Marita Krause

Revised equipartition and minimum energy formula for magnetic field strength estimates from radio synchrotron observations

The commonly used classical equipartition or minimum-energy estimate of total magnetic fields strengths from radio synchrotron intensities is of limited practical use because it is based on the hardly known ratio K of the total energies of cosmic ray protons and electrons and also has inherent problems. We present a revised formula, using the number density ratio K for which we give estimates. For particle acceleration in strong shocks K is about 40 and increases with decreasing shock strength. Our revised estimate for the field strength gives larger values than the classical estimate for flat radio spectra with spectral indices of about 0.5–0.6, but smaller values for steep spectra and total fields stronger than about 10 µG. In very young supernova remnants, for example, the classical estimate may be too large by up to 10×. On the other hand, if energy losses of cosmic ray electrons are important, K increases with particle energy and the equipartition field may be underestimated significantly. Our revised larger equipartition estimates in galaxy clusters and radio lobes are consistent with independent estimates from Faraday rotation measures, while estimates from the ratio between radio synchrotron and X-ray inverse Compton intensities generally give much weaker fields. This may be explained e.g. by a concentration of the field in filaments. Our revised field strengths may also lead to major revisions of electron lifetimes in jets and radio lobes estimated from the synchrotron break frequency in the radio spectrum. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Evolution of magnetic fields in galaxies and future observational tests with the Square Kilometre Array

Aims. In the context of models of galaxy formation and evolution, we investigate the cosmological evolution of large- and small-scale magnetic fields inside galaxies. Methods. We use the dynamo theory to derive the timescales of amplification and ordering of magnetic fields in disk and puffy galaxies. Turbulence in protogalactic halos generated by thermal virialization can drive an efficient turbulent dynamo. Results from simulations of hierarchical structure formation cosmology provide a tool to develop an evolutionary model of regular magnetic fields coupled with galaxy formation and evolution. Results. The turbulent (small-scale) dynamo was able to amplify a weak seed magnetic field in halos of protogalaxies to a few μG strength within a few 10 8 yr. This turbulent field served as a seed to the mean-field (large-scale) dynamo. Galaxies similar to the Milky Way formed their disks at z ≈ 10 and regular fields of μG strength and a few kpc coherence length were generated within 2 Gyr (at z ≈ 3), but field-ordering on the coherence scale of the galaxy size required an additional 6 Gyr (at z ≈ 0.5). Giant galaxies formed their disks at z ≈ 10, allowing more efficient dynamo generation of strong regular fields (with kpc coherence length) already at z ≈ 4. However, the age of the Universe is short for fully coherent fields in giant galaxies larger than 15 kpc to have been achieved. Dwarf galaxies should have hosted fully coherent fields at z ≈ 1. After a major merger, the strength of the turbulent field is enhanced by a factor of a few. Conclusions. This evolutionary scenario can be tested by measurements of polarized synchrotron emission and Faraday rotation with the planned Square Kilometre Array (SKA). We predict an anticorrelation between galaxy size and ratio between ordering scale and galaxy size. Weak regular fields (small Faraday rotation) in galaxies at z < 3 are signatures of major mergers. Undisturbed dwarf galaxies should host fully coherent fields, giving rise to strong Faraday rotation signals. Radio observations may serve as a clock for measuring the time since the last major merger.

Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 - I. The distribution and transport of cosmic rays

Context. Nearby edge-on galaxies showing a synchrotron halo are nearly ideal objects for studying the transport of cosmic rays (C Rs) in galaxies. Among them, the nearby starburst galaxy NGC 253 hosts a galactic wind indicated by various ISM phases in its halo. Aims. The diffusive and convective CR transport from the disk into the halo is investigated using the local CR bulk speed. The connection between the CR transport and the galactic wind is outlined. Methods. We observed NGC 253 with the VLA at�6.2 cm in a mosaic with 15 pointings. The missing zero-spacing fl ux density of the VLA mosaic was filled in using observations with the 100-m Effelsberg telescope. We also obtained a new �3.6 cm map from Effelsberg observations and reproduced VLA maps at �20 cm and �90 cm. The high dynamic range needed due to the strong nuclear point-like source was addressed with a special data calibration scheme for both the single-dish and the interferometric observations. Results. We find a thin and a thick radio disk with exponential scalehei ghts of 0.3 kpc and 1.7 kpc at �6.2 cm. The equipartition total magnetic field strength between 7 µG and 18µG in the disk is remarkably high. We use the spectral aging of the cosmic ray electrons (CREs) seen in the vertical profiles of the spectral index to d etermine a lower limit for the global CR bulk speed as 170± 70 km s −1 . The linear correlation between the scaleheights and the CRE lifetimes, as evident from the dumbbell shaped halo, requires a vertical CR transport with a bulk speed of 300± 30 km s −1 in the northeastern halo, similar to the escape velocity of 2 80 km s −1 . This shows the presence of a “disk wind” in NGC 253. In the southwestern halo, the transport is mainly diffusive with a diffusion coeffi cient of 2.0± 0.2× 10 29 cm 2 s −1 . Conclusions. Measuring the radio synchrotron scaleheight and estimating the CRE lifetime allow us to determine the bulk speed of the CR transport into the halo. The transport is convective and more effi cient in the northeastern halo, while it is diffusive in the southwestern halo. The luminous material is transported by the disk wind, which can explain the different amounts of extra-planar H I, H�, and soft X-ray emission in the two halo parts. Future low-frequency radio observations will provide the data to analyze the vertical velocity profile of galactic winds.

High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission

Context. Constraints on the origin and propagation of cosmic rays can be achieved by studying the variation in the spectral index of the synchrotron emission across external galaxies. Aims. We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M 33 at a linear resolution of 360 pc. Methods. We separated the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 µm and a standard dust model, we dereddened the Hα emission. The extinction corrected Hα emission serves as a template for the thermal free-free radio emission. Subtracting this free-free emission from the observed 3.6 cm and 20 cm emission (Effelsberg and the VLA), we obtained the nonthermal maps. A constant electron temperature used to obtain the thermal radio intensity seems appropriate for M 33, which, unlike the Milky Way, has a shallow metallicity gradient. Results. For the first time, we derive the distribution of the nonthermal spectral index across a galaxy, M 33. We detect strong nonthermal emission from the spiral arms and starforming regions. Wavelet analysis shows that at 3.6 cm the nonthermal emission is dominated by contributions from starforming regions, while it is smoothly distributed at 20 cm. For the whole galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm, respectively. The thermal emission is slightly stronger in the southern than in the northern half of the galaxy. We find a clear radial gradient of mean extinction in the galactic plane. Conclusions. The nonthermal spectral index map indicates that the relativistic electrons suffer energy loss when diffusing from their origin in starforming regions towards interarm regions and the outer parts of the galaxy. We also conclude that the radio emission is mostly nonthermal at R > 5 kpc in M 33.

Correlated optical, X-ray, and γ-ray flaring activity seen with INTEGRAL during the 2015 outburst of V404 Cygni

After 25 years of quiescence, the microquasar V404 Cyg entered a new period of activity in June 2015. This X-ray source is known to undergo extremely bright and variable outbursts seen at all wavelengths. It is therefore an object of prime interest to understand the accretion-ejection connections. These can, however, only be probed through simultaneous observations at several wavelengths. We made use of the INTEGRAL instruments to obtain long, almost uninterrupted observations from 2015 June 20, 15:50 UTC to June 25, 4:05 UTC, from the optical V band up to the soft γ-rays. V404 Cyg was extremely variable in all bands, with the detection of 18 flares with fluxes exceeding 6 Crab (20–40 keV) within three days. The flare recurrence can be as short as ~20 min from peak to peak. A model-independent analysis shows that the >6 Crab flares have a hard spectrum. A simple 10–400 keV spectral analysis of the off-flare and flare periods shows that the variation in intensity is likely to be only due to variations of a cut-off power-law component. The optical flares seem to be at least of two different types: one occurring in simultaneity with the X-ray flares, the other showing a delay greater than 10 min. The former could be associated with X-ray reprocessing by either an accretion disk or the companion star. We suggest that the latter are associated with plasma ejections that have also been seen in radio.

THE INTEGRATED POLARIZATION OF SPIRAL GALAXY DISKS

We present integrated polarization properties of nearby spiral galaxies at 4.8 GHz, and models for the integrated polarization of spiral galaxy disks as a function of inclination. Spiral galaxies in our sample have observed integrated fractional polarization in the range 1%-17.6%. At inclinations less than 50°, the fractional polarization depends mostly on the ratio of random to regular magnetic field strength. At higher inclinations, Faraday depolarization associated with the regular magnetic field becomes more important. The observed degree of polarization is lower ( 2 × 1021 W Hz–1. The polarization angle of the integrated emission is aligned with the apparent minor axis of the disk for galaxies without a bar. In our axially symmetric models, the polarization angle of the integrated emission is independent of wavelength. Simulated distributions of fractional polarization for randomly oriented spiral galaxies at 4.8 GHz and 1.4 GHz are presented. We conclude that polarization measurements, e.g., with the Square Kilometre Array, of unresolved spiral galaxies allow statistical studies of the magnetic field in disk galaxies using large samples in the local universe and at high redshift. As these galaxies behave as idealized background sources without internal Faraday rotation, they can be used to detect large-scale magnetic fields in the intergalactic medium.

Cosmic rays and the magnetic field in the nearby starburst galaxy NGC 253 III. Helical magnetic fields in the nuclear outflow

Context. Magnetic fields are good tracers of gas compression by shock w aves in the interstellar medium. These can be caused by the interaction of star-formation driven outflows from individ ual star formation sites as described in the chimney model. Integration along the line-of-sight and cosmic-ray diffusion may hamper detection of compressed magnetic fields in m any cases. Aims. We study the magnetic field structure in the central part of th e nuclear starburst galaxy NGC 253 with spatial resolutions between 40 and 150 pc to detect any filamentary emission assoc iated with the nuclear outflow. As the nuclear region is much b righter than the rest of the disc we can distinguish this emission fro m that of the disc. Methods. We used radio polarimetric observations with the VLA. New observations at �3 cm with 7. ′′ 5 resolution were combined with archive data at �� 20 and 6 cm. We created a map of the rotation measure distribution between �� 6 and 3 cm and compared it with a synthetic polarization map. Results. We find filamentary radio continuum emission in a geometrical distribution, which we interpret as the boundary of the NW nuclear outflow cone seen in projection. The scaleheight o f the continuum emission is 150± 20 pc, regardless of the observing frequency. The equipartition magnetic field strength is 46 ± 10µG for the total field and 21± 5µG for the regular field in the filaments. We find that the ordered magnetic field is aligned along the fila ments, in agreement with amplification due to compression. T he perpendicular diffusion coeffi cient across the filaments is �⊥ = 1.5× 10 28 cm 2 s −1 · E(GeV) 0.5±0.7 . In the SE part of the nuclear outflow cone the magnetic field is pointing away from the disc in form o f a helix, with an azimuthal component increasing up to at least 1200 pc height, where it is about equal to the total component. The ordered magnetic field in the disc is anisotropic within a radius of 2.2 kpc. At larger radii, the large-scale field is regular and of even parity. Conclusions. The magnetic filaments indicate an interaction of the nuclea r outflow with the interstellar medium. The magnetic field is able to collimate the outflow, which can explain the observ ed small opening angle of≈ 26 ◦ . Owing to the conservation of angular momentum by the plasma in the nuclear outflow, the field lines a re frozen into the plasma, and they wind up into a helix. Strong adiabatic losses of the cosmic-ray electrons in the acceler ated outflow can partly explain why the radio luminosity of th e nucleus lies below the radio-FIR correlation.

A detailed study of the radio-FIR correlation in NGC 6946 with Herschel-PACS/SPIRE from KINGFISH

We derive the distribution of the synchrotron spectral index across NGC 6946 and investigate the correlation between the radio continuum (synchrotron) and far-infrared (FIR) emission using the KINGFISH Herschel-PACS and SPIRE data. The radio-FIR correlation is studied as a function of star formation rate, magnetic field strength, radiation field strength, and the total gas surface density. The synchrotron emission follows both star-forming regions and the so-called magnetic arms present in the inter-arm regions. The synchrotron spectral index is steepest along the magnetic arms (α_n ~ 1), while it is flat in places of giant Hii regions and in the center of the galaxy (α_n ~ 0.6−0.7). The map of α_n provides observational evidence for aging and energy loss of cosmic ray electrons (CREs) propagating in the disk of the galaxy. Variations in the synchrotron-FIR correlation across the galaxy are shown to be a function of both star formation and magnetic field strength. We find that the synchrotron emission correlates better with cold rather than with warm dust emission, when the diffuse interstellar radiation field is the main heating source of dust. The synchrotron-FIR correlation suggests a coupling between the magnetic field and the gas density. NGC 6946 shows a power-law behavior between the total (turbulent) magnetic field strength B and the star formation rate surface density Σ_(SFR) with an index of 0.14 (0.16) ± 0.01. This indicates an efficient production of the turbulent magnetic field with the increasing gas turbulence expected in actively star forming regions. Moreover, it is suggested that the B-Σ_(SFR) power law index is similar for the turbulent and the total fields in normal galaxies. On the other hand, for galaxies interacting with the cluster environment this index is steeper for turbulent magnetic fields than it is for the total magnetic fields. The scale-by-scale analysis of the synchrotron-FIR correlation indicates that the ISM affects the propagation of old/diffused CREs, resulting in a diffusion coefficient of D_0 = 4.6 × 10^(28) cm^2 s^(-1) for 2.2 GeV CREs.

M 82 – A radio continuum and polarisation study - I. Data reduction and cosmic ray propagation

The potential role of magnetic fields and cosmic ray propagation for feedback processes in the early Universe can be probed by studies of local starburst counterparts with an equivalent star-formation rate. Archival data from the WSRT was reduced and a new calibration technique introduced to reach the high dynamic ranges needed for the complex source morphology of M82. This data was combined with archival VLA data, yielding total power maps at 3cm, 6cm, 22cm and 92cm. The data shows a confinement of the emission at wavelengths of 3/6cm to the core region and a largely extended halo reaching up to 4kpc away from the galaxy midplane at wavelengths of 22/92cm up to a sensitivity limit of 90muJy and 1.8mJy respectively. The results are used to calculate the magnetic field strength in the core region to 98muG and to 24muG in the halo regions. From the observation of free-free losses the filling factor of the ionised medium could be estimated to 2%. We find that the radio emission from the core region is dominated by very dense HII-regions and supernova remnants, while the surrounding medium is filled with hot X-ray and neutral gas. Cosmic rays radiating at frequencies higher than 1.4 GHz are suffering from high synchrotron and inverse Compton losses in the core region and are not able to reach the halo. Even the cosmic rays radiating at longer wavelengths are only able to build up the observed kpc sized halo, when several starbursting periods are assumed where the photon field density varies by an order of magnitude. These findings together with the strong correlation between Halpha, PAH+, and our radio continuum data suggests a magnetic field which is frozen into the ionised medium and driven out of the galaxy kinematically.

High-resolution radio continuum survey of M 33 III. Magnetic fields

Aims. We study the magnetic field structure, strength, and energy density in the Scd galaxy M 33. Methods. Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across M 33. We fit a 3D model of the regular magnetic field to the observed azimuthal distribution of polarization angles. We also analyze the spatial variation of depolarization across the galaxy. Results. Faraday rotation, measured between 3.6 and 6.2 cm at an angular resolution of 3 � (0.7 kpc), shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M 33 at 3.6 cm is polarized. High degrees of polarization of the synchrotron emission (>20%) and strong regular magnetic fields in the sky plane (� 6.6 μG) exist in-between two northern spiral arms. We estimate the average total and regular magnetic field strengths in M 33 as � 6.4 and 2.5 μG, respectively. Under the assumption that the disk of M 33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m = 0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m = 0 + 1) modes from 3 to 5 kpc radius. Conclusions. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m = 0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M 33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M 33’s rotation curve. Generation of interstellar magnetic fields from turbulent gas motion in M 33 is indicated by the equipartition of turbulent and magnetic energy densities.