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Noise parameters are a set of four measurable quantities which determine the noise performance of a radio frequency device under test (DUT). The noise parameters of a two-port device can be extracted by connecting a set of four or more source impedances at the device’s input, measuring the noise power of the device with each source connected, and then solving a matrix equation. However, sources with high reflection coefficients ( $|\Gamma |\approx 1$ ) cannot be used due to a singularity that arises in entries of the matrix. Here, we detail a new method of noise parameter extraction using a singularity-free matrix that is compatible with high-reflection sources. We show that open, short, load, and an open cable (OSLC) can be used to extract noise parameters and detail a practical measurement approach. The OSLC approach is particularly well-suited for low-noise amplifiers at frequencies below 1 GHz, where alternative methods require physically large apparatus.

Based on spectroscopic signatures, about one-third of known H2O maser sources in active galactic nuclei (AGNs) are believed to arise in highly inclined accretion disks around central engines. These "disk maser candidates" are of interest primarily because angular structure and rotation curves can be resolved with interferometers, enabling dynamical study. We identify five new disk maser candidates in studies with the Green Bank Telescope, bringing the total number published to 30. We discovered two (NGC 1320, NGC 17) in a survey of 40 inclined active galaxies (v sys < 20, 000 km s-1). The remaining three disk maser candidates were identified in monitoring of known sources: NGC 449, NGC 2979, and NGC 3735. We also confirm a previously marginal case in UGC 4203. For the disk maser candidates reported here, inferred rotation speeds are 130-500 km s-1. Monitoring of three more rapidly rotating candidate disks (CG 211, NGC 6264, VV 340A) has enabled measurement of likely orbital centripetal acceleration, and estimation of central masses ((2-7) x107 M ) and mean disk radii (0.2-0.4 pc). Accelerations may ultimately permit estimation of distances when combined with interferometer data. This is notable because the three AGNs are relatively distant (10,000 km s-1

Modern graphics processing units (GPUs) are inexpensive commodity hardware that offer Tflop/s theoretical computing capacity. GPUs are well suited to many compute-intensive tasks including digital signal processing. We describe the implementation and performance of a GPU-based digital correlator for radio astronomy. The correlator is implemented using the NVIDIA CUDA development environment. We evaluate three design options on two generations of NVIDIA hardware. The different designs utilize the internal registers, shared memory, and multiprocessors in different ways. We find that optimal performance is achieved with the design that minimizes global memory reads on recent generations of hardware. The GPU-based correlator outperforms a single-threaded CPU equivalent by a factor of 60 for a 32-antenna array, and runs on commodity PC hardware. The extra compute capability provided by the GPU maximizes the correlation capability of a PC while retaining the fast development time associated with using standard hardware, networking, and programming languages. In this way, a GPU-based correlation system represents a middle ground in design space between high performance, custom-built hardware, and pure CPU-based software correlation. The correlator was deployed at the Murchison Widefield Array 32-antenna prototype system where it ran in real time for extended periods. We briefly describe the data capture, streaming, and correlation system for the prototype array.

We present a new XMM-Newton spectrum of the Seyfert 2 nucleus of IC 2560, which hosts H2O maser emission from an inclined Keplerian accretion disk. The X-ray spectrum shows soft excess due to multitemperature ionized plasma, a hard continuum, and strong emission features, from Mg, Si, S, Ca, Fe, and Ni, mainly due to fluorescence. It is consistent with reflection of the continuum from a mostly neutral medium and obscuration due to a high column density, >1024 cm[?]2. The amplitude of the reflected component may exceed 10% of the central unobscured luminosity. This is higher than the reflected fraction of a few percent observed in other Seyfert 2 sources such as NGC 4945. We observe an emission line at 6.7 keV, possibly due to Fe XXV, undetected in previous Chandra observations. The absorption column density associated with this line is less than 1023 cm[?]2, which is lower than the obscuration of the central source. We hypothesize that this highly ionized Fe line emission originates in warm gas, which is also responsible for a scattered component of continuum emission that may dominate the spectrum between 1 and 3 keV. We compare X-ray and maser emission characteristics of IC 2560 and other AGNs that exhibit water maser emission originating in disk structures around central engines. The temperature for the region of the disk associated with maser action is consistent with the expected 400-1000 K range. The clumpiness of disk structures (inferred from the maser distribution) may depend on the unobscured luminosities of the central engines.

We present a Very Long Baseline Interferometry image of the water maser emission in the nuclear region of NGC 3393. The maser emission has a linear distribution oriented at a position angle of ~-34deg, perpendicular to both the kiloparsec-scale radio jet and the axis of the narrow-line region. The position-velocity diagram displays a red-blue asymmetry about the systemic velocity and the estimated dynamical center, and is thus consistent with rotation. Assuming Keplerian rotation in an edge-on disk, we obtain an enclosed mass of (3.1 +- 0.2) x 107 M within 0.36 +- 0.02 pc ( 1.48 +- 0.06 mas), which corresponds to a mean mass density of ~108.2 M pc[?]3. We also report the measurement with the Green Bank Telescope of a velocity drift, a manifestation of centripetal acceleration within the disk, of 5 +- 1 km s[?]1 yr[?]1 in the ~3880 km s[?]1 maser feature, which is most likely located along the line of sight to the dynamical center of the system. From the acceleration of this feature, we estimate a disk radius of 0.17 +- 0.02 pc, which is smaller than the inner disk radius ( 0.36 +- 0.02 pc) of emission that occurs along the midline (i.e., the line of nodes). The emission along the line of sight to the dynamical center evidently occurs much closer to the center than the emission from the disk midline, contrary to the situation in the archetypal maser systems NGC 4258 and NGC 1068. The outer radius of the disk as traced by the masers along the midline is about 1.5 pc.

We report the discovery of water maser emission in five active galactic nuclei (AGNs) with the 100 m Green Bank Telescope (GBT). The positions of the newly discovered masers, measured with the VLA, are consistent with the optical positions of the host nuclei to within 1 s (0.''3 radio and 1.''3 optical) and most likely mark the locations of the embedded central engines. The spectra of three sources, 2MASX J08362280+3327383, NGC 6264, and UGC 09618 NED02, display the characteristic spectral signature of emission from an edge-on accretion disk with maximum orbital velocity of ~700, ~800, and ~1300 km s-1, respectively. We also present a GBT spectrum of a previously known source, Mrk 0034, and interpret the narrow Doppler components reported here as indirect evidence that the emission originates in an edge-on accretion disk with orbital velocity of ~500 km s-1. We obtained a detection rate of 12% (5 out of 41) among Seyfert 2 and LINER systems with 10,000 km s-1< vsys < 15,000 km s-1. For the 30 nuclear water masers with available hard X-ray data, we report a possible relationship between unabsorbed X-ray luminosity (2-10 keV) and total isotropic water maser luminosity, L2-10 [?] Limg1.gif, consistent with the model proposed by Neufeld and Maloney, in which X-ray irradiation and heating of molecular accretion disk gas by the central engine excites the maser emission.

We present an analysis of polarimetric observations at 22 GHz of the water vapor masers in NGC 4258 obtained with the VLA and the GBT. We do not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water, a non-paramagnetic molecule. We have improved the 1 s upper limit estimate on the toroidal component of the magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from 300 to 90 mG. We have developed a new method for the analysis of spectra with blended features and derive a 1 s upper limit of 30 mG on the radial component of the magnetic field at a radius of 0.14 pc. Assuming thermal and magnetic pressure balance, we estimate an upper limit on the mass accretion rate of ~10-3.7 M yr-1 for a total magnetic field of less than 130 mG. We discuss the ramifications of our results on current maser models proposed to explain the observed maser emission structure and the consequences for current accretion theories. We find from our magnetic field limits that the thin-disk model and the jet-disk model are better candidates for accounting for the extremely low luminosity nature of NGC 4258 than models that include advection-dominated accretion flows.

We report monitoring of the 0.3-10 keV spectrum of NGC 4258 with the XMM-Newton Observatory at five epochs over 1.5 yr. We also report reprocessing of an overlapping four-epoch series of archival Chandra observations (0.5-10 keV). By including earlier ASCA and BeppoSAX observations, we present a new, 9 year time series of models fitted to the X-ray spectrum of NGC 4258. We model the Chandra and XMM-Newton data self-consistently with partially absorbed, hard power-law, soft thermal plasma, and soft power-law components. Over the 9 years, the photoelectric absorbing column (~1023 cm-2) did not vary detectably, except for a ~40% drop between two ASCA epochs separated by 3 years (in 1993 and 1996) and a ~60% rise between two XMM-Newton epochs separated by just 5 months (in 2001 and 2002). In contrast, factor of 2-3 changes are seen in absorbed flux on the timescale of years. These are uncorrelated with changes in absorbing column and indicative of central engine variability. The most rapid change in luminosity (5-10 keV) that we detect (with XMM-Newton and Chandra) is ~30% over 19 days. The warped disk, a known source of H2O maser emission in NGC 4258, is believed to cross the line of sight to the central engine. We propose that the variations in absorbing column arise from inhomogeneities sweeping across the line of sight in the rotating disk at the radius at which the disk crosses the line of sight. We estimate that the inhomogeneities are ~1015 cm in size at the crossing radius of 0.29 pc, slightly smaller than the expected scale height of the disk. This result thus provides strong evidence that the warped accretion disk is the absorber in this (and possibly other) active galactic nuclei (AGNs). This is the first direct confirmation that obscuration in type 2 AGNs may, in some cases, arise in thin, warped accretion disks rather than in geometrically thick tori. Some previous studies report detection of weak Fe Ka emission in NGC 4258. We do not detect this line emission in any of our XMM-Newton spectra with a 90% upper limit to the equivalent width of ~49 eV for one observation. Weak, time-variable Fe line absorption has also been reported for a previous Chandra study. We do not observe evidence of absorption lines in the XMM-Newton or reprocessed Chandra data. The absence of Fe line emission is consistent with the disk being optically thin to hard photons, as well as subtending a small solid angle as seen from the central engine because of the known shallowness of the warp.

We have mapped, for the first time, the full velocity extent of the water maser emission in NGC 3079. The largely north-south distribution of emission, aligned with a kiloparsec-scale molecular disk, and the segregation of blueshifted and redshifted emission on the sky are suggestive of a nearly edge-on molecular disk on parsec scales. Positions and line-of-sight velocities of blueshifted and redshifted maser emission are consistent with a central mass of ~2 x 106 M enclosed within a radius of ~0.4 pc. The corresponding mean mass density of 106.8 M pc-3 is suggestive of a central black hole, which is consistent with the detection of hard X-ray excess (20-100 keV) and an Fe Ka line from the nucleus. Because the rotation curve traced by the maser emission is flat, the mass of the parsec-scale disk is significant with respect to the central mass. Since the velocity dispersion of the maser features does not decrease with radius and constitutes a large fraction of the orbital velocity, the disk is probably thick and flared. The rotation curve and the physical conditions necessary to support maser emission imply a Toomre Q-parameter that is [?]1. Thus, the disk is most likely clumpy, and we argue that it is probably forming stars. Overall, the accretion disk in NGC 3079 stands in contrast to the compact, thin, warped, differentially rotating disk in the archetypal maser galaxy NGC 4258. We have also mapped radio continuum emission in the vicinity of the disk and identify a new, time-variable, nonthermal component (E) that is not collinear with the previously imaged putative jet. Based on the large luminosity and the unusually steep spectrum (a < -2.1), we exclude a radio supernova as the progenitor of E. However, because its spectrum is consistent with an aging electron energy distribution, E might be a rapidly cooling remnant, which may indicate that the jet axis wobbles. Alternatively, considering its location, the component might mark a shock in a wide-angle outflow that is interacting with a dense ambient medium. In this context, masers at high latitudes above the disk, mapped in this and previous studies, may be tracing an inward extension of the kiloparsec-scale bipolar wide-angle outflow previously observed along the galactic minor axis.

We report the discovery of two binary millisecond pulsars in the core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20 cm. PSR J2140-2310A (M30A) is an eclipsing 11 ms pulsar in a 4 hr circular orbit, and PSR J2140-23B (M30B) is a 13 ms pulsar in an as yet undetermined but most likely highly eccentric (e > 0.5) and relativistic orbit. Timing observations of M30A with a 20 month baseline have provided precise determinations of the pulsar's position (within 4'' of the optical centroid of the cluster) and spin and orbital parameters, which constrain the mass of the companion star to be m2 [?] 0.1 M. The position of M30A is coincident with a possible thermal X-ray point source found in archival Chandra data, which is most likely caused by emission from hot polar caps on the neutron star. In addition, there is a faint (V555 ~ 23.8) star visible in archival Hubble Space Telescope (HST) F555W data that may be the companion to the pulsar. Eclipses of the pulsed radio emission from M30A by the ionized wind from the compact companion star show a frequency-dependent duration ([?]n-a with a ~ 0.4-0.5) and delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms. Future observations of M30 may allow both the measurement of post-Keplerian orbital parameters from M30B and the detection of new pulsars through the effects of strong diffractive scintillation.

We analyze recent simultaneous Chandra/RXTE observations of the Seyfert 2 galaxy NGC 4945. The unprecedented spatial resolution of Chandra means we are able to separate the spectra of the nucleus, starburst, and superwind regions, while the RXTE data extend the spectrum to higher energies. The extreme absorbing column of NH ~ 4 x 1024 cm-2 means that the nucleus is only directly seen above 8-10 keV, while the lower energy spectrum from the nuclear region in Chandra is dominated by reflection. By contrast, the superwind is dominated by emission from hot plasma, but the starburst region contains both hot plasma and reflection signatures. Forming a reflected spectrum requires that the starburst region contain clumps of cool, optically thick material, perhaps star-forming cores, which are irradiated by 7-10 keV photons from the nucleus. Since photons of this energy are obscured along the line of sight, this confirms the result of Madejski et al. that the extreme absorption material is disklike rather than toroidal. However, the IR/optical limits on the lack of high-excitation emission lines show that, by contrast, the lower energy photons from the nucleus are obscured in all directions. We discuss the complex absorption structure revealed by these observations and propose an overall source geometry in which the nucleus is completely embedded in material with NH ~ 1023 cm-2.

We present Very Long Baseline Array (VLBA) observations of water maser emission associated with the star-forming region IRAS 21391+5802, which is embedded in a bright-rimmed cometary globule in IC 1396. The angular resolution of the maps is ~0.8 mas, corresponding to a spatial resolution of ~0.6 AU at an estimated distance of 750 pc. Proper motions are derived for 10 maser features identified consistently over three epochs, which were separated by intervals of about 1 month. The masers appear in four groups, which are aligned linearly on the sky, roughly along a northeast-southwest direction, with a total separation of ~520 AU (~07). The three-dimensional velocities of the masers have a maximum value of ~42 km s-1(~9 AU yr-1). The average error on the derived proper motions is ~4 km s-1. The overall pattern of proper motions is indicative of a bipolar outflow. Proper motions of the masers in a central cluster, with a projected extent of ~20 AU, show systematic deviations from a radial outflow. However, we find no evidence of Keplerian rotation, as has been claimed elsewhere. A nearly circular loop of masers lies near the middle of the cluster. The radius of this loop is 1 AU, and the line-of-sight velocities of the masers in the loop are within 2 km s-1of the systemic velocity of the region. These masers presumably exist at the radial distance where significant dust condensation occurs in the outflow emanating from the star.

The water masers in NGC 4258 delineate the structure and dynamics of a subparsec-diameter accretion disk around a supermassive black hole. Very Long Baseline Array (VLBA) observations provide precise information about the positions in the plane of the sky and the three-dimensional velocity vectors for the maser emission, but the positions along the line of sight must be inferred from models. Previous measurements placed an upper limit on the accelerations of the high-velocity spectral features of 1 km s-1 yr-1, suggesting that they are located near the midline (the diameter perpendicular to the line of sight), where they would have exactly zero acceleration. From similar measurements, the accelerations of the systemic velocity spectral features have been estimated to be about 9 km s-1 yr-1, indicating that they lie toward the front of the disk where the acceleration vector points directly away from the line of sight. We report acceleration measurements for 12 systemic velocity spectral features and 19 high-velocity spectral features using a total of 25 epochs of observations from Effelsberg (five epochs), the Very Large Array (15 epochs), and the VLBA (five epochs) spanning the years 1994-1997. The measured accelerations of the systemic velocity features are between 7.5 and 10.4 km s-1 yr-1, and there is no evidence for a dip in the spectrum at the systemic velocity. Such a dip has been attributed in the past to an absorbing layer of noninverted H2O. The accelerations of the high-velocity features, measured here for the first time, range from -0.77 to 0.38 km s-1 yr-1. From the line-of-sight accelerations and velocities, we infer the positions of these high-velocity masers with a simple edge-on disk model. The resulting positions fall between -13.deg6 and 9.deg3 in azimuth (measured from the midline). A model that suggests a spiral shock origin of the masers, in which changes in maser velocity are due to the outward motion of the shock wave, predicts apparent accelerations of -0.05(thp/2.deg5) km s-1 yr-1, where thp is the pitch angle of the spiral arms. Our data are not consistent with these predictions. We also discuss the physical properties of the high-velocity masers. Most notably, the strongest high-velocity masers lie near the midline, where the velocity gradient is smallest, thereby providing the longest amplification path lengths.

We have observed and imaged the SiO maser source in the late-type star VX Sgr with a single VLBI baseline at 3.5 mm wavelength. Phase-referenced channel maps with a beamwidth of ~0.4 mas show multiple components over a LSR velocity range of -1 to 24 km s-1, and many of the maser features display velocity widths smaller than 1 km s-1. The maser spots in this 86 GHz transition (v= 1, J= 2 ? 1) are arranged in an irregular ring-shaped configuration of diameter ~30 mas, echoing the structures found in the J= 1 ? 0 transition at 43 GHz around several Mira variables and VX Sgr itself. Unlike previous observations of SiO masers, in VX Sgr we detect a clear, systematic velocity gradient of ~0.6 km s-1AU-1among the strongest complex of masers on the southernmost portion of the ring structure. This velocity gradient can be interpreted as a rigid rotation of the circumstellar envelope with vsin i~ 13 km s-1. Maser emission is not detected on long (5000 km) baselines, which places a 0.1 mas lower limit on the size of strong maser spots.

We have conducted VLBI observations at subparsec resolution of water maser and radio continuum emission in the nucleus of the nearby active galaxy NGC 3079. The 22 GHz maser emission arises in compact (~0.01 pc at a distance of 16 Mpc) clumps, distributed over ~2 pc along an axis that is approximately aligned with the major axis of the galactic disk. The Doppler velocities of the water maser clumps are consistent with their lying in the inner parsec of a molecular disk with a binding mass ~106M?, rotating in the same sense as the edge-on kiloparsec-scale molecular disk observed in CO emission. However, the velocity field has a significant nonrotational component, which may indicate supersonic turbulence in the disk. This distribution is markedly different from that of water masers in NGC 4258, which trace a nearly perfectly Keplerian rotating disk with a binding mass of 3.5 × 107M?. The 22 GHz radio continuum emission in NGC 3079 is dominated by a compact (<0.1 pc) source that is offset 0.5 pc to the west of the brightest maser feature. No bright maser emission is coincident with a detected compact continuum source. This suggests that the large apparent luminosity of the maser is not caused by beamed amplification of high brightness temperature continuum emission. At 8 and 5 GHz, we confirm the presence of two compact continuum sources with a projected separation of 1.5 pc. Both have inverted spectra between 5 and 8 GHz and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet.

Radio astronomy observatories with high throughput back end instruments require real-time data processing. While computing hardware continues to advance rapidly, development of real-time processing pipelines remains difficult and time-consuming, which can limit scientific productivity. Motivated by this, we have developed Bifrost: an open-source software framework for rapid pipeline development.(a)Bifrost combines a high-level Python interface with highly efficient reconfigurable data transport and a library of computing blocks for CPU and GPU processing. The framework is generalizable, but initially it emphasizes the needs of high-throughput radio astronomy pipelines, such as the ability to process data buffers as if they were continuous streams, the capacity to partition processing into distinct data sequences (e.g. separate observations), and the ability to extract specific intervals from buffered data. Computing blocks in the library are designed for applications such as interferometry, pulsar dedispersion and timing, and transient search pipelines. We describe the design and implementation of the Bifrost framework and demonstrate its use as the backbone in the correlation and beamforming back end of the Long Wavelength Array (LWA) station in the Sevilleta National Wildlife Refuge, NM.