Donald C. Backer

The International Pulsar Timing Array project: using pulsars as a gravitational wave detector

The International Pulsar Timing Array project combines observations of pulsars from both northern and southern hemisphere observatories with the main aim of detecting ultra-low frequency (similar to 10(-9)-10(-8) Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.

The Precision Array for Probing the Epoch of Re-ionization: Eight Station Results

We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21 cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139 MHz and 174 MHz using data from both arrays that reaches down to 80 mJy (4.9 K, for a beam size of 2.15e−5 sr at 156 MHz), with a 10 mJy (620 mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (C� ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310 mK at � = 100 for a 1.46 MHz band centered at 164.5 MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21 cm emission associated with re-ionization.

Gravitational Waves Probe the Coalescence Rate of Massive Black Hole Binaries

We calculate the expected nHz-μHz gravitational wave (GW) spectrum from coalescing massive black hole (MBH) binaries resulting from mergers of their host galaxies. We consider detection of this spectrum by precision pulsar timing and a future pulsar timing array. The spectrum depends on the merger rate of massive galaxies, the demographics of MBHs at low and high redshift, and the dynamics of MBH binaries. We apply recent theoretical and observational work on all of these fronts. The spectrum has a characteristic strain hc(f) ~ 10-15(f/yr-1)-2/3, just below the detection limit from recent analysis of precision pulsar timing measurements. However, the amplitude of the spectrum is still very uncertain owing to approximations in the theoretical formulation of the model, to our lack of knowledge of the merger rate and MBH population at high redshift, and to the dynamical problem of removing enough angular momentum from the MBH binary to reach a GW-dominated regime.

Interferometric Detection of Linear Polarization from Sagittarius A* at 230 GHz

We measured the linear polarization of Sagittarius A* to be 7.2% ± 0.6% at 230 GHz using the BIMA array with a resolution of 36 × 09. This confirms the previously reported detection with the James Clerk Maxwell Telescope (JCMT) 14 m antenna. Our high-resolution observations demonstrate that the polarization does not arise from dust but from a synchrotron source associated with Sgr A*. We see no change in the polarization position angle and only a small change in the polarization fraction in four observations distributed over 60 days. We find a position angle of 139° ± 4°, which differs substantially from what was found in earlier JCMT observations at the same frequency. Polarized dust emission cannot account for this discrepancy, leaving variability and observational error as the only explanations. The BIMA observations alone place an upper limit on the magnitude of the rotation measure of 2 × 106 rad m-2. These new observations, when combined with the JCMT observations at 150, 375, and 400 GHz, suggest that the rotation measure = -4.3 ± 0.1 × 105 rad m-2. This RM may be caused by an external Faraday screen. Barring a special geometry or a high number of field reversals, this RM rules out accretion rates greater than ~10-7 M☉ yr-1. This measurement is inconsistent with high accretion rates necessary in standard advection-dominated accretion flow and Bondi-Hoyle models for Sgr A*. It argues for low accretion rates as a major factor in the overall faintness of Sgr A*.

Constructing a pulsar timing array

Arrival time data from a spatial array of millisecond pulsars can be used (1) to provide a time standard for long time scales, (2) to detect perturbations of the earths orbit, and (3) to search for a cosmic background of gravitational radiation. A polynomial time series representation for these three effects is first developed that is appropriate for analysis of the present data with its limited degrees of freedom. A pulsar timing array program is then described that has been established at the National Radio Astronomy Observatory 43 m telescope with observations of PSR 1620 - 26, PSR 181 - 24, and PSR 1937 + 21. The results cover a 2 yr period beginning in July 1987. The influence of global parameters - clock, earth location, and effects of gravitational radiation is discussed in the context of the polynomial model. 45 refs.

Arecibo pulsar survey using alfa. I. Survey strategy and first discoveries

We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth, and fast-dump digital spectrometers. The search targets low Galactic latitudes, |b| 5°, in the accessible longitude ranges 32° l 77° and 168° l 214°. The instrumentation, data processing, initial survey observations, sensitivity, and database management are described. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 μs. Analysis of the data with their full time and frequency resolutions is ongoing. Here we report the results of a preliminary, low-resolution analysis for which the data were decimated to speed up the processing. We have detected 29 previously known pulsars and discovered 11 new ones. One of these, PSR J1928+1746, with a period of 69 ms and a relatively low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another, PSR J1906+07, is a nonrecycled pulsar in a relativistic binary with an orbital period of 3.98 hr. In parallel with the periodicity analysis, we also search the data for isolated dispersed pulses. This technique has resulted in the discovery of PSR J0628+09, an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that ~1000 new pulsars will be found in our ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.

Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging

We have detected the intrinsic size of Sagittarius A*, the Galactic center radio source associated with a supermassive black hole, showing that the short-wavelength radio emission arises from very near the event horizon of the black hole. Radio observations with the Very Long Baseline Array show that the source has a size of 24 ± 2 Schwarzschild radii at 7-millimeter wavelength. In one of eight 7-millimeter epochs, we also detected an increase in the intrinsic size of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(60_{-17}^{+25}\%\) \end{document}. These observations place a lower limit to the mass density of Sagittarius A* of 1.4 × 104 solar masses per cubic astronomical unit.

The Allen Telescope Array: The First Widefield, Panchromatic, Snapshot Radio Camera for Radio Astronomy and SETI

The first 42 elements of the Allen Telescope Array (ATA-42) are beginning to deliver data at the Hat Creek Radio Observatory in northern California. Scientists and engineers are actively exploiting all of the flexibility designed into this innovative instrument for simultaneously conducting surveys of the astrophysical sky and conducting searches for distant technological civilizations. This paper summarizes the design elements of the ATA, the cost savings made possible by the use of commercial off-the-shelf components, and the cost/performance tradeoffs that eventually enabled this first snapshot radio camera. The fundamental scientific program of this new telescope is varied and exciting; some of the first astronomical results will be discussed.

Proper Motion of the Compact, Nonthermal Radio Source in the Galactic Center, Sagittarius A*

Proper motions and radial velocities of luminous infrared stars in the Galactic center have provided strong evidence for a dark mass of 2.5 × 106 M☉ in the central 0.05 pc of the Galaxy. The leading hypothesis for this mass is a black hole. High angular resolution measurements at radio wavelengths find a compact radio source, Sagittarius (Sgr) A*, that is either the faint glow from a small amount of material accreting onto the hole with low radiative efficiency or a miniature active galactic nucleus (AGN) core-jet system. This paper provides a full report on the first program that has measured the apparent proper motion of Sgr A* with respect to background extragalactic reference frame. Our current result is The observations were obtained with the NRAO Very Large Array at 4.9 GHz over 16 yr. The proper motion of Sgr A* provides an estimate of its mass based on equipartition of kinetic energy between the hole and the surrounding stars. The measured motion is largest in galactic longitude. This component of the motion is consistent with the secular parallax that results from the rotation of the solar system about the center, which is a global measure of the difference between Oorts constants (A-B), with no additional peculiar motion of Sgr A*. The current uncertainty in Oorts galactic rotation constants limits the use of this component of the proper motion for a mass inference. In latitude, we find a small, and weakly significant, peculiar motion of Sgr A*, -19 ± 7 km s-1 after correction for the motion of the solar system with respect to the local standard of rest. We consider sources of peculiar motion of Sgr A* ranging from unstable radio wave propagation through intervening turbulent plasma to the effects of asymmetric masses in the center. These fail to account for a significant peculiar motion. One can appeal to an m = 1 dynamical instability that numerical simulations have revealed. However, the measurement of a latitude peculiar proper motion of comparable magnitude and error but with opposite sign in the companion paper by Reid leads us to conclude at the present time that our errors may be underestimated and that the actual peculiar motion might therefore be closer to zero. Improvement of these measurements with further observations and resolving the differences between independent experiments will provide the accuracies of a few km s-1 in both coordinates that will provide both a black hole mass estimate and a definitive determination of Oorts galactic rotation constants on a global Galactic scale.

Arecibo Pulsar Survey Using ALFA. II. The Young, Highly Relativistic Binary Pulsar J1906+0746

We report the discovery of PSR J1906+0746, a young 144 ms pulsar in a highly relativistic 3.98 hr orbit with an eccentricity of 0.085 and expected gravitational wave coalescence time of � 300 Myr. The new pulsar was found during precursor survey observations with the Arecibo 1.4 GHz feed array system and retrospectively detected in the Parkes Multibeam plane pulsar survey data. From radio follow-up observations with Arecibo, Jodrell Bank, GreenBank,andParkes,wehavemeasuredthespin-downandbinaryparametersofthepulsaranditsbasicspectral and polarization properties. We also present evidence for pulse profile evolution, which is likely due to geodetic precession, a relativistic effect caused by the misalignment of the pulsar spin and total angular momentum vectors. Our measurements show that PSR J1906+0746 is a young object with a characteristic age of 112 kyr. From the measured rate of orbital periastron advance (7N57 � 0N03 yr � 1 ), we infer a total system mass of 2:61 � 0:02 M� . While these parameters suggest that the PSR J1906+0746 binary system might be a younger version of the double pulsar system, intensive searches for radio pulses from the companion have so far been unsuccessful. It is therefore not known whether the companion is another neutron star or a massive white dwarf. Regardless of the nature of the companion, a simple calculation suggests that the Galactic birthrate of binaries similar to PSR J1906+0746is � 60Myr � 1 .ThisimpliesthatPSRJ1906+0746willmakeasignificantcontributiontothecomputed cosmic inspiral rate of compact binary systems. Subject headingg pulsars: general — pulsars: individual (PSR J1906+0746)