Search for Electromagnetic Counterparts to LIGO-Virgo Candidates: Expanded Very Large Array

Context. The G25.65+1.04 source is one of the few known ‘super-flare’ water masers in the Milky Way, but in contrast to other super-flare sources, it remains the least studied. In fact, even the nature of the source driving the water maser is still unclear. Aims. With this project, we aim to clarify the composition and properties of the G25.65+1.04 region by means of examining the parameters of the continuum sources and establishing their association with masers of different types. Methods. Our previous VLA (Karl G. Jansky Very Large Array) observation detected four continuum peaks (VLA 1-4), three of which (VLA 1-3) were closely spaced and presented a linear orientation. However, the observation, which used the VLA B configuration, lacked the spatial resolution to resolve individual sources. A higher-resolution (A configuration) VLA observation of the continuum and spectral lines was conducted in 2019 using the L, S, C, and Ku bands. Results. For the first time, the continuum source VLA 1 – associated with the flaring water maser – is resolved into two components: VLA 1A and 1B. The component VLA 1A and the water maser are found to spatially coincide and are thought to be powered by the same source, a protostar at an early stage of evolution showing active ejection. We argue that VLA 2 pinpoints an actively ejecting high-mass protostar, as it is associated with a 6.7 GHz methanol maser and a magnetised jet traced by a 22 GHz H2O maser. Highly polarised OH maser emission is detected in the vicinity of VLA 1-2, with the brightest OH maser found in VLA 2. The magnetic field, identified from the OH maser emission, ranges from ∼ + 0.4 mG in VLA 1A to ∼ − 8 mG in VLA 2. Conclusions. The G25.65+1.04 region is found to consist of at least two young stellar objects: VLA 1A and VLA 2. Both sources are found to be at an active accretion and ejection stage of evolution.

We report centimeter continuum observations with the VLA and the VLBA as well as mid-infrared observations with COMICS/SUBARU toward the components of the YLW 15 very young binary system, VLA 1 and VLA 2. The centimeter emission of the two components traces partially thick free-free emission, likely due to collimated, ionized winds. VLA 1 is an embedded protostar, undetected in the near-IR, and possibly in the Class 0 to Class I transition and powering a Herbig-Haro outflow. Its mid-IR emission appears slightly resolved with a diameter of ~16 AU, possibly tracing circumstellar material from both the envelope and the disk. VLA 2 is a typical Class I object, unresolved in the mid-IR, and is the responsible of the strong X-ray emission associated with YLW 15. The expected centimeter ''peri-stellar'' emission associated with the X-ray emission is not detected with the VLBA at 6 cm likely due to the high optical depth of the free-free emission. Strikingly, the near to mid-IR properties of YLW 15 suggest that VLA 1 is a more embedded YSO, or alternatively, less luminous than VLA 2, whereas orbital proper motions of this binary system indicate that VLA 1 is more massive than VLA 2. This result is apparently against the expected evolutionary scenario, where one expects that the more massive YSO in a binary system is the more evolved and luminous YSO. Finally, the nearby source YLW 16A is detected with the VLA, its position coincides with reported near-IR and X-ray sources.

Begun in 2001 with a total budget of around $100M, the Expanded Very Large Array (EVLA) project is the only major upgrade to the VLA undertaken since the interferometer was dedicated in 1980. The goal of this 11-year long project is to improve all the observational capabilities of the original VLA - except for collecting area and spatial resolution - by at least an order of magnitude. To achieve this, the 28 VLA antennas have been modernized with new digital data transmission systems that link to a new, wideband, fiber optic digital LO/IF system, and eight new sets of cooled receivers are under construction that will offer full frequency coverage from 1 to 50 GHz, with instantaneous bandwidths up to 8 GHz provided by two independent dual polarization frequency pairs. The new WIDAR correlator provided by NRAO's Canadian EVLA partner replaced the old VLA correlator in early 2010 and is currently undergoing commissioning. The long duration of the EVLA construction project coupled with the need to maintain the scientific productivity and user base of the telescope obviously precluded shutting down the old array while new infrastructure was built and commissioned. Consequently, the construction plan was based on the fundamental assumption that the old VLA would continue to operate as new EVLA capabilities gradually came online; in some cases, additional complexity had to be designed into new hardware in order to maintain transitional interoperability between the old analog and new digital systems as the latter were installed and commissioned. As construction has advanced, operations has increasingly had to coexist side by side with EVLA commissioning and verification. Current commissioning plans attempt to balance making new EVLA capabilities available to the user community as soon as they have been installed and verified, and maintaining a stable and robust end-to-end data acquisition and delivery process for the user community.

We describe the world's largest synthesis radio telescope, the Very Large Array (VLA), and how it can be used to complement observations with the Solar and Heliospheric Observatory (SOHO) and the Yohkoh solar spacecraft. The VLA provides images with high spatial and temporal resolution, often across the visible solar disk. The VLA also detects nonthermal radiation that is not observed with SOHO and Yohkoh, and provides estimates for the coronal magnetic field strengths that are not directly measured by these spacecraft. The VLA data can be combined with SOHO CDS, SOHO EIT, or Yohkoh SXT observations to provide new insights to the compact, variable sources, called blinkers and bright points, in the solar transition region or low corona. A new 400 cm VLA system provides images of nonthermal burst activity associated with Coronal Mass Ejections (CMEs), and may detect thermal emission from CMEs, that can be compared with SOHO's LASCO and EIT instruments to obtain new information about the origin and evolution of CMEs.

The Expanded Very Large Array (EVLA) project is currently under way to upgrade the Very Large Array (VLA) to improve the sensitivity, bandwidth, spectral resolution and frequency coverage of the existing array of 27 radio telescopes [1]. The EVLA receivers will provide continuous frequency coverage from 1 GHz to 50 GHz. Three of the cryogenically cooled EVLA receivers will provide octave band coverage for the L-band (1 GHz to 2 GHz), S-Band (2 GHz to 4 GHz) and C-band (4 GHz to 8 GHz).

Context. Recently, the H.E.S.S. Collaboration discovered a very high energy gamma-ray point source close to the Galactic plane. They offered three possible explanations for the nature of the sourc e: a gamma-ray binary, a pulsar wind nebula, or a BL Lacertae object. They concluded that the observations favoured an extreme BL Lacertae object interpretation. Aims. We investigated the nature of the radio source reported as the counterpart of the very high energy gamma-ray source. Methods. We performed high-resolution radio interferometric observations with the European Very Long Baseline Interferometry Network (EVN) at a frequency of 1.6 GHz on 2011 May 18. We also reanalysed archival 1.4-GHz radio continuum and HI spectral line data taken with the Very Large Array (VLA). Results. The accurate position of the radio source, as observed with EVN, is∼ 4 ′′ off from the one obtained in the NRAO VLA Sky Survey (NVSS). The new position is in excellent agreement with that of the proposed X-ray counterpart of the TeV source. The brightness temperature of the compact radio source is∼8× 10 7 K. From HI absorption data, a distance of about 11.5± 1.5 kpc can be inferred for this source. The large-scale HI data unveiled t he presence of a shell-like feature with the radio/X-ray/TeV point source in its interior. We interpret this shell as the last vestige of a very old supernova that exploded in a tenuous environment created by the stellar wind of its massive stellar precursor. Conclusions. The estimated brightness temperature of the radio point source counterpart of HESS J1943+213 is well below the value expected from the Doppler-boosted radio emission of a BL Lacertae object. This fact and the discovery of traces of a dista nt supernova explosion around the location of the TeV source lead us to conclude that the most likely origin of the high-energy emission is a remote pulsar wind nebula. If this scenario is true, then the HI shel l around HESS J1943+213 may represent a population of hitherto missing Galactic supernova remnants.

Based on the two-epoch European Very Long Baseline Interferometry (VLBI) Network (EVN) archive data from OH line observations of IIZw 096, we confirm that the high-resolution OH emission in this source mainly comes from two spots (OH1 and OH2) of component (comp) D1 of this merging system. We found no significant variations in the OH 1667 MHz line emission, including flux densities and peak positions. The OH 1665 MHz line emission is detected at about the 6σ level in the OH1 region by combining two epochs of EVN observations. By using archival data from the Very Long Baseline Array (VLBA), Very Large Array (VLA), and Atacama Large Millimeter Array (ALMA) observations, we investigated the properties of the environment of this component through H I, CO(3-2), and HCO+(4-3) lines and the multi-band radio continuum emission. We found that the comp D1 shows the brightest CO, HCO+ line emission, as well as multi-band radio continuum emission. The environment around D1 shows no clear velocity structure associated with circular motions, making it different from most other OH megamasers (OHMs) in the literature, which might have been caused by an effect during the merger stage. Meanwhile, we found that the CO emission shows three velocity structures around D1, including the central broad full width at half maximum (FWHM) region, the double-peak region where the CO line profile shows two separated peaks, and the region of the high-velocity clouds where the CO line peaks at a high velocity (∼11 000 km s−1). Similarly, H Iobservations in absorption also show high-velocity clouds around the D1 region, which might be due to inflows caused by the merging of two or more galaxy components. Based on the high-resolution K-band VLA and L-band VLBA observations of the radio continuum emission, we derived the brightness temperature in the range 105 K–106 K, which is consistent with other starburst dominant OHM sources in the literature. The multi-band VLA observations show that the radio continuum emission of comp D might also have contributions from free-free emission in addition to synchrotron emission. As a consequence, these results support a starburst origin for the OHMs, without the presence of an active galactic nucleus (AGN).

For nearly two decades, the aperature synthesis telescope known as the Very Large Array (VLA) has been providing a wealth of data on all types of astronomical objects. This telescope fills the need for more powerful and more sensitive instruments to gather data on a vast range of plasma phenomena in astrophysical settings. Currently the VLA is capable of observing at eight radio frequency bands (four corresponding to important spectral lines) with sensitivities on the order of 10−29 Wm −2 Hz −1, or 10−3 Jansky (1 mJy), and noise levels ten times lower. This paper describes the VLA and its capabilities, and describes how researchers interested in plasma astrophysics can use the instrument.

The Expanded Very Large Array (EVLA) project is the next generation instrument for high resolution and high sensitivity long-millimeter to short-meter wavelength radio astronomy. It is currently funded by NSF, with completion scheduled for 2012. The EVLA will upgrade the VLA with new feeds, receivers, data transmission hardware, correlator, and a new software system to enable the instrument to achieve its full potential. This software includes both that required for controlling and monitoring the instrument and that with emphasis on the scientific functions related to the telescope. The primary goals of the software are: to maximize the scientific return of the EVLA; provide ease of use, for both novices and experts; exploit commonality amongst all NRAO telescopes where possible. The software design methodology includes detailed initial use-cases and requirements from the scientists, intimate interaction between the scientists and the programmers during design and implementation, and rapid prototyping and development cycles (as short as a week). This manuscript is an update of a similar description published for the SPIE meeting two years ago; a more full description and background can be found there, while this document concentrates on the changes since then, specifically in the area of the high level design and pre-observing software (user authentication; proposal preparation, submission, and handling; observation preparation; and observation scheduling).

The National Radio Astronomy Observatory (NRAO) operates the Very Large Array (VLA) Radio Observatory on the Plains of San Augustin in New Mexico, and the Very Long Baseline Array (VLBA) with 10 antenna locations from Hawaii to St. Croix. Harmful radio frequency interference (RFI) is a growing problem for the VLA and the VLBA, and for that matter, all Earth-based radio astronomy. The paper describes an automated monitor system that has been recently installed at the VLA for use in identifying and cataloging RFI. The operation of the VLA and VLBA are summarized and the reason interference is a problem is explained. Initial results of the monitoring are included as are plans for future expansion of the program.

We have used the Very Large Array (VLA) in C configuration to carry out a sensitive 20-cm radio survey of regions of sky that have been surveyed in the far-infrared (FIR) over the wavelength range 5--200 μm with ISO (Infrared Space Observatory) as part of the European Large-Area ISO Survey (ELAIS). As usual in surveys based on a relatively small number of overlapping VLA pointings, the flux limit varies over the area surveyed: from a 5σ limit of 0.135 mJy over an area of 0.12 deg2 to 1.15 mJy or better over the whole region covered of 4.22 deg2. In this paper we present the complete radio catalogue of 867 sources, 428 of which form a complete sample in the flux range 0.2--1.0 mJy. These regions of sky have previously been surveyed to shallower flux limits at 20 cm with the VLA as part of the VLA D configuration NVSS (full width at half-maximum, FWHM=45 arcsec) and VLA B configuration FIRST (FWHM=5 arcsec) surveys. Our whole survey has a nominal 5σ flux limit a factor of 2 below that of the NVSS; 3.4 deg2 of the survey reaches the nominal flux limit of the FIRST survey and 1.5 deg2 reaches 0.25 mJy, a factor of 4 below the nominal FIRST survey limit. In addition, our survey is at a resolution intermediate between the two surveys and thus is well suited for a comparison of the reliability and resolution-dependent surface brightness effects that affect interferometric radio surveys. We have carried out a detailed comparison of our own survey and these two independent surveys in order to assess the reliability and completeness of each. Considering the whole sample, we found that to the 5σ nominal limits of 2.3 and 1.0 mJy, respectively, the NVSS and FIRST surveys have a completeness of 96+2-3 and 89+2-3 per cent and a reliability of 99+1-2 and 94+2-2 per cent.

The goal of the Expanded Very Large Array (EVLA), project is to upgrade a world-class astronomical instrument in the meter-to-millimeter wavelength bands. The project combines modern technologies with the sound design of the existing Very Large Array (VLA) to increase by an order of magnitude the sensitivity, resolution, and frequency coverage of the existing instrument. This paper discusses the techniques used to maintain phase coherence of the EVLA system. The enhancements to the VLA system include improved feeds and receivers, new Local Oscillator (LO) and Intermediate Frequency (IF) systems, a fiber optic LO distribution system, high speed digitizers, 10Gbps digital links, a dense wavelength division multiplexed fiber transmission system, and a new high speed correlator. The phase requirement for the LO system requires that a phase stability of 2.8 picoseconds per hour at 40 GHz be maintained across the entire array. To accomplish this, a near real time continuous measurement will be made of the phase delay in the fiber optic cable distributing the LO reference signals to each antenna. This information will be used by the correlator to set the delay on each of the baselines in the array.

Excited-state OH maser emission has previously been reported in the circumstellar envelopes of only two evolved stars: the Mira star AU Geminorum and the hypergiant NML Cygni. We present Very Large Array (VLA) observations of the 1665, 1667, and excited-state 4750 MHz main-line OH transitions in AU Gem and Expanded Very Large Array (EVLA) observations of the excited-state 6030 and 6035 MHz OH main-line transitions in NML Cyg. We detect masers in both main-line transitions in AU Gem but no excited-state emission in either star. We conclude that the excited-state OH emission in AU Gem is either a transient phenomenon (such as for NML Cyg, outlined below) or possibly an artifact in the data and that the excited-state OH emission in NML Cyg was generated by an episode of enhanced shock between the stellar mass loss and an outflow of the Cyg OB2 association. With these single exceptions, it therefore appears that excited-state OH emission indeed should not be predicted nor observable in evolved stars as part of their normal structure or evolution.

Observations from Global Positioning System (GPS) receivers and the Very Large Array (VLA) radio telescope recorded traveling ionospheric disturbances (TID) from underground nuclear explosions (UNEs), detonated in September 1992. The slant TEC (STEC) data derived from GPS observations were processed for all ray paths to isolate TIDs. For the TIDs from the Hunters Trophy test on 18 September 1992 and the Divider test on 23 September 1992, the propagated mean velocities of the TIDs were about 573 m/s and 740 m/s with standard deviations of 85 m/s and 135 m/s, respectively. For the VLA observations, the spectral analysis produced three‐dimensional fluctuation spectral cubes for the Hunters Trophy event. The arrival time of the TID at the VLA implied a propagation speed of 570–710 m/s. This study suggests the global availability of GNSS tracking networks and new low‐frequency (VHF) radio telescopes may offer a method of UNE detection and characterization, which could complement the International Monitoring System (IMS).

Six cataclysmic variable stars were searched for 6-cm emission using the NRAO Very Large Array. The candidate stars were SS Cyg, U Gem, TT Ari, RW Sex, EX Hya, and A0 Psc (H2252-035). None were detected; the flux density range between 0.1 and 0.4 mJy. These limits are, respectively, between a factor of 6 and 1.5 lower than the 6-cm emission detected from the magnetic binary AM Her by Chanmugam and Dulk.