scholarly journals A Linear Polarization Survey of the Southern Sky at 408 MC/S

1965 ◽  
Vol 18 (6) ◽  
pp. 635 ◽  
Author(s):  
DS Mathewson ◽  
DK Milne
Keyword(s):  
Magnetic Field ◽  
Linear Polarization ◽  
Large Scale ◽  
Close Correlation ◽  
Great Circle ◽  
Relativistic Electrons ◽  
Sky Survey ◽  
Radiation Theory ◽  
Almost All ◽  
Spiral Arm

The results of 408 Mcts linear polarization observations of the southern sky using the 210 ft steerable reflector at Parkes are presented. Combination of this survey with the northern sky survey of the Leiden group shows that almost all of the. polarization at this frequency lies in a band about 60� wide, which contains the great circle that passes through the galactic poles and intersects the plane at lil = 340� and 160�. This large-scale distribution of linear polarization at 408 Mcts may be explained on the basis of synchrotron radiation theory if the Sun lies almost at the centre of a spiral arm that has a magnetic field directed along it towards III = 70� and 250�. The concentrations of relativistic electrons may be confined to regions of higher than average magnetic field strength (5 X 10-5 G) elongated in the direction of the arm. The observations show that the magnetic fields in these "synchrotron" regions are very ordered. The distance to the polarized regions may be about 150 pc. At high latitudes, close correlation is observed between features of the distribution of background polarization and Faraday rotation of extragalactic sources.

scholarly journals Importance of Magnetic Effects in a Two-Flow Model for Extragalactic Radio Jets

1990 ◽  
Vol 140 ◽  
pp. 445-445
Author(s):  
H. Sol ◽  
G. Pelletier ◽  
E. Asseo
Keyword(s):  
Magnetic Field ◽  
Coming Out ◽  
Large Scale ◽  
Flow Model ◽  
Minimal Distance ◽  
Relativistic Electrons ◽  
Ambient Plasma ◽  
Relaxation Zone ◽  
Radio Jets ◽  
The Magnetic Field

We propose a model for extragalactic radio jets in which two different flows of particles are taken into account, (i) a beam of relativistic electrons and positrons extracted from the funnel of accretion disc and responsible for the observed superluminal motion, (ii) a classical or mildly relativistic wind of electrons and protons coming out from all parts of the disc (Sol et al., 1989). Studying the mutual interaction of the two flows, we show that the configuration is not destroyed by the plasma-beam instability as long as the magnetic field, assumed longitudinal, is strong enough, with an electron gyrofrequency ωc = eB/mec greater than the ambient plasma frequency ωp = (4πnpe2)1/2 (Pelletier et al., 1988). When ωc < ωp, the relativistic beam loses its energy and its momentum mainly through the development of strong Langmuir turbulence in the wind, and disappears quietly after some relaxation zone where heating and entrainment of the wind occur. This emphasizes one aspect of the important role likely played by the magnetic field in the dynamics of extragalactic jets and provides one example in which the magnetic field, acting on the microscopic scale of an interaction, induces strong effects on large–scale structures. Detailed data on the closest known superluminal radio source 3C120 (Walker et al., 1987, 1988; Benson et al., 1988) allow a check on the likelihood of our model. Observational estimates of the variation along the jet of the magnetic field and of the ambient plasma density np suggest that the magnetic field reaches its critical value (corresponding to ωc = ωp) at a minimal distance of about 1.4 kpc from the central engine. This is amazingly close to the location of the 4′–radio knot, a “rather curious structure” described by Walker et al. (1987), which we interpret as the beam relaxation zone in the context of our two–flow model (Sol et al., 1989).

scholarly journals The Magnetic Field Near the Local Bubble

1997 ◽  
Vol 166 ◽  
pp. 227-238
Author(s):  
Carl Heiles
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Zeeman Splitting ◽  
Relativistic Electrons ◽  
Local Bubble ◽  
Scale Field ◽  
Large Scale Field ◽  
Field Lines ◽  
Hi Shells ◽  
The Magnetic Field

AbstractThere are almost no direct observational indicators of the magnetic field inside the local bubble. Just outside the bubble, the best tracers are stellar polarization and HI Zeeman splitting. These show that the local field does not follow the large-scale Galactic field. Here we discuss whether the deformation of the large-scale field by the local HI shells is consistent with the observations. We concentrate on the Loop 1 region, and find that the field lines are well-explained by this idea; in addition, the bright radio filaments of Radio Loop 1 delineate particular field lines that are “lit up” by an excess of relativistic electrons.

scholarly journals Magnetic field evolution in solar-type stars

2019 ◽  
Vol 15 (S354) ◽  
pp. 169-180
Author(s):  
Axel Brandenburg
Keyword(s):  
Magnetic Field ◽  
Linear Polarization ◽  
Large Scale ◽  
Rossby Number ◽  
Rotating Stars ◽  
Field Evolution ◽  
Solar Type ◽  
Activity Cycles ◽  
Rapidly Rotating Stars ◽  
The Magnetic Field

AbstractWe discuss selected aspects regarding the magnetic field evolution of solar-type stars. Most of the stars with activity cycles are in the range where the normalized chromospheric Calcium emission increases linearly with the inverse Rossby number. For Rossby numbers below about a quarter of the solar value, the activity saturates and no cycles have been found. For Rossby numbers above the solar value, again no activity cycles have been found, but now the activity goes up again for a major fraction of the stars. Rapidly rotating stars show nonaxisymmetric large-scale magnetic fields, but there is disagreement between models and observations regarding the actual value of the Rossby number where this happens. We also discuss the prospects of detecting the sign of magnetic helicity using various linear polarization techniques both at the stellar surface using the parity-odd contribution to linear polarization and above the surface using Faraday rotation.

scholarly journals Near-infrared imaging polarimetry toward M 17 SWex

2019 ◽  
Vol 71 (Supplement_1) ◽  
Author(s):  
Koji Sugitani ◽  
Fumitaka Nakamura ◽  
Tomomi Shimoikura ◽  
Kazuhito Dobashi ◽  
Quang Nguyen-Luong ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Near Infrared ◽  
Infrared Imaging ◽  
Dark Cloud ◽  
Imaging Polarimetry ◽  
Magnetic Stability ◽  
Almost All ◽  
The Magnetic Field

Abstract We conducted near-infrared ($\mathit {JHK}_{\rm s}$) imaging polarimetry toward the infrared dark cloud (IRDC) M 17 SWex, including almost all of the IRDC filaments as well as its outskirts, with the polarimeter SIRPOL on the IRSF 1.4 m telescope. We revealed the magnetic fields of M 17 SWex with our polarization-detected sources that were selected by some criteria based on their near-IR colors and the column densities toward them, which were derived from the Herschel data. The selected sources indicate not only that the ordered magnetic field is perpendicular to the cloud elongation as a whole, but also that at both ends of the elongated cloud the magnetic field appears to be bent toward its central part, i.e., a large-scale hourglass-shaped magnetic field perpendicular to the cloud elongation. In addition to this general trend, the elongations of the filamentary subregions within the dense parts of the cloud appear to be mostly perpendicular to their local magnetic fields, while the magnetic fields of the outskirts appear to follow the thin filaments that protrude from the dense parts. The magnetic strengths were estimated to be ∼70–$300\, \mu$G in the subregions, of which the lengths and average number densities are ∼3–9 pc and ∼2–7 × 103 cm−3, respectively, by the Davis–Chandrasekhar–Fermi method with the angular dispersion of our polarization data and the velocity dispersion derived from the C18O (J = 1–0) data obtained by the Nobeyama 45 m telescope. These field configurations and our magnetic stability analysis of the subregions imply that the magnetic field has controlled the formation/evolution of the M 17 SWex cloud.

scholarly journals The complex large-scale magnetic fields in the first Galactic quadrant as revealed by the Faraday depth profile disparity

2020 ◽  
Vol 497 (3) ◽  
pp. 3097-3117
Author(s):  
Y K Ma ◽  
S A Mao ◽  
A Ordog ◽  
J C Brown
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Milky Way ◽  
Broad Band ◽  
Galactic Magnetic Field ◽  
Field Reversal ◽  
Very Large Array ◽  
Large Scale Magnetic Field ◽  
Galactic Longitude ◽  
Spiral Arm

ABSTRACT The Milky Way is one of the very few spiral galaxies known to host large-scale magnetic field reversals. The existence of the field reversal in the first Galactic quadrant near the Sagittarius spiral arm has been well established, yet poorly characterized due to the insufficient number of reliable Faraday depths (FDs) from extragalactic radio sources (EGSs) through this reversal region. We have therefore performed broad-band (1–$2\, {\rm GHz}$) spectropolarimetric observations with the Karl G. Jansky Very Large Array (VLA) to determine the FD values of 194 EGSs in the Galactic longitude range of 20°–52° within ±5° from the Galactic mid-plane, covering the Sagittarius arm tangent. This factor of five increase in the EGS FD density has led to the discovery of a disparity in FD values across the Galactic mid-plane in the Galactic longitude range of 40°–52°. Combined with existing pulsar FD measurements, we suggest that the Sagittarius arm can host an odd-parity disc field. We further compared our newly derived EGS FDs with the predictions of three major Galactic magnetic field models, and concluded that none of them can adequately reproduce our observational results. This has led to our development of new, improved models of the Milky Way disc magnetic field that will serve as an important step towards major future improvements in Galactic magnetic field models.

scholarly journals The intergalactic magnetic field probed by a giant radio galaxy

2019 ◽  
Vol 622 ◽  
pp. A16 ◽  
Author(s):  
S. P. O’Sullivan ◽  
J. Machalski ◽  
C. L. Van Eck ◽  
G. Heald ◽  
M. Brüggen ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Radio Galaxy ◽  
Small Scale ◽  
Cosmological Simulations ◽  
Rotation Measure ◽  
Sky Survey ◽  
Intergalactic Magnetic Field ◽  
Using Data ◽  
The Universe

Cosmological simulations predict that an intergalactic magnetic field (IGMF) pervades the large scale structure (LSS) of the Universe. Measuring the IGMF is important to determine its origin (i.e. primordial or otherwise). Using data from the LOFAR Two Metre Sky Survey (LoTSS), we present the Faraday rotation measure (RM) and depolarisation properties of the giant radio galaxy J1235+5317, at a redshift of z = 0.34 and 3.38 Mpc in size. We find a mean RM difference between the lobes of 2.5 ± 0.1 rad m−2, in addition to small scale RM variations of ∼0.1 rad m−2 . From a catalogue of LSS filaments based on optical spectroscopic observations in the local universe, we find an excess of filaments intersecting the line of sight to only one of the lobes. Associating the entire RM difference to these LSS filaments leads to a gas density-weighted IGMF strength of ∼0.3 μG. However, direct comparison with cosmological simulations of the RM contribution from LSS filaments gives a low probability (∼5%) for an RM contribution as large as 2.5 rad m−2, for the case of IGMF strengths of 10–50 nG. It is likely that variations in the RM from the Milky Way (on 11′ scales) contribute significantly to the mean RM difference, and a denser RM grid is required to better constrain this contribution. In general, this work demonstrates the potential of the LOFAR telescope to probe the weak signature of the IGMF. Future studies, with thousands of sources with high accuracy RMs from LoTSS, will enable more stringent constraints on the nature of the IGMF.

Mapping magnetic field and relativistic electrons along a solar flare current sheet

2020 ◽  
Author(s):  
Gregory Fleishman ◽  
Bin Chen ◽  
Gary Dale ◽  
Gelu Nita et al.
Keyword(s):  
Magnetic Field ◽  
Solar Flare ◽  
Current Sheet ◽  
Energy Release ◽  
Large Scale ◽  
Local Maximum ◽  
High Energy ◽  
Relativistic Electrons ◽  
Flare Loop ◽  
Magnetic Bottle

&lt;p&gt;In the standard model of solar flares, a large-scale reconnection current sheet (RCS) is postulated as the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear due to the lack of measurements for the magnetic properties of the RCS. Here we report the first measurement of spatially-resolved magnetic field and flare-accelerated relativistic electrons along a large-scale RCS in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the RCS above the flare loop-top, referred to as a &quot;magnetic bottle&quot;. This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of over 4000 V/m is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. In contrast, the relativistic electrons concentrate at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest crucial new input to the current picture of high energy electron acceleration.&lt;/p&gt;

scholarly journals Detection of 40–48 GHz dust continuum linear polarization towards the Class 0 young stellar object IRAS 16293–2422

2018 ◽  
Vol 617 ◽  
pp. A3 ◽  
Author(s):  
Hauyu Baobab Liu ◽  
Yasuhiro Hasegawa ◽  
Tao-Chung Ching ◽  
Shih-Ping Lai ◽  
Naomi Hirano ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Linear Polarization ◽  
Large Scale ◽  
Black Body ◽  
Field Configuration ◽  
Stellar Object ◽  
Continuum Emission ◽  
Residual Polarization ◽  
Young Stellar Object ◽  
The Magnetic Field

Aims. The aims of this work are to test the feasibility of observing dust polarization at frequencies lower than 50 GHz, which is the optically thinner part of the modified black body spectrum, and to clarify whether or not the polarization mechanism is identical or similar to that for (sub)millimeter observations. Methods. We performed the new Karl G. Jansky Very Large Array (JVLA) full polarization observations at 40–48 GHz (6.3–7.5 mm) towards the nearby (d= 147 ± 3.4 pc) Class 0 young stellar object (YSO) IRAS 16293–2422, and compared these with the previous Submillimeter Array (SMA) observations. We observed the quasar J1407+2827, which is weakly polarized and can be used as a leakage term calibrator for <9 GHz observations, to gauge the potential residual polarization leakage after calibration. Results. We did not detect Stokes Q, U, and V intensities from the observations of J1407+2827, and constrain (3σ) the residual polarization leakage after calibration to be ≲0.3%. Limited by thermal noise, we only detected linear polarization from one of the two binary components of our target source, IRAS 16293–2422 B. The measured polarization percentages range from less than one percent to a few tens of percent. The derived polarization position angles from our observations are in excellent agreement with those detected from the previous observations of the SMA, implying that on the spatial scale we are probing (~50–1000 au), the physical mechanisms for polarizing the continuum emission do not vary significantly over the wavelength range of ~0.88–7.5 mm. Conclusions. We hypothesize that the observed polarization position angles trace the magnetic field, which converges from large scale to an approximately face-on rotating accretion flow. In this scenario, magnetic field is predominantly poloidal on >100 au scales, and becomes toroidal on smaller scales. However, this interpretation remains uncertain due to the high dust optical depths at the central region of IRAS 16293–2422 B and the uncertain temperature profile. We suggest that dust polarization at wavelengths comparable or longer than 7 mm may still trace interstellar magnetic field. Future sensitive observations of dust polarization in the fully optically thin regime will have paramount importance for unambiguously resolving the magnetic field configuration.

scholarly journals New constraints on the magnetization of the cosmic web using LOFAR Faraday rotation observations

2020 ◽  
Vol 495 (3) ◽  
pp. 2607-2619 ◽  
Author(s):  
S P O’Sullivan ◽  
M Brüggen ◽  
F Vazza ◽  
E Carretti ◽  
N T Locatelli ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Large Scale ◽  
Current Data ◽  
Inhomogeneous Universe ◽  
Origin And Evolution ◽  
Rotation Measure ◽  
Sky Survey ◽  
Significant Difference

ABSTRACT Measuring the properties of extragalactic magnetic fields through the effect of Faraday rotation provides a means to understand the origin and evolution of cosmic magnetism. Here, we use data from the LOFAR Two-Metre Sky Survey (LoTSS) to calculate the Faraday rotation measure (RM) of close pairs of extragalactic radio sources. By considering the RM difference (ΔRM) between physical pairs (e.g. double-lobed radio galaxies) and non-physical pairs (i.e. close projected sources on the sky), we statistically isolate the contribution of extragalactic magnetic fields to ΔRM along the line of sight between non-physical pairs. From our analysis, we find no significant difference between the ΔRM distributions of the physical and non-physical pairs, limiting the excess Faraday rotation contribution to &lt;1.9 rad m−2 (${\sim}95{{\ \rm per\ cent}}$ confidence). We use this limit with a simple model of an inhomogeneous universe to place an upper limit of 4 nG on the cosmological co-moving magnetic field strength on Mpc scales. We also compare the RM data with a more realistic suite of cosmological magnetohydrodynamical simulations that explore different magnetogenesis scenarios. Both magnetization of the large-scale structure by astrophysical processes such as galactic and AGN outflows, and simple primordial scenarios with seed magnetic field strengths &lt;0.5 nG cannot be rejected by the current data; while stronger primordial fields or models with dynamo amplification in filaments are disfavoured.

Sign in / Sign up

Export Citation Format

Share Document