ABSTRACT
Two coherent radio emission mechanisms operate in stellar coronae: plasma emission and cyclotron emission. They directly probe the electron density and magnetic field strength respectively. ...Most stellar radio detections have been made at cm-wavelengths where it is often not possible to uniquely identify the emission mechanism, hindering the utility of radio observations in probing coronal conditions. In anticipation of stellar observations from a suite of sensitive low-frequency ($\nu \sim 10^2\, {\rm MHz}$) radio telescopes, here I apply the general theory of coherent emission in non-relativistic plasma to the low-frequency case. I consider the recently reported low-frequency emission from dMe flare stars AD Leo and UV Ceti and the quiescent star GJ 1151 as test cases. My main conclusion is that unlike the cm-wave regime, for reasonable turbulence saturation regimes, the emission mechanism in metre-wave observations ($\nu \sim 10^2\, {\rm MHz}$) can often be identified based on the observed brightness temperature, emission duration, and polarization fraction. I arrive at the following heuristic: M-dwarf emission that is ≳ hour-long with ${\gtrsim}50{{\ \rm per\ cent}}$ circular polarized fraction at brightness temperatures of ${\gtrsim}10^{12}\,$K at ${\sim}100\, {\rm MHz}$ in M-dwarfs strongly favours a cyclotron maser interpretation.
ABSTRACT
A new upper limit on the 21 cm signal power spectrum at a redshift of z ≈ 9.1 is presented, based on 141 h of data obtained with the Low-Frequency Array (LOFAR). The analysis includes ...significant improvements in spectrally smooth gain-calibration, Gaussian Process Regression (GPR) foreground mitigation and optimally weighted power spectrum inference. Previously seen ‘excess power’ due to spectral structure in the gain solutions has markedly reduced but some excess power still remains with a spectral correlation distinct from thermal noise. This excess has a spectral coherence scale of 0.25–0.45 MHz and is partially correlated between nights, especially in the foreground wedge region. The correlation is stronger between nights covering similar local sidereal times. A best 2-σ upper limit of $\Delta ^2_{21} \lt (73)^2\, \mathrm{mK^2}$ at $k = 0.075\, \mathrm{h\, cMpc^{-1}}$ is found, an improvement by a factor ≈8 in power compared to the previously reported upper limit. The remaining excess power could be due to residual foreground emission from sources or diffuse emission far away from the phase centre, polarization leakage, chromatic calibration errors, ionosphere, or low-level radiofrequency interference. We discuss future improvements to the signal processing chain that can further reduce or even eliminate these causes of excess power.
Violent solar eruptions are often accompanied by relativistic beams of charged particles. In the solar context they are referred to as solar particle events and are known to generate a characteristic ...swept-frequency radio burst. Due to their ionising potential, these beams influence atmospheric chemistry and habitability. Radio observations provide a crucial discriminant between stellar flares that do and do not generate particle beams. Here I use solar empirical data and semi-quantitative theoretical estimates to gauge the feasibility of detecting the associated radio bursts. My principal conclusion is that a dedicated search for swept frequency radio bursts on second timescales in existing low-frequency (
ν
≲ 10
2
MHz) datasets, while technically challenging, will likely provide the evidence high-energy particles beams in Sun-like stars.
Intense, millisecond-duration bursts of radio waves (named fast radio bursts) have been detected from beyond the Milky Way
. Their dispersion measures-which are greater than would be expected if they ...had propagated only through the interstellar medium of the Milky Way-indicate extragalactic origins and imply contributions from the intergalactic medium and perhaps from other galaxies
. Although several theories exist regarding the sources of these fast radio bursts, their intensities, durations and temporal structures suggest coherent emission from highly magnetized plasma
. Two of these bursts have been observed to repeat
, and one repeater (FRB 121102) has been localized to the largest star-forming region of a dwarf galaxy at a cosmological redshift of 0.19 (refs.
). However, the host galaxies and distances of the hitherto non-repeating fast radio bursts are yet to be identified. Unlike repeating sources, these events must be observed with an interferometer that has sufficient spatial resolution for arcsecond localization at the time of discovery. Here we report the localization of a fast radio burst (FRB 190523) to a few-arcsecond region containing a single massive galaxy at a redshift of 0.66. This galaxy is different from the host of FRB 121102, as it is a thousand times more massive, with a specific star-formation rate (the star-formation rate divided by the mass) a hundred times smaller.
ABSTRACT
The extreme, time-variable Faraday rotation observed in the repeating fast radio burst (FRB) 121102 and its associated persistent synchrotron source demonstrates that some FRBs originate in ...dense, dynamic, and possibly relativistic magneto-ionic environments. Besides rotation of the linear polarization vector (Faraday rotation), such media can generally convert linear to circular polarization (Faraday conversion). We use non-detection of Faraday conversion, and the temporal variation in Faraday rotation and dispersion in bursts from FRB 121102 to constrain models where the progenitor inflates a relativistic nebula (persistent source) confined by a cold dense medium (e.g. supernova ejecta). We find that the persistent synchrotron source, if composed of an electron–proton plasma, must be an admixture of relativistic and non-relativistic (Lorentz factor γ < 5) electrons. Furthermore, we independently constrain the magnetic field in the cold confining medium, which provides the Faraday rotation, to be between 10 and 30 mG. This value is close to the equipartition magnetic field of the confined persistent source implying a self-consistent and overconstrained model that can explain the observations.
Direct Radio Discovery of a Cold Brown Dwarf Vedantham, H. K.; Callingham, J. R.; Shimwell, T. W. ...
Astrophysical journal. Letters,
11/2020, Letnik:
903, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Magnetospheric processes seen in gas giants such as aurorae and circularly polarized cyclotron maser radio emission have been detected from some brown dwarfs. However, previous radio observations ...targeted known brown dwarfs discovered via their infrared emission. Here we report the discovery of BDR J1750+3809, a circularly polarized radio source detected around 144 MHz with the Low-Frequency Array (LOFAR) telescope. Follow-up near-infrared photometry and spectroscopy show that BDR J1750+3809 is a cold methane dwarf of spectral type T6.5 1 at a distance of . The quasi-quiescent radio spectral luminosity of BDR J1750+3809 is 5 × 1015 erg s−1 Hz−1, which is over two orders of magnitude larger than that of the known population of comparable spectral type. This could be due to a preferential geometric alignment or an electrodynamic interaction with a close companion. In addition, as the emission is expected to occur close to the electron gyrofrequency, the magnetic field strength at the emitter site in BDR J1750+3809 is B 25 G, which is comparable to planetary-scale magnetic fields. Our discovery suggests that low-frequency radio surveys can be employed to discover substellar objects that are too cold to be detected in infrared surveys.
ABSTRACT
The Deep Synoptic Array 10-dish prototype (DSA-10) is an instrument designed to detect and localize fast radio bursts with arcsecond accuracy in real time. Deployed at Owens Valley Radio ...Observatory, it consists of ten 4.5-m diameter dishes, equipped with a 250-MHz bandwidth dual polarization receiver, centred at 1.4 GHz. The 20 input signals are digitized and field programmable gate arrays are used to transform the data to the frequency domain and transmit it over ethernet. A series of computer servers buffer both raw data samples and perform a real time search for fast radio bursts on the incoherent sum of all inputs. If a pulse is detected, the raw data surrounding the pulse are written to disc for coherent processing and imaging. The prototype system was operational from 2017 June to 2018 February conducting a drift scan search. Giant pulses from the Crab Pulsar were used to test the detection and imaging pipelines. The 10-dish prototype system was brought online again in 2019 March, and will gradually be replaced with the new DSA-110, a 110-dish system, over the next 2 yr to improve sensitivity and localization accuracy.
Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, ...has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5 ± 0.1 parsecs per cubic centimeter), exceptionally intense (120 ± 30 janskys), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 square arc minutes. On the basis of a low Faraday rotation (12.0 ± 0.7 radians per square meter), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nanogauss, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.
Visibility scintillation resulting from wave propagation through the turbulent ionosphere can be an important source of noise at low radio frequencies (ν ≲ 200 MHz). Many low-frequency experiments ...are underway to detect the power spectrum of brightness temperature fluctuations of the neutral-hydrogen 21-cm signal from the Epoch of Reionization (EoR: 12 ≳ z ≳ 7, 100 ≲ ν ≲ 175 MHz). In this paper, we derive scintillation noise power spectra in such experiments while taking into account the effects of typical data processing operations such as self-calibration and Fourier synthesis. We find that for minimally redundant arrays such as LOFAR and MWA, scintillation noise is of the same order of magnitude as thermal noise, has a spectral coherence dictated by stretching of the snapshot uv-coverage with frequency, and thus is confined to the well-known wedge-like structure in the cylindrical (two-dimensional) power spectrum space. Compact, fully redundant (d
core ≲ r
F ≈ 300 m at 150 MHz) arrays such as HERA and SKA-LOW (core) will be scintillation noise dominated at all baselines, but the spatial and frequency coherence of this noise will allow it to be removed along with spectrally smooth foregrounds.
PDF
