Bulletin of the American Physical Society
APS April Meeting 2017
Volume 62, Number 1
Saturday–Tuesday, January 28–31, 2017; Washington, DC
Session S5: From Axions to Active Galactic Nuclei |
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Sponsoring Units: DAP Chair: Masha Baryakhtar, Perimeter Institute Room: Virginia B |
Monday, January 30, 2017 1:30PM - 1:42PM |
S5.00001: Astrophysics of Collapsing Axion Stars Joshua Eby, Madelyn Leembruggen, Peter Suranyi, L.C.R. Wijewardhana Axion stars are condensed states of large numbers of axion particles, bound by self-gravitation and quantum self-interactions. The mass of weakly bound axion stars is limited by gravitational stability, with condensates exceeding the maximum mass subject to collapse. During the collapse process, the axion density increases and higher-order self-interactions become increasingly relevant. By taking these terms into account, we provide evidence that in spite of a leading attractive interaction, collapsing axion stars stabilize in a dense state which is larger than its Schwarzschild radius, and so do not form black holes. During the last moments of collapse, number changing processes take place in the axion star with a very large rate, leading to emission of many highly energetic axions which escape from galaxies and galaxy clusters. Finally, if axion stars are a significant fraction of cold dark matter, then frequent collisions with each other or with ordinary stars could catalyze this collapse process as well. [Preview Abstract] |
Monday, January 30, 2017 1:42PM - 1:54PM |
S5.00002: Emission of Photons and Relativistic Axions from Axion stars Abhishek Mohapatra, Eric Braaten, Hong Zhang The number of nonrelativistic axions can be changed by inelastic reactions that produce relativistic axions or photons. Any even number of nonrelativistic axions can scatter inelastically into two relativistic axions. Any odd number of axions can annihilate into two photons. This reaction produces a monochromatic radio-frequency signal at an odd-integer harmonic of the fundamental frequency set by the axion mass. The loss rates of axions from axion stars through these inelastic relations are calculated using the framework of a nonrelativistic effective field theory. Odd-integer harmonics of a fundamental radio-frequency signal provide a unique signature for collapsing axion stars or any dense configuration of axions. [Preview Abstract] |
Monday, January 30, 2017 1:54PM - 2:06PM |
S5.00003: Searches for Axionlike Particles with the Fermi Large Area Telescope Andrea Albert, Manuel Meyer, Miguel Sanchez-Conde, Matthew Wood Axionlike particles (ALPs) are dark-matter candidates that occur in a variety of extensions of the Standard Model. These particles could leave signatures in gamma rays, due to the coupling of ALPs to photons in external electromagnetic fields. To date, observations with Fermi Large Area Telescope (LAT) provide the strongest constraints on the photon-ALP coupling for ALP masses between 0.5 and 20 neV. Here, we summarize these constraints and present the sensitivity to detect an ALP induced gamma-ray burst from a Galactic core-collapse supernova. ALPs would be produced in the stellar medium via the Primakoff effect and convert into gamma rays in the Galactic magnetic field. Fermi LAT observations would be able to probe couplings where ALPs could constitute the entirety of dark matter. Below 1 neV, the Fermi-LAT sensitivity would surpass that of future laboratory experiments by one order of magnitude. [Preview Abstract] |
Monday, January 30, 2017 2:06PM - 2:18PM |
S5.00004: Imprint of DESI fiber assignment on anisotropic power spectrum of emission line galaxies Robert N. Cahn, Lucas Pinol, Nicholas Hand, Patrick McDonald, Uros Seljak The Dark Energy Spectroscopic Instrument (DESI), a multiplexed fiber-fed spectrograph, is a Stage-IV ground-based dark energy experiment aiming to measure redshifts for 29 million Emission-Line Galaxies (ELG), 4 million Luminous Red Galaxies (LRG), and 2 million Quasi-Stellar Objects (QSO). The survey design includes an observation strategy determined by a fiber assignment algorithm that optimizes the allocation of fibers to targets. We investigate the systematic effects of the fiber assignment coverage on the anisotropic galaxy clustering of ELGs and show that, in the absence of any corrections, it leads to discrepancies of order ten percent on large scales for the power spectrum multipoles. We introduce a method where objects in a random catalog are assigned a coverage, and the mean density is separately computed for each coverage factor, and show that this method reduces, but does not eliminate the effect. The angular dependence of the contaminating signal is mostly in purely transverse modes. We find that the effect can be mitigated by binning in angle and excluding contributions near the transverse direction. [Preview Abstract] |
Monday, January 30, 2017 2:18PM - 2:30PM |
S5.00005: Photometric Properties of Galaxy Clusters in the Dark Energy Survey Brian Welch Galaxy clusters have been observed to have a "red sequence" of tightly correlated galaxies in color-magnitude space. Many cluster finding algorithms rely on the red sequence to identify clusters and to determine their member galaxies. However, that approach may introduce unwanted selection effects. To address this issue, we have developed a probabilistic cluster membership assignment technique with less emphasis on the red sequence. We found that this technique works well when applied to simulations as well as real data. Our sample of clusters are selected by the redmapper algorithm in the Dark Energy Survey (DES) year-1 data set, and cover a redshift range of $0.1\leq z \leq 0.95$. We use our members list to make measurements of the cluster masses and radii, as well as the red sequence parameters and red-to-blue member fractions of these clusters. This is the largest study of its kind to date. Our results will inform assumptions used in cluster selection and studies in the future. [Preview Abstract] |
Monday, January 30, 2017 2:30PM - 2:42PM |
S5.00006: A Radiative Transport Model for Blazars Tiffany Lewis, Justin Finke, Peter Becker Blazars are observed across the electromagnetic spectrum, often with strong variability throughout. We start from first-principles to build up a transport model, whose solution is the electron distribution, rather than assuming a convenient functional form. Our analytical transport model considers shock acceleration, adiabatic expansion, stochastic acceleration, Bohm diffusion, and synchrotron radiation. We use this solution to give predictions for the X-ray spectrum and time lags, comparing the results with BeppoSAX observations of X-ray flares from Mrk 421. This new self-consistent model provides an unprecedented view into the jet physics at play in this source, especially the strength of the shock and stochastic acceleration components and the size of the acceleration region. More recently, we augmented the transport model to incorporate Compton scattering, including Klein-Nishina effects. Here, an analytical solution cannot be derived. Therefore we obtain the steady-state electron distribution computationally. We compare the resulting radiation spectrum with multi-wavelength data for 3C 279. We show that our new Compton + synchrotron blazar model is the first to successfully fit the FermiLAT gamma-ray data for this source based on a first-principles physical calculation. [Preview Abstract] |
Monday, January 30, 2017 2:42PM - 2:54PM |
S5.00007: Implications of Relationships between Black Hole Mass, Bolometric Luminosity, and Beam Power of AGN Ruth Daly Relationships between black hole mass, disk luminosity, and outflow beam power are studied for a large sample of AGN. The results are applied to study spins of supermassive black holes and to constrain models that relate outflow and accretion disk properties. The results of these studies will be discussed. [Preview Abstract] |
Monday, January 30, 2017 2:54PM - 3:06PM |
S5.00008: Numerical studies of SMBH magnetospheres and observational predictions for AGNs and inner jets Alex Ford, Mikhail V. Medvedev Electrodynamic, radiative and plasma processes around SMBHs in AGNs determine how relativistic jets are launched and how the black hole energy is extracted. The cornerstone process here is plasma production via the electron-position cascade in the so-called ``gap'' region of a SMBH force-free magnetosphere. This multi-stage process, involving particle acceleration, photon Compton up-scattering and production of $e^\pm$ secondaries, is explored numerically by computing the radial development of the entire cascade and accompanying plasma physical and radiative processes. Here we show how the $e^\pm$ plasma production depends on the black hole mass and spin, the amount and spectrum of the ambient photons and magnetic fields, and other parameters and provide empirical scaling relations. We also present the full structure of the gap region and make solid observational predictions for X-ray and gamma-ray fluxes and spectra, which can readily be compared with observations of AGNs and inner regions of their jets. [Preview Abstract] |
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