Aims. We exploit synchrotron radiation to constrain the low-energy interstellar electron spectrum, using various radio surveys and connecting with electron data from Fermi-LAT and other experiments. ...Methods. The GALPROP programme for cosmic-ray propagation, gamma-ray and synchrotron radiation is used. Secondary electrons and positrons are included. Propagation models based on cosmic-ray and gamma-ray data are tested against synchrotron data from 22 MHz to 94 GHz. Results. The synchrotron data confirm the need for a low-energy break in the cosmic-ray electron injection spectrum. The interstellar spectrum below a few GeV has to be lower than standard models predict, and this suggests less solar modulation than usually assumed. Reacceleration models are more difficult to reconcile with the synchrotron constraints. We show that secondary leptons are important for the interpretation of synchrotron emission. We also consider a cosmic-ray propagation origin for the low-energy break. Conclusions. Exploiting the complementary information on cosmic rays and synchrotron gives unique and essential constraints on electrons, and has implications for gamma rays. This connection is especially relevant now in view of the ongoing Planck and Fermi missions.
The Correct Sense of Faraday Rotation Ferrière, K; West, J L; Jaffe, T R
Monthly notices of the Royal Astronomical Society,
11/2021, Letnik:
507, Številka:
4
Journal Article
Recenzirano
Odprti dostop
The phenomenon of Faraday rotation of linearly polarized synchrotron emission in a magneto-ionized medium has been understood and studied for decades. But since the sense of the rotation itself is ...irrelevant in most contexts, some uncertainty and inconsistencies have arisen in the literature about this detail. Here, we start from basic plasma theory to describe the propagation of polarized emission from a background radio source through a magnetized, ionized medium in order to rederive the correct sense of Faraday rotation. We present simple graphics to illustrate the decomposition of a linearly polarized wave into right and left circularly polarized modes, the temporal and spatial propagation of the phases of those modes, and the resulting physical rotation of the polarization orientation. We then re-examine the case of a medium that both Faraday-rotates and emits polarized radiation and show how a helical magnetic field can construct or destruct the Faraday rotation. This paper aims to resolve a source of confusion that has arisen between the plasma physics and radio astronomy communities and to help avoid common pitfalls when working with this unintuitive phenomenon.
We extend previous work modelling the Galactic magnetic field in the plane using synchrotron emission in total and polarized intensity. In this work, we include a more realistic treatment of the ...cosmic ray electrons using the galprop propagation code optimized to match the existing high-energy data. This addition reduces the degeneracies in our previous analysis and when combined with an additional observed synchrotron frequency allows us to study the low-energy end of the cosmic ray electron spectrum in a way that has not previously been done. For a pure diffusion propagation, we find a low-energy injection spectrum slightly harder than generally assumed: for J(E) ∝E
α, we find α=−1.34 ± 0.12, implying a very sharp break with the spectrum above a few GeV. This then predicts a synchrotron brightness temperature spectral index, β, on the Galactic plane, that is −2.8 < β < −2.74 below a few GHz and −2.98 < β < −2.91 up to 23 GHz. We find that models including cosmic ray re-acceleration processes appear to be incompatible with the synchrotron data.
Context.
Cosmic rays (CRs) and the Galactic magnetic field (GMF) are fundamental actors in many processes in the Milky Way. The observed interaction product of these actors is Galactic synchrotron ...emission integrated over the line of sight (LOS). A comparison to simulations can be made with this tracer using existing GMF models and CR density models. This probes the GMF strength and morphology and the CR density.
Aims.
Our aim is to provide insight into the Galactic CR density and the distribution and morphology of the GMF strength by exploring and explaining the differences between the simulations and observations of synchrotron intensity.
Methods.
At low radio frequencies HII regions become opaque due to free–free absorption. Using these HII regions we can measure the synchrotron intensity over a part of the LOS through the Galaxy. The measured intensity per unit path length, that is, the emissivity, for HII regions at different distances, allows us to probe the variation in synchrotron emission not only across the sky but also in the third dimension of distance. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using a number of existing GMF models in conjunction with the Galactic CR modeling code GALPROP, we can simulate these synchrotron emissivities.
Results.
We present an updated catalog, compiled from the literature, of low-frequency absorption measurements of HII regions, their distances, and electron temperatures. We report a simulated emissivity that shows a compatible trend for HII regions that are near the observer. However, we observe a systematically increasing synchrotron emissivity for HII regions that are far from the observer, which is not compatible with the values simulated by the GMF models and GALPROP.
Conclusions.
Current GMF models plus a GALPROP generated CR density model cannot explain low-frequency absorption measurements. One possibility is that distances to all HII regions catalogued at the kinematic “far” distance are erroneously determined, although this is unlikely since it ignores all evidence for far distances in the literature. However, a detection bias due to the nature of this tracer requires us to keep in mind that certain sources may be missed in an observation. The other possibilities are an enhanced emissivity in the outer Galaxy or a diminished emissivity in the inner Galaxy.
Motivated by the large-scale asymmetry observed in the cosmic microwave background sky, we consider a specific class of anisotropic cosmological models--Bianchi type VII sub(h)--and compare them to ...the WMAP first-year data on large angular scales. Remarkably, we find evidence of a correlation that is ruled out as a chance alignment at the 3 s level. The best-fit Bianchi model corresponds to x = 0.55, sub(0) = 0.5, a rotation axis in the direction (l, b) = (222, -62), shear (s/H) sub(0) = 2.4 x 10 super(-10), and a right-handed vorticity (w/H) sub(0) = 4.3 x 10 super(-10). Correcting for this component greatly reduces the significance of the large-scale power asymmetry, resolves several anomalies detected on large angular scales (i.e., the low quadrupole amplitude and quadrupole/octopole planarity and alignment), and can account for a non-Gaussian "cold spot" on the sky. Despite the apparent inconsistency with the best-fit parameters required in inflationary models to account for the acoustic peaks, we consider the results sufficiently provocative to merit further consideration.
We present a method for parametric modelling of the physical components of the Galaxy's magnetized interstellar medium, simulating the observables and mapping out the likelihood space using a Markov ...Chain Monte Carlo analysis. We then demonstrate it using total and polarized synchrotron emission data as well as rotation measures of extragalactic sources. With these three data sets, we define and study three components of the magnetic field: the large-scale coherent field, the small-scale isotropic random field and the ordered field. In this first paper, we use only data along the Galactic plane and test a simple two-dimensional (2D) logarithmic spiral model for the magnetic field that includes a compression and a shearing of the random component giving rise to an ordered component. We demonstrate with simulations that the method can indeed constrain multiple parameters yielding measures of, for example, the ratios of the magnetic field components. Though subject to uncertainties in thermal and cosmic ray electron densities and depending on our particular model parametrization, our preliminary analysis shows that the coherent component is a small fraction of the total magnetic field and an ordered component comparable in strength to the isotropic random component is required to explain the polarization fraction of synchrotron emission. We outline further work to extend this type of analysis to study the magnetic spiral arm structure, the details of the turbulence as well as the 3D structure of the magnetic field.
As the next step towards an improved large-scale Galactic magnetic field model, we present a simple comparison of polarized synchrotron and thermal dust emission on the Galactic plane. We find that ...the field configuration in our previous model that reproduces the polarized synchrotron is not compatible with the Wilkinson Microwave Anisotropy Probe (WMAP) 94 GHz polarized emission data. In particular, the high degree of dust polarization in the outer Galaxy (90° < < 270°) implies that the fields in the dust-emitting regions are more ordered than the average of synchrotron-emitting regions. This new dust information allows us to constrain the spatial mixing of the coherent and random magnetic field components in the outer Galaxy. The inner Galaxy differs in polarization degree and apparently requires a more complicated scenario than our current model. In the scenario that each interstellar component (including fields and now dust) follows a spiral-arm modulation, as observed in external galaxies, the changing degree of ordering of the fields in dust-emitting regions may imply that the dust arms and the field component arms are shifted as a varying function of Galactocentric radius. We discuss the implications for how the spiral-arm compression affects the various components of the magnetized interstellar medium, but conclude that improved data such as that expected from the Planck satellite will be required for a thorough analysis.
Wilkinson Microwave Anisotropy Probe (WMAP) data when combined with ancillary data on free-free, synchrotron and dust allow an improved understanding of the spectrum of emission from each of these ...components. Here, we examine the sky variation at intermediate latitudes using a cross-correlation technique. In particular, we compare the observed emission in 15 selected sky regions to three 'standard' templates.
The free-free emission of the diffuse ionized gas is fitted by a well-known spectrum at K and Ka band, but the derived emissivity corresponds to a mean electron temperature of ∼4000-5000 K. This is inconsistent with estimates from Galactic H ii regions although a variation in the derived ratio of Hα to free-free intensity by a factor of ∼2 is also found from region to region. The origin of the discrepancy is unclear.
The anomalous emission associated with dust is clearly detected in most of the 15 fields studied. The anomalous emission correlates well with the Finkbeiner, Davis & Schlegel model 8 predictions (FDS8) at 94 GHz, with an effective spectral index between 20 and 60 GHz, of β∼−2.85. Furthermore, the emissivity varies by a factor of ∼2 from cloud to cloud. A modestly improved fit to the anomalous dust at K band is provided by modulating the template by an estimate of the dust colour temperature, specifically FDS8 × T
n
. We find a preferred value n∼ 1.6, although there is a scatter from region to region. Nevertheless, the preferred index drops to zero at higher frequencies where the thermal dust emission dominates.
The synchrotron emission steepens between GHz frequencies and the WMAP bands. There are indications of spectral index variations across the sky but the current data are not precise enough to accurately quantify this from region to region.
Our analysis of the WMAP data indicates strongly that the dust-correlated emission at the low WMAP frequencies has a spectrum which is compatible with spinning dust; we find no evidence for a synchrotron component correlated with dust. The importance of these results for the correction of cosmic microwave background data for Galactic foreground emission is discussed.
Context. Cosmic rays (CRs) and magnetic fields are dynamically important components in the Galaxy, and their energy densities are comparable to that of the turbulent interstellar gas. The interaction ...of CRs and Galactic magnetic fields (GMF) produces synchrotron radiation clearly visible in the radio regime. Detailed measurements of synchrotron radiation averaged over the line-of-sight (LOS), so-called synchrotron emissivities, can be used as a tracer of the CR density and GMF strength. Aims. Our aim is to model the synchrotron emissivity in the Milky Way using a three-dimensional dataset instead of LOS-integrated intensity maps on the sky. Methods. Using absorbed HII regions, we measured the synchrotron emissivity over a part of the LOS through the Galaxy, changing from a two-dimensional to a three-dimensional view. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using various simple axisymmetric emissivity models and a number of GMF-based emissivity models, we were able to simulate the synchrotron emissivities and compare them to the observed values in the catalog. Results. We present a catalog of low-frequency absorption measurements of HII regions, their distances and electron temperatures, compiled from literature. These data show that the axisymmetric emissivity models are not complex enough, but the GMF-based emissivity models deliver a reasonable fit. These models suggest that the fit can be improved by either an enhanced synchrotron emissivity in the outer reaches of the Milky Way or an emissivity drop near the Galactic center. Conclusions. Current GMF models plus a constant CR density model cannot explain low-frequency absorption measurements, but the fits improved with slight (ad hoc) adaptations. It is clear that more detailed models are needed, but the current results are very promising.
STUDY QUESTION
When a chromosome aneuploidy is detected in the first polar body and a reciprocal loss or gain of the same chromosome is detected in the second polar body, is the resulting embryo ...usually aneuploid for that chromosome?
SUMMARY ANSWER
When reciprocal aneuploidy occurs in polar bodies, the resulting embryo is usually normal for that chromosome, indicating that premature separation of sister chromatids (PSSC)—not non-disjunction—likely occurred in meiosis I.
WHAT IS KNOWN ALREADY
Single-nucleotide polymorphism-based microarray analysis can be used to accurately determine the chromosomal status of polar bodies and embryos. Sometimes, the only abnormality found is a reciprocal gain or loss of one or two chromosomes in the two polar bodies. Prediction of the status of the resulting embryo in these cases is problematic.
STUDY DESIGN, SIZE, DURATION
Blinded microarray analysis of previously diagnosed aneuploid embryos that had reciprocal polar body aneuploidy.
MATERIALS, SETTING, METHODS
IVF cycles were performed between 2008 and 2011 in patients aged 40 ± 3 years (range 35–47 years) with an indication for polar body-based aneuploidy screening. Thirty-five aneuploid vitrified Day 3 embryos were warmed, cultured to Day 5 and biopsied for microarray analysis. Predictions were made for the ploidy status of the embryo if PSSC or non-disjunction had occurred. The signal intensity for the aneuploid chromosome in the first polar body was compared between those that resulted in euploid and aneuploid embryos.
MAIN RESULTS AND THE ROLE OF CHANCE
Among 34 embryos with evaluable results, 31 were euploid on re-analysis. Of 43 chromosomes that had reciprocal aneuploidy in the polar bodies, 41 were disomic in the embryo, indicating that PSSC was likely to have occurred 95% (95% confidence interval 85–99%) of the time. The log 2 ratio signal intensity from the chromosomes that underwent non-disjunction, resulting in unbalanced embryos, were outliers when compared with those that underwent PSSC.
LIMITATIONS, REASONS FOR CAUTION
Although most embryos with reciprocal aneuploid polar bodies were euploid, it is unknown whether they maintain equivalent reproductive potential when transferred. Further study is needed to determine whether these embryos should be re-biopsied and considered for transfer.
WIDER IMPLICATIONS OF THE FINDINGS
This study is consistent with increasing evidence that PSSC is the primary cause of meiosis I errors in embryos from women of advanced reproductive age. Clinicians should be cautious in interpreting results from polar body aneuploidy screening, especially when only the first polar body is tested.
STUDY FUNDING/COMPETING INTEREST(S)
None.
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