We investigate six supernova remnant (SNR) candidates-G51.21+0.11, G52.37-0.70, G53.07+0.49, G53.41+0.03, G53.84-0.75, and the possible shell around G54.1+0.3-in the Galactic plane using newly ...acquired Low-Frequency Array High-band Antenna observations, as well as archival Westerbork Synthesis Radio Telescope and Very Large Array Galactic Plane Survey mosaics. We find that G52.37-0.70, G53.84-0.75, and the possible shell around pulsar wind nebula G54.1+0.3 are unlikely to be SNRs, while G53.07+0.49 remains a candidate SNR. G51.21+0.11 has a spectral index of = − 0.7 0.21, but lacks X-ray observations and as such requires further investigation to confirm its nature. We confirm one candidate, G53.41+0.03, as a new SNR because it has a shell-like morphology, a radio spectral index of = − 0.6 0.2 and it has the X-ray spectral characteristics of a 1000-8000 year old SNR. The X-ray analysis was performed using archival XMM-Newton observations, which show that G53.41+0.03 has strong emission lines and is best characterized by a nonequilibrium ionization model, consistent with an SNR interpretation. Deep Arecibo radio telescope searches for a pulsar associated with G53.41+0.03 resulted in no detection, but placed stringent upper limits on the flux density of such a source if it was beamed toward Earth.
ABSTRACT
We report on the detection of MKT J174641.0−321404, a new radio transient found in untargeted searches of wide-field MeerKAT radio images centred on the black hole X-ray binary H1743−322. ...MKT J174641.0−321404 is highly variable at 1.3 GHz and was detected three times during 11 observations of the field in late 2018, reaching a maximum flux density of 590 ± 60 µJy. We associate this radio transient with a high proper motion, M dwarf star SCR 1746−3214 12 pc away from the Sun. Multiwavelength observations of this M dwarf indicate flaring activity across the electromagnetic spectrum, consistent with emission expected from dMe stars, and providing upper limits on quiescent brightness in both the radio and X-ray regimes. TESS photometry reveals a rotational period for SCR 1746−3214 of 0.2292 ± 0.0025 d, which at its estimated radius makes the star a rapid rotator, comparable to other low-mass systems. Dedicated spectroscopic follow up confirms the star as a mid-late spectral M dwarf with clear magnetic activity indicated by strong H α emission. This transient’s serendipitous discovery by MeerKAT, along with multiwavelength characterization, make it a prime demonstration of both the capabilities of the current generation of radio interferometers and the value of simultaneous observations by optical facilities such as MeerLICHT. Our results build upon the literature of M dwarfs’ flaring behaviour, particularly relevant to the habitability of their planetary systems.
Abstract
We present the detection of rotationally modulated, circularly polarized radio emission from the T8 brown dwarf WISE J062309.94−045624.6 between 0.9 and 2.0 GHz. We detected this ...high-proper-motion ultracool dwarf with the Australian SKA Pathfinder in 1.36 GHz imaging data from the Rapid ASKAP Continuum Survey. We observed WISE J062309.94−045624.6 to have a time and frequency averaged Stokes
I
flux density of 4.17 ± 0.41 mJy beam
−1
, with an absolute circular polarization fraction of 66.3% ± 9.0%, and calculated a specific radio luminosity of
L
ν
∼ 10
14.8
erg s
−1
Hz
−1
. In follow-up observations with the Australian Telescope Compact Array and MeerKAT we identified a multipeaked pulse structure, used dynamic spectra to place a lower limit of
B
> 0.71 kG on the dwarf’s magnetic field, and measured a
P
= 1.912 ± 0.005 hr periodicity, which we concluded to be due to rotational modulation. The luminosity and period we measured are comparable to those of other ultracool dwarfs observed at radio wavelengths. This implies that future megahertz to gigahertz surveys, with increased cadence and improved sensitivity, are likely to detect similar or later-type dwarfs. Our detection of WISE J062309.94−045624.6 makes this dwarf the coolest and latest-type star observed to produce radio emission.
We present a method for identifying radio stellar sources using their proper-motion. We demonstrate this method using the FIRST, VLASS, RACS-low and RACS-mid radio surveys, and astrometric ...information from Gaia Data Release 3. We find eight stellar radio sources using this method, two of which have not previously been identified in the literature as radio stars. We determine that this method probes distances of
$\sim$
90pc when we use FIRST and RACS-mid, and
$\sim$
250pc when we use FIRST and VLASS. We investigate the time baselines required by current and future radio sky surveys to detect the eight sources we found, with the SKA (6.7 GHz) requiring
$<$
3 yr between observations to find all eight sources. We also identify nine previously known and 43 candidate variable radio stellar sources that are detected in FIRST (1.4 GHz) but are not detected in RACS-mid (1.37 GHz). This shows that many stellar radio sources are variable, and that surveys with multiple epochs can detect a more complete sample of stellar radio sources.
A new era of radio transients Driessen, Laura
Astronomy & geophysics : the journal of the Royal Astronomical Society,
10/2020, Letnik:
61, Številka:
5
Journal Article
Recenzirano
Abstract
Laura Driessen reviews the history of radio transient astronomy, details current investigations of a wide range of radio transients with Square Kilometre Array pathfinder and precursor ...instruments, and discusses what the SKA will reveal about the changing radio sky.
ABSTRACT
We present results from a radio survey for variable and transient sources on 15-min time-scales, using the Australian SKA Pathfinder (ASKAP) pilot surveys. The pilot surveys consist of 505 h ...of observations conducted at around 1 GHz observing frequency, with a total sky coverage of 1476 deg2. Each observation was tracked for approximately 8 – 10 h, with a typical rms sensitivity of ∼30 μJy beam−1 and an angular resolution of ∼12 arcsec. The variability search was conducted within each 8 – 10 h observation on a 15-min time-scale. We detected 38 variable and transient sources. Seven of them are known pulsars, including an eclipsing millisecond pulsar, PSR J2039−5617. Another eight sources are stars, only one of which has been previously identified as a radio star. For the remaining 23 objects, 22 are associated with active galactic nuclei or galaxies (including the five intra-hour variables that have been reported previously), and their variations are caused by discrete, local plasma screens. The remaining source has no multiwavelength counterparts and is therefore yet to be identified. This is the first large-scale radio survey for variables and transient sources on minute time-scales at a sub-mJy sensitivity level. We expect to discover ∼1 highly variable source per day using the same technique on the full ASKAP surveys.
We present the first sub-arcsecond localised Fast Radio Burst (FRB) detected using MeerKAT. The FRB, FRB 20210405I, was detected in the incoherent beam using the MeerTRAP pipeline on 2021 April 05 ...with a signal to noise ratio of 140.8 and a dispersion measure of 565.17 pc cm$^{-3}$. It was detected while MeerTRAP was observing commensally with the ThunderKAT large survey project, and was sufficiently bright that we could use the ThunderKAT 8s images to localise the FRB. Two different models of the dispersion measure in the Milky Way and halo suggest that the source is either right at the edge of the Galaxy, or outside. However, we use the combined localisation, dispersion measure, scattering, specific luminosity and chance coincidence probability information to find that the origin is most likely extragalactic and identify the likely host galaxy of the FRB: 2MASS J1701249$-$4932475. Using SALT spectroscopy and archival observations of the field, we find that the host is a disk/spiral galaxy at a redshift of $z=0.066$.
ABSTRACT
The newest generation of radio telescopes is able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient ...sky. Here, we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ∼89 000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1–0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, while we use the classifications from volunteers to develop machine learning techniques for finding transients.
Radio astronomy is heading into a new era where we can use the wide field of view and excellent sensitivity of instruments such as LOFAR, the MWA, ASKAP, and MeerKAT to commensally search for and ...monitor radio variable and transient sources. In this thesis we present searches for, and investigations of, radio variable and transient sources in images from the “more” Karoo Array Telescope (MeerKAT). We describe the fast imaging pipeline for the more Transients and Pulsars (MeerTRAP) commensal search for fast radio transients and demonstrate its use on observations of the Vela pulsar. We will use the rapid imaging pipeline to localise the transients to determine their properties and to enable rapid follow-up with other instruments. We present the discovery and investigation of the first radio transient discovered with MeerKAT: MKT J170456.2−482100. This source was found commensally in ThunderKAT observations of the low-mass X-ray binary GX 339−4. We find that MKT J170456.2−482100 is an RS CVn-like stellar binary consisting of a K type sub-giant and companion of unknown type, and that the radio flares that we observe are likely due to magnetic activity of the K star. Further investigation is needed to determine the cause of faint features in the optical spectra and to determine the nature of the companion. We detect radio emission from X-ray flaring star EXO 040830−7134.7, the first radio star detected with MeerKAT. We find that this star is a magnetically active, rotating M-dwarf and that the radio emission we observe is likely caused by the magnetic activity. We investigate and present a solution to systematic flux density variability in the light curves of MeerKAT detected sources over time, though further investigation is required to determine the source of the systematics. Finally, we present the discovery of 25, 2.6 per cent of the radio sources in the field, new long-term variable sources in images made from two years of weekly monitoring observations of GX 339−4 with MeerKAT. This is an unprecedented data set, and the discoveries we present are only scratching the surface of what we can find out using these data. We present the radio light curves and spectral index variability of the new sources, and speculate on their nature. Many of the new variable sources are consistent with refractive scintillation of active galactic nuclei but other sources require further multi-wavelength studies to ascertain their nature. This thesis represents the new and exciting research into the radio variable sky that we can now perform using MeerKAT.
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