Astrophysical magnetic fields and nonlinear dynamo theory

Data from the Fermi-LAT reveal two large gamma-ray bubbles, extending 50° above and below the Galactic center (GC), with a width of about 40° in longitude. The gamma-ray emission associated with these bubbles has a significantly harder spectrum (dN/dE ~ E –2) than the inverse Compton emission from electrons in the Galactic disk, or the gamma rays produced by the decay of pions from proton-interstellar medium collisions. There is no significant spatial variation in the spectrum or gamma-ray intensity within the bubbles, or between the north and south bubbles. The bubbles are spatially correlated with the hard-spectrum microwave excess known as the WMAP haze; the edges of the bubbles also line up with features in the ROSAT X-ray maps at 1.5-2 keV. We argue that these Galactic gamma-ray bubbles were most likely created by some large episode of energy injection in the GC, such as past accretion events onto the central massive black hole, or a nuclear starburst in the last ~10 Myr. Dark matter annihilation/decay seems unlikely to generate all the features of the bubbles and the associated signals in WMAP and ROSAT; the bubbles must be understood in order to use measurements of the diffuse gamma-ray emission in the inner Galaxy as a probe of dark matter physics. Study of the origin and evolution of the bubbles also has the potential to improve our understanding of recent energetic events in the inner Galaxy and the high-latitude cosmic ray population.

/pdf/giant-gamma-ray-bubbles-from-fermi-lat-active-galactic-wlgfmuke59.pdf

Giant gamma-ray bubbles from fermi-lat: active galactic nucleus activity or bipolar galactic wind?

Black hole (BH) accretion flows and jets are qualitatively affected by the presence of ordered magnetic fields. We study fully three-dimensional global general relativistic magnetohydrodynamic (MHD) simulations of radially extended and thick (height H to cylindrical radius R ratio of |H/R| {approx} 0.2-1) accretion flows around BHs with various dimensionless spins (a/M, with BH mass M) and with initially toroidally-dominated ({phi}-directed) and poloidally-dominated (R-z directed) magnetic fields. Firstly, for toroidal field models and BHs with high enough |a/M|, coherent large-scale (i.e. >> H) dipolar poloidal magnetic flux patches emerge, thread the BH, and generate transient relativistic jets. Secondly, for poloidal field models, poloidal magnetic flux readily accretes through the disk from large radii and builds-up to a natural saturation point near the BH. While models with |H/R| {approx} 1 and |a/M| {le} 0.5 do not launch jets due to quenching by mass infall, for sufficiently high |a/M| or low |H/R| the polar magnetic field compresses the inflow into a geometrically thin highly non-axisymmetric 'magnetically choked accretion flow' (MCAF) within which the standard linear magneto-rotational instability is suppressed. The condition of a highly-magnetized state over most of the horizon is optimal for the Blandford-Znajek mechanism that generates persistent relativistic jets with and 100% efficiency for |a/M| {approx}> 0.9. A magnetic Rayleigh-Taylor and Kelvin-Helmholtz unstable magnetospheric interface forms between the compressed inflow and bulging jet magnetosphere, which drives a new jet-disk oscillation (JDO) type of quasi-periodic oscillation (QPO) mechanism. The high-frequency QPO has spherical harmonic |m| = 1 mode period of {tau} {approx} 70GM/c{sup 3} for a/M {approx} 0.9 with coherence quality factors Q {approx}> 10. Overall, our models are qualitatively distinct from most prior MHD simulations (typically, |H/R| << 1 and poloidal flux is limited by initial conditions), so they should prove useful for testing accretion-jet theories and measuring a/M in systems such as SgrA* and M87.

General relativistic magnetohydrodynamic simulations of magnetically choked accretion flows around black holes

Single-bubble sonoluminescence (SBSL) results from the extreme temperatures and pressures achieved during bubble compression; calculations have predicted the existence of a hot, optically opaque plasma core with consequent bremsstrahlung radiation. Recent controversial reports claim the observation of neutrons from deuterium-deuterium fusion during acoustic cavitation. However, there has been previously no strong experimental evidence for the existence of a plasma during single- or multi-bubble sonoluminescence. SBSL typically produces featureless emission spectra that reveal little about the intra-cavity physical conditions or chemical processes. Here we report observations of atomic (Ar) emission and extensive molecular (SO) and ionic (O2+) progressions in SBSL spectra from concentrated aqueous H2SO4 solutions. Both the Ar and SO emission permit spectroscopic temperature determinations, as accomplished for multi-bubble sonoluminescence with other emitters. The emissive excited states observed from both Ar and O2+ are inconsistent with any thermal process. The Ar excited states involved are extremely high in energy (>13 eV) and cannot be thermally populated at the measured Ar emission temperatures (4,000-15,000 K); the ionization energy of O2 is more than twice its bond dissociation energy, so O2+ likewise cannot be thermally produced. We therefore conclude that these emitting species must originate from collisions with high-energy electrons, ions or particles from a hot plasma core.

https://zenodo.org/record/895438/files/article.pdf

Plasma formation and temperature measurement during single-bubble cavitation

Context. Our position inside the Galaxy requires 3D-modelling to obtain the distribution of the Galactic magnetic field, cosmic- ray (CR) electrons and thermal electrons. Aims. Our intention is to find a Galactic 3D-model which agrees best with available radio observations. Methods. We constrain simulated all-sky maps in total intensity, lin ear polarization, and rotation measure (RM) by observations. For the simulated maps as a function of frequency we integrate in 15 ′ wide cones the emission along the line of sight calculated from Galactic 3D-models. We test a number of large-scale magnetic field configurations and take the properties of the warm inte rstellar medium into account. Results. From a comparison of simulated and observed maps we are able to constrain the regular large-scale Galactic magnetic field in the disk and the halo of the Galaxy. The local regular fi eld is 2� G and the average random field is about 3 � G. The known local excess of synchrotron emission originating either fr om enhanced CR electrons or random magnetic fields is able to e xplain the observed high-latitude synchrotron emission. The thermal electron model (NE2001) in conjunction with a proper fill ing factor accounts for the observed optically thin thermal emission and low frequency absorption by optically thick emission. A coupling factor between thermal electrons and the random magnetic field comp onent is proposed, which in addition to the small filling fact or of thermal electrons increases small-scale RM fluctuations an d thus accounts for the observed depolarization at 1.4 GHz. Conclusions. We conclude that an axisymmetric magnetic disk field configur ation with reversals inside the solar circle fits available observations best. Out of the plane a strong toroidal magnetic field with di fferent signs above and below the plane is needed to account for the observed high-latitude RMs. The large field strength is a consequence of the small thermal electron scale height of 1 kpc, which also limits the CR electron extent up to a height of 1 kpc not to contradict with the observed synchrotron emission out of the plane. Our preferred 3D-model fits the observed Galactic total intensi ty and polarized emission better than other models over a wide frequency range and also agrees with the observed RM from extragalactic sources.

/pdf/radio-observational-constraints-on-galactic-3d-emission-245m3npx21.pdf

Radio observational constraints on Galactic 3D-emission models

Compact polarimetry is a technique that allows construction of pseudo quad-pol information from dual-polarization synthetic aperture radar (SAR) systems. Compact polarimetry showed promise of being able to reduce the complexity, cost, mass, and data rate of a SAR system while attempting to maintain many capabilities of a fully polarimetric system. In this paper, we study different transmit/receive configurations to determine which polarimetric configurations allow for superior reconstruction of the fully polarimetric data. We discuss modifications of the original reconstruction algorithm proposed by Souyris , which show potential to better reconstruct fully polarimetric data.

Comparison of Compact Polarimetric Synthetic Aperture Radar Modes

The Long Wavelength Array

New low-frequency 74 and 330 MHz observations of the Galactic center (GC) region reveal the presence of a large-scale ($6\arcdeg\times 2\arcdeg$) diffuse source of nonthermal synchrotron emission. A minimum energy analysis of this emission yields a total energy of $\sim (\phi^{4/7}f^{3/7})\times 10^{52}$ ergs and a magnetic field strength of $\sim 6(\phi/f)^{2/7}$ \muG (where $\phi$ is the proton to electron energy ratio and $f$ is the filling factor of the synchrotron emitting gas). The equipartition particle energy density is $1.2(\phi/f)^{2/7}$ \evcm, a value consistent with cosmic-ray data. However, the derived magnetic field is several orders of magnitude below the 1 mG field commonly invoked for the GC. With this field the source can be maintained with the SN rate inferred from the GC star formation. Furthermore, a strong magnetic field implies an abnormally low GC cosmic-ray energy density. We conclude that the mean magnetic field in the GC region must be weak, of order 10 \muG (at least on size scales $\ga 125\arcsec$).

/pdf/evidence-for-a-weak-galactic-center-magnetic-field-from-4mj07lcicq.pdf

Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission

We present a wide-field, subarcminute-resolution VLA image of the Galactic center region at 330 MHz. With a resolution of ~7'' × 12'' and an rms noise of 1.6 mJy beam-1, this image represents a significant increase in resolution and sensitivity over the previously published VLA image at this frequency. The improved sensitivity has more than tripled the census of small-diameter sources in the region, has resulted in the detection of two new nonthermal filaments (NTFs), 18 NTF candidates, and 30 pulsar candidates, reveals previously known extended sources in greater detail, and has resulted in the first detection of Sagittarius A* in this frequency range.

/pdf/high-resolution-wide-field-imaging-of-the-galactic-center-4ziq65mwxk.pdf

High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz

New high-resolution, wide-field 90 cm VLA observations of the Galactic center (GC) region by Nord and coworkers have revealed 20 nonthermal filament (NTF) candidates. We report 6 cm polarization observations of six of these. All of the candidates have the expected NTF morphology, and two show extended polarization, confirming their identification as NTFs. One of the new NTFs appears to be part of a system of NTFs located in the Sgr B region, 64 pc in projection north of Sgr A. These filaments cross the Galactic plane with an orientation similar to the filaments in the Galactic center radio arc. They extend the scale over which the NTF phenomena is known to occur to almost 300 pc along the Galactic plane. Another NTF was found in the Galactic plane south of the Sgr C filament but with an orientation of 45° to the Galactic plane. This is only the second of 12 confirmed NTFs that is not oriented perpendicular to the Galactic plane. An additional candidate in the Sgr C region was resolved into multiple filamentary structures. Polarization was detected only at the brightness peak of one of the filaments. Several of these filaments run parallel to the Galactic plane and can be considered additional evidence for nonpoloidal magnetic fields at the GC. Together the 90 and 6 cm observations indicate that the GC magnetic field may be more complex than a simple globally ordered dipolar field.

/pdf/new-nonthermal-filaments-at-the-galactic-center-are-they-48uqboycl1.pdf

Michael E. Nord

Papers

Comparison of Compact Polarimetric Synthetic Aperture Radar Modes

The Long Wavelength Array

Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission

High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz

New nonthermal filaments at the galactic center: are they tracing a globally ordered magnetic field?