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Register a new userSimulating polarized Galactic synchrotron emission at all frequencies, the Hammurabi code
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A. Waelkens
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We present a publicly available code called Hammurabi for generating mock polarized observations of Galactic synchrotron emission for telescopes like LOFAR, SKA, Planck and WMAP, based on model inputs for the Galactic magnetic field (GMF), the cosmic-ray density distribution and the thermal electron density. We also present mock UHECR deflection measure (UDM) maps based on model inputs for the GMF. In future, when UHECR sources are identified, this will allow us to define UDM as a GMF probe in a similar way as polarized radio sources permit us to define rotation measures. To demonstrate the codes abilities mock observations are compared to real data as a means to constrain the input parameters of our simulations with a focus on large-scale magnetic field properties. As expected, attempts at trying to model the synchrotron, UHECR deflection and RM input parameters, show that any additional observational data set greatly increases the constraints on the models. The hammurabi code addresses this by allowing to perform simulations of several different data sets simultaneously, providing the means for a more reliable constraint of the magnetized inter-stellar-medium.
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We present version X of the hammurabi package, the HEALPix-based numeric simulator for Galactic polarized emission. Improving on its earlier design, we have fully renewed the framework with modern C++ standards and features. Multi-threading support has been built in to meet the growing computational workload in future research. For the first time, we present precision profiles of hammurabi line-of-sight integral kernel with multi-layer HEALPix shells. In addition to fundamental improvements, this report focuses on simulating polarized synchrotron emission with Gaussian random magnetic fields. Two fast methods are proposed for realizing divergence-free random magnetic fields either on the Galactic scale where a field alignment and strength modulation are imposed, or on a local scale where more physically motivated models like a parameterized magneto-hydrodynamic (MHD) turbulence can be applied. As an example application, we discuss the phenomenological implications of Gaussian random magnetic fields for high Galactic latitude synchrotron foregrounds. In this, we numerically find B/E polarization mode ratios lower than unity based on Gaussian realizations of either MHD turbulent spectra or in spatially aligned magnetic fields.
Dark matter annihilations in the Galactic halo inject relativistic electrons and positrons which in turn generate a synchrotron radiation when interacting with the galactic magnetic field. We calculate the synchrotron flux for various dark matter annihilation channels, masses, and astrophysical assumptions in the low-frequency range and compare our results with radio surveys from 22 MHz to 1420 MHz. We find that current observations are able to constrain particle dark matter with thermal annihilation cross-sections, i.e. (sigma v) = 3 x 10^-26 cm^3/s, and masses M_DM < 10 GeV. We discuss the dependence of these bounds on the astrophysical assumptions, namely galactic dark matter distribution, cosmic rays propagation parameters, and structure of the galactic magnetic field. Prospects for detection in future radio surveys are outlined.
The LWA will be well suited to address many important questions about the physics and astrophysics of extragalactic synchrotron sources. Good low-frequency data will enable major steps forward in our understanding of radio galaxy physics, of the plasma in clusters of galaxies, and of active objects in the high-redshift universe. Such data will also be important in answering some basic questions about the physics of synchrotron-emitting plasmas.
Diffuse Galactic emission at low frequencies is a major contaminant for studies of redshifted $21$ cm line studies. Removal of these foregrounds is essential for exploiting the signal from neutral hydrogen at high redshifts. Analysis of foregrounds and its characteristics is thus of utmost importance. It is customary to test efficacy of foreground removal techniques using simulated foregrounds. Most simulations assume that the distribution of the foreground signal is a Gaussian random field. In this work we test this assumption by computing the binned bispectrum for the all-sky $408$ MHz map. This is done by applying different brightness temperature ($T$) thresholds in order to assess whether the cooler parts of the sky have different characteristics. We find that regions with a low brightness temperature $T < 25$ K indeed have smaller departures from a Gaussian distribution. Therefore, these regions of the sky are ideal for future H{sc i} intensity mapping surveys.
The Fan Region is one of the dominant features in the polarized radio sky, long thought to be a local (distance < 500 pc) synchrotron feature. We present 1.3-1.8 GHz polarized radio continuum observations of the region from the Global Magneto-Ionic Medium Survey (GMIMS) and compare them to maps of Halpha and polarized radio continuum intensity from 0.408-353 GHz. The high-frequency (> 1 GHz) and low-frequency (< 600 MHz) emission have different morphologies, suggesting a different physical origin. Portions of the 1.5 GHz Fan Region emission are depolarized by about 30% by ionized gas structures in the Perseus Arm, indicating that this fraction of the emission originates >2 kpc away. We argue for the same conclusion based on the high polarization fraction at 1.5 GHz (about 40%). The Fan Region is offset with respect to the Galactic plane, covering -5{deg} < b < +10{deg}; we attribute this offset to the warp in the outer Galaxy. We discuss origins of the polarized emission, including the spiral Galactic magnetic field. This idea is a plausible contributing factor although no model to date readily reproduces all of the observations. We conclude that models of the Galactic magnetic field should account for the > 1 GHz emission from the Fan Region as a Galactic-scale, not purely local, feature.
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