arXiv subjects

D. Engels

Publications and source records attributed to D. Engels.

At least 19 recordsLinked to original sources

arXiv2025

Considerations on the Origin of IRAS 19312+1950 Based on Long-Term Maser Observations

IRAS source 19312+1950 (hereafter I19312) is an infrared point source with maser emissions of SiO, H$_2$O, and OH molecules. Although initial observations suggested that I19312 might be an evolved star, its characteristics are not fully consistent with this classification. This study aims to further investigate the nature of I19312 by conducting long-term monitoring of its maser emissions and comparing the results with other known astrophysical objects. We conducted long-term monitoring of SiO, H$_2$O, and OH maser emissions using single-dish radio telescopes. The results were then compared with historical maser data and the characteristics of similar objects to infer the possible origin of I19312. The SiO maser emissions from I19312 were detected over a wide velocity range and exhibited significant time variability. The OH maser lines suggest characteristics of an evolved star, while the H$_2$O maser lines indicate molecular outflows. These features suggest that I19312 could be a candidate for a Water Fountain (WF) star, though there are inconsistencies, such as the large molecular gas mass, that challenge this hypothesis. The possibility of I19312 being a Red Nova Remnant (RNR) is also considered, but this remains speculative due to the lack of direct evidence. The evolutionary stage of I19312 remains unclear, but it shares multiple characteristics with both evolved stars with peculiar properties and RNRs. Further long-term monitoring and high-resolution interferometric observations are required to better constrain the nature of this object.

astro-ph.GA↗

arXiv2024

Water vapour masers in long-period variable stars III. Mira variables U Her and RR Aql

Within the 'Medicina/Effelsberg H2O maser monitoring program' we observed U Her and RR Aql at 22-GHz for about two decades between 1990 and 2011, with a gap between 1997 and 2000 in the case of RR Aql. In addition, maps were obtained in the period 1990-1992 of U Her with the Very Large Array. We find that the strongest emission in U Her is located in a shell with boundaries 11-25 AU. The gas crossing time is 8.5 years. We derive lifetimes for individual maser clouds of less than 4 years, based on the absence of detectable line-of-sight velocity drifts of the maser emission. The shell is not evenly filled, and its structure is maintained on timescales much longer than those of individual maser clouds. Both stars show brightness variability on several timescales. The prevalent variation is periodic, following the optical variability of the stars with a lag of 2-3 months. Superposed are irregular fluctuations, of a few months' duration, of increased or decreased excitation at particular locations, and long-term systematic variations on timescales of a decade or more. The properties of the maser emission are governed by those of the stellar wind while traversing the water maser shell. Inhomogeneities in the wind affecting the excitation conditions and prevalent beaming directions likely cause the variations seen on timescales longer than the stellar pulsation period. We propose the existence of long-living regions in the shells, which maintain favourable excitation conditions on timescales of the wind crossing times through the shells or orbital periods of (sub-)stellar companions.

astro-ph.GA↗

arXiv2023

Systematic Search for Water Fountain Candidates using the Databases of Circumstellar Maser Sources

Water fountains (WFs) are thought to be objects in the morphological evolution of the circumstellar envelopes of low- and intermediate-mass evolved stars, transitioning from spherically symmetric to asymmetric shapes. We used databases of circumstellar 1612 MHz OH and 22.235 GHz H$_2$O maser sources to search for new WF candidates using the criterion of a larger velocity range of the H$_2$O maser emission compared to that of the OH maser emission. Thus, it is in principle possible to identify WFs with an H$_2$O velocity ranges smaller than those for the previously known WFs. For the OH maser line, we analyzed database entries of 8,474 observations from 2,195 sources, and 6,085 observations from 3,642 sources for H$_2$O maser line. After a close examination of the velocity ranges and line profiles, we identified 11 sources that meet the criterion mentioned above. We examined the IRAS colors of the selected sources and found that two of them (IRAS 19069+0916 and IRAS 19319+2214) are in the color region for post-AGB stars. We find that the maser velocity criterion can discover other astrophysically interesting objects than just WFs. Such objects may include peculiar planetary nebulae with maser emissions and stellar merger remnants.

astro-ph.SR↗

arXiv2023

Rotation-tunneling spectrum and astrochemical modeling of dimethylamine, CH$_3$NHCH$_3$, and searches for it in space

Methylamine has been the only simple alkylamine detected in the interstellar medium for a long time. With the recent secure and tentative detections of vinylamine and ethylamine, respectively, dimethylamine has become a promising target for searches in space. Its rotational spectrum, however, has been known only up to 45 GHz until now. Here we investigate the rotation-tunneling spectrum of dimethylamine in selected regions between 76 and 1091 GHz using three different spectrometers in order to facilitate its detection in space. The quantum number range is extended to $J = 61$ and $K_a = 21$, yielding an extensive set of accurate spectroscopic parameters. To search for dimethylamine, we refer to the spectral line survey ReMoCA carried out with the Atacama Large Millimeter/submillimeter Array toward the high-mass star-forming region Sagittarius B2(N) and a spectral line survey of the molecular cloud G+0.693$-$0.027 employing the IRAM 30 m and Yebes 40 m radio telescopes. We report nondetections of dimethylamine toward the hot molecular cores Sgr B2(N1S) and Sgr B2(N2b) as well as G+0.693$-$0.027 which imply that dimethylamine is at least 14, 4.5 and 39 times less abundant than methylamine toward these sources, respectively. The observational results are compared to computational results from a gas-grain astrochemical model. The modeled methylamine to dimethylamine ratios are compatible with the observational lower limits. However, the model produces too much ethylamine compared with methylamine which could mean that the already fairly low levels of dimethylamine in the models may also be too high.

astro-ph.GA↗

arXiv2021

Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.

astro-ph.IM↗

arXiv2021

An infrared study of Galactic OH/IR stars. III. Variability properties of the Arecibo sample

We present the results of a near-infrared (NIR) monitoring program carried out between 1999 and 2005 to determine the variability properties of the Arecibo sample of OH/IR stars. The sample consists of 385 IRAS-selected Asymptotic Giant Branch (AGB) candidates, for which their O-rich chemistry has been proven by the detection of 1612 MHz OH maser emission. The monitoring data was complemented by data collected from public optical and NIR surveys. We fitted the light curves obtained in the optical and NIR bands with a model using an asymmetric cosine function, and derived a period for 345 sources (90% of the sample). Based on their variability properties, most of the Arecibo sources are classified as long-period large-amplitude variable stars (LPLAV), 4% as (candidate) post-AGB stars, and 3% remain unclassified although they are likely post-AGB stars or highly obscured AGB stars. The period distribution of the LPLAVs peaks at 400 days, with periods between 300 and 800 days for most of the sources, and has a long tail up to 2100 days. Typically, the amplitudes are between 1 and 3 mag in the NIR and between 2 and 6 mag in the optical. We find correlations between periods and amplitudes, with larger amplitudes associated to longer periods, as well as between the period and the infrared colours, with the longer periods linked to the redder sources. Among the post-AGB stars, the light curve of IRAS 19566+3423 was exceptional, showing a large systematic increase in K-band brightness over 7 years.

astro-ph.SR↗

arXiv2021

LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cut-off in the electron energy distribution, as has been previously suggested. Thermal (free-free) absorption or synchrotron self absorption models are able to describe the low-frequency spectral shape of the hotspots, however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic.

astro-ph.GA↗

arXiv2020

Water vapour masers in long-period variable stars II. The semi-regular variables R Crt and RT Vir

Within the 'Medicina/Effelsberg H2O maser monitoring program' we have observed the maser emission of R Crt and RT Vir for more than two decades. To get insight in the distribution and longevity of maser spots in the circumstellar envelopes, we have collected interferometric data, taken in the same period, from the literature. We confirm short-time variations of individual maser features on timescales of months to up to 1.5 years. Also decade-long variations of the general brightness level independent from individual features were seen in both stars. These are due to brightness variations occurring independently from each other in selected velocity ranges, and are independent of the optical lightcurves. Expected drifts in velocity of individual features are usually masked by blending. However, in RT Vir we found an exceptional case of a feature with a constant velocity over 7.5 years (<0.06 km/s/yr). We attribute the long-term brightness variations to the presence of regions with higher-than-average density in the stellar wind, which host several clouds which emit maser radiation on the short time scales. These regions typically need ~20 years to cross the H2O maser shell, where the right conditions to excite H2O masers are present. The constant velocity feature (11 km/s) is likely to come from a single maser cloud, which moved through about half of RT Vir's H2O maser shell without changing velocity. From this we infer that its path was located in the outer part of the H2O maser shell, where RT Vir's stellar wind apparently has already reached its terminal outflow velocity. This conclusion is corroborated by the observation that the highest H2O maser outflow velocity in RT Vir approaches the terminal outflow velocity as given by OH and CO observations. This is generally not observed in other semi-regular variable stars.

astro-ph.GA↗

arXiv2020

Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies

The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (<100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15 arcsec with a high dynamic range and good image fidelity. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna (LBA) system. We reduced the datasets and obtained an image for each A-team source. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.

astro-ph.HE↗

arXiv2019

Reduction of the maximum mass-loss rate of OH/IR stars due to unnoticed binary interaction

In 1981, the idea of a superwind that ends the life of cool giant stars was proposed. Extreme OH/IR-stars develop superwinds with the highest mass-loss rates known so far, up to a few 10^(-4) Msun/yr, informing our understanding of the maximum mass-loss rate achieved during the Asymptotic Giant Branch (AGB) phase. A condundrum arises whereby the observationally determined duration of the superwind phase is too short for these stars to become white dwarfs. Here, we report on the detection of spiral structures around two cornerstone extreme OH/IR-stars, OH26.5+0.6 and OH30.1-0.7, identifying them as wide binary systems. Hydrodynamical simulations show that the companion's gravitational attraction creates an equatorial density enhancement mimicking a short extreme superwind phase, thereby solving the decades-old conundrum. This discovery restricts the maximum mass-loss rate of AGB stars around the single-scattering radiation-pressure limit of a few 10^(-5) Msun/yr. This brings about crucial implications for nucleosynthetic yields, planet survival, and the wind-driving mechanism.

astro-ph.SR↗

arXiv2017

The LOFAR window on star-forming galaxies and AGN - curved radio SEDs and IR-radio correlation at $0 < z < 2.5$

We present a study of the low-frequency radio properties of star forming (SF) galaxies and active galactic nuclei (AGN) up to redshift $z=2.5$. The new spectral window probed by the Low Frequency Array (LOFAR) allows us to reconstruct the radio continuum emission from 150 MHz to 1.4 GHz to an unprecedented depth for a radio-selected sample of $1542$ galaxies in $\sim 7~ \rm{deg}^2$ of the LOFAR Bo\"otes field. Using the extensive multi-wavelength dataset available in Bo\"otes and detailed modelling of the FIR to UV spectral energy distribution (SED), we are able to separate the star-formation (N=758) and the AGN (N=784) dominated populations. We study the shape of the radio SEDs and their evolution across cosmic time and find significant differences in the spectral curvature between the SF galaxy and AGN populations. While the radio spectra of SF galaxies exhibit a weak but statistically significant flattening, AGN SEDs show a clear trend to become steeper towards lower frequencies. No evolution of the spectral curvature as a function of redshift is found for SF galaxies or AGN. We investigate the redshift evolution of the infrared-radio correlation (IRC) for SF galaxies and find that the ratio of total infrared to 1.4 GHz radio luminosities decreases with increasing redshift: $ q_{\rm 1.4GHz} = (2.45 \pm 0.04) \times (1+z)^{-0.15 \pm 0.03} $. Similarly, $q_{\rm 150MHz}$ shows a redshift evolution following $ q_{\rm 150GHz} = (1.72 \pm 0.04) \times (1+z)^{-0.22 \pm 0.05}$. Calibration of the 150 MHz radio luminosity as a star formation rate tracer suggests that a single power-law extrapolation from $q_{\rm 1.4GHz}$ is not an accurate approximation at all redshifts.

astro-ph.GA↗

arXiv2017

Investigating the Unification of LOFAR-detected powerful AGN in the Bo\"otes Field

Low radio frequency surveys are important for testing unified models of radio-loud quasars and radio galaxies. Intrinsically similar sources that are randomly oriented on the sky will have different projected linear sizes. Measuring the projected linear sizes of these sources provides an indication of their orientation. Steep-spectrum isotropic radio emission allows for orientation-free sample selection at low radio frequencies. We use a new radio survey of the Bo\"otes field at 150 MHz made with the Low Frequency Array (LOFAR) to select a sample of radio sources. We identify 44 radio galaxies and 16 quasars with powers $P>10^{25.5}$ W Hz$^{-1}$ at 150 MHz using cross-matched multi-wavelength information from the AGN and Galaxy Evolution Survey (AGES), which provides spectroscopic redshifts. We find that LOFAR-detected radio sources with steep spectra have projected linear sizes that are on average 4.4$\pm$1.4 larger than those with flat spectra. The projected linear sizes of radio galaxies are on average 3.1$\pm$1.0 larger than those of quasars (2.0$\pm$0.3 after correcting for redshift evolution). Combining these results with three previous surveys, we find that the projected linear sizes of radio galaxies and quasars depend on redshift but not on power. The projected linear size ratio does not correlate with either parameter. The LOFAR data is consistent within the uncertainties with theoretical predictions of the correlation between the quasar fraction and linear size ratio, based on an orientation-based unification scheme.

astro-ph.GA↗

arXiv2017

Recurring OH Flares towards o Ceti: I. location and structure of the 1990s' and 2010s' events

We present the analysis of the onset of the new 2010s' OH flaring event detected in the OH ground-state main line at 1665~MHz towards o Ceti and compare its characteristics with those of the 1990s' flaring event. This is based on a series of complementary single-dish and interferometric observations both in OH and H2O obtained with the Nancay Radio telescope (NRT), the Medicina and Effelsberg Telescopes, the European VLBI Network (EVN), and (e)Multi-Element Radio Linked Interferometer Network ((e)MERLIN). We compare the overall characteristics of o Ceti's flaring events with those which have been observed towards other thin-shell Miras, and explore the implication of these events with respect to the standard OH circumstellar-envelope model. The role of binarity in the specific characteristics of o Ceti's flaring events is also investigated. The flaring regions are found to be less than ~400$ +/- 40 mas (i.e., ~40 +/- 4$ AU) either side of o Ceti, with seemingly no preferential location with respect to the direction to the companion Mira B. Contrary to the usual expectation that the OH maser zone is located outside the H2O maser zone, the coincidence of the H2O and OH maser velocities suggests that both emissions arise at similar distances from the star. The OH flaring characteristics of Mira are similar to those observed in various Mira variables before, supporting the earlier results that the regions where the transient OH maser emission occurs are different from the standard OH maser zone.

astro-ph.SR↗

arXiv2016

The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release

The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey that will eventually cover the entire Northern sky. Each of the 3170 pointings will be observed for 8 hrs, which, at most declinations, is sufficient to produce ~5arcsec resolution images with a sensitivity of ~0.1mJy/beam and accomplish the main scientific aims of the survey which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Due to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully-automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-area low-frequency survey. In excess of 44,000 sources are detected in the images that have a resolution of 25arcsec, typical noise levels of less than 0.5 mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45d00m00s to 57d00m00s).

astro-ph.IM↗

arXiv2016

LBCS: the LOFAR Long-Baseline Calibrator Survey

(abridged). We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (50-100mJy) at frequencies around 110--190~MHz on scales of a few hundred mas. At least for the 200--300-km international baselines, we find around 1 suitable calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include calibrator source lists and fringe-rate and delay maps of wide areas -- typically a few degrees -- around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200--600~km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per square degree. Coherence times decrease from 1--3 minutes on 200-km baselines to about 1 minute on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 minutes, is seen on the DE609 baseline, which at 227km is close to being the shortest. We see median coherence times of between 80 and 110 seconds on the four longest baselines (580--600~km), and about 2 minutes for the other baselines. The success of phase transfer from calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 degree.

astro-ph.IM↗

arXiv2016

LOFAR 150-MHz observations of the Bo\"otes field: Catalogue and Source Counts

We present the first wide area (19 deg$^2$), deep ($\approx120-150$ {\mu}Jy beam$^{-1}$), high resolution ($5.6 \times 7.4$ arcsec) LOFAR High Band Antenna image of the Bo\"otes field made at 130-169 MHz. This image is at least an order of magnitude deeper and 3-5 times higher in angular resolution than previously achieved for this field at low frequencies. The observations and data reduction, which includes full direction-dependent calibration, are described here. We present a radio source catalogue containing 6276 sources detected over an area of $19$\,deg$^2$, with a peak flux density threshold of $5\sigma$. As the first thorough test of the facet calibration strategy, introduced by van Weeren et al., we investigate the flux and positional accuracy of the catalogue. We present differential source counts that reach an order of magnitude deeper in flux density than previously achieved at these low frequencies, and show flattening at 150 MHz flux densities below 10 mJy associated with the rise of the low flux density star-forming galaxies and radio-quiet AGN.

astro-ph.CO↗

arXiv2016

A large light-mass component of cosmic rays at 10^{17} - 10^{17.5} eV from radio observations

Cosmic rays are the highest energy particles found in nature. Measurements of the mass composition of cosmic rays between 10^{17} eV and 10^{18} eV are essential to understand whether this energy range is dominated by Galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal comes from accelerators capable of producing cosmic rays of these energies. Cosmic rays initiate cascades of secondary particles (air showers) in the atmosphere and their masses are inferred from measurements of the atmospheric depth of the shower maximum, Xmax, or the composition of shower particles reaching the ground. Current measurements suffer from either low precision, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays is a rapidly developing technique, suitable for determination of Xmax with a duty cycle of in principle nearly 100%. The radiation is generated by the separation of relativistic charged particles in the geomagnetic field and a negative charge excess in the shower front. Here we report radio measurements of Xmax with a mean precision of 16 g/cm^2 between 10^{17}-10^{17.5} eV. Because of the high resolution in $Xmax we can determine the mass spectrum and find a mixed composition, containing a light mass fraction of ~80%. Unless the extragalactic component becomes significant already below 10^{17.5} eV, our measurements indicate an additional Galactic component dominating at this energy range.

astro-ph.HE↗

arXiv2015

The LOFAR Multifrequency Snapshot Sky Survey (MSSS) I. Survey description and first results

We present the Multifrequency Snapshot Sky Survey (MSSS), the first northern-sky LOFAR imaging survey. In this introductory paper, we first describe in detail the motivation and design of the survey. Compared to previous radio surveys, MSSS is exceptional due to its intrinsic multifrequency nature providing information about the spectral properties of the detected sources over more than two octaves (from 30 to 160 MHz). The broadband frequency coverage, together with the fast survey speed generated by LOFAR's multibeaming capabilities, make MSSS the first survey of the sort anticipated to be carried out with the forthcoming Square Kilometre Array (SKA). Two of the sixteen frequency bands included in the survey were chosen to exactly overlap the frequency coverage of large-area Very Large Array (VLA) and Giant Metrewave Radio Telescope (GMRT) surveys at 74 MHz and 151 MHz respectively. The survey performance is illustrated within the "MSSS Verification Field" (MVF), a region of 100 square degrees centered at J2000 (RA,Dec)=(15h,69deg). The MSSS results from the MVF are compared with previous radio survey catalogs. We assess the flux and astrometric uncertainties in the catalog, as well as the completeness and reliability considering our source finding strategy. We determine the 90% completeness levels within the MVF to be 100 mJy at 135 MHz with 108" resolution, and 550 mJy at 50 MHz with 166" resolution. Images and catalogs for the full survey, expected to contain 150,000-200,000 sources, will be released to a public web server. We outline the plans for the ongoing production of the final survey products, and the ultimate public release of images and source catalogs.

astro-ph.IM↗