Magnetic field at the inner disk edge

AbstractOur present understanding of the coronal structure of T Tauri stars is fragmentary and observations in different wavelength regimes often appear to give contradictory results. X-ray data suggest the presence of magnetic loops on a variety of scales, from compact loops of size less than a stellar radius, up to very large loops of up to 10 stellar radii which may connect to the disk. While some stars show a clear rotational modulation in X-rays, implying distinct bright and dark regions, many do not. This picture is complicated by the accretion process itself, which also contributes to the X-ray emission. The location of the inner edge of the accretion disk and the nature of the magnetic field there are still hotly-contested issues. Accretion indicators often suggest the presence of discrete accretion funnels. This has implications for the structure of the corona, as does the presence of an outflowing wind. All of these factors are linked to the structure of the magnetic field, which we are now beginning to unravel through Zeeman-Doppler imaging. In this review I will describe the present state of our understanding of the magnetic structure of T Tauri coronae and the impact this has during such an early evolutionary stage.

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X-rays from accretion shocks in classical T Tauri stars: 2D MHD modeling and the role of local absorption

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314001707 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 48-49

Author(s):

C. Argiroffi ◽

R. Bonito ◽

S. Orlando ◽

M. Miceli ◽

F. Reale ◽

...

Keyword(s):

Magnetic Field ◽

T Tauri Stars ◽

X Rays ◽

X Ray ◽

T Tauri ◽

Shock Region ◽

Post Shock ◽

The Magnetic Field ◽

Accretion Shocks

AbstractIn classical T Tauri stars (CTTS) strong shocks are formed where the accretion funnel impacts with the denser stellar chromosphere. Although current models of accretion provide a plausible global picture of this process, some fundamental aspects are still unclear: the observed X-ray luminosity in accretion shocks is order of magnitudes lower than predicted; the observed density and temperature structures of the hot post-shock region are puzzling and still unexplained by models.To address these issues we performed 2D MHD simulations describing an accretion stream impacting onto the chromosphere of a CTTS, exploring different configurations and strengths of the magnetic field. From the model results we then synthesized the X-ray emission emerging from the hot post-shock, taking into account the local absorption due to the pre-shock stream and surrounding atmosphere.We find that the different configurations and strengths of the magnetic field profoundly affect the hot post-shock properties. Moreover the emerging X-ray emission strongly depends also on the viewing angle under which accretion is observed. Some of the explored configuration are able to reproduce the observed features of X-ray spectra of CTTS.

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X-Ray Emission from Cataclysmic Variables

International Astronomical Union Colloquium ◽

10.1017/s0252921100076582 ◽

1979 ◽

Vol 53 ◽

pp. 508-508

Author(s):

D. Q. Lamb

Keyword(s):

Magnetic Field ◽

Field Strength ◽

Magnetic Field Strength ◽

Classification Scheme ◽

Accretion Rate ◽

Cataclysmic Variables ◽

X Rays ◽

X Ray ◽

Stellar Radius ◽

The Magnetic Field

Many cataclysmic variables have been found to be hard, as well as soft, X-ray sources. Emission from the boundary layer of an optically thick accretion disk extending down to the stellar surface can, at outburst, produce soft X-rays, but the production of hard X-rays from such a disk is difficult to understand. We therefore conjecture that the sources which emit hard X-rays have magnetic fields and are, in general, rotating. We then propose a classification scheme for cataclysmic variables based on the size of the Alfven radius rA relative to the stellar radius R of the degenerate dwarf and the separation α of the binary system. We show that many of the varied characteristics displayed by the cataclysmic variable X-ray sources can be understood in terms of this ordering. We suggest that the AM Her Class (AM Her, AN UMa, W Pup, and 2A0311-23) have R ≪ α ≪ rA , the DQ Her Class (DQ Her, V533 Her, and AE Aqr) have R ≪ rA ≪ α, while the SS Cyg Class (SS Cyg, U Gem, EX Hya, and GK Per) have rA ≲ R ≪ α. Although rA depends on both the magnetic field strength of the degenerate dwarf ana the accretion rate, for comparable rates of accretion the ordering that we propose is essentially one of decreasing magnetic field strength.

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Thermal and non-thermal connection in radio mini-halos

Proceedings of the International Astronomical Union ◽

10.1017/s1743921318007020 ◽

2018 ◽

Vol 14 (S342) ◽

pp. 137-140

Author(s):

A. Ignesti ◽

G. Brunetti ◽

M. Gitti ◽

S. Giacintucci

Keyword(s):

Magnetic Field ◽

Surface Brightness ◽

X Rays ◽

X Ray ◽

Point Analysis ◽

Spectrum Radio ◽

Point To Point ◽

The Magnetic Field ◽

Relativistic Particles ◽

Open Issues

AbstractSeveral cool-core clusters are known to host a radio mini-halo, a diffuse, steep-spectrum radio source located in their cores, thus probing the presence of non-thermal components as magnetic field and relativistic particles on scales not directly influenced by the central AGN. The nature of the mechanism that produces a population of radio-emitting relativistic particles on the scale of hundreds of kiloparsecs is still unclear. At the same time, it is still debated if the central AGN may play a role in the formation of mini-halos by providing the seed of the relativistic particles. We aim to investigate these open issues by studying the connection between thermal and non-thermal components of the intra-cluster medium. We performed a point-to-point analysis of the radio and the X-ray surface brightness of a compilation of mini-halos. We find that mini-halos have super-linear scalings between radio and X-rays, with radio brightness declining more steeply than the X-ray brightness. This trend is opposite to that generally observed in giant radio halos, thus marking a possible difference in the physics of the two radio sources. Finally, using the scalings between radio and X-rays and assuming a hadronic origin of mini-halos we derive constraints on the magnetic field in the core of the hosting clusters.

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Dust entrainment in photoevaporative winds: The impact of X-rays

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936615 ◽

2020 ◽

Vol 635 ◽

pp. A53 ◽

Cited By ~ 3

Author(s):

R. Franz ◽

G. Picogna ◽

B. Ercolano ◽

T. Birnstiel

Keyword(s):

Extreme Ultraviolet ◽

Mass Loss Rate ◽

Disk Surface ◽

Maximum Height ◽

X Rays ◽

X Ray ◽

T Tauri ◽

Dust Entrainment ◽

Disk Evolution ◽

The Impact

Context. X-ray- and extreme ultraviolet (XEUV) driven photoevaporative winds acting on protoplanetary disks around young T Tauri stars may crucially impact disk evolution, affecting both gas and dust distributions. Aims. We investigate the dust entrainment in XEUV-driven photoevaporative winds and compare our results to existing magnetohydrodynamic and EUV-only models. Methods. We used a 2D hydrodynamical gas model of a protoplanetary disk irradiated by both X-ray and EUV spectra from a central T Tauri star to trace the motion of passive Lagrangian dust grains of various sizes. The trajectories were modelled starting at the disk surface in order to investigate dust entrainment in the wind. Results. For an X-ray luminosity of LX = 2 × 1030 erg s−1 emitted by a M* = 0.7 M⊙ star, corresponding to a wind mass-loss rate of Ṁw ≃ 2.6 × 10−8 M⊙ yr−1, we find dust entrainment for sizes a0 ≲ 11 μm (9 μm) from the inner 25 AU (120 AU). This is an enhancement over dust entrainment in less vigorous EUV-driven winds with Ṁw ≃ 10−10 M⊙ yr−1. Our numerical model also shows deviations of dust grain trajectories from the gas streamlines even for μm-sized particles. In addition, we find a correlation between the size of the entrained grains and the maximum height they reach in the outflow. Conclusions. X-ray-driven photoevaporative winds are expected to be dust-rich if small grains are present in the disk atmosphere.

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On Special Features of Non-Thermal Solar X Radiation above 10 keV

International Astronomical Union Colloquium ◽

10.1017/s0252921100089958 ◽

1972 ◽

Vol 14 ◽

pp. 761-762

Author(s):

G. Elwert ◽

E. Haug

Keyword(s):

Magnetic Field ◽

Angular Distribution ◽

Power Law ◽

Impulsive Phase ◽

X Rays ◽

Energetic Electrons ◽

X Ray ◽

Preferred Direction ◽

Field Lines ◽

The Magnetic Field

The polarization and angular distribution of solar hard X radiation above 10 keV was calculated under the assumption that the X rays originate as bremsstrahlung from energetic electrons moving in a preferred direction. The source electrons are supposed to have a power-law spectrum. These conditions are to be expected in the impulsive phase of an X-ray burst. The spiral orbits of the electrons around the magnetic field lines are taken into account.

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Multiepoch, multiwavelength study of accretion onto T Tauri

Astronomy and Astrophysics ◽

10.1051/0004-6361/201731613 ◽

2018 ◽

Vol 618 ◽

pp. A55 ◽

Cited By ~ 2

Author(s):

P. C. Schneider ◽

H. M. Günther ◽

J. Robrade ◽

J. H. M. M. Schmitt ◽

M. Güdel

Keyword(s):

Near Infrared ◽

Accretion Rate ◽

Strong Shock ◽

Field Configuration ◽

Optical Data ◽

X Rays ◽

Coronal Emission ◽

X Ray ◽

T Tauri ◽

The Impact

Classical T Tauri stars (CTTSs) accrete matter from the inner edge of their surrounding circumstellar disks. The impact of the accretion material on the stellar atmosphere results in a strong shock, which causes emission from the X-ray to the near-infrared (NIR) domain. Shock velocities of several 100 km s−1 imply that the immediate post shock plasma emits mainly in X-rays. Indeed, two X-ray diagnostics, the so-called soft excess and the high densities observed in He-like triplets, differentiate CTTSs from their non-accreting siblings. However, accretion shock properties derived from X-ray diagnostics often contradict established ultraviolet (UV)–NIR accretion tracers and a physical model simultaneously explaining both, X-ray and UV–NIR accretion tracers, is not yet available. We present new XMM-Newton and Chandra grating observations of the CTTS T Tauri combined with UV and optical data. During all epochs, the soft excess is large and the densities derived from the O VII and Ne IX He-like triplets are compatible with coronal densities. This confirms that the soft X-ray emission cannot originate in accretion funnels that carry the bulk of the accretion rate despite T Tauri’s large soft excess. Instead, we propose a model of radially density stratified accretion columns to explain the density diagnostics and the soft excess. In addition, accretion rate and X-ray luminosity are inversely correlated in T Tauri over several epochs. Such an anti-correlation has been observed in samples of stars. Hence the process causing it must be intrinsic to the accretion process, and we speculate that the stellar magnetic field configuration on the visible hemisphere affects both the accretion rate and the coronal emission, eventually causing the observed anti-correlation.

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A compact permanent-magnet system for measuring magnetic circular dichroism in resonant inelastic soft X-ray scattering

Journal of Synchrotron Radiation ◽

10.1107/s1600577517000923 ◽

2017 ◽

Vol 24 (2) ◽

pp. 449-455 ◽

Cited By ~ 4

Author(s):

Jun Miyawaki ◽

Shigemasa Suga ◽

Hidenori Fujiwara ◽

Hideharu Niwa ◽

Hisao Kiuchi ◽

...

Keyword(s):

Magnetic Field ◽

Permanent Magnets ◽

Magnetic Circular Dichroism ◽

X Rays ◽

Magnet System ◽

X Ray ◽

X Ray Scattering ◽

Magnetic Dichroism ◽

The Magnetic Field ◽

Ray Scattering

A compact and portable magnet system for measuring magnetic dichroism in resonant inelastic soft X-ray scattering (SX-RIXS) has been developed at the beamline BL07LSU in SPring-8. A magnetic circuit composed of Nd–Fe–B permanent magnets, which realised ∼0.25 T at the center of an 11 mm gap, was rotatable around the axis perpendicular to the X-ray scattering plane. Using the system, a SX-RIXS spectrum was obtained under the application of the magnetic field at an angle parallel, nearly 45° or perpendicular to the incident X-rays. A dedicated sample stage was also designed to be as compact as possible, making it possible to perform SX-RIXS measurements at arbitrary incident angles by rotating the sample stage in the gap between the magnetic poles. This system enables facile studies of magnetic dichroism in SX-RIXS for various experimental geometries of the sample and the magnetic field. A brief demonstration of the application is presented.

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MASSIVE PHOTON PAIRS AND FEATURES OF CHANGES IN THE X-RAY BACKGROUND OF THE SOLAR CROWN AND EARTH'S MAGNETOSPHERE

EurasianUnionScientists ◽

10.31618/esu.2413-9335.2020.2.76.902 ◽

2020 ◽

Vol 2 (7(76)) ◽

pp. 42-46

Author(s):

I.K. Mirzoeva

Keyword(s):

Magnetic Field ◽

Solar Corona ◽

X Rays ◽

Earth’S Magnetosphere ◽

X Ray ◽

Massive Photon ◽

Photon Pairs ◽

Earth's Magnetosphere ◽

X Ray Background ◽

The Magnetic Field

The analysis of the x-ray background of the solar corona in the range of 2-25 Kev for three months of 2003 was carried out.the integrated energy spectrum was obtained according to the RHESSI project. Comparison with the data of the x-ray background of The earth's magnetosphere according to the XMM-Newton project in the soft range of x-rays allowed us to draw a conclusion about the common nature of the features of seasonal variations of the x-ray background of The earth's magnetosphere and the thermal x-ray background of the solar corona. The main reason for these changes is the splitting of massive photon pairs born from vacuum in the magnetic field of the solar corona and in the magnetic field of the Earth. According to the RHESSI, XMM-Newton, and Plank projects, theoretical and experimental evidence for the existence of massive photon pairs (ultralight scalar bosons) is provided.

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Der Energiegewinn von Elektronen und die durch sie erzeugte harte Röntgen-Strahlung bei Thetapinch-Entladungen vor der Zündung

Zeitschrift für Naturforschung A ◽

10.1515/zna-1962-1104 ◽

1962 ◽

Vol 17 (11) ◽

pp. 977-989

Author(s):

R. Chodura ◽

M. Keilhacker

Keyword(s):

Magnetic Field ◽

Electric Field ◽

Energy Distribution ◽

Charged Particles ◽

Zero Magnetic Field ◽

Time Interval ◽

Spatial Distributions ◽

X Rays ◽

X Ray ◽

The Magnetic Field

The following article deals with measurements on hard X-rays produced in thetapinch discharges before breakdown of the gas which often last for several halfcycles of the magnetic field. In order to explain the timedependent intensity and energy of the X-rays, at first two possible spatial distributions of the electric field in a thetapinch-coil are discussed and the gain of energy of charged particles is calculated. The calculation shows that the adiabatic invariant μ = m2 ν2/(2 mo Β) which gives the gain of energy as a function of the magnetic field Β is proportional to (| ω̇g ½to)—3 where ω̇g is the time-derivative of the gyrofrequency which is assumed to be constant and to is the time between zero magnetic field and the start of the particle. Therefore the hard X-rays can be produced only by electrons which were existing in a small time interval around zero magnetic field of the order |ω̇g|–½. Because of the dependence of µ on the initial position of the particle the elecrtons have an energy distribution which is calculated under the assumption that all electrons are uniformely distributed initially over the cross-section of the coil.From the measured time dependence of X-ray intensity the spatial distribution of the electric field in halfcycles before breakdown can be infered. The ratio of the X-ray intensities with and without absorbers has been measured for different values of the timedependent magnetic field and also for different steady bias magnetic fields by which the starting conditions (to) of the particles are altered. These ratios are in good agreement with corresponding theoretical values which are derived from the calculated energy distribution of the electrons. The experimental results show that in halfcycles before breakdown there exist different spatial distributions of the electric field depending on the rising density of charged particles.

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Gas shells and magnetic fields in the Orion-Eridanus superbubble

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936239 ◽

2019 ◽

Vol 631 ◽

pp. A52 ◽

Cited By ~ 4

Author(s):

T. Joubaud ◽

I. A. Grenier ◽

J. Ballet ◽

J. D. Soler

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Expansion Velocity ◽

X Rays ◽

X Ray ◽

Stellar Feedback ◽

Hot Plasma ◽

Formation History ◽

X Ray Absorption ◽

The Magnetic Field

Aims. The Orion-Eridanus superbubble has been blown by supernovae and supersonic winds of the massive stars in the Orion OB associations. It is the nearest site at which stellar feedback on the interstellar medium that surrounds young massive clusters can be studied. The formation history and current structure of the superbubble are still poorly understood, however. It has been pointed out that the picture of a single expanding object should be replaced by a combination of nested shells that are superimposed along the line of sight. We have investigated the composite structure of the Eridanus side of the superbubble in the light of a new decomposition of the atomic and molecular gas. Methods. We used H I 21 cm and CO (J = 1−0) emission lines to separate coherent gas shells in space and velocity, and we studied their relation to the warm ionised gas probed in Hα emission, the hot plasma emitting X-rays, and the magnetic fields traced by dust polarised emission. We also constrained the relative distances to the clouds using dust reddening maps and X-ray absorption. We applied the Davis–Chandrasekhar–Fermi method to the dust polarisation data to estimate the plane-of-sky components of the magnetic field in several clouds and along the outer rim of the superbubble. Results. Our gas decomposition has revealed several shells inside the superbubble that span distances from about 150–250 pc. One of these shells forms a nearly complete ring filled with hot plasma. Other shells likely correspond to the layers of swept-up gas that is compressed behind the expanding outer shock wave. We used the gas and magnetic field data downstream of the shock to derive the shock expansion velocity, which is close to ~20 km s−1. Taking the X-ray absorption by the gas into account, we find that the hot plasma inside the superbubble is over-pressured compared to plasma in the Local Bubble. The plasma comprises a mix of hotter and cooler gas along the lines of sight, with temperatures of (3–9) and (0.3 − 1.2) × 106 K, respectively. The magnetic field along the western and southern rims and in the approaching wall of the superbubble appears to be shaped and compressed by the ongoing expansion. We find plane-of-sky magnetic field strengths from 3 to 15 μG along the rim.

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