Gyrosynchrotron Emission Research Articles

A combined multiwavelength VLA/ALMA/Chandra study unveils the complex magnetosphere of the B-type star HR5907

We present new radio/millimeter measurements of the hot magnetic star HR5907 obtained with the VLA and ALMA interferometers. We find that HR5907 is the most radio luminous early type star in the cm-mm band among those presently known. Its multi-wavelength radio light curves are strongly variable with an amplitude that increases with radio frequency. The radio emission can be explained by the populations of the non-thermal electrons accelerated in the current sheets on the outer border of the magnetosphere of this fast rotating magnetic star. We classify HR5907 as another member of the growing class of strongly magnetic fast rotating hot stars where the gyro-synchrotron emission mechanism efficiently operates in their magnetospheres. The new radio observations of HR5907 are combined with archival X-ray data to study the physical condition of its magnetosphere. The X-ray spectra of HR5907 show tentative evidence for the presence of non-thermal spectral component. We suggest that non-thermal X-rays originate a stellar X-ray aurora due to streams of non-thermal electrons impacting on the stellar surface. Taking advantage of the relation between the spectral indices of the X-ray power-law spectrum and the non-thermal electron energy distributions, we perform 3-D modeling of the radio emission for HR5907. The wavelength-dependent radio light-curves probe magnetospheric layers at different heights above the stellar surface. A detailed comparison between simulated and observed radio light-curves leads us to conclude that the stellar magnetic field of HR5907 is likely non-dipolar, providing further indirect evidence of the complex magnetic field topology of HR5907.

Estimation of a coronal mass ejection magnetic field strength using radio observations of gyrosynchrotron radiation

Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the low solar corona into interplanetary space. These eruptions are often associated with the acceleration of energetic electrons which produce various sources of high intensity plasma emission. In relatively rare cases, the energetic electrons may also produce gyrosynchrotron emission from within the CME itself, allowing for a diagnostic of the CME magnetic field strength. Such a magnetic field diagnostic is important for evaluating the total magnetic energy content of the CME, which is ultimately what drives the eruption. Here, we report on an unusually large source of gyrosynchrotron radiation in the form of a type IV radio burst associated with a CME occurring on 2014-September-01, observed using instrumentation from the Nançay Radio Astronomy Facility. A combination of spectral flux density measurements from the Nançay instruments and the Radio Solar Telescope Network (RSTN) from 300 MHz to 5 GHz reveals a gyrosynchrotron spectrum with a peak flux density at ~1 GHz. Using this radio analysis, a model for gyrosynchrotron radiation, a non-thermal electron density diagnostic using the Fermi Gamma Ray Burst Monitor (GBM) and images of the eruption from the GOES Soft X-ray Imager (SXI), we were able to calculate both the magnetic field strength and the properties of the X-ray and radio emitting energetic electrons within the CME. We find the radio emission is produced by non-thermal electrons of energies >1 MeV with a spectral index of δ ~ 3 in a CME magnetic field of 4.4 G at a height of 1.3 R⊙, while the X-ray emission is produced from a similar distribution of electrons but with much lower energies on the order of 10 keV. We conclude by comparing the electron distribution characteristics derived from both X-ray and radio and show how such an analysis can be used to define the plasma and bulk properties of a CME.

A Jansky VLA Survey of Magnetic Cataclysmic Variable Stars. I. The Data

Abstract The Jansky Very Large Array was used to observe 121 magnetic cataclysmic variables (MCVs). We report radio detections of 18 stars. Thirteen are new radio sources, increasing the number of MCVs that are radio sources by more than twofold, from 8 to 21. Most detections are at 8.7 GHz (X-band) with a lesser number at 5.4 and 21.1 GHz (C- and K-bands). With the exception of AE Aqr, whose flux density is typically >5 mJy, the flux densities are in the range of 24–780 μJy. Thirteen of the detections show highly circularly polarized emission, which is characteristic of electron-cyclotron maser emission. The data suggest that MCVs could possibly be divided into two classes of radio emitters: those dominated by weakly polarized gyrosynchrotron emission and those by highly polarized electron-cyclotron maser emission.

Flare SOL2012-07-06: On the Origin of the Circular Polarization Reversal Between 17 GHz and 34 GHz

The new generations of multiwavelength radioheliographs with high spatial resolution will employ microwave imaging spectropolarimetry to recover flare topology and plasma parameters in the flare sources and along the wave propagation paths. The recorded polarization depends on the emission mechanism and emission regime (optically thick or thin), the emitting particle properties, and propagation effects. Here, we report an unusual flare, SOL2012-07-06T01:37, whose optically thin gyrosynchrotron emission of the main source displays an apparently ordinary mode sense of polarization in contrast to the classical theory that favors the extraordinary mode. This flare produced copious nonthermal emission in hard X-rays and in high-frequency microwaves up to 80 GHz. It is found that the main flare source corresponds to an interaction site of two loops with greatly different sizes. We have investigated the three possible known reasons of the circular polarization sense reversal - mode coupling, positron contribution, and the effect of beamed angular distribution. We excluded polarization reversal due to contribution of positrons because there was no relevant response in the X-ray emission. We find that a beam-like electron distribution can produce the observed polarization behavior, but the source thermal density must be much higher than the estimate from to the X-ray data. We conclude that the apparent ordinary wave emission in the optically thin mode is due to radio wave propagation across the quasi-transverse (QT) layer. The abnormally high transition frequency (above 35 GHz) can be achieved reasonably low in the corona where the magnetic field value is high and transverse to the line of sight. This places the microwave source below this QT layer, i.e. very low in the corona.

LSPM J1314+1320: An Oversized Magnetic Star with Constraints on the Radio Emission Mechanism

Abstract LSPM J1314+1320 (=NLTT 33370) is a binary star system consisting of two nearly identical pre-main-sequence stars of spectral type M7. The system is remarkable among ultracool dwarfs for being the most luminous radio emitter over the widest frequency range. Masses and luminosities are at first sight consistent with the system being coeval at age ∼80 Myr according to standard (nonmagnetic) evolutionary models. However, these models predict an average effective temperature of ∼2950 K, which is 180 K hotter than the empirical value. Thus, the empirical radii are oversized relative to the standard models by ≈13%. We demonstrate that magnetic stellar models can quantitatively account for the oversizing. As a check on our models, we note that the radio emission limits the surface magnetic field strengths: the limits depend on identifying the radio emission mechanism. We find that the field strengths required by our magnetic models are too strong to be consistent with gyrosynchrotron emission but are consistent with electron cyclotron maser emission.

Double-coronal X-Ray and Microwave Sources Associated with a Magnetic Breakout Solar Eruption

Abstract Double-coronal hard X-ray (HXR) sources are believed to be critical observational evidence of bi-directional energy release through magnetic reconnection in large-scale current sheets in solar flares. Here, we present a study on double-coronal sources observed in both HXR and microwave regimes, revealing new characteristics distinct from earlier reports. This event is associated with a footpoint-occulted X1.3-class flare (2014 April 25, starting at 00:17 UT) and a coronal mass ejection that were likely triggered by the magnetic breakout process, with the lower source extending upward from the top of the partially occulted flare loops and the upper source co-incident with rapidly squeezing-in side lobes (at a speed of ∼250 km s−1 on both sides). The upper source can be identified at energies as high as 70–100 keV. The X-ray upper source is characterized by flux curves that differ from those of the lower source, a weak energy dependence of projected centroid altitude above 20 keV, a shorter duration, and an HXR photon spectrum slightly harder than those of the lower source. In addition, the microwave emission at 34 GHz also exhibits a similar double-source structure and the microwave spectra at both sources are in line with gyrosynchrotron emission given by non-thermal energetic electrons. These observations, especially the co-incidence of the very-fast squeezing-in motion of side lobes and the upper source, indicate that the upper source is associated with (and possibly caused by) this fast motion of arcades. This sheds new light on the origin of the corona double-source structure observed in both HXRs and microwaves.

Young, active radio stars in the AB Doradus moving group

Context. Precise determination of stellar masses is necessary to test the validity of pre-main-sequence (PMS) stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 Msun. To improve such a test, and based on our previous studies, we selected the AB Doradus moving group (AB Dor-MG) as the best-suited association on which to apply radio-based high-precision astrometric techniques to study binary systems. Aims. We seek to determine precise estimates of the masses of a set of stars belonging to the AB Dor-MG using radio and infrared observations. Methods. We observed in phase-reference mode with the Very Large Array (VLA) at 5 GHz and with the European VLBI Network (EVN) at 8.4 GHz the stars HD 160934, EK Dra, PW And, and LO Peg. We also observed some of these stars with the near-infrared CCD AstraLux camera at the Calar Alto observatory to complement the radio observations. Results. We determine model-independent dynamical masses of both components of the star HD 160934, A and c, which are 0.70+/-0.07 Msun and 0.45+/-0.04 Msun , respectively. We revised the orbital parameters of EK Dra and we determine a sum of the masses of the system of 1.38+/-0.08 Msun. We also explored the binarity of the stars LO Peg and PW And. Conclusions. We found observational evidence that PMS evolutionary models underpredict the mass of PMS stars by 10%-40%, as previously reported by other authors. We also inferred that the origin of the radio emission must be similar in all observed stars, that is, extreme magnetic activity of the stellar corona that triggers gyrosynchrotron emission from non-thermal, accelerated electrons.

JVLA Observations of Young Brown Dwarfs

Abstract We present sensitive 3.0 cm JVLA radio continuum observations of six regions of low-mass star formation that include twelve young brown dwarfs (BDs) and four young BD candidates. We detect a total of 49 compact radio sources in the fields observed, of which 24 have no reported counterparts and are considered new detections. Twelve of the radio sources show variability in timescales of weeks to months, suggesting gyrosynchrotron emission produced in active magnetospheres. Only one of the target BDs, FU Tau A, was detected. However, we detected radio emission associated with two of the BD candidates, WL 20S and CHLT 2. The radio flux densities of the sources associated with these BD candidates are more than an order of magnitude larger than expected for a BD and suggest a revision of their classification. In contrast, FU Tau A falls on the well-known correlation between radio luminosity and bolometric luminosity, suggesting that the emission comes from a thermal jet and that this BD seems to be forming as a scaled-down version of low-mass stars.

Forward Modeling of EUV and Gyrosynchrotron Emission from Coronal Plasmas with FoMo

The FoMo code was developed to calculate the EUV emission from optically thin coronal plasmas. The input data for FoMo consists of the coronal density, temperature and velocity on a 3D grid. This is translated to emissivity on the 3D grid, using CHIANTI data. Then, the emissivity is integrated along the line-of-sight to calculate the emergent spectral line that could be observed by a spectrometer. Moreover, the code has been extended to model also the radio emission from plasmas with a population of non-thermal particles. In this case, also optically thick plasmas may be modelled. The radio spectrum is calculated over a large wavelength range, allowing for the comparison with data from a wide range of radio telescopes.

Radio detections of southern ultracool dwarfs

We report the results of a volume-limited survey using the Australia Telescope Compact Array to search for transient and quiescent radio emission from 15 southern hemisphere ultracool dwarfs. We detect radio emission from 2MASSW J0004348-404405 increasing the number of radio loud ultracool dwarfs to 22. We also observe radio emission from 2MASS J10481463-3956062 and 2MASSI J0339352-352544, two sources with previous radio detections. The radio emission from the three detected sources shows no variability or flare emission. Modelling this quiescent emission we find that it is consistent with optically thin gyrosynchrotron emission from a magnetosphere with an emitting region radius of (1 - 2)$R_*$, magnetic field inclination 20$^{\circ}$ - 80$^{\circ}$, field strength $\sim$10 - 200 G, and power-law electron density $\sim$10$^4$ - 10$^8$ cm$^{-3}$. Additionally, we place upper limits on four ultracool dwarfs with no previous radio observations. This increases the number of ultracool dwarfs studied at radio frequencies to 222. Analysing general trends of the radio emission for this sample of 15 sources, we find that the radio activity increases for later spectral types and more rapidly rotating objects. Furthermore, comparing the ratio of the radio to X-ray luminosities for these sources, we find 2MASS J10481463-3956062 and 2MASSI J0339352-352544 violate the Guedel-Benz relation by more than two orders of magnitude.

Modeling the distribution of circular polarization degree of microwave emission along the flare loops in event July 19, 2012

The distribution of the degree of circular polarization of microwave emission along the solar flare loops during the event on July 19, 2012, was analyzed with the use of data from the Nobeyama Radioheliograph (NoRH). It was found that the polarization degree in the loop footpoints remains negative (R–L 0 ) and also does not change over time. In the northern and southern legs of the loop, the polarization degree sign changes over time. The proposed explanation of this effect is based on modeling the kinetics of nonthermal electrons and the calculation of their gyrosynchrotron (GS) emission for the model loop located on the solar limb. It was shown that the observed change in the polarization sign in the event on July 19, 2012, can be caused by the presence of longitudinal anisotropy of energy electrons at the upper part of the flare loop.

RADIO MONITORING OF THE PERIODICALLY VARIABLE IR SOURCE LRLL 54361: NO DIRECT CORRELATION BETWEEN THE RADIO AND IR EMISSIONS

LRLL 54361 is an infrared source located in the star forming region IC 348 SW. Remarkably, its infrared luminosity increases by a factor of 10 during roughly one week every 25.34 days. To understand the origin of these remarkable periodic variations, we obtained sensitive 3.3 cm JVLA radio continuum observations of LRLL 54361 and its surroundings in six different epochs: three of them during the IR-on state and three during the IR-off state. The radio source associated with LRLL 54361 remained steady and did not show a correlation with the IR variations. We suggest that the IR is tracing the results of fast (with a timescale of days) pulsed accretion from an unseen binary companion, while the radio traces an ionized outflow with an extent of $\sim$100 AU that smooths out the variability over a period of order a year. The average flux density measured in these 2014 observations, 27$\pm$5 $\mu$Jy, is about a factor of two less than that measured about 1.5 years before, $53\pm$11 $\mu$Jy, suggesting that variability in the radio is present, but over larger timescales than in the IR. We discuss other sources in the field, in particular two infrared/X-ray stars that show rapidly varying gyrosynchrotron emission.

RADIO ASTROMETRY OF THE CLOSE ACTIVE BINARY HR 5110

The close active binary HR 5110 was observed at six epochs over 26 days using a global very long baseline interferometry array at 15.4 GHz. We used phase referencing to determine the position of the radio centroid at each epoch with an uncertainty significantly smaller than the component separation. After correcting for proper motion and parallax, we find that the centroid locations of all six epochs have barycenter separations consistent with an emission source located on the KIV secondary, and not in an interaction region between the stars or on the F primary. We used a homogeneous power-law gyrosynchrotron emission model to reproduce the observed flux densities and fractional circular polarization. The resulting ranges of mean magnetic field strength and relativistic electron densities are of the order of 10 G and 105 cm−3, respectively, in the source region.

SHORT- AND LONG-TERM RADIO VARIABILITY OF YOUNG STARS IN THE ORION NEBULA CLUSTER AND MOLECULAR CLOUD

ABSTRACT We have used the Karl G. Jansky Very Large Array (VLA) to carry out multi-epoch radio continuum monitoring of the Orion Nebula Cluster (ONC) and the background Orion Molecular Cloud (OMC; 3 epochs at Q band and 11 epochs at Ka band). Our new observations reveal the presence of 19 radio sources, mainly concentrated in the Trapezium Cluster and the Orion Hot Core (OHC) regions. With the exception of the Becklin–Neugebauer object and source C (which we identify here as dust emission associated with a proplyd) the sources all show radio variability between the different epochs. We have found tentative evidence of variability in the emission from the massive object related to source I. Our observations also confirm radio flux density variations of a factor >2 on timescales of hours to days in five sources. One of these flaring sources, OHC-E, has been detected for the first time. We conclude that the radio emission can be attributed to two different components: (i) highly variable (flaring) non-thermal radio gyrosynchrotron emission produced by electrons accelerated in the magnetospheres of pre-main-sequence low-mass stars and (ii) thermal emission due to free–free radiation from ionized gas and/or heated dust around embedded massive objects and proplyds. Combining our sample with other radio monitoring at 8.3 GHz and the X-ray catalog provided by Chandra, we have studied the properties of the entire sample of radio/X-ray stars in the ONC/OMC region (51 sources). We have found several hints of a relation between the X-ray activity and the mechanisms responsible for (at least some fraction of) the radio emission. We have estimated a radio flaring rate of ∼0.14 flares day−1 in the dense stellar cluster embedded in the OHC region. This suggests that radio flares are more common events during the first stages of stellar evolution than previously thought. The advent of improved sensitivity with the new VLA and ALMA will dramatically increase the number of stars in young clusters detected at radio wavelengths, which will help us improve our understanding of the origin and nature of the radio emission.

WIDEBAND DYNAMIC RADIO SPECTRA OF TWO ULTRA-COOL DWARFS

A number of radio-loud ultra cool dwarf stars (UCD) exhibit both continuous broadband and highly polarized pulsed radio emission. In order to determine the nature of the emission and the physical characteristics in the source region, we have made multi-epoch, wideband spectral observations of TVLM 0513-46 and 2M 0746+20. We combine these observations with archival radio data to fully characterize both the temporal and spectral properties of the radio emission. The continuum spectral energy distribution can be well modeled using gyrosynchrotron emission from mildly relativistic electrons in a dipolar field. The pulsed emission exhibits a variety of time-variable characteristics, including frequency drifts, frequency cutoffs, and multiple pulses per period. For 2M 0746+20 we determine a pulse period consistent with previously determined values. We modeled locations of pulsed emission using an oblique rotating magnetospheric model with beamed electron cyclotron maser (ECM) sources. The best-fit models have narrow ECM beaming angles aligned with the local source magnetic field direction, except for one isolated burst from 2M 0746+20. For TVLM 0513-46, the best-fit rotation axis inclination is nearly orthogonal to the line of sight. For 2M 0746+20 we found a good fit using a fixed inclination i=36 deg, determined from optical observations. For both stars the ECM sources are located near feet of magnetic loops with radial extents 1.2Rs-2.7 Rs and surface fields 2.2 - 2.5 kG. These results support recent suggestions that radio over-luminous UCDs have a global `weak field' non-axisymmetric magnetic topologies.

Perturbations of gyrosynchrotron emission polarization from solar flares by sausage modes: forward modeling

We examined the polarization of the microwave flaring emission and its modulation by the fast sausage standing wave using a linear 3D magnetohydrodynamic model of a plasma cylinder. We analyzed the effects of the line-of-sight angle on the perturbations of the gyrosynchrotron intensity for two models: a base model with strong Razin suppression and a low-density model in which the Razin effect was negligible. The circular polarization (Stokes V) oscillation is in phase with the intensity oscillation, and the polarization degree (Stokes V/I) oscillates in phase with the magnetic field at the examined frequencies in both models. The two quantities experience a periodical reversal of their signs with a period equal to half of the sausage wave period when seen at a 90 ◦ viewing angle, in this case, their modulation depth reaches 100%.

THREE-DIMENSIONAL RADIO AND X-RAY MODELING AND DATA ANALYSIS SOFTWARE: REVEALING FLARE COMPLEXITY

We have undertaken a major enhancement of our IDL-based simulation tools developed earlier for modeling microwave and X-ray emission. The object-based architecture provides an interactive graphical user interface that allows the user to import photospheric magnetic field maps and perform magnetic field extrapolations to almost instantly generate 3D magnetic field models, to investigate the magnetic topology of these models by interactively creating magnetic field lines and associated magnetic flux tubes, to populate the flux tubes with user-defined nonuniform thermal plasma and anisotropic, nonuniform, nonthermal electron distributions; to investigate the spatial and spectral properties of radio and X-ray emission calculated from the model, and to compare the model-derived images and spectra with observational data. The application integrates shared-object libraries containing fast gyrosynchrotron emission codes developed in FORTRAN and C++, soft and hard X-ray codes developed in IDL, a FORTRAN-based potential-field extrapolation routine and an IDL-based linear force free field extrapolation routine. The interactive interface allows users to add any user-defined radiation code that adheres to our interface standards, as well as user-defined magnetic field extrapolation routines. Here we use this tool to analyze a simple single-loop flare and use the model to constrain the 3D structure of the magnetic flaring loop and 3D spatial distribution of the fast electrons inside this loop. We iteratively compute multi-frequency microwave and multi-energy X-ray images from realistic magnetic fluxtubes obtained from an extrapolation of a magnetogram taken prior to the flare, and compare them with imaging data obtained by SDO, NoRH, and RHESSI instruments. We use this event to illustrate use of the tool for general interpretation of solar flares to address disparate problems in solar physics.

Intersunspot Microwave Sources

We studied a number of solar active regions using two-dimensional spatially resolved microwave observations. Data from the Nobeyama Radioheliograph and the Siberian Solar Radio Telescope together with observations by the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric Observatory (SOHO) have allowed us to identify long-lived intersunspot sources (ISSs) in most of the investigated active regions. Their centers are often located above the line-of-sight magnetic field inversion line that separates the leading and following polarities of a full active region (first type of ISS) or above the inversion line that separates magnetic polarities inside of a complex of sunspots (second type of ISS). ISSs of the first type are extended and, in general, they are sources of bremsstrahlung emission. ISSs of the second type are compact and are, most likely, sources of gyroresonance or gyrosynchrotron emission. We propose a qualitative model involving three types of magnetic connectivity to explain how long-lasting ISSs may be generated.

JVLA OBSERVATIONS OF IC 348 SW: COMPACT RADIO SOURCES AND THEIR NATURE

We present sensitive 2.1 and 3.3 cm JVLA radio continuum observations of the region IC 348 SW. We detect a total of 10 compact radio sources in the region, of which seven are first reported here. One of the sources is associated with the remarkable periodic time-variable infrared source LRLL 54361, opening the possibility of monitoring this object at radio wavelengths. Four of the sources appear to be powering outflows in the region, including HH 211 and HH 797. In the case of the rotating outflow HH 797 we detect at its center a double radio source, separated by $\sim3"$. Two of the sources are associated with infrared stars that possibly have gyrosynchrotron emission produced in active magnetospheres. Finally, three of the sources are interpreted as background objects.

FORWARD MODELING OF GYROSYNCHROTRON INTENSITY PERTURBATIONS BY SAUSAGE MODES

To determine the observable radio signatures of the fast sausage standing wave, we examine gyrosynchrotron (GS) emission modulation using a linear three-dimensional magnetohydrodynamic model of a plasma cylinder. Effects of the line-of-sight angle and instrumental resolution on perturbations of the GS intensity are analyzed for two models: a base model with strong Razin suppression and a low-density model in which the Razin effect was unimportant. Our finding contradicts previous predictions made with simpler models: an in-phase variation of intensity between low (f f {sub peak}) frequencies is found for the low-density model and an anti-phase variation for the base model in the case of a viewing angle of 45°. The spatially inhomogeneous character of the oscillating emission source and the spatial resolution of the model are found to have a significant effect on the resulting intensity.