Radio Brightness Research Articles

MWA Observations of the EOR1 Field

AbstractThe EOR1 field at (RA,DEC)=(4hrs, −30.0°) is one of the main targets of the MWA EOR experiment. It is notable for being in one of the coldest regions of the southern radio sky, as well as for containing the bright radio galaxy Fornax A. We report an early demonstration that the distance of this field from the Galactic Centre may make it a prime field for EOR observations.

The Detection of an Extremely Bright Fast Radio Burst in a Phased Array Feed Survey

Abstract We report the detection of an ultra-bright fast radio burst (FRB) from a modest, 3.4-day pilot survey with the Australian Square Kilometre Array Pathfinder. The survey was conducted in a wide-field fly’s-eye configuration using the phased-array-feed technology deployed on the array to instantaneously observe an effective area of 160 deg2, and achieve an exposure totaling 13200 deg2 hr . We constrain the position of FRB 170107 to a region in size (90% containment) and its fluence to be 58 ± 6 Jy ms. The spectrum of the burst shows a sharp cutoff above 1400 MHz, which could be due to either scintillation or an intrinsic feature of the burst. This confirms the existence of an ultra-bright ( Jy ms) population of FRBs.

A possible time-delayed brightening of the Sgr A* accretion flow after the pericenter passage of the G2 cloud

Abstract A possibility of time-delayed radio brightenings of Sgr A* triggered by the pericenter passage of the G2 cloud is studied by carrying out global three-dimensional magnetohydrodynamic simulations, taking into account the radiative cooling of the tidal debris of the G2 cloud. Magnetic fields in the accretion flow are strongly perturbed and reorganized after the passage of G2. We have found that the magnetic energy in the accretion flow increased by a factor of 3–4 in 5–10 yr after the passage of G2 through a dynamo mechanism driven by the magneto-rotational instability. Since this B-field amplification enhances the synchrotron emission from the disk and the outflow, the radio and the infrared luminosity of Sgr A* are expected to increase some time, around 2020. The time delay of the radio brightening enables us to determine the rotation axis of the preexisting disk.

Asymmetric Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3

Abstract The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded ∼1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011–2015) of 0.58% yr−1, but large spatial variations are present. Using the nonparametric “Demons” method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from to yr−1. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities v s as low as 3600 km s−1 (for an assumed distance of 8.5 kpc), much less than v s = 12,000–13,000 km s−1 along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at yr−1. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion.

SPECIAL FIELDS OF THE GALAXY RADIO EMISSION

The brightness temperatures of radio emission were measured with the UTR-2 low-frequency radio telescope at frequencies 12.6, 14.7, 16.7, 20 and 25 MHz for the following Galaxy regions: the North Pole, the area of minimum radio brightness and the anticenter region. The brightness temperature spectrums of the galactic background radiation were constructed for the selected areas using the data at other different wavelengths. The brightness temperature spectral indices were determined for these areas at wavelengths ranging from meter to decameter.

Search for accelerated electron anisotropy signatures based on observed polarization of the flaring loop microwave emission

The distribution maps of the circular polarization degree and radio brightness have been analyzed for more than 40 flares based on the Nobeyama Radioheliograph data. It has been shown that the observed microwave emission is polarized in the ordinary mode in some flaring loop parts in six events. Based on a joint analysis of the photospheric magnetic field maps obtained from the HMI/SDO and MDI/SOHO magnetograph’s and the radio emission dynamics in different source parts, it has been concluded that the ordinary mode predominance in all six selected events can be connected with implementation of the longitudinal pitch-angle anisotropy of emitting electrons.

Distribution of the surface radio brightness of asteroid Golevka constructed from the radar data

A method for construction of a 2D radio image of an asteroid from the radar data obtained with the use of a continuous monochromatic signal has been tested. The method is based on the inverse focused synthesis of the antenna aperture due to the rotational motion of the object and reconstructive computeraided tomography. On the basis of the analysis of the echo signals from asteroid Golevka, a 2D distribution of its surface radio brightness has been constructed. Reconstruction of the asteroid silhouette confirmed that Golevka has a not very elongated angular body whose maximum diameter is 620 m and the minimum diameter is 460 m.

Shocks in nova outflows – II. Synchrotron radio emission

The discovery of GeV gamma-rays from classical novae indicates that shocks and relativistic particle acceleration are energetically key in these events. Further evidence for shocks comes from thermal keV X-ray emission and an early peak in the radio light curve on a timescale of months with a brightness temperature which is too high to result from freely expanding photo-ionized gas. Paper I developed a one dimensional model for the thermal emission from nova shocks. This work concluded that the shock-powered radio peak cannot be thermal if line cooling operates in the post-shock gas at the rate determined by collisional ionization equilibrium. Here we extend this calculation to include non-thermal synchrotron emission. Applying our model to three classical novae, we constrain the amplification of the magnetic field $\epsilon_B$ and the efficiency $\epsilon_e$ of accelerating relativistic electrons of characteristic Lorentz factor $\gamma \sim 100$. If the shocks are radiative (low velocity $v_{\rm sh} \lesssim 1000$ km s$^{-1}$) and cover a large solid angle of the nova outflow, as likely characterize those producing gamma-rays, then values of $\epsilon_e \sim 0.01-0.1$ are required to achieve the peak radio brightness for $\epsilon_B = 10^{-2}$. Such high efficiencies exclude secondary pairs from pion decay as the source of the radio-emitting particles, instead favoring the direct acceleration of electrons at the shock. If the radio-emitting shocks are instead adiabatic (high velocity), as likely characterize those responsible for the thermal X-rays, then much higher brightness temperatures are possible, allowing the radio-emitting shocks to cover a smaller outflow solid angle.

Comparison of solar radio and extreme ultraviolet synoptic limb charts during the present solar maximum

The present solar cycle is particular in many aspects: it had a delayed rising phase, it is the weakest of the last 100 years, and it presents two peaks separated by more than one year. To understand the impact of these characteristics on the solar chromosphere and coronal dynamics, images from a wide wavelength range are needed. In this work we use the 17~GHz radio continuum, formed in the upper chromosphere and the EUV lines 304 and 171~{\AA}, that come from the transition region (He II) and the corona (Fe IX, X), respectively. We analyze daily images at 304 and 171~{\AA} obtained by the Atmospheric Imaging Assembly (AIA). The 17~GHz maps were obtained by the Nobeyama Radioheliograph (NoRH). To construct synoptic limb charts, we calculated the mean emission of delimited limb areas with 100" wide and angular separation of $5^\circ$. At the equatorial region, the results show an hemispheric asymmetry of the solar activity. The northern hemisphere dominance is coincident with the first sunspot number peak, whereas the second peak occurs concurrently with the increase in the activity at the south. The polar emission reflects the presence of coronal holes at both EUV wavelengths, moreover, the 17~GHz polar brightenings can be associated with the coronal holes. Until 2013, both EUV coronal holes and radio polar brightenings were more predominant at the south pole. Since then they have not been apparent in the north, but thus appear in the beginning of 2015 in the south as observed in the synoptic charts. This work strengthens the association between coronal holes and the 17~GHz polar brightenings as it is evident in the synoptic limb charts, in agreement with previous case study papers. The enhancement of the radio brightness in coronal holes is explained by the presence of bright patches closely associated with the presence of intense unipolar magnetic fields.

High-energy gamma-ray observations of the accreting black hole V404 Cygni during its 2015 June outburst

Abstract We report on Fermi/Large Area Telescope observations of the accreting black hole low-mass X-ray binary V404 Cygni during its outburst in 2015 June–July. Detailed analyses reveal a possible excess of γ-ray emission on 2015 26 June, with a very soft spectrum above 100 MeV, at a position consistent with the direction of V404 Cyg (within the 95 per cent confidence region and a chance probability of 4 × 10−4). This emission cannot be associated with any previously known Fermi source. Its temporal coincidence with the brightest radio and hard X-ray flare in the light curve of V404 Cyg, at the end of the main active phase of its outburst, strengthens the association with V404 Cyg. If the γ-ray emission is associated with V404 Cyg, the simultaneous detection of 511 keV annihilation emission by INTEGRAL reqires that the high-energy γ-rays originate away from the corona, possibly in a Blandford–Znajek jet. The data give support to models involving a magnetically arrested disc where a bright γ-ray jet can re-form after the occurrence of a major transient ejection seen in the radio.

Shock-powered radio emission from V5589 Sagittarii (Nova Sgr 2012 #1)

Since the Fermi discovery of $\gamma$-rays from novae, one of the biggest questions in the field has been how novae generate such high-energy emission. Shocks must be a fundamental ingredient. Six months of radio observations of the 2012 nova V5589 Sgr with the VLA and 15 weeks of X-ray observations with Swift/XRT show that the radio emission consisted of: 1) a shock-powered, non-thermal flare; and 2) weak thermal emission from $10^{-5}$ M$_\odot$ of freely expanding, photoionized ejecta. Absorption features in the optical spectrum and the peak optical brightness suggest that V5589 Sgr lies 4 kpc away (3.2-4.6 kpc). The shock-powered flare dominated the radio light curve at low frequencies before day 100. The spectral evolution of the radio flare, its high radio brightness temperature, the presence of unusually hard ($kT_x > 33$ keV) X-rays, and the ratio of radio to X-ray flux near radio maximum all support the conclusions that the flare was shock-powered and non-thermal. Unlike most other novae with strong shock-powered radio emission, V5589 Sgr is not embedded in the wind of a red-giant companion. Based on the similar inclinations and optical line profiles of V5589 Sgr and V959 Mon, we propose that shocks in V5589 Sgr formed from collisions between a slow flow with an equatorial density enhancement and a subsequent faster flow. We speculate that the relatively high speed and low mass of the ejecta led to the unusual radio emission from V5589 Sgr, and perhaps also to the non-detection of $\gamma$-rays.

Statistical synthesis of multiantenna ultrawideband radiometric complexes

An optimum signal processing algorithm of radiometric imaging has been synthesized for the first time using multiantenna ultrawideband (UWB) radiometric complexes (RMCs). Radiometric images (RMI) are interpreted physically as intensity depending on the angular coordinates or the spectral radio brightness averaged in the operation frequency band. In accordance with the synthesized algorithm, a structural scheme of ultrawideband radiometric complexes has been developed. An analytical expression for the ambiguity function of radiometric complexes has been obtained. The ambiguity function is modeled in the case of processing narrowband and ultrawideband radiometric signals. As follows from the analysis of the results, new elements of the theory of optimum processing of UWB radiometric signals with the involvement of multielement antenna systems are an important tool in creating highly accurate, biologically and ecologically safe complexes for studying various media and objects.

Synthetic Radio Views of Simulated Solar Flux Ropes

In this paper, we produce synthetic radio views on simulated flux ropes in the solar corona, where finite-beta magnetohydrodynamic (MHD) simulations serve to mimic the flux rope formation stages, as well as their stable endstates. These endstates represent twisted flux ropes where balancing Lorentz forces, gravity and pressure gradients determine the full thermodynamic variation throughout the flux rope. The obtained models are needed to quantify radiative transfer in radio bands, and allow us to contrast weak to strong magnetic field conditions. Field strengths of up to 100 G in the flux rope yield the radio views dominated by optically thin free-free emission. The forming flux rope shows clear morphological changes in its emission structure as it deforms from an arcade to a flux rope, both on disk and at the limb. For an active region filament channel with a field strength of up to 680 G in the flux rope, gyroresonance emission (from the third-fourth gyrolayers) can be detected and even dominates over free-free emission at the frequencies of up to 7 GHz. Finally, we also show synthetic views on a simulated filament embedded within a (weak-field) flux rope, resulting from an energetically consistent MHD simulation. For this filament, synthetic views at the limb show clear similarities with actual observations, and the transition from optically thick (below 10 GHz) to optically thin emission can be reproduced. On the disk, its dimension and temperature conditions are as yet not realistic enough to yield the observed radio brightness depressions.

Suzaku observations of a shock front tracing the western edge of the giant radio halo in the Coma Cluster

Abstract We present the results of new Suzaku observations of the Coma Cluster, the X-ray brightest, nearby, merging system hosting a well-studied, typical giant radio halo. It has been previously shown that, on the western side of the cluster, the radio brightness shows a much steeper gradient compared to other azimuths. XMM-Newton and Planck revealed a shock front along the southern half of the region associated with this steep radio gradient, suggesting that the radio emission is enhanced by particle acceleration associated with the shock passage. Suzaku demonstrates for the first time that this shock front extends northwards, tracing the entire length of the western edge of the Coma radio halo. The shock is detected both in the temperature and X-ray surface brightness distributions and has a Mach number of around $\mathcal {M}\sim 1.5$. The locations of the surface brightness edges align well with the edge of the radio emission, while the obtained temperature profiles seem to suggest shocks located 125–185 kpc further out in radius. In addition, the shock strengths derived from the temperature and density jumps are in agreement when using extraction regions parallel to the radio halo edge, but become inconsistent with each other when derived from radial profiles centered on the Coma Cluster core. It is likely that, beyond mere projection effects, the geometry of the shock is more complex than a front with a single, uniform Mach number and an approximately spherically symmetric shape.

CHROMOSPHERIC SUNSPOTS IN THE MILLIMETER RANGE AS OBSERVED BY THE NOBEYAMA RADIOHELIOGRAPH

ABSTRACT We investigate the upper chromosphere and the transition region of the sunspot umbra using the radio brightness temperature at 34 GHz (corresponding to 8.8 mm observations) as observed by the Nobeyama Radioheliograph (NoRH). Radio free–free emission in the longer millimeter range is generated around the transition region, and its brightness temperature yields the region's temperature and density distribution. We use the NoRH data at 34 GHz by applying the Steer-CLEAN image synthesis. These data and the analysis method enable us to investigate the chromospheric structures in the longer millimeter range with high spatial resolution and sufficient visibilities. We also perform simultaneous observations of one sunspot using the NoRH and the Nobeyama 45 m telescope operating at 115 GHz. We determine that 115 GHz emission mainly originates from the lower chromosphere while 34 GHz emission mainly originates from the upper chromosphere and transition region. These observational results are consistent with the radio emission characteristics estimated from current atmospheric models of the chromosphere. On the other hand, the observed brightness temperature of the umbral region is almost the same as that of the quiet region. This result is inconsistent with current sunspot models, which predict a considerably higher brightness temperature of the sunspot umbra at 34 GHz. This inconsistency suggests that the temperature of the region at which the 34 GHz radio emission becomes optically thick should be lower than that predicted by the models.

Identification of a new γ-ray-emitting narrow-line Seyfert 1 galaxy, at redshift ∼1

Abstract We report on the identification of a new γ-ray-emitting narrow-line Seyfert 1 (NLS1) galaxy, SDSS J122222.55+041315.7, which increases the number of known objects of this remarkable but rare type of active galactic nuclei to seven. Its optical spectrum, obtained in the Sloan Digital Sky Survey-Baryon Oscillation Spectroscopic Survey, reveals a broad H β emission line with a width (FWHM) of 1734 ± 104 km s−1. This, along with strong optical Fe ii multiplets [R4570 = 0.9] and a weak [O iii] λ5007 emission line, makes the object a typical NLS1. On the other hand, the source exhibits a high radio brightness temperature, rapid infrared variability, and a flat X-ray spectrum extending up to ∼200 keV. It is associated with a luminous γ-ray source detected significantly with Fermi/LAT. Correlated variability with other wavebands has not yet been tested. The spectral energy distribution can be well modelled by a one-zone leptonic jet model. This new member is by far the most distant γ-ray-emitting NLS1, at a redshift of z = 0.966.

RADIO IMAGING OBSERVATIONS OF PSR J1023+0038 IN AN LMXB STATE

The transitional millisecond pulsar binary system PSR J1023+0038 re-entered an accreting state in 2013 June, in which it bears many similarities to low-mass X-ray binaries (LMXBs) in quiescence or near-quiescence. At a distance of just 1.37 kpc, PSR J1023+0038 offers an unsurpassed ability to study low-level accretion onto a highly-magnetized compact object. We have monitored PSR J1023+0038 intensively using radio imaging with the Karl G. Jansky Very Large Array, the European VLBI Network and LOFAR, seeing rapidly variable, flat spectrum emission that persists over a period of 6 months. The flat spectrum and variability are indicative of synchrotron emission originating in an outflow from the system, most likely in the form of a compact, partially self-absorbed jet, as is seen in LMXBs at higher accretion rates. The radio brightness, however, greatly exceeds extrapolations made from observations of more vigorously accreting neutron star LMXB systems. We postulate that PSR J1023+0038 is undergoing radiatively inefficient 'propeller-mode' accretion, with the jet carrying away a dominant fraction of the liberated accretion luminosity. We confirm that the enhanced gamma-ray emission seen in PSR J1023+0038 since it re-entered an accreting state has been maintained; the increased gamma-ray emission in this state can also potentially be associated with propeller-mode accretion. Similar accretion modes can be invoked to explain the radio and X-ray properties of the other two known transitional millisecond pulsar systems XSS J12270-4859 and PSR J1824-2452I (M28I), suggesting that radiatively inefficient accretion may be an ubiquitous phenomenon amongst (at least one class of) neutron star binaries at low accretion rates.

Polarized radio filaments outside the Galactic plane

We used data from the \wmap satellite at 23, 33 and 41 GHz to study the diffuse polarised emission over the entire sky. The emission originates mostly from filamentary structures with well-ordered magnetic fields. Some of these structures have been known for decades in radio continuum maps. Their origin is not clear and there are many filaments that are visible for the first time. We have identified and studied 11 filaments. The polarisation fraction of some of them can be as high as 40\%, which is a signature of a well ordered magnetic field. The polarisation spectral indices, averaged over 18 regions in the sky is $\beta = -3.06 \pm 0.02$, consistent with synchrotron radiation. There are significant variations in $\beta$ over the sky ($\Delta\beta\approx0.2$). We explore the link between the large-scale filaments and the local ISM, using the model of an expanding shell in the solar vicinity. We compared observed polarisation angles with the predictions from the model and found good agreement. This strongly suggests that many large scale filaments and loops are nearby structures. This is important in the context of the Galactic magnetic field as these structures are normally included in global models, neglecting the fact that they might be local. We also studied the level of contamination added by the diffuse filaments to the CMB polarisation power spectra. We conclude that, even though these filaments present low radio brightness, a careful removal will be necessary for future all-sky CMB polarisation analysis.

Low-frequency polarization measurements of the diffuse radio emission of the galaxy

Polarization measurements of diffuse Galactic radio emission at 151.5, 198, 217, 237, and 290 MHz have been carried out in the direction of the North Celestial Pole, North Galactic Pole, one region of the North Polar Spur, minimum radio brightness of the Northern sky (l = 190°, b = 50°), and in the direction l = 147°, b = 9° in the so-called FAN region with enhanced polarization. The results obtained testify to the presence of low spatial frequencies in the angular distribution of the Stokes parameters Q and U of the diffuse Galactic synchrotron emission that are not detectable in interferometric observations. The spectra of the brightness temperature of the polarized component, rotation measures, and intrinsic polarization position angles of the radio emission in the studied regions are presented.

Radio afterglows and host galaxies of gamma-ray bursts

Considering the contribution of emission from the host galaxies of gamma-ray bursts (GRBs) to radio afterglows, we investigate the effect of host galaxies on observations statistically. For the three types of event, i.e. low-luminosity, standard and high-luminosity GRBs, it is found that a tight correlation exists between the ratio of the radio flux (RRF) of the host galaxy to the total radio peak emission and the observational frequency. Towards lower frequencies, in particular, the contribution from the host increases significantly. The correlation can be used to obtain a useful estimate for the radio brightness of those host galaxies that only have very limited radio afterglow data. Using this prediction, we reconsidered the theoretical radio afterglow light curves for four kinds of event: high-luminosity, low-luminosity, standard and failed GRBs, taking into account the contribution from host galaxies and aiming to explore the detectability of these events by the Five-hundred-metre Aperture Spherical radio Telescope (FAST). Lying at a typical redshift of z = 1, most of the events can be detected easily by FAST. For the less fierce low-luminosity GRBs, their radio afterglows are not strong enough to exceed the sensitivity limit of FAST at such distances. However, since a large number of low-luminosity bursts actually happen very near to us, it is expected that FAST will still be able to detect many of them.