Search

We present search results of 22 high-latitude (∣b∣ > 25°) sightlines for OH 18 cm emission using the 305 m radio telescope at the Arecibo Observatory. These sightlines appear in neutral hydrogen emission at intermediate velocities −90 ≤ V LSR ≤ −20 km s−1 and are predicted to have a sufficient molecular composition so as to be detectable in molecular emission. Such objects, known as intermediate-velocity molecular clouds (IVMCs), have historically been detected through 12CO emission. Recent studies indicate that IVMCs may be widespread in the galaxy and have important implications for models of the interstellar medium and star formation. However, we report nondetections of OH emission toward the 22 sightlines and provide stringent upper limits on the OH column density. Using available H i and A v data in combination with existing state-of-the-art PDR models, we estimate H2 column densities and find that they are more than an order of magnitude lower than the predicted values. We also find that the hydrogen volume density of these clouds is ≲25 cm−3. In addition, we discuss the known IVMCs with previous 12CO detections in the context of the PDR models. Our analysis of these clouds indicates that the structure of molecular material in IVMCs is morphologically clumpy. These results motivate the need for future sensitive, on-the-fly searches (rather than targeted searches) for CO emission from IVMCs with resolutions on the order of ∼1′. High angular resolution (∼1′) H i and A v data will also be helpful to better constrain the structure and composition of IVMCs.

We present results from a search for radio recombination lines in three H i self-absorbing (HISA) clouds at 750 MHz and 321 MHz with the Robert C. Byrd Green Bank Telescope, and in three Galactic plane positions at 327 MHz with the Arecibo Telescope. We detect carbon recombination lines (CRRLs) in the direction of DR4 and DR21, as well as in the Galactic plane position G34.94 + 0.0. We additionally detect hydrogen recombination lines in emission in five of the six sightlines, and a Helium line at 750 MHz toward DR21. Combining our new data with 150 MHz Low Frequency Array detections of CRRL absorption toward DR4 and DR21, we estimate the electron densities of the line-forming regions by modeling the line width as a function of frequency. The estimated densities are in the range 1.4 → 6.5 cm−3 toward DR4, for electron temperatures 200 → 20 K. A dual line-forming region with densities between 3.5 → 24 cm−3 and 0.008 → 0.3 cm−3 could plausibly explain the observed line width as a function of frequency on the DR21 sight line. The central velocities of the CRRLs compare well with CO emission and HISA lines in these directions. The cloud densities estimated from the CO lines are smaller (at least a factor of five) than those of the CRRL-forming regions. It is likely that the CRRL-forming and HISA gas is located in a denser, shocked region either at the boundary of or within the CO emitting cloud.

A recently proposed concept for the Next Generation Arecibo Telescope involves an array of tightly packed small dishes [1]. In support, we present mitigation methods for grating lobe effects inherent in regularly spaced aperture synthesis.

We present preliminary results from an automated radio frequency interference (RFI) excision method being developed at the Arecibo Observatory. We recently implemented and tested a new channel weighting scheme to excise RFI confined to a relatively narrow range in frequency and time space. This excision scheme was proved to be a tremendous success in detecting weak radio spectral lines from data taken with the Arecibo (see Fig. 1a ) and the Green Bank telescopes. Currently, the RFI excision is done manually. As the next step to automate the RFI excision, we are applying Machine Learning techniques. We are also exploring the powerful tools provided by Deep Learning to increase the efficacy of the excision scheme.

The Arecibo Observatory (AO) scientific and engineering staff and the AO user community have proposed a new telescope concept, referred to as the ‘Next Generation Arecibo Telescope’ (NGAT) [1] . The proposal is for an array of dishes to be ‘tightly’ packed on a tiltable platform, on a single plane. This configuration was envisaged to obtain maximum surface brightness sensitivity. The design satisfies the following key science requirements: (a) 5 MW of continuous-wave transmitting power at 2 - 6 GHz, 1-2 arcmin beamwidth at these frequencies, and increased sky coverage for planetary defense; (b) 0° to 45° or more sky coverage from zenith to observe both parallel and perpendicular directions to the geomagnetic field, 10 MW peak transmitting power at 430 MHz (also at 220 MHz under consideration) and very high surface brightness sensitivity for incoherent scattering experiments and (c) excellent sensitivity over 200 MHz to 30 GHz frequency range and increased sky coverage and telescope pointing up to 48° from zenith to observe the Galactic Center for astronomical research.

The 44.7 ms X-ray pulsar in the supernova remnant G12.82-0.02/HESS J1813-178 has the second highest spin-down luminosity of known pulsars in the Galaxy, with E-dot=5.6e37 erg/s. Using the Green Bank Telescope, we have detected radio pulsations from PSR J1813-1749 at 4.4-10.2 GHz. The pulse is highly scattered, with an exponential decay timescale \tau longer than that of any other pulsar at these frequencies. A point source detected at this position by Dzib et al. in several observations with the Jansky Very Large Array can be attributed to the pulsed emission. The steep dependence of \tau on observing frequency explains why all previous pulsation searches at lower frequencies failed (\tau~0.25 s at 2 GHz). The large dispersion measure, DM=1087 pc/cc, indicates a distance of either 6.2 or 12 kpc according to two widely used models of the electron density distribution in the Galaxy. These disfavor a previously suggested association with a young stellar cluster at the closer distance of 4.8 kpc. The high X-ray measured column density of ~1e23/cm^2 also supports a large distance. If at d~12 kpc, HESS J1813-178 would be one of the most luminous TeV sources in the Galaxy., Comment: Accepted by ApJ; 8 pages, 7 figures

We present a model for a noise-matched phased array feed (PAF) system and compare model predictions with the measurement results. The PAF system consists of an array feed, a receiver, a beamformer, and a parabolic reflector. The novel aspect of our model is the characterization of the PAF system by a single matrix. This characteristic matrix is constructed from the open-circuit voltage covariance (OCVC) at the output of the PAF due to a signal from the observing source, ground spillover noise, sky background noise, and (low-noise) amplifier (LNA) noise. The best signal-to-noise ratio (SNR) on the source achievable with the PAF system will be the maximum eigenvalue of the characteristic matrix. The voltage covariance due to signal and spillover noise is derived by applying the Lorentz reciprocity theorem. The receiver noise covariance and noise temperature are obtained in terms of Lange invariants such that they are suitable for noise matching the array feed. The model predictions are compared with the measured performance of a 1.4 GHz, 19-element, dual-polarized PAF on the Robert C. Byrd Green Bank Telescope. We show that the model predictions, obtained with an additional noise contribution due to the measured losses ahead of the LNA, compare well with the measured ratio of system temperature to aperture efficiency as a function of frequency and as a function of offset from the boresight. Furthermore, our modeling indicates that the bandwidth over which this ratio is optimum can be improved by a factor of at least two by noise matching the PAF with the LNA.

We conducted a drift-scan observation campaign using the 305-m Arecibo telescope in January and March 2020 when the observatory was temporarily closed during the intense earthquakes and the initial outbreak of the COVID-19 pandemic, respectively. The primary objective of the survey was to search for fast radio transients, including Fast Radio Bursts (FRBs) and Rotating Radio Transients (RRATs). We used the 7-beam ALFA receiver to observe different sections of the sky within the declination region $\sim$(10$-$20) deg on 23 nights and collected 160 hours of data in total. We searched our data for single-pulse transients, covering up to a maximum dispersion measure of 11 000 pc cm$^{-3}$ at which the dispersion delay across the entire bandwidth is equal to the 13 s transit length of our observations. The analysis produced more than 18 million candidates. Machine learning techniques sorted the radio frequency interference and possibly astrophysical candidates, allowing us to visually inspect and confirm the candidate transients. We found no evidence for new astrophysical transients in our data. We also searched for emission from repeated transient signals, but found no evidence for such sources. We detected single pulses from two known pulsars in our observations and their measured flux densities are consistent with the expected values. Based on our observations and sensitivity, we estimated the upper limit for the FRB rate to be $, Comment: 11 pages, 8 figures. Accepted 2021 October 27 for publication in MNRAS

Brigham Young University (BYU), in collaboration with Cornell University, is developing the Advanced Cryogenic L-band Phased Array Camera for Arecibo (ALPACA) as a user provided facility instrument on the Arecibo 305 m radio telescope. This instrument will consist of a fully cryogenic 69-element phased array feed (PAF) and real-time digital beamformer back end capable of producing 40 simultaneous dual polarized beams with approximately 305 MHz of instantaneous bandwidth at 1.4 GHz. The target design goal is to operate with a system noise temperature of 25 K. We report on simulation results and a preliminary overview of the design and progress of the instrument’s development.

We report the detection of emission from the OH 18 cm Λ-doublet transitions toward Comet C/2020 F3 NEOWISE using the Arecibo Telescope. The antenna temperatures are 113 ± 3 mK for the 1667 MHz line and 57 ± 3 mK for the 1665 MHz line. The beam-averaged OH column density (centered on the comet nucleus) derived from the 1667 transition is N OH = (1.11 ± 0.06) × 1013 cm−2. We implemented the Haser model to derive an OH production rate. The estimated OH production rate using the 1667 transition is Q OH = (3.6 ± 0.6) × 1028 s−1, a factor of 2.4 lower than optically derived values for the same observing day, the difference of which is likely explained by quenching.

The Survey of Ionized Gas of the Galaxy, Made with the Arecibo telescope (SIGGMA) provides a fully-sampled view of the radio recombination line (RRL) emission from the portion of the Galactic plane visible by Arecibo. Observations use the Arecibo L-band Feed Array (ALFA), which has a FWHM beam size of 3.4 arcmin. Twelve hydrogen RRLs from H163$\alpha$ to H174$\alpha$ are located within the instantaneous bandpass from 1225 MHz to 1525 MHz. We provide here cubes of average ("stacked") RRL emission for the inner Galaxy region $32 \le \ell \le 70$ degrees, $|b|\le1.5$ degree, with an angular resolution of 6 arcmin. The stacked RRL rms at 5.1 km/s velocity resolution is $\sim0.65$ mJy beam$^{-1}$, making this the most sensitive large-scale fully-sampled RRL survey extant. We use SIGGMA data to catalogue 319 RRL detections in the direction of 244 known HII regions, and 108 new detections in the direction of 79 HII region candidates. We identify 11 Carbon RRL emission regions, all of which are spatially coincident with known HII regions. We detect RRL emission in the direction of 14 of the 32 supernova remnants (SNRs) found in the survey area. This RRL emission frequently has the same morphology as the SNRs. The RRL velocities give kinematic distances in agreement with those found in the literature, indicating that RRLs may provide an additional tool to constrain distances to SNRs. Finally, we analyze the two bright star-forming complexes: W49 and W51. We discuss the possible origins of the RRL emission in directions of SNRs W49B and W51C.

Using Green Bank Telescope radio recombination line (RRL) data, we analyze the role of leaking radiation from HII regions in maintaining the ionization of the interstellar medium. We observed a sample of eight Galactic HII regions of various sizes, morphologies, and luminosities. For each region the hydrogen RRL intensity decreases roughly as a power-law with distance from the center of the region. This suggests that radiation leaking from the HII region is responsible for the majority of surrounding ionized gas producing RRL emission. Our results further indicate that the hydrogen RRL intensity appears to be fundamentally related to the HII region sizes traced by their photodissociation regions, such that physically smaller HII regions show a steeper decrease in intensity with increasing distance from the region centers. As a result, giant HII regions may have a much larger effect in maintaining the ionization of the interstellar medium. For six of the eight observed HII regions we find a decrease in the 4He+/H+ abundance ratio with increasing distance, indicating that He-ionizing photons are being absorbed within the ionization front of the HII region. There is enhanced carbon RRL emission toward directions with strong continuum background, suggesting that the carbon emission is amplified by stimulated emission., 46 pages, 11 figures, 5 tables. Accepted for publication in ApJS

An FPGA-based digital-receiver has been developed for a low-frequency imaging radio interferometer, the Murchison Widefield Array (MWA). The MWA, located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia, consists of 128 dual-polarized aperture-array elements (tiles) operating between 80 and 300\,MHz, with a total processed bandwidth of 30.72 MHz for each polarization. Radio-frequency signals from the tiles are amplified and band limited using analog signal conditioning units; sampled and channelized by digital-receivers. The signals from eight tiles are processed by a single digital-receiver, thus requiring 16 digital-receivers for the MWA. The main function of the digital-receivers is to digitize the broad-band signals from each tile, channelize them to form the sky-band, and transport it through optical fibers to a centrally located correlator for further processing. The digital-receiver firmware also implements functions to measure the signal power, perform power equalization across the band, detect interference-like events, and invoke diagnostic modes. The digital-receiver is controlled by high-level programs running on a single-board-computer. This paper presents the digital-receiver design, implementation, current status, and plans for future enhancements., Comment: 14 pages, 7 figures

Polycyclic Aromatic Hydrocarbons (PAHs) are believed to be the small-size tailof the interstellar carbonaceous dust grain population. Their vibrational emissionis the most widely accepted source of the aromatic near-infrared features, andtheir rotational radiation is a likely explanation for the dust-correlated anomalousmicrowave emission (AME). Yet, no individual interstellar PAH molecule has beenidenti ed to date. It was recently recognised that quasi-symmetric planar PAHs oughtto have a well identi able comb-like rotational spectrum, and suggested to search forthem in spectroscopic data with matched- ltering techniques. We report the resultsof the rst such search, carried out with the Green Bank Telescope, and targetingthe star-forming region IC348 in the Perseus molecular cloud, a known source ofAME. Our observations amounted to 16.75 hours and spanned a 3 GHz-wide bandextending from 23.3 to 26.3 GHz. Using frequency switching, we achieved a sensitivityof 0.4 mJy per 0.4 MHz channel (1˙). The non-detection of comb-like spectra allowedus to set upper bounds on the abundance of speci c quasi-symmetric PAH molecules(speci ed uniquely by their moments of inertia) of approximately 0.1% of the totalPAH abundance. This bound generically applies to PAHs with approximately 15 to100 carbon atoms.Key words: ISM: dust, molecules, lines and bands

Phased Array Feed (PAF) technology is the next major advancement in radio astronomy in terms of combining high sensitivity and large field of view. The Focal L-band Array for the Green Bank Telescope (FLAG) is one of the most sensitive PAFs developed so far. It consists of 19 dual-polarization elements mounted on a prime focus dewar resulting in seven beams on the sky. Its unprecedented system temperature of$\sim$17 K will lead to a 3 fold increase in pulsar survey speeds as compared to contemporary single pixel feeds. Early science observations were conducted in a recently concluded commissioning phase of the FLAG where we clearly demonstrated its science capabilities. We observed a selection of normal and millisecond pulsars and detected giant pulses from PSR B1937+21., Comment: 2 pages, 3 figures, submitted for publication in the proceedings of IAU Symposium 337. Pulsar Astrophysics: The Next Fifty years

We model Carbon Recombination Line (CRL) emission from the Photo Dissociation Region (PDR) surrounding the Ultra-Compact (UC) HII region W48A. Our modelling shows that the inner regions ($A_V \sim 1$) of the CII layer in the PDR contribute significantly to the CRL emission. The dependence of line ratios of CRL emission with the density of the PDR and the far ultra-violet (FUV) radiation incident on the region is explored over a large range of these parameters that are typical for the environments of UCHII regions. We find that by observing a suitable set of CRLs it is possible to constrain the density of the PDR. If the neutral density in the PDR is high ($\gtrsim 10^7$ \cmthree) CRL emission is bright at high frequencies ($\gtrsim 20$ GHz), and absorption lines from such regions can be detected at low frequencies ($\lesssim 10$ GHz). Modelling CRL observations towards W48A shows that the UCHII region is embedded in a molecular cloud of density of about $4 \times$ 10$^7$ \cmthree., Comment: Accepted for publication in MNRAS. 6 pages 5 figures

We empirically evaluate the scheme proposed by Lieu & Duan (2013) in which the light curve of a time-steady radio source is predicted to exhibit increased variability on a characteristic timescale set by the sightline's electron column density. Application to extragalactic sources is of significant appeal as it would enable a unique and reliable probe of cosmic baryons. We examine temporal power spectra for 3C 84 observed at 1.7 GHz with the Karl G. Jansky Very Large Array and the Robert C. Byrd Green Bank Telescope. These data constrain the ratio between standard deviation and mean intensity for 3C 84 to less than 0.05% at temporal frequencies ranging between 0.1-200 Hz. This limit is 3 orders of magnitude below the variability predicted by Lieu & Duan (2013) and is in accord with theoretical arguments presented by Hirata & McQuinn (2014) rebutting electron density dependence. We identify other spectral features in the data consistent with the slow solar wind, a coronal mass ejection, and the ionosphere., 19 pages, 2 figures, ApJ in press

We measure carbon radio recombination line (RRL) emission at 5.3 GHz toward four HII regions with the Green Bank Telescope (GBT) to determine the magnetic field strength in the photodissociation region (PDR) that surrounds the ionized gas. Roshi (2007) suggests that the non-thermal line widths of carbon RRLs from PDRs are predominantly due to magneto-hydrodynamic (MHD) waves, thus allowing the magnetic field strength to be derived. We model the PDR with a simple geometry and perform the non-LTE radiative transfer of the carbon RRL emission to solve for the PDR physical properties. Using the PDR mass density from these models and the carbon RRL non-thermal line width we estimate total magnetic field strengths of B ~ 100-300 micro Gauss in W3 and NGC6334A. Our results for W49 and NGC6334D are less well constrained with total magnetic field strengths between B ~ 200-1000 micro Gauss. HI and OH Zeeman measurements of the line-of-sight magnetic field strength (B_los), taken from the literature, are between a factor of ~0.5-1 of the lower bound of our carbon RRL magnetic field strength estimates. Since |B_los, Accepted for publication in the ApJ

In the first paper of the series, Roshi, Kantharia & Anantharamaiah published the Galactic plane survey of carbon recombination lines (CRRLs) at 327 MHz. CRRLs were extensively detected from the inner Galaxy (longitudes

The Radio Ammonia Mid-Plane Survey (RAMPS) is a molecular line survey that aims to map a portion of the Galactic midplane in the first quadrant of the Galaxy (l = 10 deg - 40 deg, |b| < 0.4 deg) using the Green Bank Telescope. We present results from the pilot survey, which has mapped approximately 6.5 square degrees in fields centered at l = 10 deg, 23 deg, 24 deg, 28 deg, 29 deg, 30 deg, 31 deg, 38 deg, 45 deg, and 47 deg. RAMPS observes the NH3 inversion transitions NH3(1, 1) - (5, 5), the H2O 6(1,6) - 5(2,3) maser line at 22.235 GHz, and several other molecular lines. We present a representative portion of the data from the pilot survey, including NH3(1,1) and NH3(2,2) integrated intensity maps, H2O maser positions, maps of NH3 velocity, NH3 line width, total NH3 column density, and NH3 rotational temperature. These data and the data cubes from which they were produced are publicly available on the RAMPS website (http://sites.bu.edu/ramps/)., 86 pages, 55 figures

A new 1.4 GHz 19-element, dual-polarization, cryogenic phased array feed (PAF) radio astronomy receiver has been developed for the Robert C. Byrd Green Bank Telescope (GBT) as part of FLAG (Focal L-band Array for the GBT) project. Commissioning observations of calibrator radio sources show that this receiver has the lowest reported beamformed system temperature ($T_{\rm sys}$) normalized by aperture efficiency ($\eta$) of any phased array receiver to date. The measured $T_{\rm sys}/\eta$ is $25.4 \pm 2.5$ K near 1350 MHz for the boresight beam, which is comparable to the performance of the current 1.4 GHz cryogenic single feed receiver on the GBT. The degradation in $T_{\rm sys}/\eta$ at $\sim$ 4 arcmin (required for Nyquist sampling) and $\sim$ 8 arcmin offsets from the boresight is, respectively, $\sim$ 1\% and $\sim$ 20\% of the boresight value. The survey speed of the PAF with seven formed beams is larger by a factor between 2.1 and 7 compared to a single beam system depending on the observing application. The measured performance, both in frequency and offset from boresight, qualitatively agree with predictions from a rigorous electromagnetic model of the PAF. The astronomical utility of the receiver is demonstrated by observations of the pulsar B0329+54 and an extended HII region, the Rosette Nebula. The enhanced survey speed with the new PAF receiver will enable the GBT to carry out exciting new science, such as more efficient observations of diffuse, extended neutral hydrogen emission from galactic in-flows and searches for Fast Radio Bursts., Comment: 24 pages, 16 figures, to appear in Astronomical Journal

We propose a method to study the epoch of reionization based on the possible observation of 2p--2s fine structure lines from the neutral hydrogen outside the cosmological H {\sc ii} regions enveloping QSOs and other ionizing sources in the reionization era. We show that for parameters typical of luminous sources observed at $z \simeq 6.3$ the strength of this signal, which is proportional to the H {\sc i} fraction, has a brightness temperature $\simeq 20 \mu K$ for a fully neutral medium. The fine structure line from this redshift is observable at $\nu \simeq 1 \rm GHz$ and we discuss prospects for the detection with several operational and future radio telescopes. We also compute the characteristics of this signal from the epoch of recombination: the peak brightness is expected to be $\simeq 100 \mu K$; this signal appears in the frequency range 5-10 MHz. The signal from the recombination era is nearly impossible to detect owing to the extreme brightness of the Galactic emission at these frequencies., Comment: 17 pages, 1 figure, to appear in ApJ

Several indirect evidences indicate a magnetic origin for the nonthermal width of spectral lines observed toward molecular clouds. In this Letter, I suggest that the origin of the nonthermal width of carbon recombination lines (CRLs) observed from photodissociation regions (PDRs) near ultracompact H II regions is magnetic and that the magnitude of the line width is an estimate of the Alfven speed. The magnetic field strengths estimated based on this suggestion compare well with those measured toward molecular clouds with densities similar to PDR densities. I conclude that multifrequency CRL observations have the potential to form a new tool to determine the field strength near star-forming regions.

A low-noise cryogenic phased array feed (PAF) is in development for the Green Bank Telescope (GBT). The feed consists of electrically small elements tuned to operate near 1.4 GHz and optimized for active impedance matching to cooled front end low noise amplifiers (LNAs). A prototype cryogenic PAF with analog fiber link, down-converters and streaming data acquisition system was recently tested on the GBT. Preliminary results are presented. These efforts form an important step towards the development of a new receiver system, the focal L-band array for the GBT (FLAG).

The VEGAS multi-beam spectrometer (VEGAS) was built for the Green Bank Telescope (GBT) through a partnership between the National Radio Astronomy Observatory (NRAO) and the University of California at Berkeley. VEGAS is based on a Field Programmable Gate Array (FPGA) frontend and a heterogeneous computing backend comprised of Graphical Processing Units (GPUs) and CPUs. This system provides processing power to analyze up to 8 dual-polarization or 16 single-polarization inputs at bandwidths of up to 1.25 GHz per input. VEGAS was released for “shared-risk” observing in March 2014 and it became the default GBT spectral line backend in August 2014. Some of the early VEGAS observations include the Radio Ammonia Mid-Plane Survey, mapping of HCN/HCO+ in nearby galaxies, and a variety of radio-recombination line and pulsar projects. We will present some of the latest VEGAS science highlights.

We have detected C91α (8.5891 GHz) emission toward four ultracompact H II regions (UCHs; W49G, J, L, and C) in the W49 North massive star-forming region with the Very Large Array (VLA) at 3'' resolution. No carbon line emission was detected toward UCHs W49F, A, O, S, and Q at this frequency to a 3 σ level of 2 mJy. We also observed the same region in the C75α line (15.3 GHz) with no detection at a 3 σ level of 6 mJy with a 17 beam. Detection of line emission toward these sources add supporting data to the 2005 result of Roshi et al. that many UCHs have an associated photodissociation region (PDR). The similarity of the LSR velocities of carbon recombination lines and H2CO absorption toward UCHs in W49 North suggests that the PDRs reside in the dense interface zone surrounding these H II regions. Combining the observed carbon line parameters at 8.6 GHz with the upper limits on line emission at 15.3 GHz, we obtain constraints on the physical properties of the PDRs associated with W49G and J. The upper limit on the number density of hydrogen molecule obtained from carbon line models is ~5 × 106 cm-3.

Using the Very Large Array (VLA) the C76α and C53α recombination lines (RLs) have been detected toward the ultracompact H II region (UC H II region) G35.20-1.74. We also obtained upper limits to the carbon RLs at 6 cm (C110α and C111α) and 3.6 cm (C92α) wavelengths with the VLA. In addition, continuum images of the W48A complex (which includes G35.20-1.74) are made with angular resolutions in the range 14''-2''. Modeling the multiwavelength line and continuum data has provided the physical properties of the UC H II region and the photodissociation region (PDR) responsible for the carbon RL emission. The gas pressure in the PDR, estimated using the derived physical properties, is at least 4 times larger than that in the UC H II region. The dominance of stimulated emission of carbon RLs near 2 cm, as implied by our models, is used to study the relative motion of the PDR with respect to the molecular cloud and ionized gas. Our results from the kinematical study are consistent with a pressure-confined UC H II region with the ionizing star moving with respect to the molecular cloud. However, based on the existing data, other models to explain the extended lifetime and morphology of UC H II regions cannot be ruled out.

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution - 2 2 and 2 6 0 ) of radio recombination lines (RRLs) near 327 MHz in the longitude range l = 332 ! 89 using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between l = 358! 20 and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in "diuse" Cii regions. The l-v diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from Hii regions indicating that the distribution of the diuse Cii regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the (Cii) 158m emission from diuse Cii regions and find that upto 95% of the total observed (Cii) 158m emission can arise in diuse Cii regions if the temperature of the latter80 K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale 6 0 . Analysis of the dual-resolution observations toward a 2 wide field centered at l= 13:9 and toward the longitude range l= 1:75 to 6:75 shows the presence of narrow (V 15 km s 1 ) carbon line emitting regions extending over several degrees in l and b. The physical size perpendicular to the line-of-sight of an individual diuse Cii region in these directions is>200 pc.

A complete survey of radio recombination lines (RRLs) near 327 MHz from the galactic plane (l = 330° − 0°-89°, b = 0°) was carried out using a section of the Ooty Radio Telescope (ORT) with an angular resolution of 2° × 2°. A subset of regions in the same area was observed using the whole telescope which has a beam of 2° × 6'. Hydrogen RRLs were detected in most of the positions that were observed. The lv diagram and radial distribution computed from the observed spectra and their comparison with other species in the galactic plane indicate that the low density gas detected in the survey is distributed similar to the star forming regions. For an assumed temperature of 7000 K, we estimate that the densities and sizes of the regions are in the range 1 — 10 cm−3 and 20 — 200 pc respectively. Our data suggests that the low density ionized gas is in the form of outer envelopes of normal HII regions.

We present measurements of the singly ionized helium to hydrogen ratio ($n_{He^+}/n_{H^+}$) toward diffuse gas surrounding three Ultra-Compact HII (UCHII ) regions: G10.15-0.34, G23.46-0.20 \& G29.96-0.02. We observe radio recombination lines (RRLs) of hydrogen and helium near 5 GHz using the GBT to measure the $n_{He^+}/n_{H^+}$ ratio. The measurements are motivated by the low helium ionization observed in the warm ionized medium (WIM) and in the inner Galaxy diffuse ionized regions (DIR). Our data indicate that the helium is not uniformly ionized in the three observed sources. Helium lines are not detected toward a few observed positions in sources G10.15-0.34 \& G23.46-0.20 and the upper limits of the $n_{He^+}/n_{H^+}$ ratio obtained are 0.03 and 0.05 respectively. The selected sources harbor stars of type O6 or hotter as indicated by helium line detection toward the bright radio continuum emission from the sources with mean $n_{He^+}/n_{H^+}$ value 0.06$\pm$0.02. Our data thus show that helium in diffuse gas located a few pc away from the young massive stars embedded in the observed regions is not fully ionized.We investigate the origin of the non-uniform helium ionization and rule out the possibilities : (a) that the helium is doubly ionized in the observed regions and (b) that the low $n_{He^+}/n_{H^+}$ values are due to additional hydrogen ionizing radiation produced by accreting low-mass stars (Smith 2014). We find that selective absorption of ionizing photons by dust can result in low helium ionization but needs further investigation to develop a self-consistent model for dust in HII regions., 43 pages, 11 figures, 5 tables accepted to ApJ

The Graphics Processing Unit (GPU) has become an integral part of astronomical instrumentation, enabling high-performance online data reduction and accelerated online signal processing. In this paper, we describe a wide-band reconfigurable spectrometer built using an off-the-shelf GPU card. This spectrometer, when configured as a polyphase filter bank (PFB), supports a dual-polarization bandwidth of up to 1.1 GHz (or a single-polarization bandwidth of up to 2.2 GHz) on the latest generation of GPUs. On the other hand, when configured as a direct FFT, the spectrometer supports a dual-polarization bandwidth of up to 1.4 GHz (or a single-polarization bandwidth of up to 2.8 GHz)., 5 pages, 3 figures, Accepted for publication in PASA

A cryogenic phased array feed operating at L-band is in development for the Green Bank Telescope (GBT). The feed consists of electrically small feed elements optimized for active impedance matching to cooled front end low noise amplifiers (LNAs), down-converters, a real-time streaming data acquisition system, and beamforming algorithms applied in post-processing. A prototype cryogenic array feed was recently tested on the GBT. This results will be an important step towards the development of a new receiver instrument, the focal L-band array for the GBT (FLAG).

Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASA's w-projection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released., Accepted for publication in MNRAS

We present new low-frequency observations of the nearby radio galaxy Fornax A at 154 MHz with the Murchison Widefield Array, microwave flux-density measurements obtained from WMAP and Planck data, and gamma-ray flux densities obtained from Fermi data. We also compile a comprehensive list of previously published images and flux-density measurements at radio, microwave and X-ray energies. A detailed analysis of the spectrum of Fornax A between 154 MHz and 1510 MHz reveals that both radio lobes have a similar spatially-averaged spectral index, and that there exists a steep-spectrum bridge of diffuse emission between the lobes. Taking the spectral index of both lobes to be the same, we model the spectral energy distribution of Fornax A across an energy range spanning eighteen orders of magnitude, to investigate the origin of the X-ray and gamma-ray emission. A standard leptonic model for the production of both the X-rays and gamma-rays by inverse-Compton scattering does not fit the multi-wavelength observations. Our results best support a scenario where the X-rays are produced by inverse-Compton scattering and the gamma-rays are produced primarily by hadronic processes confined to the filamentary structures of the Fornax A lobes., Comment: 16 pages, 11 figures

We present the results of a survey for low frequency radio emission from 17 known exoplanetary systems with the Murchison Widefield Array. This sample includes 13 systems that have not previously been targeted with radio observations. We detected no radio emission at 154 MHz, and put 3 sigma upper limits in the range 15.2-112.5 mJy on this emission. We also searched for circularly polarised emission and made no detections, obtaining 3 sigma upper limits in the range 3.4-49.9 mJy. These are comparable with the best low frequency radio limits in the existing literature and translate to luminosity limits of between 1.2 x 10^14 W and 1.4 x 10^17 W if the emission is assumed to be 100% circularly polarised. These are the first results from a larger program to systematically search for exoplanetary emission with the MWA., Comment: Accepted for publication in MNRAS. Updated author list from last revision

We report on the detection of the millisecond pulsar PSR J0437-4715 with the Murchison Widefield Array (MWA) at a frequency of 192 MHz. Our observations show rapid modulations of pulse intensity in time and frequency that arise from diffractive scintillation effects in the interstellar medium (ISM), as well as prominent drifts of intensity maxima in the time-frequency plane that arise from refractive effects. Our analysis suggests that the scattering screen is located at a distance of $\sim$80-120 pc from the Sun, in disagreement with a recent claim that the screen is closer ($\sim$10 pc). Comparisons with higher frequency data from Parkes reveals a dramatic evolution of the pulse profile with frequency, with the outer conal emission becoming comparable in strength to that from the core and inner conal regions. As well as demonstrating high time resolution science capabilities currently possible with the MWA, our observations underscore the potential to conduct low-frequency investigations of timing-array millisecond pulsars, which may lead to increased sensitivity for the detection of nanoHertz gravitational waves via the accurate characterisation of ISM effects., Comment: 6 pages, 4 figures, 1 table, Accepted for publication in ApJ Letters

We present constraints on the physical properties of the ionized gas responsible for hydrogen radio recombination lines (RRLs) near 327 MHz detected in a recent Galactic plane survey made with the Ooty Radio Telescope. To obtain these constraints, we combined the data at 327 MHz with previously published RRL observations near 1.4 GHz. The density of the ionized gas is well constrained and is in the range of 1 to 10 cm-3. The data implies upper limits to the temperature and size of the line emitting regions of ~12,000 K and ~500 pc, respectively. Assuming an electron temperature of 7000 K, the derived path lengths of the line emitting region are in the range of 20 to 200 pc. The derived properties of the ionized gas responsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 μm emission and a considerable fraction of the [C II] 158 μm emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The Hα emission from these ionized gases is mostly undetected in the existing Hα surveys because of large interstellar extinction. About 50% of the free-free absorption of the Galactic nonthermal radiation observed at frequencies less than 100 MHz can be accounted for by the same ionized gas. We also discuss the origin of this low-density ionized gas in the inner Galaxy. The derived low line-of-sight filling factor (

We present the results of a low-resolution (2i ) 2i) survey of radio recombination lines (RRLs) near 327 MHz in the Galactic plane made with the Ooty Radio Telescope (ORT). Although the angular resolution is coarse, these observations represent the —rst contiguous survey of low-frequency RRL emis- sion in the longitude range l \ 330i¨0i¨89i (inner Galaxy). Hydrogen RRLs were detected in almost all directions in the inner Galaxy and carbon lines in several positions. In the outer Galaxy (l \ 172i¨252i), an unbiased set of 14 positions were observed and lines were detected toward three of them. To study the extent of the ionized gas above the Galactic disk, we have observed RRLs along the Galactic latitude at two speci—c longitudes and RRLs were detected up to b \^ 3i. The l-v diagram and (l \ 0i.0 13i.9). the radial distribution, obtained from RRL emission near 327 MHz, show good similarity with that of RRL emission near 1.4 GHz, ii intense ˇˇ 12CO emission and to some extent with the RRLs observed near 3 cm from normal H II region. These distributions are distinctly diUerent from those of Ha and H I emission from the Galactic disk. Based on a comparison of the radial distribution of diUerent com- ponents in the Galactic disk, we conclude that the diUuse RRL emission is associated with star-forming regions and possibly with a low-density component of known H II regions in the inner Galaxy. Subject headings: diUuse radiationH II regionsISM: moleculesISM: structure ¨ radio lines: ISM

A Survey of Ionized Gas in the Galaxy, made with the Arecibo telescope (SIGGMA) uses the Arecibo L-band Feed Array (ALFA) to fully sample the Galactic plane (30 < l < 75 and -2 < b < 2; 175 < l < 207 and -2 < b < 1) observable with the telescope in radio recombination lines (RRLs). Processed data sets are being produced in the form of data cubes of 2 degree (along l) x 4 degree (along b) x 151 (number of channels), archived and made public. The 151 channels cover a velocity range of 600 km/s and the velocity resolution of the survey changes from 4.2 km/s to 5.1 km/s from the lowest frequency channel to the highest frequency channel, respectively.RRL maps with 3.4 arcmin resolution and line flux density sensitivity of 0.5 mJy will enable us to identify new HII regions, measure their electron temperatures, study the physics of photodissociation regions (PDRs) with carbon RRLs, and investigate the origin of the extended low density medium (ELDM). Twelve Hn�� lines fall within the 300 MHz bandpass of ALFA; they are resampled to a common velocity resolution to improve the signal-to-noise ratio (SN) by a factor of 3 or more and preserve the line width. SIGGMA will produce the most sensitive fully sampled RRL survey to date. Here we discuss the observing and data reduction techniques in detail. A test observation toward the HII region complex S255/S257 has detected Hn�� and Cn�� lines with SN>10.

A new multi-beam spectrometer for the Green Bank Telescope (GBT) is being designed and built by a partnership between the National Radio Astronomy Observatory (NRAO) and the University of California at Berkeley, and partially funded by a National Science Foundation Advanced Technology Instrumentation grant. The spectrometer is a heterogeneous computing device based on a Field Programmable Gate Array (FPGA) front-end and a heterogeneous compute server backend, comprised of Graphical Processing Units (GPUs) and x86 CPUs. Working together, the hardware in this system provides processing power to analyse up to 8 dual-polarization or 16 single-polarization inputs, at bandwidths of up to 1.25 GHz per input. An aggregate of up to 10 GHz of bandwidth, dual polarization, may be processed with the spectrometer.

We present new wide-field observations of Centaurus A (Cen A) and the surrounding region at 118 MHz with the Murchison Widefield Array (MWA) 32-tile prototype, with which we investigate the spectral-index distribution of Cen A's giant radio lobes. We compare our images to 1.4 GHz maps of Cen A and compute spectral indices using temperature–temperature plots and spectral tomography. We find that the morphologies at 118 MHz and 1.4 GHz match very closely apart from an extra peak in the southern lobe at 118 MHz, which provides tentative evidence for the existence of a southern counterpart to the northern middle lobe of Cen A. Our spatially averaged spectral indices for both the northern and southern lobes are consistent with previous analyses, however we find significant spatial variation of the spectra across the extent of each lobe. Both the spectral-index distribution and the morphology at low radio frequencies support a scenario of multiple outbursts of activity from the central engine. Our results are consistent with inverse-Compton modelling of radio and gamma-ray data that support a value for the lobe age of between 10 and 80 Myr., National Science Foundation (U.S.) (Grant AST-0457585), National Science Foundation (U.S.) (Grant PHY-0835713), National Science Foundation (U.S.) (Grant CAREER-0847753), National Science Foundation (U.S.) (Grant AST-0908884), United States. Air Force Office of Scientific Research (Grant FA9550-0510247), Smithsonian Astrophysical Observatory, MIT School of Science

A primary advantage of phased array feeds (PAFs) is the ability to operate as a “radio camera,” forming an image with a single dish pointing. Each pixel corresponds to one of many simultaneously formed beams covering the field of view. However, variations in gain and noise levels across these beams hinder weak source detection. We present new calibration and beamformer design approaches to reduce these beam-to-beam variations, including a dual constraint beamformer which simultaneously satisfies beam gain and noise response constraints, and a dish rim calibration system to control spillover pattern variations.

A new spectrometer for the Green Bank Telescope (GBT) is being built jointly by the NRAO and the CASPER, University of California, Berkeley. The spectrometer uses 8 bit ADCs and will be capable of processing up to 1.25 GHz bandwidth from 8 dual polarized beams. This mode will be used to process data from focal plane arrays. The spectrometer supports observing mode with 8 tunable digital sub-bands within the 1.25 GHz bandwidth. The spectrometer can also be configured to process a bandwidth of up to 10 GHz with 64 tunable sub-bands from a dual polarized beam. The vastly enhanced backend capabilities will support several new science projects with the GBT., To appear in the Proceedings of the XXXth URSI General Assembly in Istanbul, August 2011, 4 pages; 3 figures

The Murchison Widefield Array (MWA) is a new low-frequency, wide-field-of-view radio interferometer under development at the Murchison Radio-astronomy Observatory in Western Australia. We have used a 32 element MWA prototype interferometer (MWA-32T) to observe two 50° diameter fields in the southern sky, covering a total of ~2700 deg[superscript 2], in order to evaluate the performance of the MWA-32T, to develop techniques for epoch of reionization experiments, and to make measurements of astronomical foregrounds. We developed a calibration and imaging pipeline for the MWA-32T, and used it to produce ~15' angular resolution maps of the two fields in the 110-200 MHz band. We perform a blind source extraction using these confusion-limited images, and detect 655 sources at high significance with an additional 871 lower significance source candidates. We compare these sources with existing low-frequency radio surveys in order to assess the MWA-32T system performance, wide-field analysis algorithms, and catalog quality. Our source catalog is found to agree well with existing low-frequency surveys in these regions of the sky and with statistical distributions of point sources derived from Northern Hemisphere surveys; it represents one of the deepest surveys to date of this sky field in the 110-200 MHz band., National Science Foundation (U.S.) (Grant AST-0457585), National Science Foundation (U.S.) (Grant AST-0821321), National Science Foundation (U.S.) (Grant AST-0908884), National Science Foundation (U.S.) (Grant AST-1008353), National Science Foundation (U.S.) (Grant PHY-0835713), United States. Air Force Office of Scientific Research (Grant FA9550-0510247), MIT School of Science, Smithsonian Astrophysical Observatory, Massachusetts Institute of Technology (Marble Astrophysics Fund), Raman Research Institute

Efforts have been underway for the last several years to develop phased array feed antennas for large reflectors such as the Green Bank Telescope (GBT) as well as mid-size reflectors for Square Kilometer Array demonstration instruments. We report on recent work on a cryogenic L-band phased array feed with the goal of reducing the beam equivalent system noise temperature so that it is competitive with that of traditional horn type feeds.

We report multifrequency radio continuum and hydrogen radio recombination line observations of HII regions near l=24.8d b=0.1d using the Giant Metrewave Radio Telescope (GMRT) at 1.28 GHz (n=172), 0.61 GHz (n=220) and the Very Large Array (VLA) at 1.42 GHz (n=166). The region consists of a large number of resolved HII regions and a few compact HII regions as seen in our continuum maps, many of which have associated infrared (IR) point sources. The largest HII region at l=24.83d and b=0.1d is a few arcmins in size and has a shell-type morphology. It is a massive HII region enclosing ~ 550 solar mass with a linear size of 7 pc and an rms electron density of ~ 110 cm^-3 at a kinematic distance of 6 kpc. The required ionization can be provided by a single star of spectral type O5.5. We also report detection of hydrogen recombination lines from the HII region at l=24.83d and b=0.1d at all observed frequencies near Vlsr=100 km/s. We model the observed integrated line flux density as arising in the diffuse HII region and find that the best fitting model has an electron density comparable to that derived from the continuum. We also report detection of hydrogen recombination lines from two other HII regions in the field.

We have mapped the [C I] emission at 492 GHz toward the supernova remnant Cas A. We detect [C I] emission from the periphery of the diffuse Photon Dominated Region (PDR) covering the disk of Cas A, as traced by the carbon recombination lines, as well as from the denser PDRs associated with the molecular clouds towards the south-east. [C I] emission is detected from both the Perseus and Orion arm molecular clouds, with the -47 km s^-1 Perseus arm feature being strong enough to be detected at all positions. We estimate the C/CO relative abundance to be 0.2 at the position of the identified CO clouds and >1 for most of the cloud. Here we show that the distribution of [C I] emitting regions compared to the C+ region and molecular cloud is consistent with a scenario involving PDRs. Using physical models for PDRs we constrain the physical properties of the [C I] line-forming regions. We estimate the densities of the [C I] emitting regions to be between 10^2 and 10^3 cm^-3. Based on rather high volume filling factors (~50%) we conclude that [C I] emission mainly arises from diffuse neutral gas in the Perseus arm., Accepted for publication in MNRAS

We report here on a survey of carbon recombination lines (RLs) near 8.5 GHz toward 17 ultra-compact \HII regions (\UCHII s). Carbon RLs are detected in 11 directions, indicating the presence of dense photodissociation regions (PDRs) associated with the \UCHII s. In this paper, we show that the carbon RLs provide important, complementary information on the kinematics and physical properties of the ambient medium near \UCHII s. Non-LTE models for the carbon line forming region are developed, assuming that the PDRs surround the \UCHII s, and we constrained the model parameters by multi-frequency RL data. Modeling shows that carbon RL emission near 8.5 GHz is dominated by stimulated emission and hence we preferentially observe the PDR material that is in front of the \UCHII continuum. We find that the relative motion between ionized gas and the associated PDR is about half that estimated earlier, and has an RMS velocity difference of 3.3 \kms. Our models also give estimates for the PDR density and pressure. We found that the neutral density of PDRs is typically $>$ 5 $\times$ 10$^5$ \cmthree and \UCHII s can be embedded in regions with high ambient pressure. Our results are consistent with a pressure confined \HII region model where the stars are moving relative to the cloud core. Other models cannot be ruled out, however. Interestingly, in most cases, the PDR pressure is an order of magnitude larger than the pressure of the ionized gas. Further investigation is needed to understand this large pressure difference., 28 pages, 7 figures, 5 tables (accepted for publication in ApJ)

We present the results of a search for the ground state hyperfine transition of the OH radical near 53 MHz using the National MST Radar Facility at Gadanki, India. The observed position was G48.4$-$1.4 near the Galactic plane. The OH line is not detected. We place a 3$\sigma$ upper limit for the line flux density at 39 Jy from our observations. We also did not detect recombination lines (RLs) of carbon, which were within the frequency range of our observations. The 3$\sigma$ upper limit of 20 Jy obtained for the flux density of carbon RL, along with observations at 34.5 and 327 MHz are used to constrain the physical properties of the line forming region. Our upper limit is consistent with the line emission expected from a partially ionized region with electron temperature, density and path lengths in the range 20 -- 300 K, 0.03 -- 0.3 cm$^{-3}$ and 0.1 -- 170 pc respectively., Comment: 6 pages, 6 figures