Linear polarization structures in LOFAR observations of the interstellar medium in the 3C 196 field

This study aims to characterize linear polarization structures in LOFAR observations of the interstellar medium (ISM) in the 3C196 field, one of the primary fields of the LOFAR-Epoch of Reionization key science project. We have used the high band antennas (HBA) of LOFAR to image this region and Rotation Measure (RM) synthesis to unravel the distribution of polarized structures in Faraday depth. The brightness temperature of the detected Galactic emission is 5−15 K in polarized intensity and covers the range from -3 to +8 rad m−2 in Faraday depth. The most interesting morphological feature is a strikingly straight filament at a Faraday depth of +0.5 rad m−2 running from north to south, right through the centre of the field and parallel to the Galactic plane. There is also an interesting system of linear depolarization canals conspicuous in an image showing the peaks of Faraday spectra. We used the Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same region. For the first time, we see some common morphology in the RM cubes made at 150 and 350~{; ; \rm MHz}; ; . There is no indication of diffuse emission in total intensity in the interferometric data, in line with results at higher frequencies and previous LOFAR observations. Based on our results, we determined physical parameters of the ISM and proposed a simple model that may explain the observed distribution of the intervening magneto- ionic medium. The mean line-of-sight magnetic field component, B∥, is determined to be 0.3±0.1 μG and its spatial variation across the 3C196 field is 0.1 μG. The filamentary structure is probably an ionized filament in the ISM, located somewhere within the Local Bubble. This filamentary structure shows an excess in thermal electron density (neB∥>6.2 cm−3μG) compared to its surroundings

Magnetic Fields in the Milky Way

This chapter presents a review of observational studies to determine the magnetic field in the Milky Way, both in the disk and in the halo, focused on recent developments and on magnetic fields in the diffuse interstellar medium. I discuss some terminology which is confusingly or inconsistently used and try to summarize current status of our knowledge on magnetic field configurations and strengths in the Milky Way. Although many open questions still exist, more and more conclusions can be drawn on the large-scale and small-scale components of the Galactic magnetic field. The chapter is concluded with a brief outlook to observational projects in the near future.Comment: 22 pages, 5 figures, to appear in "Magnetic Fields in Diffuse Media", eds. E.M. de Gouveia Dal Pino and A. Lazaria

Simulations of galactic dynamos

We review our current understanding of galactic dynamo theory, paying particular attention to numerical simulations both of the mean-field equations and the original three-dimensional equations relevant to describing the magnetic field evolution for a turbulent flow. We emphasize the theoretical difficulties in explaining non-axisymmetric magnetic fields in galaxies and discuss the observational basis for such results in terms of rotation measure analysis. Next, we discuss nonlinear theory, the role of magnetic helicity conservation and magnetic helicity fluxes. This leads to the possibility that galactic magnetic fields may be bi-helical, with opposite signs of helicity and large and small length scales. We discuss their observational signatures and close by discussing the possibilities of explaining the origin of primordial magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria

The LOFAR radio environment

Aims: This paper discusses the spectral occupancy for performing radio astronomy with the Low-Frequency Array (LOFAR), with a focus on imaging observations. Methods: We have analysed the radio-frequency interference (RFI) situation in two 24-h surveys with Dutch LOFAR stations, covering 30– 78 MHz with low-band antennas and 115–163 MHz with high-band antennas. This is a subset of the full frequency range of LOFAR. The surveys have been observed with a 0.76 kHz / 1 s resolution. Results: We measured the RFI occupancy in the low and high frequency sets to be 1.8% and 3.2% respectively. These values are found to be representative values for the LOFAR radio environment. Between day and night, there is no significant difference in the radio environment. We find that lowering the current observational time and frequency resolutions of LOFAR results in a slight loss of flagging accuracy. At LOFAR’s nominal resolution of 0.76 kHz and 1 s, the false-positives rate is about 0.5%. This rate increases approximately linearly when decreasing the data frequency resolution. Conclusions:Currently, by using an automated RFI detection strategy, the LOFAR radio environment poses no perceivable problems for sensitive observing. It remains to be seen if this is still true for very deep observations that integrate over tens of nights, but the situation looks promising. Reasons for the low impact of RFI are the high spectral and time resolution of LOFAR; accurate detection methods; strong filters and high receiver linearity; and the proximity of the antennas to the ground. We discuss some strategies that can be used once low-level RFI starts to become apparent. It is important that the frequency range of LOFAR remains free of broadband interference, such as DAB stations and windmills

The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum in the EDGES absorption trough

International audienceThe 21-cm absorption feature reported by the EDGES collaboration is several times stronger than that predicted by traditional astrophysical models. If genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm signal on degree scales (up to 1 K in rms), allowing these fluctuations to be detectable in nearly 50 times shorter integration times compared to previous predictions. We commenced the ‘AARTFAAC Cosmic Explorer’ (ACE) program, which employs the AARTFAAC wide-field image, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range z = 17.9–18.6 (Δν = 72.36–75.09 MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5–23.75 and 23.75–24.00 h, each with 2 h of data, recorded in ‘semi drift-scan’ mode. We demonstrate the application of the new ACE data-processing pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe that noise estimates from the channel and time-differenced Stokes V visibilities agree with each other. After 2 h of integration and subtraction of bright foregrounds, we obtain 2σ upper limits on the 21-cm power spectrum of |$\Delta _{21}^2 \lt (8139~\textrm {mK})^2$| and |$\Delta _{21}^2 \lt (8549~\textrm {mK})^2$| at |$k = 0.144~h\, \textrm {cMpc}^{-1}$| for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of |$\Delta _{21}^2 \lt (7388~\textrm {mK})^2$| at |$k = 0.144~h\, \textrm {cMpc}^{-1}$|⁠. These are the deepest upper limits thus far at these redshifts

The AARTFAAC Cosmic Explorer: observations of the 21-cm power spectrum in the EDGES absorption trough

The 21-cm absorption feature reported by the EDGES collaboration is several times stronger than that predicted by traditional astrophysical models. If genuine, a deeper absorption may lead to stronger fluctuations on the 21-cm signal on degree scales (up to 1 K in rms), allowing these fluctuations to be detectable in nearly 50 times shorter integration times compared to previous predictions. We commenced the ‘AARTFAAC Cosmic Explorer’ (ACE) program, which employs the AARTFAAC wide-field image, to measure or set limits on the power spectrum of the 21-cm fluctuations in the redshift range z = 17.9–18.6 (Δν = 72.36–75.09 MHz) corresponding to the deep part of the EDGES absorption feature. Here, we present first results from two LST bins: 23.5–23.75 and 23.75–24.00 h, each with 2 h of data, recorded in ‘semi drift-scan’ mode. We demonstrate the application of the new ACE data-processing pipeline (adapted from the LOFAR-EoR pipeline) on the AARTFAAC data. We observe that noise estimates from the channel and time-differenced Stokes V visibilities agree with each other. After 2 h of integration and subtraction of bright foregrounds, we obtain 2σ upper limits on the 21-cm power spectrum of Δ221<(8139 mK)2 and Δ221<(8549 mK)2 at k=0.144 hcMpc−1 for the two LST bins. Incoherently averaging the noise bias-corrected power spectra for the two LST bins yields an upper limit of Δ221<(7388 mK)2 at k=0.144 hcMpc−1⁠. These are the deepest upper limits thus far at these redshifts

A numerical study of 21-cm signal suppression and noise increase in direction-dependent calibration of LOFAR data

International audienceWe investigate systematic effects in direction-dependent gain calibration in the context of the Low-Frequency Array (LOFAR) 21-cm Epoch of Reionization (EoR) experiment. The LOFAR EoR Key Science Project aims to detect the 21-cm signal of neutral hydrogen on interferometric baselines of 50–250 λ. We show that suppression of faint signals can effectively be avoided by calibrating these short baselines using only the longer baselines. However, this approach causes an excess variance on the short baselines due to small gain errors induced by overfitting during calibration. We apply a regularized expectation–maximization algorithm with consensus optimization (sagecal-co) to real data with simulated signals to show that overfitting can be largely mitigated by penalising spectrally non-smooth gain solutions during calibration. This reduces the excess power with about a factor of 4 in the simulations. Our results agree with earlier theoretical analysis of this bias-variance trade off and support the gain-calibration approach to the LOFAR 21-cm signal data