Bulletin of the American Physical Society
APS April Meeting 2016
Volume 61, Number 6
Saturday–Tuesday, April 16–19, 2016; Salt Lake City, Utah
Session R18: Pulsar Timing Arrays and Supermassive Black Hole Binaries 
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Sponsoring Units: GGR DAP Chair: Curt Cutler, NASA Jet Propulsion Laboratory Room: 251F 
Monday, April 18, 2016 10:45AM  10:57AM 
R18.00001: Detecting gravitational waves with pulsartiming arrays: a case of astrophysical forensics Michele Vallisneri Pulsartiming arrays have recently reached maturity as the "third way" to gravitationalwave (GW) detection, besides groundbased interferometers and future spacebased observatories. PTA campaigns target the verylowfrequency band centered around $10^8$ Hz, so they will yield science complementary to the other two programs. For this speaker, much of the fascination with PTAs lies in the fact that they represent a grand experiment in precision measurement that was set up by Nature herself, so we have rather little control on it, and few knobs to turn. Improvements in sensitivity will come as much from ever more powerful radiotelescopes as from a better understanding of the "detectors" (neutron stars, their dynamics in binaries, the interstellar medium, ...), and from deeper, more probing analyses of the data we already have. A positive GW detection claim will require making a watertight case of astrophysical forensics, proving beyond any reasonable doubt that systematics are under control, and designing the complex inference chain that points to the presence GWs in its most unequivocal and defensible form. I discuss how these goals and concerns informed the development of recently published constraints on the astrophysical population of supermassive blackhole binaries. [Preview Abstract] 
Monday, April 18, 2016 10:57AM  11:09AM 
R18.00002: The NANOGrav Nineyear Data Set: Limits on the Anisotropic Gravitational Wave Background Chiara Mingarelli Pulsar Timing Arrays are sensitive to gravitational waves (GWs) in the 1 nHz  100 nHz frequency band. In this lowfrequency regime, we expect to measure a stochastic GW background originating from the superposition of GWs from the cosmic population of supermassive black hole binaries. Previous NANOGrav limits on the stochastic GW background have assumed an isotropic distribution of the GW power. Here we look for power in higher order modes using a spherical harmonic decomposition of the GW power on the sky, and show that we can inject single sources of GWs and recover them with this formalism. We present new limits on the power in multipole moments up to l$=$5; the angular resolution of NANOGrav. [Preview Abstract] 
Monday, April 18, 2016 11:09AM  11:21AM 
R18.00003: Are we there yet? Time to detection of nanohertz gravitational waves based on pulsartiming array limits Stephen Taylor, Michele Vallisneri, Justin Ellis, Chiara Mingarelli, Joseph Lazio, Rutger van Haasteren Pulsar timing arrays have placed highly constraining upper limits on the amplitude of the nanohertz gravitationalwave stochastic signal from the mergers of supermassive blackhole binaries ($\sim 10^{15}$ strain at $f = 1/\mathrm{yr}$). These limits suggest that binary merger rates may have been overestimated, or that environmental influences from nuclear gas or stars accelerate orbital decay, reducing the gravitationalwave signal at the lowest, most sensitive frequencies. This prompts the question whether nanohertz gravitational waves are likely to be detected in the near future. In this talk, we answer this question by deriving the range of true signal amplitudes that are compatible with current upper limits, and computing expected detection probabilities as a function of further observation time. We conclude that small arrays consisting of the pulsars with the least timing noise, which nevertheless yield the tightest upper limits, have discouraging prospects of making a detection in the next two decades. By contrast, we find large arrays have an $\sim 80\%$ probability of detection within the next ten years, even in the most pessimistic source modeling scenarios. [Preview Abstract] 
(Author Not Attending)

R18.00004: Towards solving the pulsar timing sampling problem Rutger Van Haasteren, Justin Ellis, Michele Vallisneri Bayesian data analysis of Pulsar Timing Array (PTA) has proved to be a computationally challenging problem, with scaling relations that are superlinear in both the number of pulsars and the number of model parameters. Thus far, our best models cannot be used when analyzing full (international) pulsar timing array datasets in the search for gravitational waves, and shortcuts always need to be made. A promising approach in the literature, based on Hamiltonian sampling techniques, has been shown to be infeasible in realistic datasets due to phase transition behavior of the likelihood. We have introduced a coordinate transformation that mitigates this phase transition behavior, and makes Hamiltonian sampling efficient. This makes a full (stochastic) gravitationalwave search in pulsar timing data feasible with our most uptodate models. This method scales almost linearly with the number of pulsars. [Preview Abstract] 
Monday, April 18, 2016 11:33AM  11:45AM 
R18.00005: A Transdimensional Bayesian Approach to Pulsar Timing Noise Analysis Justin Ellis, Neil Cornish The modeling of intrinsic noise in pulsar timing residual data is of crucial importance for Gravitational Wave (GW) detection and pulsar timing (astro)physics in general. The noise budget in pulsars is a collection of several well studied effects including radiometer noise, pulsephase jitter noise, dispersion measure (DM) variations, and low frequency spin noise. However, as pulsar timing data continues to improve, nonstationary and nonpowerlaw noise terms are beginning to manifest which are not well modeled by current noise analysis techniques. In this talk we present a transdimensional approach to model these nonstationary and nonpowerlaw effects through the use of a wavelet basis and an interpolation based adaptive spectral modeling. In both cases, the number of wavelets and the number of control points in the interpolated spectrum are free parameters that are constrained by the data and then marginalized over in the final inferences, thus fully incorporating our ignorance of the noise model. We show that these new methods outperform standard techniques when nonstationary and nonpowerlaw noise is present. [Preview Abstract] 
Monday, April 18, 2016 11:45AM  11:57AM 
R18.00006: Detecting gravitational wave bursts with Pulsar Timing Neil Cornish, Justin Ellis The 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 multiwavelet approach for the analysis of timing residuals from a pulsar timing array. [Preview Abstract] 
Monday, April 18, 2016 11:57AM  12:09PM 
R18.00007: Null Stream Approach for finding Sky Position of Pulsar Timing Array sources Jeffrey Hazboun, Shane Larson A null stream is constructed from the timing residuals of three pulsars by noting that the same source polarization amplitudes appear in the data stream from each pulsar. Null stream mapping of gravitational wave sources has been described for LIGO and LISA, relying on the correlated gravitational wave signals between detectors. For a collection of pulsars observing the same source, the gravitational wave signal is common to all pulsars in the array, but modified by geometric factors related to the relative position of the source on the sky. Linear combinations of a set of individual pulsar data streams can be shown to be a twoparameter family (the two sky position angles of the source) that can be minimized to determine the location of the source on the sky. Overlaying a number of null streams allows for an even stronger localization of the gravitational waves source. This presents a large advantage in a PTA where there are more independent signals than interferometric detectors. We show how multiple subarrays of pulsars affect the pointing accuracy. Additionally, a simple noise model is used to demonstrate how the presence of noise will change the character of the spectrum, suppressing features related to the gravitational wave signal. [Preview Abstract] 
Monday, April 18, 2016 12:09PM  12:21PM 
R18.00008: Gas Dynamics of the Central Cavity during Black Hole Binary Inspiral Dennis Bowen During galaxy mergers, as a result of dynamical friction (stars, gas, etc.) and gravitational slingshot, the supermassive black holes (SMBHs) from each galaxy will become gravitationally bound and eventually merge due to gravitational radiation. It is expected that gas will form a circumbinary accretion disk around the SMBH binary that will persistently feed individual “minidisks” via dense streams out to their tidal truncation radii. We present simulations of SMBH binaries in this astrophysical environment during the general relativistic inspiral regime. We place particular emphasis on the dynamics of the individual minidisks where violent shocks via diskdisk and diskstream interactions will likely produce intense electromagnetic emission. This signal emanating from the minidisks will likely prove instrumental in direct detection of SMBH binaries with currently available observatories. [Preview Abstract] 
Monday, April 18, 2016 12:21PM  12:33PM 
R18.00009: Detecting alternative polarization states of stochastic gravitational waves with pulsar timing arrays Logan O'Beirne, Neil Cornish, Nicolas Yunes We have developed simulated gravitational wave backgrounds from a population of supermassive black holes with the full range of polarization states possible in alternative theories of gravity. We apply Bayesian inference to explore how well the full polarization content can be inferred from the simulated data, and use analytic calculations of the variance in the correlation functions for each polarization state to understand the results. [Preview Abstract] 
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