Drift-scan Observations Research Articles

The Tracking Tapered Gridded Estimator for the 21-cm power spectrum from MWA drift scan observations I: Validation and preliminary results

The Tracking Tapered Gridded Estimator for the 21-cm power spectrum from MWA drift scan observations I: Validation and preliminary results

The Effects of the Local Environment on a Compact Radio Interferometer I: Cross-Coupling in the Tianlai Dish Pathfinder Array

The visibilities measured by radio astronomical interferometers include non-astronomical correlated signals that arise from the local environment of the array. These correlated signals are especially important in compact arrays such as those under development for 21[Formula: see text]cm intensity mapping. The amplitudes of the contaminated visibilities can exceed the expected 21[Formula: see text]cm signal and represent a significant systematic effect. We study the receiver noise radiated by antennas in compact arrays and develop a model for how it couples to other antennas. We apply the model to the Tianlai Dish Pathfinder Array (TDPA), a compact array of 16, 6-m dish antennas. The coupling model includes electromagnetic simulations, measurements with a network analyzer, and measurements of the noise of the receivers. We compare the model to drift-scan observations with the array and set requirements on the level of antenna cross-coupling for 21[Formula: see text]cm intensity mapping instruments. We find that for the TDPA, cross-coupling would have to be reduced by three orders of magnitude in order to contribute negligibly to the visibilities.

The tracking tapered gridded estimator for the power spectrum from drift scan observations

ABSTRACT Intensity mapping with the redshifted 21-cm line is an emerging tool in cosmology. Drift scan observations, where the antennas are fixed to the ground and the telescope’s pointing centre (PC) changes continuously on the sky due to earth’s rotation, provide broad sky coverage and sustained instrumental stability needed for 21-cm intensity mapping. Here, we present the Tracking Tapered Grided Estimator (TTGE) to quantify the power spectrum of the sky signal estimated directly from the visibilities measured in drift scan radio interferometric observations. The TTGE uses the data from the different PC to estimate the power spectrum of the signal from a small angular region located around a fixed tracking centre (TC). The size of this angular region is decided by a suitably chosen tapering window function that serves to reduce the foreground contamination from bright sources located at large angles from the TC. It is possible to cover the angular footprint of the drift scan observations using multiple TC, and combine the estimated power spectra to increase the signal-to-noise ratio. Here, we have validated the TTGE using simulations of $154 \, {\rm MHz}$ Murchison Wide-field Array drift scan observations. We show that the TTGE can recover the input model angular power spectrum Cℓ within $20 {{\ \rm per\ cent}}$ accuracy over the ℓ range 40 < ℓ < 700.

An in-depth investigation of 11 pulsars discovered by FAST

ABSTRACT We present timing solutions and analyses of 11 pulsars discovered by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These pulsars were discovered using an ultrawide bandwidth receiver in drift-scan observations made during the commissioning phase of FAST, and were then confirmed and timed using the 64-m Parkes Radio Telescope. Each pulsar has been observed over a span of at least one year. Highlighted discoveries include PSR J0344−0901, which displays mode-changing behaviour and may belong to the class of so-called swooshing pulsars (alongside PSRs B0919+06 and B1859+07); PSR J0803−0942, whose emission is almost completely linearly polarized; and PSRs J1900−0134 and J1945+1211, whose well-defined polarization angle curves place stringent constraints on their emission geometry. We further discuss the detectability of these pulsars by earlier surveys, and highlight lessons learned from our work in carrying out confirmation and monitoring observations of pulsars discovered by a highly sensitive telescope, many of which may be applicable to next-generation pulsar surveys. This paper marks one of the first major releases of FAST-discovered pulsars, and paves the way for future discoveries anticipated from the Commensal Radio Astronomy FAST Survey.

Active Optics in Astronomy: Freeform Mirror for the MESSIER Telescope Proposal

Active optics techniques in astronomy provide high imaging quality. This paper is dedicated to highly deformable active optics that can generate non-axisymmetric aspheric surfaces—or freeform surfaces—by use of a minimum number of actuators. The aspheric mirror is obtained from a single uniform load that acts over the surface of a closed-form substrate whilst under axial reaction to its elliptical perimeter ring during spherical polishing. MESSIER space proposal is a wide-field low-central-obstruction folded-two-mirror-anastigmat or here called briefly three-mirror-anastigmat (TMA) telescope. The optical design is a folded reflective Schmidt. Basic telescope features are 36 cm aperture, f/2.5, with 1.6° × 2.6° field of view and a curved field detector allowing null distortion aberration for drift-scan observations. The freeform mirror is generated by spherical stress polishing that provides super-polished freeform surfaces after elastic relaxation. Preliminary analysis required use of the optics theory of 3rd-order aberrations and elasticity theory of thin elliptical plates. Final cross-optimizations were carried out with Zemax raytracing code and Nastran FEA elasticity code in order to determine the complete geometry of a glass ceramic Zerodur deformable substrate.

Dipole Anisotropy as an Essential Qualifier for the Monopole Component of the Cosmic-dawn Spectral Signature, and the Potential of Diurnal Pattern for Foreground Estimation

Abstract While the importance of detecting the global spectral signatures of the redshifted 21 cm line of atomic hydrogen from the very early epochs cannot be overstated, the associated challenges are not limited to isolating the weak signal of interest from the orders of magnitude brighter foregrounds, and extend equally to reliably establishing the origin of the apparent global signal to the very early epochs. This Letter proposes a critical dipole test that the measurements of the monopole component of the spectrum of interest should necessarily pass. Our criterion is based on a unique correspondence between the intrinsic monopole spectrum and the differential spectrum as an imprint of dipole anisotropy (DA) resulting from the motion of observer with respect to the rest frame of our source (such as that of our solar system, interpreted from the DA in the cosmic microwave background radiation (CMBR)). More importantly, the spectral manifestation of the DA gets amplified by a significant factor, depending on the monopole spectral slopes, rendering it feasible to measure. We describe the details of such a test, and illustrate its application with the help of simulations. The Letter also alludes to a novel model-independent path toward isolating the foreground contribution, using the diurnal pattern readily apparent in drift-scan observations. Such a dipole qualifier for the monopole spectrum, when combined with reliable foreground estimation, is expected to pave way for in situ validation of spectral signatures from early epochs, which are important to presently reported and future detections of Epoch of Reionization (EoR) signal.

Spectral-Line Observations Using a Phased Array Feed on the Parkes Telescope

AbstractWe present first results from pilot observations using a phased array feed (PAF) mounted on the Parkes 64-m radio telescope. The observations presented here cover a frequency range from 1 150 to 1 480 MHz and are used to show the ability of PAFs to suppress standing wave problems by a factor of ~10, which afflict normal feeds. We also compare our results with previous HIPASS observations and with previous H i images of the Large Magellanic Cloud. Drift scan observations of the GAMA G23 field resulted in direct H i detections at z = 0.0043 and z = 0.0055 of HIPASS galaxies J2242-30 and J2309-30. Our new measurements generally agree with archival data in spectral shape and flux density, with small differences being due to differing beam patterns. We also detect signal in the stacked H i data of 1 094 individually undetected galaxies in the GAMA G23 field in the redshift range 0.05 ⩽ z ⩽ 0.075. Finally, we use the low standing wave ripple and wide bandwidth of the PAF to set a 3σ upper limit to any positronium recombination line emission from the Galactic Centre of <0.09 K, corresponding to a recombination rate of <3.0 × 1045 s−1.

DELAY SPECTRUM WITH PHASE-TRACKING ARRAYS: EXTRACTING THE H i POWER SPECTRUM FROM THE EPOCH OF REIONIZATION

ABSTRACT The detection of redshifted 21 cm emission from the epoch of reionization (EoR) is a challenging task owing to strong foregrounds that dominate the signal. In this paper, we propose a general method, based on the delay spectrum approach, to extract H i power spectra that are applicable to tracking observations using an imaging radio interferometer ("Delay Spectrum with Imaging Arrays"). Our method is based on modeling the H i signal taking into account the impact of wide field effects such as the w-term, which are then used as appropriate weights in cross-correlating the measured visibilities. Our method is applicable to any radio interferometer that tracks a phase center and could be utilized for arrays such as the Murchison Widefield Array (MWA), Low Frequency Array (LOFAR), Giant Meterwave Radio Telescope (GMRT), Donald C. Backer Precision Array for Probing the Epoch of Reionization (PAPER), and HERA. In the literature the delay spectrum approach has been implemented for near-redundant baselines using drift scan observations. In this paper we explore the scheme for non-redundant tracking arrays. This is the first application of delay spectrum methodology to such data to extract the H i signal. We analyze 3 hr of MWA tracking data on the EoR1 field. We present both two-dimensional ( ) and one-dimensional (k) power spectra from the analysis. Our results are in agreement with the findings of other pipelines developed to analyze the MWA EoR data.

Wide-bandwidth drift-scan pulsar surveys of globular clusters: application to early science observations with FAST

The Five-hundred-meter Aperture Spherical Telescope (FAST) will begin its early-science operations during 2016. Drift-scan pulsar surveys will be carried out during this period using an ultra-wide-band receiver system (covering ∼ 270 to 1620 MHz). We describe a method for accounting for the changes in the telescope beam shape and the pulsar parameters when searching for pulsars over such a wide bandwidth. We applied this method to simulated data sets of pulsars in globular clusters that are visible to FAST and found that a representative observation would have a sensitivity of ∼ 40 μJy. Our results showed that a single drift-scan (lasting less than a minute) is likely to find at least one pulsar for observations of four globular clusters. Repeated observations will increase the likely number of detections. We found that pulsars in ∼16 clusters are likely to be found if the data from 100 drift-scan observations of each cluster are incoherently combined.

AMI Galactic Plane Survey at 16 GHz - I. Observing, mapping and source extraction

The AMI Galactic Plane Survey (AMIGPS) is a large area survey of the outer Galactic plane to provide arcminute resolution images at milli-Jansky sensitivity in the centimetre-wave band. Here we present the first data release of the survey, consisting of 868 deg^2 of the Galactic plane, covering the area 76 deg \lessapprox l \lessapprox 170 deg between latitudes of |b| \lessapprox 5 deg, at a central frequency of 15.75 GHz (1.9 cm). We describe in detail the drift scan observations which have been used to construct the maps, including the techniques used for observing, mapping and source extraction, and summarise the properties of the finalized datasets. These observations constitute the most sensitive Galactic plane survey of large extent at centimetre-wave frequencies greater than 1.4 GHz.

DISCOVERY OF TWO RADIO TRANSIENTS AT A HIGH GALACTIC LATITUDE IN A 1.4 GHz DRIFT-SCAN SURVEY

We report two new radio transients at high Galactic latitude, WJN J0951+3300 (α = 09h51m22s ± 10s, δ = 33°00' ± 0°.4, b = 50°542) and WJN J1039+3300 (α = 10h39m26s ± 10s, δ = 33°00' ± 0°.4, b = 60°585), which were detected by interferometric drift-scan observations at 1.4 GHz at the Waseda Nasu Pulsar Observatory. WJN J0951+3300 was detected at 16:49:32 UT on 2006 January 12 with the flux density of approximately 1760.5 ± 265.9 mJy, and WJN J1039+3300 was detected at 17:13:32 UT on 2006 January 18 with the flux density of approximately 2242.5 ± 228.7 mJy. Both of them lasted for a short duration (≤2 days). The possibility that the distribution of the WJN radio transients is isotropic was suggested in a previous study. Having re-evaluated the log N-log S relation with the addition of the two new objects reported in this paper, we find that the slope is consistent with a slope of –1.5 and the previous result. Additionally, although there are several counterparts to WJN radio transients, we found that one of the quasar counterparts within the positional error of WJN J0951+3300 could be a radio-loud quasar. We have discussed whether or not WJN J0951+3300 could be of this quasar origin.

Quasar Detection via Variability in a High Galactic Latitude Drift‐Scan Survey

This dissertation studies the intrinsic variability of quasars as a means of quasar detection. The detection of quasars via a parameter that is independent of quasar color allows for comparison with other color-selected samples of quasars, giving a more complete understanding of any possible selection effects in present and future quasar catalogs. Using the 16 CCD QUEST Phase 1 camera on the 1 m Schmidt telescope at the Venezuelan National Astronomical Observatory, 69 drift-scan observations of a high Galactic latitude region along the celestial equator were obtained. These observations, taken over the course of three observing seasons (1999 February–March, 2000 February–March, and 2001 March–April), cover the region between and , centered at a declination of 1 . h h m 10 15 30 Four broadband filters (B, V, and two R filters) were placed over the four rows of four CCDs, allowing the capture of four images of each object passing across the camera field of view. A large region of the sky ( ) was sampled over a range 2 194 deg of timescales (from hourly to biennially) for a duration of 26 months. This time sampling allows a region of the time domain distinct from other large sky surveys to be investigated. Following standard data reduction using a suite of custom software, incomplete ensemble photometry was used to correct for varying weather conditions and to compile a list of candidate objects that are variable at a statistically significant level. A confidence level for every object in each bandpass was calculated. An object was required to be variable at the 85% confidence level or greater in at least the two R filters to remain viable. Candidate lists were compiled from the data using both a 13 and 26 month time baseline. The quasars published in the ninth edition of the catalog of Veron-Cetty & Veron (2000, A Catalogue of Quasars and Active Galactic Nuclei [ESO Sci. Rep. 19; Garching: ESO]) were used to benchmark the variability characteristics of quasars over both timescales. After 13 months, 38% of the Veron-Cetty & Veron quasars within the survey were seen to vary at greater than the 85% confidence level. Utilizing the full 26 month baseline, 61% were variable at greater than the 85% confidence level. As expected, increasing the time baseline increased the percentage of known quasars in the field, which were seen to vary. A subset of the candidates on both timescales were chosen for spectroscopic confirmation. Low-resolution spectroscopy was obtained for these candidates in 2001 April and 2002 February using the Hydra multiobject spectrograph on the WIYN 3.5 m telescope at Kitt Peak National Observatory. Seventeen objects were confirmed as quasars by means of their distinctive broadened emission features during the 2001 April campaign. An additional 15 quasars, 2 of which overlap with the 2001 April spectroscopic campaign because of slight overlaps between two fields in each observing campaign, were discovered during the 2002 February campaign. As of 2003 October, 19 of the 30 discoveries were independently confirmed via a literature search, and 11 remain new discoveries (A. W. Rengstorf et al. 2004, in preparation). It is confirmed that increasing the time baseline of observations increases the completeness (i.e., the percentage of quasars that will be seen to vary) of the survey. While the density of candidate objects on the sky did not necessitate any cuts other than variability with regard to the number of available spectrograph fibers on Hydra, an a posteriori investigation into the broadband colors of the candidate objects reveal that color cuts in B–V and V–R would have increased the efficiency (i.e., the percentage of the candidate list that are quasars) of the spectroscopic confirmation.

A Series of Faint Spectrophotometric Standard Stars at +33o Declination

We have obtained spectrophotometry in the wavelength range 350-920 nm for 21 15th magnitude stars. These stars have right ascension at 30 minute intervals in the range 8-18 hr and declination near +33°. Mean magnitudes are presented at 5 nm intervals, with calibration derived from Kitt Peak secondary spectrophotometric standards. Typical internal and absolute errors are estimated to be less than 0.02 and 0.05 mag, respectively. The sequence was established in order to provide calibration stars for drift-scan observations of a strip of sky centered at this declination. Because the stars are relatively faint and span the north Galactic cap, they should be well suited to the calibration of other data sets.

The Sunyaev‐Zeldovich Infrared Experiment: A Millimeter‐Wave Receiver for Cluster Cosmology

Measurements of the Sunyaev-Zeldovich (S-Z) effect toward distant clusters of galaxies can be used to determine the Hubble constant and the radial component of cluster peculiar velocities. Determination of the cluster peculiar velocity requires the separation of the two components of the S-Z effect, which are due to the thermal and bulk velocities of the intracluster plasma. The two components can be separated practically only at millimeter wavelengths. Measurements of the S-Z effect at millimeter wavelengths are subject to minimal astrophysical confusion and, therefore, provide an important test of results obtained at longer wavelengths. We describe the instrument used to make the first significant detections of the S-Z effect at millimeter wavelengths. This instrument employs new filter, detector, and readout technologies to produce sensitive measurements of differential sky brightness stable on long timescales. These advances allow drift-scan observations that achieve high sensitivity while minimizing common sources of systematic error.

THE GREAT-CIRCLE CAMERA: A NEW DRIFT-SCANNING INSTRUMENT

ABSTRACT We discuss the design, construction, and use of a new class of scanning camera that eliminates a critical limitation of standard CCD drift-scan observations. A standard scan, which involves no correction for the differential drift rates and curved stellar paths across the field-of-view, suffers from severe image degradation even when one observes at moderate declinations. Not only does this effect limit the area of the sky over which drift scanning is viable, but as detector sizes increase, CCD mosaics become standard, and dome/telescope seeing improves, the area of sky for which scanning is acceptable (image degradation <~ seeing) will be further reduced unless some action is taken. By modifying the scan path (the path on the sky traced by signal accumulated along a single CCD column) to lie along a great circle on the sky rather than along a path of constant declination, image degradation is minimized. In this paper, we discuss the design and implementation of a stage that rotates and translates the CCD during a drift-scan exposure so that the scan path is along a great circle on the sky. Data obtained during the commissioning run of the great Circle Camera at the Las Campanas 1-m telescope are presented.