Presentation

In a few dozen merging galaxy clusters diffuse extended radio emission has been found, implying the presence of relativistic particles and magnetic fields in the intracluster medium. A major question is how these particles are accelerated up to such extreme energies. In this talk I will present LOFAR and JVLA radio observations of the Toothbrush galaxy cluster. The Toothbrush cluster hosts diffuse 2 Mpc extended radio emission in the form of a radio relic and halo. Our deep LOFAR and JVLA observations allow a radio spectral study to test the shock origin of the relic and underlying particle acceleration mechanisms.

Electron acceleration to non-thermal energies is known to occur in low Mach number (M<5) high plasma beta shocks in low-luminosity accretion disks and galaxy clusters, but the acceleration mechanism remains poorly understood. Using two and three dimensional particle-in-cell plasma simulations, we find that electrons are efficiently accelerated in low Mach number quasi-perpendicular shocks via a Fermi-like process, by bouncing between the upstream region and the shock front. The upstream scattering is provided by oblique magnetic waves, which are self-generated by the electrons escaping ahead of the shock via the electron firehose instability. We find that our mechanism works for shocks with plasma beta >20 at nearly all magnetic field obliquities, and for electron temperatures in the range relevant for low-luminosity accretion flows and galaxy clusters.

Presentation

Mechanical ("radio mode") AGN feedback is now routinely invoked in galaxy evolution models to inhibit star formation where it is predicted by theory but forbidden by observations. Yet despite strong circumstantial evidence in support of the model, we still poorly understand the details of how the kinetic energy of radio jets and lobes couples with the hot X-ray, warm ionized, neutral, and cold molecular phases of the environment through which they propagate. I will present new Chandra, Hubble, and ALMA results that advance our understanding of this issue.

Presentation

Numerical simulations of galaxy formation provide a unique opportunity to test our knowledge of the interactions between galactic ingredients. At the same time, however, it is equally imperative to verify that astrophysical assumptions are accountable for any success in simulations, not an artifact of particular implementations. While numerical experiments have become one of the most powerful tools in formulating theories of galaxy formation, it is this requirement of reproducibility that precludes theorists from drawing a definitive conclusion based on a single kind of simulation technique. The numerical community's collective response to such a challenge is the AGORA High-resolution Galaxy Simulations Comparison Project, promoting a multi-platform approach to various problems in galaxy formation. We will perform cosmological simulations with force resolutions of ~100 physical pc or better on a variety of code platforms. Participants share project infrastructures including common initial conditions (4 halo masses and 2 assembly histories), common astrophysics packages, and common analysis platform, all to be available to the community. Cosmological runs will be systematically compared with each other and with observations to raise the realism of numerical experiments collectively.

Presentation

Relativistic magnetic reconnection can potentially explain dissipation and particle acceleration in high-energy astrophysical systems characterized by extreme flares of gamma-ray radiation. I will present preliminary efforts to characterize the radiative signatures of relativistic reconnection from kinetic plasma simulations. I will also describe a recent observation of arguably the most extreme gamma-ray flare detected by the Fermi Large Area telescope.

Presentation

The current paradigm of the ISM is that it is a multiphase turbulent environment, with turbulence affecting many important processes including star formation, cosmic ray acceleration, and the evolution of ISM structure. The origin and driver of ISM turbulence on large scales is likely related to the injection of energy from supernova explosions and, at smaller scales, to winds. In order to understand ISM turbulence and ISM structure evolution, comparisons between observations, simulations and theoretical insights must be made. I shall discuss progress that has been made towards the understanding turbulence in the diffuse and molecular ISM with the development of new techniques for comparing radio observations with MHD simulations. I will highlight upcoming work on understanding turbulence in galaxy clusters.

Presentation

Plasma evolution in many astrophysical systems is dominated by magnetohydrodynamics. Specifically of interest to this talk are collimated outflows from accretion systems. Away from the central object, the Euler equations can represent the plasma dynamics well and may be scaled to a laboratory system. We have performed experiments to investigate the effects of a background magnetic field on an otherwise hydrodynamically collimated plasma. Laser-irradiated, cone targets produce hydrodynamically collimated plasma jets and a pulse-powered solenoid provides a constant background magnetic field. The application of this field is shown to completely disrupt the original flow and a new magnetically-collimated, hollow envelope is produced. Results from these experiments and potential implications for their astrophysical analogs will be discussed.

Presentation

We study how black holes can accrete above the Eddington limit using a global three dimensional radiation magneto-hydrodynamic simulation without ad-hoc assumptions. The simulation reaches an accretion rate ~ 220L_Edd/c^2 and forms a radiation driven outflow along the rotation axis. The radiative luminosity of this flow is ~ 10L_Edd. This yields a radiative efficiency ~ 4.5%, which is comparable to the value in a standard thin disk model. In our simulation, vertical advection of radiation caused by magnetic buoyancy transports energy faster than photon diffusion, allowing a significant fraction of the photons to escape from the surface of the disk before being advected into the black hole. We contrast our results with the lower radiative efficiencies inferred in slim disk model, which neglect vertical advection. The results have important implications for the growth of supermassive black holes in the early universe, tidal disruption events and ultra-luminous X-ray sources.

Presentation

X-ray spectra and the powerful plasma diagnostics they contain give unique quantitative information on physical conditions occurring during the accretion process. We consider TW Hya, the nearest accreting T Tauri star as an example where time-domain multi-wavelength spectroscopy offers a suprising new view of mass accretion and its consequences.

The Spectral Energy Distribution (SED) Machine is an Integral Field Unit (IFU) spectrograph designed specifically to classify transients. It is comprised of two subsystems. A lenselet based IFU, with a 26" x 26" Field of View (FoV) and 0.75" spaxels feeds a constant resolution (R~100) triple-prism. The dispersed rays are than imaged onto an off-the-shelf CCD detector. The second subsystem, the Rainbow Camera (RC), is a 4-band seeing-limited imager with a 12.5' x 12.5' FoV around the IFU that will allow real time spectrophotometric calibrations with ~ 5% accuracy. Data from both subsystems is processed in real time using a dedicated reduction pipeline. The SED Machine is mounted on the Palomar 60-inch robotic telescope (P60), covers a wavelength range of 370 - 920nm at high throughput and will classify transients from on-going and future surveys at a high rate. This will provide good statistics for common types of transients, and a better ability to discover and study rare and exotic ones.

Presentation

The afterglows of gamma-ray bursts (GRBs) provide a unique way to study the explosion properties and sub-parsec environments of these catastrophic events. Indeed, observational campaigns to characterize the afterglows of long GRBs (duration > 2 sec) have lent crucial insight to their massive star progenitors. Short GRBs, which are linked to the mergers of two compact objects, are discovered at a significantly lower rate and have faint afterglows, thus making an understanding of their basic explosion properties more challenging. In this talk, I describe an observational campaign to characterize the afterglows of short GRBs over the past decade, spanning radio to X-ray wavelengths. I use the temporal and spectral behavior of their afterglows to quantify their kinetic energy scales, circumburst densities, and jet opening angles for the first time. I explore any trends between these explosion properties and their host galaxies. Finally, since compact object mergers are the premier candidates for Advanced LIGO detection, I assess the implications for electromagnetic counterparts to gravitational waves.

Presentation

Gamma-ray bursts (GRB) have been long considered to be the sources of ultra high energy cosmic rays. If GRB jets are, indeed, sites of proton acceleration at high energies, then photohadronic processes, i.e. interactions between protons and photons, become relevant. In this talk, I will discuss some of their consequences for GRB models. First, I will present how we can constrain the physical conditions of the GRB emitting region by using indirect information from the copious neutrino emission that is naturally produced via photohadronic interactions on an ad-hoc Band photon spectrum. Second, I will present a model for the formation of Band-like photon spectra from first principles. This has been built on a recently discovered radiative instability, known as "spontaneous photon quenching." I will show that for a wide parameter range the instability sets in and establishes an efficient energy transfer from protons to secondaries produced through photohadronic interactions. It is then the interplay between photons and secondary electron-positron pairs, through purely leptonic processes, that actually determines the shape of the gamma-ray spectrum at steady-state.

Presentation

Long-duration gamma-ray bursts (GRBs) are thought to come from the core-collapse of Wolf-Rayet stars. Whereas their stellar masses have a rather narrow distribution, the population of GRBs is very diverse, with gamma-ray luminosities spanning several orders of magnitude. This suggests the existence of a "hidden" stellar variable whose burst-to-burst variation leads to a spread in their luminosity. Whatever this hidden variable is, its variation should not noticeably affect the shape of GRB lightcurves, which display a constant luminosity (in a time-average sense) followed by a sharp drop at the end of the burst seen with Swift/XRT. We argue that such a hidden variable is progenitor star's large-scale magnetic flux. Shortly after the core collapse, most of stellar magnetic flux accumulates near the black hole (BH) and remains there. The flux extracts BH rotational energy and powers jets of roughly a constant luminosity, L. However, once BH mass accretion rate Mdot falls below ~ L/c2, the flux becomes dynamically important and diffuses outwards, with the jet luminosity set by the rapidly declining mass accretion rate, L ~ Mdot c2. This provides a potential explanation for the sharp end of GRBs and the universal shape of their lightcurves. During the GRB, gas infall translates spatial variation of stellar magnetic flux into temporal variation of L. We make use of the deviations from constancy in L to perform stellar magnetic flux "tomography". Using this method, we infer the presence of magnetized tori in the outer layers of progenitor stars for GRB 920513 and GRB 940210.

Presentation

Black hole-neutron star and neutron star-neutron star mergers are among the main sources of gravitational waves which will be detected in the coming years by the Advanced LIGO/VIRGO/KAGRA observatories. In some cases, these mergers can also power bright electromagnetic emissions: they are the most likely progenitors of short gamma-ray bursts, and the radioactive decay of neutron-rich material ejected by the merger can power optical/infrared transients days after the merger. Finally, they may provide important constraints on the equation of state of cold dense matter, and on the source of heavy elements in the universe. I will discuss the general relativistic simulations which are required to properly model these events, and what they have told us so far about the outcome of neutron star mergers. I will also discuss efforts to improve the physical realism of the simulations by improving the treatment of the most important effects beyond general relativistic hydrodynamics: magnetic fields, neutrinos, and the properties of nuclear matter.

Presentation

The origin of the heavy r-process elements, is the biggest unsolved question in our understanding of chemical evolution in the Milky Way. The two most likely scenarios for the formation of the r-process nuclei involve dynamical events in the lives of neutron stars where nuclear physics plays a paramount role: the inner most regions of massive, collapsed stars and the merger of two neutron stars or a neutron star and a stellar mass black hole. In this talk, I will discuss binary neutron star mergers as a possible astrophysical site of the r-process, what theoretical uncertainties exist in this scenario, and observables that may give us a direct window into the formation of the r-process elements.

Presentation

Rapidly rotating black holes are a prime arena for understanding corrections to Einstein's theory of general relativity (GR). We construct solutions for rapidly rotating black holes in dynamical Chern-Simons (dCS) gravity, a useful and motivated example of a post-GR correction. We treat dCS as an effective theory and thus work in the decoupling limit, where we apply a perturbation scheme using the Kerr metric as the background solution. Using the solutions to the scalar field and the trace of the metric perturbation, we determine the regime of validity of our perturbative approach. We find that the maximal spin limit may be divergent, and the decoupling limit is strongly restricted for rapid rotation. Rapidly-rotating stellar-mass BHs can potentially be used to place strong bounds on the coupling parameter L of dCS. In order for the black hole observed in GRO J1655-40 to be within the decoupling limit we need L <= 22 km, a value 7 orders of magnitude smaller than present Solar System bounds on dynamical Chern-Simons gravity.

Presentation

Sgr A* is the poster child for profoundly quiescent accretion flows. Forty years after its discovery in the radio and fifteen years after its discovery in X-rays with Chandra, the extreme X-ray faintness of the closest supermassive black hole remains an important puzzle in black hole accretion. To study this remarkable source, Chandra (in concert with numerous ground- and space-based observatories) undertook a 3 Ms campaign on Sgr A* in 2012, providing an excellent opportunity to probe the physics of accretion in the Galactic Center. I will present an update to our work on the X-ray variability of Sgr A*, expanding a statistical analysis of its daily flares into a more comprehensive picture of its variable processes. Finally, I will discuss the exciting physical implications of this variability for the connection between X-ray and infrared emission and our understanding of the radiation physics of Sgr A*.

Presentation

The merger of two supermassive black holes (SMBHs) imparts a gravitational-wave (GW) recoil kick to the remnant SMBH, and in extreme cases SMBHs may be ejected from their host galaxies. An accreting, recoiling SMBH may be observable as a spatially or kinematically offset quasar. Prior to the advent of a space-based GW observatory, offset quasars may offer the best evidence of recent SMBH mergers. Indeed, promising candidates have already been identified. However, systematic searches for recoils are hampered by large uncertainties, including how often and in which host galaxies offset quasars should be observable, and whether BH spin alignment prior to merger is efficient at suppressing large recoils. Motivated by this, we have developed a model for recoiling quasars in a cosmological framework, utilizing information about the progenitor galaxies from cosmological hydrodynamic simulations. Varying degrees of BH spin alignment are considered. We find that the observability of offset quasars, and their preferred host galaxies, depend strongly on the efficiency of pre-merger spin alignment, with promising indications that discoveries of recoils could distinguish between at least the extreme limits of spin alignment models. These findings will inform the design of dedicated searches for recoiling quasars.

Presentation

The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly 1,000 km/s has provided strong evidence towards the existence of a massive compact object at the galaxy's center. HVS ejected via the disruption of stellar binaries can occasionally yield a star with v ~ 10,000 km/s, here we show that this mechanism can be extended to massive black hole (MBH) mergers, where the secondary star is replaced by a MBH. We find that stars that are originally bound to the secondary MBH are frequently ejected with v > 10,000 km/s, and occasionally with velocities up to one third the speed of light, for this reason we refer to stars ejected from these systems as ``semi-relativistic'' hypervelocity stars (SHS). Bound to no galaxy, the velocities of these stars are so great that they can cross a significant fraction of the observable universe in the time since their ejection (several Gpc). Tens will be giant stars that could be detected by future all-sky infrared surveys such as WFIRST or Euclid and proper motion surveys such as LSST, with spectroscopic follow-up being possible with JWST.

Presentation

The recent discovery of the G2 gas cloud in the Galactic center has led to speculation that we will observe an accretion event onto the massive black hole, Sgr A*, when it disrupts near its periapse passage. Accordingly, G2 has been closely monitored in an intensive, ongoing observing campaign. Current hydrodynamic simulations have failed however to reproduce even the present properties of the cloud. In particular, the cloud disrupts after only a fraction of its orbit, which imposes unrealistically tight restrictions on its origin. This casts doubt on detailed predictions for its accretion onto the black hole. In this talk I will show how magnetic fields (which are significant in the Galactic center) can strongly inhibit the disruption of such gas clouds. I will describe ongoing work to correctly model the propagation of the G2 through the magnetized medium of the Galactic center. With these simulations we aim to understand the size and survival time of magnetized gas clouds orbiting massive black holes, and to use the orbital deviation of G2 due to drag forces as a probe of the accretion flow around SgrA*.

I will give an update on the so-called "Fermi bubble" structure discovered using data from the Fermi Gamma-ray Space Telescope. This gigantic gamma-ray emitting structure could be evidence for past accretion events of the central supermassive black hole. I will present recent simulations of the Fermi bubbles and discuss what can be learned about the physical properties of the bubbles and what's the implication to past accretion activities of the Sgr A*. I will end up with a discussion of future gamma-ray space missions, especially our recent proposal of a dedicated gamma-ray satellite mapping the sky from ~10 MeV to ~1 GeV with much improved PSF, and how such future gamma-ray experiment would help us to learn about the Fermi bubbles.

Presentation

In scenarios where dark matter particles can annihilate to produce standard model particles, the galactic center of the Milky Way is expected to provide the highest flux from dark matter in the sky. This has allowed galactic center observations to set extremely stringent limits on the parameters of the dark matter particle. Recently, we have worked on gamma-ray observations from the Fermi-LAT telescope, and have detected a significant extended excess, which is spherically symmetric around the position of the galactic center, and does not trace any known astrophysical emission profile. In this talk, I will summarize the current status of these observations and discuss dark matter and astrophysical interpretations of the data. I will show forthcoming results which strongly constrain the properties and the possible interpretations of the observed excess. Finally, I will posit upcoming tests which will strongly suggest, or rule out, a dark matter interpretation.

Presentation

So far, ~10 fast radio bursts (FRBs) have been reported by the Parks radio telescope and the Arecibo observatory. The dispersion measures indicate that the sources are at cosmological distance, and the full sky event rate can be quite large ~10,000/day. If this is really the case, the FRBs are a promising target of multi-messenger astronomy in the coming years. I will present our cosmological binary white dwarf merger model, and also discuss future prospects of FRB astrophysics

Presentation

Pulsar timing arrays (PTAs) offer a unique opportunity to detect low frequency gravitational waves (GWs) in the near future. In this frequency band, the expected source of GWs are Supermassive Black Hole Binaries (SMBHBs) and they will most likely form in an ensemble creating a stochastic GW background with the possibility of a few nearby/massive sources that will be individually resolvable. In this talk we will discuss the prospects of detecting both stochastic background and continuous wave GW sources as a function of time. Here we use simulated datasets making reasonable assumptions about the quality and number of pulsars added to our PTA and we use the most recent estimates for the amplitude of the stochastic background. We compute the sensitivity to GWs as a function of time taking into account uncertainties in: telescope time, intrinsic noise properties of the pulsars, and in the overall shape of the GW power spectrum. We show that a detection of the stochastic GW background is likely within the next decade and could come as soon as 2017 Furthermore we show that the detection of an individual GW source is less likely but still hopeful within the next decade.

Presentation

High-precision timing of Galactic millisecond pulsars with radio telescopes holds great promise for the detection of astrophysical gravitational-waves in frequency range 10--100 nHz. Modern Bayesian data analysis methods rely mostly on Markov Chain Monte Carlo (MCMC) to explore the model parameter space when searching for signals in the pulsar timing data. Current challenges involve parameter spaces with large dimensionality, and linear algebra of high-dimensional systems. I will present sampling methods (taken from the Planck analysis team), and rank-reduction methods for large linear systems, that have enabled us to decrease the dimensionality of such problems. These methods are now being used to search for gravitational-waves in pulsar timing array projects. Especially our rank-reduction techniques are useful for any data analysis problem that involve large linear least-squares systems.

Accretion discs occur in many astrophysical systems from star and planet formation to active galactic nuclei. Most accretion disc theory assumes that discs are highly symmetric, e.g. planar, prograde and circular. There are many cases where these assumptions are likely to be violated in reality. I will present ongoing work that systematically removes these symmetries to uncover new and unforseen effects in accretion discs. These range from tearing up discs into discrete rings which drive shocks and cancellation of angular momentum, to disc eccentricity growth in binary systems.

Presentation

Simulations of the formation and evolution of compact objects with all necessary physical effects in multi-D remain challenging. An example are collapsing massive stars ad central engines for long gamma ray bursts. An accurate treatment of neutrino/photon radiation transport, general relativistic gravity, nuclear reactions, and magnetohydrodynamics with a microphysical equation of state must be included. Due to the nature of turbulence and hydrodynamic instabilities, these systems are intrinsically three-dimensional, and involve multiple length and time scales. To account for all required input physics in three dimensions, massively parallel simulation codes are required. Apart from algorithmic complexity, however, most simulations are largely effected by the scaling bottleneck. In this talk, I discuss the paradigm shift in massively parallel astrophysical simulations codes that will be necessary in order to harness current and future computing resources efficiently.

Presentation

If only there is enough mass near the black hole, accretion rate can easily exceed the Eddington limit. Tidal disruption events and mergers of supermassive black holes in the early Universe are good candidates. Such accretion flows are studied numerically with radiation magnetohydrodynamical codes. In this talk I will describe simulations performed in general relativity with code KORAL. I will show that the isotropic equivalent luminosities for observer looking almost perpendicular along the axis can be as large as 10^46 - 10^47 erg/s for a 1e5 Msun non-rotating black hole. Implications of this fact will be discussed.

Presentation

One of the most important discoveries of the Fermi Gamma-ray Space Telescope is the detection of two giant bubbles extending 50 degrees above and below the Galactic center (GC). The symmetry about the GC of the Fermi bubbles suggests some episode of energy injection from the GC, possibly related to past jet activity of the central active galactic nuclei (AGN). Thanks to the proximity to the GC, the Fermi Bubbles are excellent laboratories for studying cosmic rays (CRs), Galactic magnetic field, and AGN feedback in general. Using three-dimensional magnetohydrodynamic simulations that include relevant CR physics, I will show how leptonic AGN jets can explain the key characteristics of the Fermi bubbles and the spatially correlated features observed in the X-ray, microwave, and radio wavelengths. I will also discuss how we use our simulations in combination with the multi-wavelength data to obtain constraints on the composition of the Fermi bubbles.