Advanced Diagnostics for the Study of Linearly Polarized Emission. II: Application to Diffuse Interstellar Radio Synchrotron Emission [GA]

Diagnostics of polarized emission provide us with valuable information on the Galactic magnetic field and the state of turbulence in the interstellar medium, which cannot be obtained from synchrotron intensity alone. In Paper I (Herron et al. 2017b), we derived polarization diagnostics that are rotationally and translationally invariant in the $Q$-$U$ plane, similar to the polarization gradient. In this paper, we apply these diagnostics to simulations of ideal magnetohydrodynamic turbulence that have a range of sonic and Alfv\’enic Mach numbers. We generate synthetic images of Stokes $Q$ and $U$ for these simulations, for the cases where the turbulence is illuminated from behind by uniform polarized emission, and where the polarized emission originates from within the turbulent volume. From these simulated images we calculate the polarization diagnostics derived in Paper I, for different lines of sight relative to the mean magnetic field, and for a range of frequencies. For all of our simulations, we find that the polarization gradient is very similar to the generalized polarization gradient, and that both trace spatial variations in the magnetoionic medium for the case where emission originates within the turbulent volume, provided that the medium is not supersonic. We propose a method for distinguishing the cases of emission coming from behind or within a turbulent, Faraday rotating medium, and a method to partly map the rotation measure of the observed region. We also speculate on statistics of these diagnostics that may allow us to constrain the physical properties of an observed turbulent region.

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C. Herron, B. Burkhart, B. Gaensler, et. al.
Fri, 16 Feb 18

Comments: 34 pages, 25 figures, accepted for publication in ApJ

Measurement of the solar system acceleration using the Earth scale factor [GA]

We propose an alternative method to detect the secular aberration drift induced by the solar system acceleration due to the attraction to the Galaxy centre. This method is free of the individual radio source proper motion caused by intrinsic structure variation. We developed a procedure to estimate the scale factor directly from very long baseline interferometry (VLBI) data analysis in a source-wise mode within a global solution. The scale factor is estimated for each reference radio source individually as a function of astrometric coordinates (right ascension and declination). This approach splits the systematic dipole effect and uncorrelated motions on the level of observational parameters. We processed VLBI observations from 1979.7 to 2016.5 to obtain the scale factor estimates for more than 4,000 reference radio sources. We show that the estimates highlight a dipole systematics aligned with the direction to the centre of the Galaxy. With this method we obtained a Galactocentric acceleration vector with an amplitude of 5.2 $\pm$ 0.2 \mu as/yr and direction $\alpha_G = 281\deg \pm 3\deg$ and $\delta_G = -35\deg \pm 3\deg$.

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O. Titov and H. Krasna
Fri, 16 Feb 18

Comments: accepted to A&A

Two- and Multi-dimensional Curve Fitting using Bayesian Inference [CL]

Fitting models to data using Bayesian inference is quite common, but when each point in parameter space gives a curve, fitting the curve to a data set requires new nuisance parameters, which specify the metric embedding the one-dimensional curve into the higher-dimensional space occupied by the data. A generic formalism for curve fitting in the context of Bayesian inference is developed which shows how the aforementioned metric arises. The result is a natural generalization of previous works, and is compared to oft-used frequentist approaches and similar Bayesian techniques.

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A. Steiner
Fri, 16 Feb 18

Comments: N/A

Solar-System Studies with Pulsar Timing Arrays [IMA]

High-precision pulsar timing is central to a wide range of astrophysics and fundamental physics applications. When timing an ensemble of millisecond pulsars in different sky positions, known as a pulsar timing array (PTA), one can search for ultra-low-frequency gravitational waves (GWs) through the spatial correlations that spacetime deformations by passing GWs are predicted to induce on the pulses’ times-of-arrival (TOAs). A pulsar-timing model, requires the use of a solar-system ephemeris (SSE) to properly predict the position of the solar-system barycentre, the (quasi-)inertial frame where all TOAs are referred. Here, I discuss how while errors in SSEs can introduce correlations in the TOAs that may interfere with GW searches, one can make use of PTAs to study the solar system. I discuss work done within the context of the European Pulsar Timing Array and the International Pulsar Timing Array collaborations. These include new updates on the masses of planets from PTA data, first limits on masses of the most massive asteroids, and comparisons between SSEs from independent groups. Finally, I discuss a new approach in setting limits on the masses of unknown bodies in the solar system and calculate mass sensitivity curves for PTA data.

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R. Caballero, EPTA. and IPTA. Collaboration
Fri, 16 Feb 18

Comments: 4 pages, 1 figure, accepted for publication in the proceedings of IAU Symposium 337 – Pulsar Astrophysics: The Next Fifty Years

A dynamical approach in exploring the unknown mass in the Solar system using pulsar timing arrays [IMA]

The error in the Solar system ephemeris will lead to dipolar correlations in the residuals of pulsar timing array for widely separated pulsars. In this paper, we utilize such correlated signals, and construct a Bayesian data-analysis framework to detect the unknown mass in the Solar system and to measure the orbital parameters. The algorithm is designed to calculate the waveform of the induced pulsar-timing residuals due to the unmodelled objects following the Keplerian orbits in the Solar system. The algorithm incorporates a Bayesian-analysis suit used to simultaneously analyse the pulsar-timing data of multiple pulsars to search for coherent waveforms, evaluate the detection significance of unknown objects, and to measure their parameters. When the object is not detectable, our algorithm can be used to place upper limits on the mass. The algorithm is verified using simulated data sets, and cross-checked with analytical calculations. We also investigate the capability of future pulsar-timing-array experiments in detecting the unknown objects. We expect that the future pulsar timing data can limit the unknown massive objects in the Solar system to be lighter than $10^{-11}$ to $10^{-12}$ $M_{\odot}$, or measure the mass of Jovian system to fractional precision of $10^{-8}$ to $10^{-9}$.

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Y. Guo, K. Lee and R. Caballero
Fri, 16 Feb 18

Comments: 10 pages, 10 figures, 2 tables

Hierarchical multi-stage MCMC follow-up of continuous gravitational wave candidates [IMA]

Leveraging Markov chain Monte Carlo (MCMC) optimization of the F-statistic, we introduce a method for the hierarchical follow-up of continuous gravitational wave candidates identified by wide-parameter space semi-coherent searches. We demonstrate parameter estimation for continuous wave sources and develop a framework and tools to understand and control the effective size of the parameter space, critical to the success of the method. Monte Carlo tests of simulated signals in noise demonstrate that this method is close to the theoretical optimal performance.

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G. Ashton and R. Prix
Fri, 16 Feb 18

Comments: 15 pages, 9 figures, 3 tables

TIFR Near Infrared Imaging Camera-II on the 3.6-m Devasthal Optical Telescope [IMA]

TIFR Near Infrared Imaging Camera-II is a closed-cycle Helium cryo-cooled imaging camera equipped with a Raytheon 512 x 512 pixels InSb Aladdin III Quadrant focal plane array having sensitivity to photons in the 1-5 microns wavelength band. In this paper, we present the performance of the camera on the newly installed 3.6-m Devasthal Optical Telescope (DOT) based on the calibration observations carried out during 2017 May 11-14 and 2017 October 7-31. After the preliminary characterization, the camera has been released to the Indian and Belgian astronomical community for science observations since 2017 May. The camera offers a field-of-view of ~86.5 arcsec x 86.5 arcsec on the DOT with a pixel scale of 0.169 arcsec. The seeing at the telescope site in the near-infrared bands is typically sub-arcsecond with the best seeing of ~0.45 arcsec realized in the near-infrared K-band on 2017 October 16. The camera is found to be capable of deep observations in the J, H and K bands comparable to other 4-m class telescopes available world-wide. Another highlight of this camera is the observational capability for sources up to Wide-field Infrared Survey Explorer (WISE) W1-band (3.4 microns) magnitudes of 9.2 in the narrow L-band (nbL; lambda_{cen} ~3.59 microns). Hence, the camera could be a good complementary instrument to observe the bright nbL-band sources that are saturated in the Spitzer-Infrared Array Camera ([3.6] <= 7.92 mag) and the WISE W1-band ([3.4] <= 8.1 mag). Sources with strong polycyclic aromatic hydrocarbon (PAH) emission at 3.3 microns are also detected. Details of the observations and estimated parameters are presented in this paper.

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T. Baug, D. Ojha, S. Ghosh, et. al.
Thu, 15 Feb 18

Comments: 16 pages, 12 figures, 1 table, Accepted for publication in the Journal of Astronomical Instrumentation