Gravitational lensing is a potentially powerful tool for elucidating the origin of gamma-ray emission from distant sources. Cosmic lenses magnify the emission and produce time delays between mirage images. Gravitationally-induced time delays depend on the position of the emitting regions in the source plane. Temporal resolution at gamma-ray energies can be used to measure these time delays, which, in turn, can be used to resolve the origin of the gamma-ray flares spatially. As a prototypical example of the power of lensing combined with long, uniformly sampled light curves provided by the Fermi satellite, we investigated the spatial origin of gamma-ray flares from two known gravitationally lensed sources: PKS 1830-211 and B2 0218+35.

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Dynamical interactions between stars in dense stellar systems have very important observational and dynamical consequences. Binary stars, e.g., prevent globular clusters from collapsing through triple interactions, where dynamically assembled neutron stars are believed to be related to short GRBs and the main source of GWs. In my talk I will present a detailed study on binary-single stellar interactions, and how general relativistic corrections and tidal effects affect the dynamics which can lead to new exiting observables.

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Laboratory astrophysics is the colloquial term for experimental work performed in the lab motivated by astrophysical questions. In general, this type of work complements observational and numerical astronomy; from spectroscopic studies, to astrochemistry, to high-energy physics, and others. It is the high-energy-density (HED) environments, or systems with pressures >1Mbar, that I am interested in exploring. Much work goes into developing a platform, where a well-scaled experiment may be performed with respect to a specific object of interest. In many cases, however, direct scaling is not possible. Rather, achievement of specific dimensionless parameters in the lab is required to reach a relevant physics regime and measurements of these systems may be used as benchmarks for astrophysical codes and to provide insight into the microphysics of astrophysical systems. I will discuss a number of experimental platforms currently under development to investigate: the formation of collisionless shocks and the role of self-generated magnetic fields, the dynamics and evolution of supersonic magnetized jets, and the production of relativistic electron-positron jets.

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The GMT CfA Large Earth Finder G-CLEF is a general purpose visible echelle spectrograph that also provides precision radial velocity capabilities. G-CLEF will reside in a gravity invariant and temperature controlled environment on the azimuth disk of the telescope. An optical and fiber relay system will transfer light from the telescope to G-CLEF and will scramble the light within each of the seven sub-apertures in the process. The instrument will support several observing modes with a range of spectral resolutions and input aperture sizes.In my presentation I will go over some of the science cases for G-CLEF, as well as its optical and mechanical design.

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The massive neutrino background makes up a component of the dark matter, and as such affects the growth of large-scale structure, such as galaxy clusters. This affords us an opportunity to measure the neutrino mass. I will describe our method for simulating massive neutrinos and as an example show how they affect galaxy clusters.

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The separate universe conjecture states that in General Relativity a long-wavelength density perturbation affects physics locally such that short-scale gravitational clustering takes place in a separate universe with different background density and curvature. We construct Conformal Fermi Coordinates to verify this conjecture for small cosmological scalar perturbations on arbitrary scales. In this case, the isotropic part of the perturbation is entirely absorbed into a modified local expansion history and spatial geometry. The anisotropic part, on the other hand, is exactly captured by a tidal field in the Newtonian form. With the aid of this formalism, it can be shown that nonlinear gravitational evolution does not generate clustering signature that mimics local-type primordial non-Gaussianity. Rather, contamination to f_NL type non-Gaussianity only arises from relativistic projection effects on photon propagation, which depend on the specific large-scale structure tracer and observable considered, and are in principle distinguishable from nonlinear gravitational clustering.

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The relation between Mstar and Mhalo has important implications for galaxy formation, particularly when comparing simulations to observations. For Mstar 1e8 Msun, the abundance matching AM technique is remarkably accurate, even correctly predicting galaxy counts around Milky Way-analogues in SDSS. However, the cosmological stellar mass function is unconstrained at Mstar 1e8 Msun. I therefore use the Local Group LG, where counts are complete for Mstar 1e5 Msun, to test AM extrapolations, focusing specifically on the impact of scatter. I show that scatter increases counts at fixed Mstar, requiring a more rapid fall-off with Mhalo to avoid overproducing the LG dwarfs. I also briefly explore the implications of these models on outstanding questions in dwarf galaxy formation.

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Recent studies highlight the importance of including and resolving multiple stellar feedback mechanisms. With the sophisticated treatment of ISM and stellar physics at unprecedented numerical resolution, we are now ready to study the formation mechanisms of stars and star clusters in a galactic context. We have investigated the active star formation sites and the ensuing evolution of star clusters in high-redshift merging proto-galaxies, and examined one of the formation hypotheses of old, metal-poor, blue globular clusters GCs. The high dynamic range in our simulation allows us to track the relaxation and evolution of star clusters for up to 0.5 Gyrs, and to potentially identify some long-lasting star clusters as candidates for present-day metal-poor GCs.

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The evolution of clusters, groups, and early-type galaxies is governed by mergers and by feedback from supermassive black holes (SMBH). Chandra's sensitivity and high angular resolution have revolutionized our understanding of these processes, revealing details of how clusters grow through mergers of subclusters over cosmic time and the important role that SMBHs play in keeping early-type galaxies "red and dead." I will discuss examples of Chandra observations that show shocks and bubbles in the cores of clusters produced by SMBH outbursts which reheat the radiatively cooling gas. On large scales, cluster mergers contribute to the evolution of galaxies as well as the large scale structures in which they reside. I will discuss examples of cluster mergers that illustrate merger shocks, cold fronts and sloshing of the hot plasma, as well as the transformations of cluster galaxies and the generation of radio halos and relics.

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The properties of the hot X-ray gas, in the halos of galaxies, are sensitive to the dark matter halo mass and a range of baryonic processes. Therefore, many scaling relations linking X-ray observables with other galaxy properties can provide powerful observational links between the mass distribution, supermassive black hole SMBH feedback, supernovae SNe feedback, and environmental effects that govern the evolution of galaxies. We will report on an X-ray and radio survey of early-type galaxies intended to provide an observational anchor for feedback models and identify the mechanism that is responsible for feeding active galactic nuclei in massive early-type galaxies.

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AGN Feedback is one of the most promising heating mechanisms to circumvent the cooling-flow problem in galaxy clusters. However, this picture is complicated by the field-line wrapping effect of the heat-flux driven buoyancy instability HBI caused by anisotropic conduction. I will present results from our 3D MHD simulations that include anisotropic conduction, radiative cooling, and AGN feedback. I will address the following questions: (1) whether AGN jet-driven turbulence can randomize field lines and halt the HBI, (2) what the relative importance of conductive heating and direct AGN heating is, and (3) what the impacts of conduction on AGN feedback and ICM properties are and whether they are observable.

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Precision observations by the Fermi-LAT telescope have uncovered a significant gamma-ray excess emanating from the Galactic Center. This result is particularly exciting, since its spectrum and morphology match predictions from annihilating dark matter. However, several reasonable astrophysical explanations also exist, including emission from a yet-undiscovered population of gamma-ray pulsars. I will discuss the current state of Fermi-LAT observations and theoretical models of the Galactic Center, focusing on recent and upcoming studies which may definitively determine the source of the gamma-ray excess.

A stochastic superposition of gravitational waves (GWs) from supermassive black holes SMBHs is expected to produce a stochastic GW background that will leave a unique signature in the correlations of arrival times of pulses from a collection of radio pulsars. Here I present on the first analysis that places constraints on the amplitude and shape of the stochastic GW background. We find that the data favor a turnover in the GW strain spectrum for current models of SMBH merger rates. This result indicates that environmental factors, other than GWs from circular binaries, are influencing the GW spectrum.

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The merger of two neutron stars forms a rapidly spinning and highly magnetized neutron star magnetar, which subsequently undergoes collapse to a black hole and produces a short gamma-ray burst (GRB). One of the biggest uncertainties is how long the magnetar survives prior to collapse. If the magnetar persists, the magnetic spin-down can provide an enormous reservoir of energy, which is not the case in the prompt collapse to a black hole. One prediction of the magnetar scenario is the production of a radio transient from the interaction of the magnetar with the surrounding medium. In this talk, I present observations which directly test the presence of long-lived magnetars following short GRBs. These observations are among the first tests to identify the remnants of neutron star mergers.

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I will present global relativistic simulations of the interaction between a rotating stars magnetic field and an accretion disk, showing the opening of field lines, the formation and reconnection of current sheets, and a large increase in the spin-down torque applied by the pulsar wind. The principal simulation results can be captured by a simple analytic model. For conditions relevant to accreting millisecond pulsars, the enhanced pulsar wind torque can be substantially larger than that predicted by existing models, and is in principle capable of maintaining spin equilibrium at frequencies less than 1 kHz. If the pulsar wind is collimated by the surrounding environment, the resulting jet can satisfy the power requirements of the highly relativistic outflows from Cir X-1 and Sco X-1.

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Pulsar timing arrays have proved to be great tools in the hunt for gravitational waves. A key ingredient for the success of pulsar timing arrays is many precisely timed millisecond pulsars. New pulsars are being discovered at an ever growing rate, and with the Square Kilometre Array upcoming, this rate is going to increase drastically. Initially, creating a timing model for a newly discovered pulsar is manual labour, and observing schedules are not made optimally, which wastes lost of observing time. I have created a method that can optimize observing schedules, and automatically provide timing solutions for newly discovered pulsars.

I discuss how AGN feedback is tightly coupled with the formation of multiphase gas and the newly probed chaotic cold accretion in massive galaxies, groups, and clusters. In a turbulent and heated atmosphere, cold clouds and filaments condense out of the hot X-ray plasma and rain toward the SMBH. The recurrent chaotic collisions between the cold filaments promote angular momentum cancellation and boost the accretion rate, triggering powerful AGN outflows which quench the cooling flow. AGN feedback thus subsides and the hot halo is allowed to cool again, reloading a new cycle. Ultimately, chaotic cold accretion creates a symbiotic link between the SMBH and the host system, leading to a tight self-regulated feedback loop which preserves the hot halo in thermal equilibrium through cosmic time.

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Echoes from the broad line region in active galactic nuclei AGN allow for the measurement of supermassive black hole masses outside the local Universe. However, the detailed structure of the broad line region is difficult to constrain due to the very small scales involved. With a new generation of high-quality reverberation mapping datasets, we can substitute time resolution for spatial resolution and begin to model echoes from the broad line region directly. I will discuss the development of a direct modeling approach for reverberation mapping data capable of measuring the absolute black hole mass and the geometry and dynamics of the broad line region. As an example, I will show an application of this approach to the Lick AGN Monitoring Project 2008 reverberation mapping sample.

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In this talk I will focus on the possible association of IceCube IC neutrinos with BL Lacs, a sub-class of radio loud AGN. I will present the results of a hadronic model applied on six individual BL Lacs. These were selected as probable counterparts of the IC neutrinos through a model-independent joint positional and energetic diagnostic. By employing Monte Carlo simulations, which reproduce all the main properties of blazars in the radio, X-ray, and gamma-ray bands, the findings from individual sources can be extrapolated to the whole BL Lac class. I will present a detailed calculation of the cumulative neutrino emission from BL Lacs, and discuss its specific predictions that should be testable in the next few years.

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In my talk I will present simulations of a close encounter of a red dwarf with a supermassive BH. Using Kerr-Shield coordinates one is able to track the accretion rate through the BH horizon. I will show how the gas forms a quasi disk and discuss how the accreted gas makes its way to the BH. A comparison between non- and magnetized stars will be given.

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Massive stars play an important role in many astrophysical systems. Based on three dimensional radiation hydrodynamic simulations, I will show how massive stars get their super-Eddington luminosity in different regimes. When efficient convection develops, convection reduces the radiation acceleration. The simulations provide the first numerical calibration of mixing length theory in the radiation dominated regime. When convection becomes inefficient, turbulent velocity exceeds isothermal sound speed, driving shocks and large density fluctuations. The simulation results have important implications for the evolutions of massive stars.

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Relatively massive B-type stars with closely orbiting stellar companions can evolve to produce Type Ia supernovae, X-ray binaries, millisecond pulsars, mergers of neutron stars, gamma ray bursts, and sources of gravitational waves. However, the formation mechanism, intrinsic frequency, and evolutionary processes of B-type binaries are poorly understood. We have utilized large data sets of eclipsing binaries to measure the physical properties of B-type binaries with low metallicities, extreme mass ratios, and intermediate orbital periods. The updated multiplicity statistics provide valuable insight into the formation of massive stars and binaries as well as reliable initial conditions for population synthesis studies of Type Ia supernovae and low-mass X-ray binaries.

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The tidal disruption of stars by supermassive black holes SMBHs holds great promise as a method for probing SMBH demographics, but there are several aspects of these events that we do not yet understand fully. I will discuss an apparent conflict between my own theoretically calculated tidal disruption event rates, and the rates inferred from the observational sample of roughly a dozen optically selected flares. Specifically, even highly conservative theoretical calculations predict a volumetric event rate roughly an order of magnitude above common observational estimates. Several possible resolutions to this conflict exist, and I will briefly examine two: a bimodality in optical emission from tidal disruption flares, and exotic stellar dynamics in galactic nuclei.

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I will present simulations in which magnetic fields of different strengths and configurations were included within a star that is tidally disrupted by a supermassive black hole. We find that such fields play only a mild dynamical role in the disruption itself, but imprint a field strength upon the resulting tidal debris that is comparable to the original stars. In disruptions where the star survives the encounter, the spin-up of the core results in an order of magnitude enhancement in the magnetic field strength within it. I will discuss how these outcomes are likely to affect the accretion disk that forms out of the tidal debris, and the implications for jet production.

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New ALMA observations of giant galaxies with cooling hot haloes reveals that a supermassive black hole can act much like a mechanical pump in a water fountain. I will show multiwavelength evidence for these galaxy-spanning phenomena, wherein the black holes bipolar jets propel billion solar mass molecular outflows, while giant molecular clouds are simultaneously falling inwards toward the center of the galaxy, ultimately merging with the black hole accretion reservoir. The findings suggest that cold molecular gas can couple to black hole growth via both feeding and feedback, in alignment with cold chaotic accretion models for the regulation of star formation in large galaxies.

Stellar-mass black holes are now ubiquitously found in X-ray binaries. Such black holes are formed in collapsing massive stars, but many questions remain about how the progenitor properties connect to those of the resulting black holes: Which progenitors produce black holes not neutron stars? What is the initial mass function of black holes? What is the initial spin distribution? I will discuss possible signatures of newborn black holes in different types of collapsars and how to identify them in the era of time-domain astronomy.

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We present our high resolution Chandra HETG spectra of V404 Cygni as it went into outburst in June 2015. We find strong detection of both emission and absorption features in the time-resolved spectra in species of Si, S, Mg and Fe. Based on the He-like triplets of these ions, we estimate the gas is residing at distance between 1e10-1e12 cm from the source, with the higher ionization ions residing closer to the black hole than the lower ionization ions. Finally, we find the kinetic power of the wind is extremely high, and rivals that of the radiative component.

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One 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. There are two primary techniques by which spin is being measured: the reflection and continuum-fitting methods. In both cases, spin is obtained by measuring the inner radius of the accretion disk, which corresponds to the innermost stable circular orbit ISCO. I will describe my work on measuring the spins of stellar-mass black holes.

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In accretion disks of slowly accreting black holes, such as in SgrA* and M87, the plasma become collisionless, and the temperatures of protons and electrons decouple. In these cases the extent to which the simulations can be applied to observations is limited by the simple assumptions made about electron thermodynamics. I will present the results of black hole accretion simulations that treat electrons as a separate fluid and show how physically-motivated electron heating rates result in simulated electron temperature distributions that are significantly different from the constant electron-to-proton temperature ratio assumed in previous work.

Magnetic fields and turbulence are vital components in galactic processes, including cosmic ray transport, ISM structure formation and star formation. However turbulence is difficult to measure observationally and the role of simulations is vital for both testing theories of ISM turbulence and gauging observational diagnostics via synthetic observations. In this talk I will discuss the origins of turbulence in galaxies, and its connection to the star formation process, both from observations and the Illustris AREPO cosmological simulation. I will also highlight how turbulence can be measured in spectral line observations of molecular clouds and diffuse gas in galaxies in order to constrain and test simulations as well as obtain important properties of turbulence such as the injection scale

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Core-collapse in rapidly rotating, strongly magnetized progenitors may power some of the most luminous supernovae we observe and possibly set the stage for a subsequent long gamma-ray burst. I will present results from three-dimensional 3D general-relativistic magnetohydrodynamic MHD simulations that show how an instability of the ultra-strong toroidal magnetic field affects jet stability and the dynamics, geometry and nucleosynthetic signature of MHD-driven explosions. I will also discuss new 3D simulations of MHD turbulence in rapidly rotating protoneutron stars.

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The neutrino mechanism for powering core collapse supernovae (CCSN), where neutrinos emitted from deep in the core of the collapsed star deposit a fraction of their energy behind the stalled CCSN accretion shock, is thought to be the most likely means of producing observed Type II supernovae. Still, there is significant uncertainty whether this mechanism can actually reinvigorate the shock and if it can explain the energetics of observed CCSNe. Previous works using simplified neutrino treatments have shown that the evolution of accretion shock front formed during a CCSN differs significantly when axial symmetry is imposed, relative to full three dimensional simulations with no symmetries imposed. Recently, we have performed some of the first fully three dimensional, multi-group radiation hydrodynamic simulations of these events. For our chosen 27 solar mass progenitor model, which sets the initial conditions for our simulations, we are finding that shock runaway occurs on timescales of hundreds of milliseconds, which is indicative of a successful neutrino powered explosion. I will discuss our radiation transport methods, results from the simulations, the resolution and symmetry dependence of the results, and future work.

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Most supermassive black holes, including those being imaged by the Event Horizon Telescope, have accretion discs in which the Coulomb mean free path is expected to be very large. Existing ideal fluid models may not describe well the evolution of these discs. I will discuss an extended MHD model in which deviations from ideal MHD are taken into account through an anisotropic heat flux and viscosity. I will then present axisymmetric simulations of thick discs using a new code capable of integrating that model. The simulations show that the viscous shear has an O1 effect on the evolution, increasing angular momentum transport, outflows, and heating of the plasma. By comparison, the heat flux has a much more modest impact on the evolution of the system, at least in axisymmetric simulations.

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