Rotation Measure RM Research Articles

Fine frequency structure of interstellar scintillation pattern in radio emission of the PSR B1133+16 at 111 MHz

The B1133+16 pulsar was observed at a frequency of 111 MHz with the LPA PRAO radio telescope from October 2022 to March 2023. Observations were made twice a week for two days in a row. In total 38 measurements of the scattering parameters were carried out with a high frequency resolution (up to 65 Hz). We used continuous recording of voltage in the frequency band 2.5 MHz with 8-bit digitization. The signal was reconstructed using the coherent dedispersion method. Dynamic spectra (DSP) were constructed for each observing session. Then, for each DSP, we calculated a two-dimensional autocorrelation function (2DACF) and analyzed its frequency and time sections. We have studied the fine frequency structure of pulsar scintillations by analyzing both the dynamic spectra, and the spectra of individual pulses. It has been found that the intrinsic shape of diffraction distortion on average can be represented by a sharp two-sided function with a characteristic frequency width of 1–2 kHz. The long-term variability of the following parameters has been carefully studied: characteristic scales in frequency and time (fdifandtdif), and rotation measureRM. Based on the analysis of long-term variations, it is suggested that the true frequency form of scintillations may be distorted by ionospheric effects. We also compared the scintillation parameters separately for the two mean profile components, and no differences between the parameters were found.

Pulsar‐based Galactic Magnetic Map: A Large‐Scale Clockwise Magnetic Field with an Anticlockwise Annulus

The rotation measure RM and location L of over 350 pulsars in the Milky Way are used to constrain the structure of the large-scale magnetic field in the Milky Way disk. Various magnetic field directions in various concentric circles are employed in the mapping to fit the RM and L values, allowing the testing of various models. This approach can build up signal-to-noise ratios by averaging across large rings (of size 1 kpc); at worst, this approach may miss a (double) field reversal occurring within a ring. My current work here begins with the assumption of concentric rings. In the best pulsar-based fit, an overall clockwise-going magnetic field (as seen from the north Galactic pole) extends radially at least from 1 to 12 kpc from the Galactic center, except for a 2 kpc wide anticlockwise magnetic field located in an annulus between 4 and 6 kpc from the Galactic center. The best fit has a very special feature in the form of a string of H II regions located in the anticlockwise annulus. For the mean pulsar RM as averaged over radial rings, the system of a clockwise-going Galactic magnetic field with only one anticlockwise-going field in an annulus produces the lowest reduced χ2. All other fits with a different number of field-reversed annuli have a 4 times larger reduced χ2 or worse. Thus a completely clockwise fit (no reversed ring) has a 5 times larger reduced χ2. Fits do not support more than one magnetically reversed annulus in the Galaxy, coming up with an 8 times larger reduced χ2 for two opposite rings, 4 times larger for four opposite rings, 4 times larger for six opposite rings, and 9 times larger for 12 alternating zones. Splitting the pulsars above from those below the Galactic plane, the best fit with a single reversed ring holds true separately for pulsars above the plane and for pulsars below the plane. The origin of the anticlockwise annulus could be due to a number of factors (internal or external) or could be primordial (regular or chaotic).

Systematic bias in interstellar magnetic field estimates

Faraday rotation of the polarization plane in magnetized thermal plasma provides one of the most efficient methods to deduce regular magnetic fields from radio astronomical observations. Since the Faraday rotation measure RM is proportional to an integral, along the line of sight, of magnetic field weighted with thermal electron density, RM is believed to yield the regular magnetic field averaged over large volume. Here we show that this is not the case in a turbulent medium where fluctuations in magnetic field and electron density are not statistically independent, and so contribute to RM. For example, in the case of pressure equilibrium, magnetic field can be anticorrelated with plasma density to produce a negative contribution. As a result, the strength of the regular magnetic field obtained from RM can be underestimated if the fluctuations in electron density and magnetic field are neglected. The anticorrelation also reduces the standard deviation of RM. We further discuss the effect of the positive correlations where the standard treatment of RM leads to an overestimated magnetic field. Because of the anisotropy of the turbulent magnetic field, the regular magnetic fields strength, obtained from synchrotron emission using standard formulae, can be overestimated. A positive correlation between cosmic-ray number density and magnetic field leads to an overestimate of the strengths of the regular and total fields. These effects can explain the difference between the strengths of the regular Galactic magnetic field as indicated by RM and synchrotron emissivity data and reconcile the magnetic field strength in the Solar vicinity with typical strength of regular magnetic fields in external galaxies.

Erratum: Faraday ghosts: depolarization canals in the Galactic radio emission

Narrow, elongated regions of very low polarized intensity -- so-called canals -- have recently been observed by several authors at decimeter wavelengths in various directions in the Milky Way, but their origin remains enigmatic. We show that the canals arise from depolarization by differential Faraday rotation in the interstellar medium and that they represent level lines of Faraday rotation measure RM, a random function of position in the sky. Statistical properties of the separation of canals depend on the autocorrelation function of RM, and so provide a useful tool for studies of interstellar turbulence.

Faraday ghosts: depolarization canals in the Galactic radio emission

Narrow, elongated regions of very low polarized intensity ‐ so-called ‘canals’ ‐ have recently been observed by several authors at decimetre wavelengths in various directions in the Milky Way, but their origin remains enigmatic. We show that the canals arise from depolarization by differential Faraday rotation in the interstellar medium and that they represent level lines of Faraday rotation measure RM, a random function of position in the sky. Statistical properties of the separation of canals depend on the autocorrelation function of RM, and so provide a useful tool for studies of interstellar turbulence.

Multi-frequency polarimetry of the Galactic radio background around 350 MHz

We study a conspicuous ring-like structure with a radius of about 1.4° which was observed with the Westerbork Synthesis Radio Telescope (WSRT) at 5 frequencies around 350 MHz. This ring is very prominent in Stokes Q and U, and less so in polarized intensity P. No corresponding structure is visible in total intensity Stokes I, which indicates that the ring is created by Faraday rotation and depolarization processes. The polarization angle changes from the center of the ring outwards to a radius . Thus, the structure in polarization angle is not ring-like but resembles a disk, and it is larger than the ring in P. The rotation measure RM decreases almost continuously over the disk, from RM rad m-2 at the edge, to -8 rad m-2 in the center, while outside the ring the RM is slightly positive. This radial variation of RM yields stringent constraints on the nature of the ring-like structure, because it rules out any spherically symmetrical magnetic field configuration, such as might be expected from supernova remnants or wind-blown bubbles. We discuss several possible connections between the ring and known objects in the ISM, and conclude that the ring is a predominantly magnetic funnel-like structure. This description can explain both the field reversal from outside to inside the ring, and the increase in magnetic field, probably combined with an electron density increase, towards the center of the ring. The ring-structure in P is most likely caused by a lack of depolarization due to a very uniform RM distribution at that radius. Beyond the ring, the RM gradient increases, depolarizing the polarized emission, so that the polarized intensity decreases. In the southwestern corner of the field a pattern of narrow filaments of low polarization, aligned with Galactic longitude, is observed, indicative of beam depolarization due to abrupt changes in RM. This explanation is supported by the observed RM.

Characteristics of the structure in the Galactic polarized radio background at 350 MHz

Angular power spectra and structure functions of the Stokes parameters Q and U and polarized intensity P are derived from three sets of radio polarimetric observations. Two of the observed fields have been studied at multiple frequencies, allowing determination of power spectra and structure functions of rotation measure RM as well. The third field extends over a large part of the northern sky, so that the variation of the power spectra over Galactic latitude and longitude can be studied. The power spectra of Q and U are steeper than those of P, probably because a foreground Faraday screen creates extra structure in Q and U, but not in P. The extra structure in Q and U occurs on large scales, and therefore causes a steeper spectrum. The derived slope of the power spectrum of P is the multipole spectral index , and is consistent with earlier estimates. The multipole spectral index decreases with Galactic latitude (i.e. the spectrum becomes flatter), but is consistent with a constant value over Galactic longitude. Power spectra of the rotation measure RM show a spectral index , while the structure function of RM is approximately flat. The structure function is flatter than earlier estimates from polarized extragalactic sources, which could be due to the fact that extragalactic source RM probes the complete line of sight through the Galaxy, whereas as a result of depolarization diffuse radio polarization only probes the nearby ISM.

Depolarization and Faraday effects in galaxies

Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto-ionic medium and relativistic electrons are non-uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non-uniformity of the magneto-ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line-of-sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies. We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations. We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.

Discussion following the Reports by Weaver and Field

This long, very intensive, and partially very confused discussion, has been rearranged in six sections: (1) Direct and Indirect Evidence of X-rays and Low-Energy Cosmic Rays; (2) The Boundary Layer between the Stable Gas Phases; (3) Theoretical Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (4) Observational Aspects of Interstellar Gas Dynamics and the Formation of Clouds; (5) Observations of the Rarefied, Neutral Intercloud Medium and of the Interstellar Electron Density; (6) The Dynamical Theory of Hii Regions. Section 2 has been transferred from the Discussion on Monday, September 8 (Chapter 2). To Section 6 have been added remarks made during various discussions. A couple of remarks have been transferred to other Discussions. Part of the Discussion (in Section 5) was very confused; an attempt has been made to condense and to make as much sense as possible out of what was said. For the convenience of the reader I recapitulate a few concepts, the (mis)-use of which lead partially to the confusion: (i)The hydrogen surface density or column density NH = ʃ nH dl (NH is sometimes called the hydrogen measure HM).(ii)The dispersion measure DM = ʃ ne dl (DM is often called the electron surface density Ne).(iii)The rotation measure RM = c1 ʃ neB‖ dl.(iv)The emission measure EM = ʃ ne2 dl.In these definitions, nH represents the hydrogen density, ne the electron density, B‖ the component of the magnetic field strength along the line of sight and l the distance along the line of sight. Conventionally nH and ne are expressed in cm−3, B in μG and l in pc. In addition there are two combinations of these quantities (and of the electron temperature T) involved in the Discussion:(v)The free-free absorption coefficient k(v) = c2(v)ne2T−3/2 (v is the frequency).(vi)The free-free emissivity ɛ(v)=c3(v)ne2T−1/2 (only at radio wavelengths). c2 and c3 depend also on T, but only rather weakly.

Magnetic fields and spiral structure

The most valuable new information on galactic magnetic fields is derived from the study of Faraday effects in extragalactic radio sources and pulsars. The rotation of the plane of polarization of an electromagnetic wave with a wavelength of 1 m – the so called Rotation Measure RM – which traverses a medium with electron density ne (cm−3) and magnetic field intensity H (in μG) is given by where dl is the element of length along the line of sight expressed in pc. In the case of the pulsars, the delayed arrival of the pulses at the longer wavelengths immediately yields the integrated electron density along the line of sight, that is