We present the results of a recent re-reduction of the data from the Very Large Array (VLA) Low-frequency Sky Survey (VLSS). We used the VLSS catalogue as a sky model to correct the ionospheric ...distortions in the data and create a new set of sky maps and corresponding catalogue at 73.8 MHz. The VLSS Redux (VLSSr) has a resolution of 75 arcsec, and an average map rms noise level of σ ∼ 0.1 Jy beam−1. The clean bias is 0.66 × σ and the theoretical largest angular size is 36 arcmin. Six previously unimaged fields are included in the VLSSr, which has an unbroken sky coverage over 9.3 sr above an irregular southern boundary. The final catalogue includes 92 964 sources. The VLSSr improves upon the original VLSS in a number of areas including imaging of large sources, image sensitivity, and clean bias; however the most critical improvement is the replacement of an inaccurate primary beam correction which caused source flux errors which vary as a function of radius to nearest pointing centre in the VLSS.
Gravitational waves have been detected from a binary neutron star merger event, GW170817. The detection of electromagnetic radiation from the same source has shown that the merger occurred in the ...outskirts of the galaxy NGC 4993, at a distance of 40 megaparsecs from Earth. We report the detection of a counterpart radio source that appears 16 days after the event, allowing us to diagnose the energetics and environment of the merger. The observed radio emission can be explained by either a collimated ultrarelativistic jet, viewed off-axis, or a cocoon of mildly relativistic ejecta. Within 100 days of the merger, the radio light curves will enable observers to distinguish between these models, and the angular velocity and geometry of the debris will be directly measurable by very long baseline interferometry.
We consider an initial value problem for a class of nonlinear fractional differential equations involving Hilfer fractional derivative. We prove existence and uniqueness of global solutions in the ...space of weighted continuous functions. The stability of the solution for a weighted Cauchy-type problem is also analyzed.
Clusters of galaxies are the largest gravitationally bound objects in the Universe, containing about 10sup 15 solar masses of hot (10sup 8 K) gas, galaxies and dark matter in a typical volume of 10 ...Mpcsup 3. Magnetic fields and relativistic particles are mixed with the gas as revealed by giant 'radio haloes', which arise from diffuse, megaparsec-scale synchrotron radiation at cluster centre. Radio haloes require that the emitting electrons are accelerated in situ (by turbulence), or are injected (as secondary particles) by proton collisions into the intergalactic medium. They are found only in a fraction of massive clusters that have complex dynamics, which suggests a connection between these mechanisms and cluster mergers. Here we report a radio halo at low frequencies associated with the merging cluster Abell 521. This halo has an extremely steep radio spectrum, which implies a high frequency cut-off; this makes the halo difficult to detect with observations at 1.4 GHz (the frequency at which all other known radio haloes have been best studied). The spectrum of the halo is inconsistent with a secondary origin of the relativistic electrons, but instead supports turbulent acceleration, which suggests that many radio haloes in the Universe should emit mainly at low frequencies. PUBLICATION ABSTRACT
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We present new images and continuum spectral analysis for 14 resolved Galactic supernova remnants (SNRs) selected from the 74 MHz Very Large Array Low-Frequency Sky Survey Redux (VLSSr). We combine ...new integrated measurements from the VLSSr with, when available, flux densities extracted from the Galactic and Extragalactic All-Sky Murchison Widefield Array Survey and measurements from the literature to generate improved integrated continuum spectra sampled from ~15 MHz to ~217 GHz. We present the VLSSr images. When possible we combine them with publicly available images at 1.4 GHz, to analyse the resolved morphology and spectral index distribution across each SNR. We interpret the results and look for evidence of thermal absorption caused by ionised gas either proximate to the SNR itself, or along its line of sight. Three of the SNRs, G4.5+6.8 (
Kepler
), G28.6−0.1, and G120.1+1.4 (
Tycho
), have integrated spectra which can be adequately fit with simple power laws. The resolved spectral index map for
Tycho
confirms internal absorption which was previously detected by the Low Frequency Array, but it is insufficient to affect the fit to the integrated spectrum. Two of the SNRs are pulsar wind nebulae, G21.5−0.9 and G130.7+3.1 (3C 58). For those we identify high-frequency spectral breaks at 38 and 12 GHz, respectively. For the integrated spectra of the remaining nine SNRs, a low frequency spectral turnover is necessary to adequately fit the data. In all cases we are able to explain the turnover by extrinsic thermal absorption. For G18.8+0.3 (Kes 67), G21.8−0.6 (Kes 69), G29.7−0.3 (Kes 75), and G41.1−0.3 (3C 397), we attribute the absorption to ionised gas along the line of sight, possibly from extended H
II
region envelopes. For G23.3−0.3 (W41) the absorption can be attributed to H
II
regions located in its immediate proximity. Thermal absorption from interactions at the ionised interface between SNR forward shocks and the surrounding medium were previously identified as responsible for the low frequency turnover in SNR G31.9+0.0 (3C 391); our integrated spectrum is consistent with the previous results. We present evidence for the same phenomenon in three additional SNRs G27.4+0.0 (Kes 73), G39.2–0.3 (3C 396), and G43.3–0.2 (W49B), and derive constraints on the physical properties of the interaction. This result indicates that interactions between SNRs and their environs should be readily detectable through thermal absorption by future low frequency observations of SNRs with improved sensitivity and resolution.
Motivated by the Hilfer fractional derivative (which interpolates the Riemann-Liouville derivative and the Caputo derivative), we consider a new type of fractional derivative (which interpolates the ...Hadamard derivative and its Caputo counterpart). We prove the well-posedness for a basic Cauchy type fractional differential equation involving this kind of derivative. This is established in an appropriate underlying space after proving the equivalence of this problem with a certain corresponding Volterra integral equation.
We report the discovery of up to 35 new supernova remnants (SNRs) from a 42" resolution, 90 cm multiconfiguration Very Large Array survey of the Galactic plane covering 4 5 < l < 22 0 and "b" < 1 25. ...Archival 20 cm, 11 cm, and 8 km data have also been used to identify the SNRs and constrain their properties. The 90 cm image is sensitive to SNRs with diameters 2.'5 to 50' and down to a surface brightness limit of 610 super(-21) W m super(-2) Hz super(-1)sr super(-1). This survey has nearly tripled the number of SNRs known in this part of the Galaxy and represents an overall 15% increase in the total number of Galactic SNRs. These results suggest that further deep, low-frequency surveys of the inner Galaxy will resolve the discrepancy between the expected number of Galactic SNRs and the significantly smaller number of currently known SNRs.
Abstract
The study of stripped-envelope core-collapse supernovae (SNe), with evidence for strong interaction of SN ejecta with the circumstellar medium (CSM), provides insights into the pre-supernova ...progenitor, and a fast-forwarded view of the progenitor mass-loss history. In this context, we present late-time radio observations of SN 2004dk, a Type Ibc supernova located in the galaxy NGC 6118, at a distance of
d
L
≈ 23 Mpc. About 10 yr after explosion, SN 2004dk has shown evidence for H
α
emission, possibly linked to the SN ejecta interacting with a H-rich CSM. Using data from the VLA Low Band Ionosphere and Transient Experiment (VLITE), we confirm the presence of a late-time radio rebrightening accompanying the observed H
α
emission. We model the SN 2004dk radio light curves within the (spherically symmetric) synchrotron-self-absorption (SSA) model. Within this model, our VLITE observations combined with previously collected VLA data favor an interpretation of SN 2004dk as a strongly CSM-interacting radio SN going through a complex environment shaped by nonsteady mass loss from the SN progenitor.
Recent analyses of the excess of gamma-ray radiation emanating from the Galactic center (GC) region suggest an origin in a population of thousands of undetected millisecond pulsars (MSPs). We have ...conducted a search for pulsar candidates using new high-sensitivity, wide-field radio observations of the GC covering 5 deg2. We conducted the search at a low frequency of ∼320 MHz in order to take advantage of the very steep spectra typical of pulsars. Additional observations at 6 GHz of the most steep-spectrum, compact sources resulted in a list of seven candidate pulsars. No pulsations were detected for any of the candidates in a search conducted with the GBT at 1.5, 2, and 6 GHz, presumably due to severe temporal scattering in the GC region or along the line of sight. We discuss the implications of the nondetections on pulse period and distance estimates using two different models of the Galactic distribution of ionized gas. For our best candidate, C1748−2827, located 43′ from Sgr A*, we estimate that pulsations from a normal pulsar would have been detected up to a distance of ∼8 kpc and from an MSP up to ∼4.5 kpc.
The VLA Low-Frequency Sky Survey Cohen, A. S; Lane, W. M; Cotton, W. D ...
The Astronomical journal,
09/2007, Letnik:
134, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The VLA Low-frequency Sky Survey (VLSS) has imaged 95% of the 3p sr of sky north of d = -30° at a frequency of 74 MHz (4 m wavelength). The resolution is 80'' (FWHM) throughout, and the typical rms ...noise level is s 0.1 Jy beam-1. The typical point-source detection limit is 0.7 Jy beam-1, and so far nearly 70,000 sources have been cataloged. This survey used the 74 MHz system added to the VLA in 1998. It required new imaging algorithms to remove the large ionospheric distortions at this very low frequency throughout the entire ~11.9° field of view. This paper describes the observation and data reduction methods used for the VLSS and presents the survey images and source catalog. All of the calibrated images and the source catalog are available from the VLSS Web site for use by the astronomical community.
PDF
