ViolentI will review recent work on ALMA, Chandra, HST, and MUSE observations of mechanical ("radio mode") black hole feedback signatures in the Universe's largest galaxies. The results suggest that jets launched by black holes can couple to all phases of the ambient ISM, including the cold molecular phase, from which all stars are born. Yet rather than an explosive, violent process that drives all gas out of the galaxy - as frequently invoked in galaxy evolution models - the process is likely more subtle, and suggestive of a self-regulating feedback loop mediated by the cold molecular phase.
- 2:50 - 3:20 Coffee Break
- Session 4: Accretion, Cosmology
- Chair: Fred Baganoff
- 3:20 - 3:40
Detecting Gravitational Wave Bursts with Pulsar TimingThe history of astronomy has shown that the Universe is full of suprises. One of the great hopes for gravitational wave astronomy is the discovery of unanticipated phenomena. To accomplish this we need to develop flexible analysis techniques that are able to detect signals with arbitrary waveform morphology. Here I will describe a multi-wavelet approach for the analysis of timing residuals from a pulsar timing array.
- 3:40 - 4:00
Advancements in Multiphase Halo Astrophysics and Chaotic Cold AccretionI highlight major advancements in the astrophysics of multiphase gas in massive galaxies. The new high-resolution 3D simulations show the X-ray plasma cools at the interface of the turbulent eddies, stimulating a top-down condensation cascade of warm filaments. The kpc-scale ionized filaments form a skin enveloping the neutral filaments. The peaks condense in cold molecular clouds, which inelastically collide in the core leading to Chaotic Cold Accretion. CCA can be modeled as spherical viscous accretion with large self-similar variance, which can explain the rapid AGN variability and is key for AGN feedback. CCA reproduces the observations of cospatial multiphase gas in massive galaxies, as Chandra X-ray images, SOAR Hα filaments, Herschel [CII] emission, and ALMA molecular associations.
- 4:00 - 4:20
Simulating State Transitions in Black Hole Accretion FlowsIn this talk I will review most recent progress in simulating transitions from radiatively inefficient, optically thin mode of black hole accretion, through luminous-hot mode, into optically thick, but geometrically thin.
- 4:20 - 4:40
Stumbling Towards Simulations of Collisionless Black Hole Accretion FlowsThe accreting plasmas surrounding many black holes, including those at the centres of our galaxy and M87, are in the collisionless regime. Here magnetohydrodynamics is not necessarily a satisfactory approximation, and full plasma kinetics is, in principle, required. I'll describe some recent progress on methods to enable plasma-kinetic particle-in-cell simulations in arbitrary strongly curved spacetimes.
- 4:40 - 5:00
Reproducibility: An Insight from the AGORA High-resolution Galaxy Simulations ComparisonWhen experimenting with stars and galaxies is humanly impossible, numerical experiments are often the only means to test our galaxy formation theory. As the theory's success relies on robust numerical experiments, it is only reasonable that the same scientific standard of reproducibility is applied to galaxy simulations. Yet, the task of reproducing the galaxy simulations has not received high priority. In this talk, I will first highlight the effort to recognize and overcome the “reproducibility crisis” in other scientific disciplines. Then I will discuss the collective response by the numerical galaxy formation community to such a challenge, the AGORA Initiative. I will present AGORA's second report using an isolated galaxy disk to compare 9 state-of-the-art hydrodynamics codes.
- 5:00 - 5:20
Combining Spectroscopy with Photometry to Probe Higher Redshift ClusteringOn large scales, massive galaxies are used to examine the underlying matter distribution of the Universe. Spectroscopic surveys like SDSS-III's BOSS obtain redshifts to measure the clustering of these luminous tracers, providing a powerful cosmological probe. Several large imaging surveys are underway to provide target selection for a next generation baryon acoustic oscillations (BAO) experiment. I will discuss prospects for combining existing spectroscopic data with the high-quality public imaging and photometry provided by these surveys to study higher redshift clustering in advance of the next generation of galaxy redshift surveys.
- 6:30pm Dinner for Fellows at the NuBar
- Wednesday, October 19
- Session 5: Gravity Waves
- Chair: Matthew Evans
- 9:00 - 9:20
Did LIGO Detect Dark Matter?There is a possibility that the recent LIGO detection of gravitational waves originated from the merger of two primordial black holes, making up the dark matter. Thirty solar mass black holes, as detected by LIGO, lie within an allowed mass window for primordial black hole dark matter. Interestingly, our best estimates of the number of observable mergers fall within the range implied by current LIGO data. I will explain these estimates, discuss the (considerable!) theoretical uncertainties, and finish with prospects for testing the model.
- 9:20 - 9:40
Getting the Most Out of Gravitational-Wave Observations: Kicks and Spin PrecessionThe age of gravitational-wave astronomy has begun. The first detections of gravitational waves constitute a tremendous breakthrough in fundamental physics and astronomy: they are not only the first direct detections of such elusive signals, but also the first irrefutable observations of black-hole binary systems. Careful theoretical modeling of the astrophysical sources of gravitational-waves is crucial to maximize the scientific outcome of current and future detectors. Here we present advances on gravitational-wave source modeling, studying in particular: (i) the precessional dynamics of spinning black-hole binaries using advanced multi-timescale techniques, and (ii) the prospects for direct detection of black-hole kicks.
- 9:40 - 10:00
Electromagnetic Counterparts to Gravitational WavesThanks to the advent of the Advanced LIGO/VIRGO network, the era of gravitational wave discovery is upon us. Their continued discovery will bring unprecedented information about the basic properties of compact object mergers. However, the detection of a counterpart at electromagnetic wavelengths will significantly leverage the gravitational wave event by providing a precise position, enabling redshift determination, association to a host galaxy, and invaluable astrophysical context. Here, I describe ongoing observational efforts to place constraints on counterparts to gravitational waves across the electromagnetic spectrum.
- 10:00 - 10:20
Fake it 'til you Make it: Fast and Accurate Predictions for Dark Matter Substructure around Milky Way-Mass GalaxiesThe abundance and distributions of subhalos around Milky May galaxies, particularly at the low mass end, provides a key test of the LCDM paradigm of structure formation. However, accurate statistical predictions for substructure populations remain problematic: fully hydrodynamic simulations are prohibitively expensive, while dark matter only simulations fail to capture crucial interactions with the central galaxy. I demonstrate the importance of these interactions, which eliminate all subhalos within the central ~20 kpc, and present the results of "embedded disk" simulations, which employ an analytic potential for the galactic disk and which reproduce subhalo populations from hydrodynamic simulations to within 25% at ~1% of the computational cost.
- 10:20 - 10:40 Coffee Break
- Session 6: Multiple Compact Systems
- Chair: Salvatore Vitale
- 10:40 - 11:00
Not-so Runaway Growth and the Formation of Massive Black HolesNuclear star clusters (NSCs) and supermassive black holes (SMBHs) both inhabit galactic nuclei, coexisting in a range of bulge masses, but excluding each other in the largest or smallest galaxies. We propose that the transformation of NSCs into SMBHs occurs via runaway tidal captures, once NSCs exceed a certain critical central density and velocity dispersion. The bottleneck in this process, as with all collisional runaways, is growing the first e-fold in black hole mass. The growth of a stellar mass black hole past this bottleneck occurs as tidally captured stars are consumed from repeated mass transfer at pericenter. The runaway slows once the seed SMBH consumes the core of its host NSC. The smallest SMBHs have probably grown primarily from strong tidal encounters with NSC stars.
- 11:00 - 11:20
Formation of Tidal Captures and Gravitational Wave Inspirals in Binary-Single InteractionsI present the first systematic study on how dynamical stellar tides and general relativistic (GR) effects affect the dynamics and outcomes of binary-single interactions. For this, I have constructed an N-body code that includes tides and GR. Using this numerical formalism I show that the main effect from tides is the formation of two-body tidal captures that form during the chaotic and resonant evolution of the triple system. Using a corresponding analytical framework I find that the rate of tidal inspirals relative to collisions is largest for triple systems hosting white dwarfs and neutron stars. As a result, one finds that the rate of highly eccentric white dwarf - neutron star mergers is greatly dominated by tidal capture inspirals. This has several EM and GW observable consequences.
- 11:20 - 11:40
Tips for Finding Massive Black Hole BinariesUnderstanding the processes by which massive black hole binaries (MBHBs) come together and merge is important for understanding the mutual build up of galaxies and their central black holes, the environments of galactic nuclei, and the low frequency gravitational wave sky. I will discuss electromagnetic signatures of MBHBs at sub-pc separations that can help to characterize the MBHB population and understand the uncertain processes which do or do not bring the black holes close enough for gravitational wave driven merger. I will present predictions for electromagnetic signatures of MBHBs surrounded by a gas disk. I will then discuss the application of this theoretical work in the context of a recently discovered set of MBHB candidates identified via their periodic optical continua.
- 11:40 - 12:00
Electromagnetic Counterparts from Long-Lived Binary Neutron Star Merger RemnantsBinary neutron star mergers are among the most promising events for multimessenger astronomy. Recent studies indicate that in a large fraction of binary neutron star mergers a long-lived neutron star may be formed rather than a black hole. In this talk, I will present a model that provides a self-consistent evolution of such a post-merger system and its electromagnetic (EM) emission. The resulting EM signals provide unique information about the post-merger evolution on timescales that are inaccessible to GWs. Lightcurves and spectra from this model will be discussed in the context of multimessenger astronomy and be compared to other potentially promising EM counterparts. Implications for short gamma-ray bursts and their X-ray afterglows are also pointed out.
- 12:00 - 12:20
Numerical Simulations of Merging Black Holes and Neutron StarsThe recent detection of gravitational waves emitted by merging black holes opened a new way to observe the universe. Soon, gravitational wave detectors will also observe binary neutron star and black hole-neutron star mergers. Then, the gravitational waves will sometimes be followed by bright electromagnetic signals, which can provide additional information about merging binaries. Neutron star mergers also help us understand nuclear interactions, and may be the main production site of many heavy elements. I will discuss what we can learn from these mergers, how numerical simulations can help us interpret observations, and current frontiers in the simulation of these mergers.
- 12:20 - 12:40
A NICER Look at Spinning Black HolesOne of the most remarkable properties of an astrophysical black hole is that it can be completely described by just its mass and spin. Knowledge of spin is important for testing models of black hole formation, relativistic jets, GRBs, and more. I will discuss recent work measuring the spins of stellar-mass black holes and describe a new and forthcoming X-ray mission, NICER, which has important capabilities for improving our understanding of black holes. In particular, for measuring their spins.
- 12:40 - 1:40 Lunch
- 1:40 - 2:20
A New Era of Compact Objects
- Session 7: Lensing, Mostly
- Chair: Paul Green
- 2:20 - 2:40
Accurate, Physical Photometric Redshifts for Deep Galaxy Surveys with Heterogeneous Training DataTesting cosmological models with deep imaging surveys such as the Dark Energy Survey and LSST requires accurate photometric redshifts of millions of galaxies in extended redshift ranges. However, very few spectroscopic observations of faint, high redshift galaxies are available. As a result, photometric redshifts rise as a major source of uncertainty in the exploitation of current and upcoming surveys. I will present a new approach for robustly estimating photometric redshifts even in the presence of limited, highly heterogeneous spectroscopic training data. This approach combines the advantages of machine learning and template fitting methods, and delivers valid and robust probability distributions.
- 2:40 - 3:00
The Bright and Dark Side of Weak Gravitational LensingThe Dark Energy Survey (DES) in in the process of imaging 5000 sq. deg. of southern extragalactic sky with the primary goal of constraining cosmology with weak gravitational lensing, clustering of galaxies, galaxy cluster counts and distant supernovae. I present early results from weak gravitational lensing, with a focus on (1) the most massive galaxy clusters and (2) novel constraints on the least dense lines of sight (dubbed galaxy troughs). I will also discuss prospects of these measurements with upcoming data releases.
- 3:00 - 3:20
Mapping Mass Across the Universe: CMB Lensing Measurements Present and FutureMeasurements of gravitational lensing in the CMB directly probe the projected distribution of mass out to high redshifts, and thus encode a wealth of information about cosmology. In my talk, I will explain how CMB lensing measurements will allow us to precisely determine the masses of neutrinos, probe the relation between dark and luminous matter at high redshifts, and constrain the properties of inflation and the early universe. After reviewing first measurements of CMB lensing, I will discuss new studies of the lensing effect with CMB data from the ACTPol experiment. I will show the remarkable precision achievable by CMB lensing surveys over just the next five years and explain the great scientific returns and challenges we can expect from these surveys.
- 3:20 - 3:40
Caustic Crossing at Giga-Parsecs Away and Micro-LensingRecently, an intriguing transient emerged in the cluster lensing system of SN Refusal. It has been interpreted as a caustic-crossing event in which a background star at z~1.5 underwent extreme magnification as it was crossing a caustic due to gravitational lensing by the foreground cluster at z ~ 0.5. We investigate the effect of micro-lensing by intracluster stars. Using analytical and numerical studies, we predict that micro-lensing, being greatly enhanced near the macro-lensing critical curve, can turn a smooth critical curve into a corrugated network, and hence induce significant fluctuations in the light curve. The wide-band color along the line of sight suggests a surface mass density 10^4 times larger than the critical density required to disrupt the smooth critical curve.
- 3:40 - 4:00 Coffee Break
- Session 8: Jets
- Chair: Dan Schwartz
- 4:00 - 4:20
How AGN Jets Heat the Intracluster MediumFeedback from active galactic nuclei (AGN) is believed to prevent catastrophic cooling in galaxy clusters. However, how the feedback energy is transformed into heat, and how the AGN jets heat the intracluster medium (ICM) isotropically, still remain elusive. In our recent work, we gain insights into these processes using state-of-the-art simulations of AGN feedback in clusters. I will present what we have learned about the relative importance of all relevant mechanisms, including radiative cooling, advection, adiabatic compression/expansion, turbulence, shock heating, and mixing with hot bubble gas. I will also show how the heat provided by the bipolar AGN jets is isotropized by the process of 'gentle circulation' within the cluster core.
- 4:20 - 4:40
The Story of Strongly Lensed Jets and CosmologyExtragalactic jets are the largest particle accelerators in the universe. Spatial origin of gamma-ray radiation from these sources cannot be fathom due to the poor angular resolution of the detectors. We propose to investigate gravitationally lensed blazars. Cosmic lenses magnify the emission and produce time delays between mirage images. These time delays depend on the position of the emitting regions in the source plane. We combine the precisely measured time delays at gamma rays, well-resolved positions of radio images, a model of the lens and the Hubble constant to elucidate the origin of gamma-ray flares from bright blazar B2 0218+35. We achieve 1 mas spatial resolution at gamma-ray energies. We find that the gamma-ray flares do not originate from the radio core as commonly assumed.
- 4:40 - 5:00
The Power Within: Revealing Structure, Flaring and Jet Launching within the Coronae of Seyfert GalaxiesDetailed analysis of the reflection and reverberation of X-rays from the innermost regions of AGN accretion discs reveals, in detail, the structure and processes within the corona that produce the intense X-ray continuum and extreme variability that is seen. Detailed analysis of X-ray observations of Seyfert galaxies spanning more than a decade have enabled the measurement of the geometry of the corona and changes it undergoes giving rise to orders of magnitude variation in luminosity. Reverberation studies are allowing structures within the corona to be resolved for the first time, including collimated structures in the innermost regions that may point to the mechanism by which jets are launched and the process liberating energy from the accretion flow to power these extreme objects.
- 5:00 - 5:20
Plasmoids in Relativistic Magnetic Reconnection: the Blobs of Blazar Emission Models?
Powerful flares from blazars with minute- timescale variability are challenging for current models of blazar emission. I will present a physically motivated model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. I will show that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. Multiple flares with a range of flux-doubling time-scales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization.
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