Einstein Fellows Symposium 2018

October 2-3

Phillips Auditorium

Harvard-Smithsonian Center for Astrophysics, Cambridge, MA

Watch the talks on our channel, Day 1 and Day 2.

Download a PDF of the program.

  • Tuesday, October 2
  • 8:45 - 9:15 Coffee & Continental Breakfast for Fellows
  • 9:15 - 9:30
  • Paul Green & Belinda Wilkes
  • Welcome
  • Session 1: Gravity & Lensing
    9:30 - 10:45
  • Chair: Rosanne Distefano
  • 9:30 - 9:55
  • Davide Gerosa (California Institute of Technology)
  • Getting ready: exploiting LISA to improve LIGO's tests of General Relativity

    The early inspiral of massive stellar-mass black-hole binaries merging in LIGO's sensitivity band will be detectable at low frequencies by the upcoming space mission LISA. LISA will predict, with years of forewarning, the time and frequency with which binaries will be observed by LIGO. We will, therefore, find ourselves in the position of knowing that a binary is about to merge, with the unprecedented opportunity to optimize ground-based operations to increase their scientific payoff. We apply this idea to detections of multiple ringdown modes, or black-hole spectroscopy. Narrowband tunings can boost the detectors' sensitivity at frequencies corresponding to the first subdominant ringdown mode and double our prospects to experimentally test the Kerr nature of astrophysical black holes.


  • 9:55 - 10:20
  • Liang Dai (Institute for Advanced Study)
  • Detecting lensing diffraction effects in gravitational waves from compact binary coalescence

    Forthcoming observatories will detect numerous gravitational wave emissions from compact binary mergers from the distant Universe. In the ground-based frequency band ~ 10 Hz - 1000 Hz, due to wave diffraction effects, the waveform can be perturbed by small intervening mass clumps ~ 100 - 1000 Msun. Even when multiple lensed images do not form and magnification is insignificant, wave diffraction imprints amplitude and phase modulations in the frequency domain waveform. This can be exploited for probing the clustering of the dark matter on very small scales and therefore constraining micro-physics models for the dark matter. We devise and demonstrate an agnostic method based on dynamic programming to search for diffraction induced modulations.


  • 10:20 - 10:45
  • Maximiliano Isi (Massachusetts Institute of Technology)
  • Gravitational-wave polarizations beyond general relativity: recent measurements and future prospects

    Polarizations are a fundamental property of the geometry of gravitational waves (GWs), determining the directions in which space is stretched and squeezed as the waves whiz by. With the detections of GW170814 and GW170817, we have obtained some direct evidence that GW polarizations are as Einstein predicted---but there is much more to be learned with future measurements. In this talk, I will review the basics of GW polarizations, summarize the implications of recent compact-binary detections, and outline future ways to study this basic property of gravity with ground-based detectors.


  • 10:45 - 11:10 Coffee Break
  • Session 2: Supernovae, Kilonovae, GRBs & Pulsars
    11:10 - 12:50
  • Chair: Edo Berger
  • 11:10 - 11:35
  • Ben Margalit (University of California, Berkeley)
  • FRBs as flaring magnetars, and their connection to SLSNe and LGRBs

    The repeating fast radio burst (FRB) 121102 was recently localized to a star forming region in a metal poor dwarf galaxy, similar to the host environment of superluminous supernovae (SLSNe) and long gamma-ray bursts (LGRBs). This FRB was furthermore shown to have associated quiescent radio emission and an extremely large rotation measure (RM). I present a model for FRB 121102 as a flaring magnetar in an electron-ion nebula, which can match both the large RM and the luminous quiescent synchrotron source co-located with the FRB emission. I will additionally discuss calculations for the photo-ionization of the surrounding supernova ejecta by magnetar spindown in such cases, illustrating that FRB 121102 is consistent with the population of magnetars associated with SLSNe.


  • 11:35 - 12:00
  • Daniel Siegel (Columbia University)
  • The cosmic origin of the heavy elements

    The recent detection of the binary neutron star merger GW170817 by LIGO and Virgo was followed by a firework of electromagnetic counterparts across the entire electromagnetic spectrum. The ultraviolet, optical, and near-infrared emission is consistent with a kilonova that provided strong evidence for the formation of heavy elements in the merger ejecta by the rapid neutron capture process (r-process). Employing general-relativistic magnetohydrodynamic simulations, I will argue that the astrophysical site that generated the heavy elements and the red kilonova of GW170817 are winds from a post-merger accretion disk. I will mention potential problems in explaining the galactic r-process abundances solely with neutron star mergers and discuss possible alternatives.


  • 12:00 - 12:25
  • Jennifer Barnes (Columbia University)
  • Diagnosing nucleosynthesis in kilonova light curves

    The gravitational wave event GW170817 and its associated optical/infrared electromagnetic emission (the kilonova) provided the first direct evidence that neutron star mergers produce at least some heavy elements via r-process nucleosynthesis. However, an outstanding questions is whether and how observations can distinguish between an event that produces the heaviest elements the r-process can synthesize from one that burns only a lighter subset. I will discuss recent work showing that a handful of long-live alpha-decaying or fissioning nuclei can have an outsize effect on the kilonova's late-time luminosity. Precise measurements of nebular-phase kilonova light curves could detect these effects, enabling more detailed estimates of the composition synthesized in these mergers.


  • 12:25 - 12:50
  • Kate Alexander (Northwestern University)
  • Radio Observations of GW170817: Probing the Structure of Relativistic Jets

    Radio observations provide a unique window into a wide variety of astrophysical transients, from gamma-ray bursts to tidal disruptions of stars by supermassive black holes. In these high-energy laboratories, the longer timescale of radio emission allows for extensive followup, probing the physics underlying the formation and growth of relativistic jets and outflows. I will present radio observations of GW170817, the first binary neutron star merger observed in gravitational waves. Our full multi-wavelength dataset (Alexander et al. 2018) is consistent with a successful structured jet expanding into a low-density circumbinary medium. If future observations continue to track our predictions, we expect that the radio and X-ray emission will remain detectable until 1000 days post-merger.


  • 12:50 - 2:00 Lunch
  • 2:00 - 2:45
  • KEYNOTE SPEAKER: Cecilia Garraffo (Harvard Institute for Applied Computational Science)
  • Stellar Activity and Rotation in Cool Stars

  • 2:45 - 3:10 Coffee
  • Session 3: Cosmology
    3:10 - 5:15
  • Chair: Doug Finkbeiner
  • 3:10 - 3:35
  • Benedikt Diemer (Harvard University)
  • The Splashback Radius of Dark Matter Halos

    The density profiles of dark matter halos are commonly thought not to have a well-defined edge. In this talk, I will show that the situation is more complicated: the outer profiles of halos depend on their dynamical state, and exhibit a novel feature called the splashback radius. I will argue for this radius as a physically motivated halo boundary and discuss recent observational and theoretical results supporting this notion. The splashback radius contains complex information about halos that cannot be derived from conventional radius definitions.


  • 3:35 - 4:00
  • Daniel Gruen (Stanford University)
  • The future of photometric redshifts

    Tests of cosmology with lensing from present and future surveys like DES and LSST will only be reliable if the uncertainty in source galaxy redshift distributions is small and fully quantified. I will report on recent progress on this front and outline how stringent requirements on photometric redshifts can be met with a combination of wide field, extended multi-band deep-field, and sufficient spectroscopic/narrow-band data.


  • 4:00 - 4:25
  • Boris Leistedt (New York University)
  • Forward modeling photometric surveys

    I will describe a new global calibration approach based on hierarchical modeling that delivers unprecedented precise redshifts and SED models for photometric galaxy and quasar surveys. I will show applications to data from the DES and DECALS data and how it opens new perspectives for the robust exploitation of ongoing and upcoming surveys and testing cosmological models.


  • 4:25 - 4:50
  • Philip Mocz (Princeton University)
  • Small-scale features in fuzzy dark matter cosmology

    Fuzzy dark matter (FDM) is an alternative dark matter model proposed to solve the small-scale problems of cold dark matter. The FDM particle is an ultra-light boson that forms a condensate with a kpc-scale de Broglie wavelength that suppresses small-scale structure by quantum mechanical effects. In this talk we explore several aspects of FDM, such as interference waves and soliton cores. We characterize departures from classical predictions, by looking at the dynamical heating effects of quantum fluctuations, the soliton mass--halo mass relation, and cosmological box comparisons with and without quantum mechanics.


  • 4:50 - 5:15
  • Zachary Slepian (Lawrence Berkeley National Laboratory)
  • On Decoupling the Integrals of Cosmological Perturbation Theory

    Cosmological perturbation theory is a way to compute late-time clustering statistics, such as the galaxy 2-point correlation function or power spectrum, given the statistics of the linear density field, which we infer from the CMB. It expresses nonlinear contributions to the late-time statistics, produced by gravitational evolution, as convolution integrals of linear power spectra against kernels. These integrals quickly become high-dimensional: the next-to-leading contributions involve 12-D integrals. These are very costly to evaluate numerically. I present a new approach to reduce these to a sequence of 1-D integrals by exploiting the isotropy of the power spectrum to perform all angular integrals analytically. This approach should substantially accelerate computation of the late-time nonlinear power spectrum for a large set of input cosmological parameters, as will be required for an MCMC analysis of upcoming large surveys such as DESI, Euclid, or LSST.


  • 6:30 Dinner for Fellows at the NuBar
  • Wednesday, October 3
  • 9:00 - 9:30 Coffee and Continental Breakfast for Fellows
  • Session 4: Stars
    9:30 - 10:20
  • Chair: Eric Keto
  • 9:30 - 9:55
  • Morgan MacLeod (Harvard-Smithsonian Center for Astrophysics)
  • Accretion onto compact objects during common envelope phases

    Common envelope phases occur in binary systems when one star evolves to engulf its companion. These interactions transform binaries through the tightening of orbits by drag forces, and they may transform the objects themselves through accretion. This talk will summarize recent findings from hydrodynamic simulations about the nature of gas flow around objects embedded within a common envelope and use these results to discuss the degree of accretion that results. Accretion during the common envelope phase is significant in that it may modify the masses and spins of compact objects passing through such an interaction, potentially with observable signatures for gravitational wave detectors like the LIGO-VIRGO network.


  • 9:55 - 10:20
  • Anna Rosen (Harvard University)
  • How Stellar Feedback Limits Accretion onto Massive Stars

    The injection of energy and momentum into the ISM by young massive stars' intense radiation fields, collimated protostellar outflows, and their fast, isotropic winds can have a profound influence on their formation. Yet, this stellar feedback remains as one of the largest uncertainties in massive star formation. In this talk, I will present a suite of 3D radiation-magnetohydrodynamic simulations of the collapse of massive pre-stellar cores that include these feedback processes to determine how stellar feedback can limit accretion. Inclusion of feedback from outflows punches holes in the ISM along the star's polar directions, thereby increasing the size of optically thin regions where radiation can escape while winds shock heat the infalling gas thereby reducing accretion onto the star.


  • 10:20 - 10:45 Coffee
  • Session 5: Photons in Galaxies
    10:45 - 11:35
  • Chair: Belinda Wilkes
  • 10:45 - 11:10
  • Rahul Kannan (Smithsonian Astrophysical Observatory)
  • Impact of radiation fields in Galaxies

    Radiation fields are thought to play an important role in regulating star formation in Galaxies. A variety of physical mechanisms such as photoheating, radiation pressure and trapped IR radiation have been invoked to disperse the stellar birth clouds and even drive large scale outflows. However, the modelling of these processes have so far been necessarily crude due to the lack of an efficient radiation hydrodynamics (RHD) implementation. I will present a series of high resolution RHD simulations using Arepo-RT, that self-consistently model these radiative processes in order to quantify the importance of each of them in a variety of environments.


  • 11:10 - 11:35
  • Aaron Smith (Massachusetts Institute of Technology)
  • The Physics of Lyman-alpha Escape from High-Redshift Galaxies

    Lyman-alpha (Lyα) photons from ionizing sources and cooling radiation undergo a complex resonant scattering process that generates unique spectral signatures in high-redshift galaxies. I will present a radiative transfer study of cosmological zoom-in simulations from the Feedback In Realistic Environments (FIRE) project, focusing on the time, spatial, and angular properties of the Lyα emission. Many of the interesting features can be understood in terms of the galaxy's star formation history. I will also discuss my new resonant Discrete Diffusion Monte Carlo (rDDMC) method designed to break the efficiency barrier of Monte Carlo Ly╬▒ radiative transfer in the high optical depth regime, which will enable fully coupled 3D Lyα radiation hydrodynamics in the near future.


  • Session 6: Active Galaxies
    11:35 - 12:25
  • Chair: Paul Nulsen
  • 11:35 - 12:00
  • Daniel D'Orazio (Harvard University)
  • Tracking the orbits of supermassive black hole binaries at sub-parsec separations

    I will discuss two new techniques for directly tracking the orbits of supermassive black hole binaries (SBHBs) in galactic nuclei. Specifically, I will show that up to ~100's of putative SBHBs in nearby AGN have separations resolvable by mm-wavelength VLBI while also having orbital periods less than 10 years. Hence, orbital motion can be directly probed. In optical wavelengths, the astrometric precision of Gaia could similarly detect binary orbital motion by the end of its 5 year mission. Such observations could provide the first, and possibly only, electromagnetic form of definitive SBHB detection and also allow a precise measurement of the binary mass, or a novel measurement of the Hubble constant.


  • 12:00 - 12:25
  • Ke Fang (Stanford University)
  • Black hole jets in galaxy clusters as common origins of high-energy cosmic particles

    It has been a mystery that with ten orders of magnitude difference in energy, high-energy neutrinos, ultrahigh-energy cosmic rays, and sub-TeV gamma rays all present comparable energy injection rate, hinting an unknown common origin. Here we show that black hole jets embedded in clusters of galaxies may work as sources of all three messengers. Specifically, the highest-energy particles leave the source rectilinearly and contribute to the observed cosmic rays above 0.1 EeV, the intermediate-energy particles interact with the ICM gas and produce neutrinos and gamma rays, and the lowest-energy particles are cooled due to the expansion of the radio lobes inflated by the jets. The energy output required to explain the measurements of all three messengers is consistent with observations.


  • 12:25 - 1:40 Lunch
  • Session 7: AGN Variability
    1:40 - 2:30
  • Chair: Grant Tremblay
  • 1:40 - 2:05
  • Vivienne Baldassare (Yale University)
  • Variability as a tool for identifying AGNs in low-mass galaxies

    The population of massive black holes (BHs) at the centers of nearby low-mass galaxies provides some of the best observational constraints on the masses of BH seeds at high redshift. However, BHs in low-mass galaxies are observationally challenging to detect. I will discuss recent work searching for AGNs in low-mass galaxies via optical photometric variability. We analyzed the nuclear variability of 28,000 nearby galaxies with Sloan Digital Sky Survey spectroscopy in Stripe 82. While most of the variability-selected AGNs have narrow emission lines consistent with the presence of an AGN, a small fraction have narrow lines dominated by star formation. The star-forming systems tend to be low-mass, and may be AGNs missed by other selection techniques due to star formation dilution.


  • 2:05 - 2:30
  • Krista Lynne Smith (Stanford University)
  • High-Energy Astrophysics with Exoplanet Satellites: AGN and Blazars

    Exoplanet-hunting spacecraft have great potential for high energy astrophysics but are often beset by dangerous systematics that mimic the source's true variability. Using careful diagnostics, many of these effects can be mitigated, but results indicate that simultaneous ground-based monitoring is key. One result of our study after mitigation was the discovery of an optical quasi-periodic oscillation that is now being followed up with NICER. X-ray confirmation of the same period or the corresponding higher frequency QPO can inform models of the relationship between optical and X-ray emitting regions. I will also discuss my new project to simultaneously monitor blazars with Swift and the exoplanet hunter TESS. Finally, I will highlight some of my VLA results regarding AGN feedback.


  • 2:30 - 3:00 Coffee
  • Session 8: Accretion
    3:00 - 4:35
  • Chair: Malgosia Sobolewska
  • 3:00 -3:25
  • Dan Wilkins (Stanford University)
  • Venturing Beyond the ISCO with X-ray Reverberation

    X-ray reverberation, where variability in the continuum emission is seen to echo off the inner regions of the accretion disc, represents a unique probe of the extreme environment around black holes. Through these studies, a picture is emerging of how the corona is structured, how it evolves during dramatic X-ray flares and how it may be linked to the launching of jets. X-ray reverberation studies probe the accretion flow, by combining general relativistic ray tracing with radiative transfer descriptions of X-ray reflection. This probes not just the structure of the disc, but of the innermost stable orbit, showing how the behavior of material in its final moments as it plunges into the black hole can be elucidated through X-ray observations and providing a unique test of General Relativity


  • 3:25 - 3:50
  • Vladimir Zhdankin (Princeton University)
  • Turbulent particle energization in radiatively inefficient accretion flows

    Radiatively inefficient accretion flows (RIAFs) onto black holes are heavily influenced by collisionless plasma physics. To explain their stability, models require that RIAFs consist of a two-temperature plasma in which ions are much hotter than electrons. Whether the ambient turbulence can establish such a two-temperature state remains under debate. In a step toward resolving this debate, I will describe recent results on electron and ion energization by turbulence in particle-in-cell simulations of collisionless plasmas, with temperatures such that ions (protons) are sub-relativistic and electrons are relativistic. This work is the initial step towards building a first-principles understanding of RIAFs and other high-energy astrophysical systems.


  • 3:50 - 4:15
  • Eric Coughlin (Columbia University)
  • Self-similar Shock Propagation with Accretion

    Shocks are ubiquitous in astrophysics, and understanding how they propagate and affect their environments can reveal useful information about many seemingly dinstinct phenomena. I will describe a new regime of self-similar shock propagation that, in contrast to the well-known, Sedov-Taylor blastwave, is valid when the shock is only marginally supersonic, accounts for the gravitational field of a central object, and results in accretion onto that central object. I will compare the predictions of the self-similar solution to numerical simulations of failed supernovae, wherein a weak shock is generated within and propagates through the hydrogen envelope of a massive star.


  • 4:15 - 4:35
  • Dheeraj Pasham (Massachusetts Institute of Technology)
  • A Remarkably Loud Quasi-Periodicity after a Star is Disrupted by a Massive Black Hole

    The immense tidal forces of massive black holes can rip apart stars that come too close to them. As the resulting stellar debris spirals inwards, it heats up and emits x-rays when near the black hole. I will talk about our recent discovery of an exceptionally stable 131-second x-ray quasi-periodicity from a black hole after it disrupted a star. Using a black hole mass indicated from host galaxy scaling relations implies that, (1) this periodicity originates from very close to the black hole's event horizon, and (2) the black hole is rapidly spinning. Our findings suggest that other disruption events with similar highly sensitive observations likely also exhibit quasi-periodicities that encode information about the fundamental properties of their black holes.


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