arXiv subjects

J. Lazio

Publications and source records attributed to J. Lazio.

32 records · Page 2Linked to original sources

arXiv2012

Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves

We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.

astro-ph.CO↗

arXiv2009

The exploration of the unknown

The discovery of cosmic radio emission by Karl Jansky in the course of searching for the source of interference to telephone communications and the instrumental advances which followed, have led to a series of new paradigm changing astronomical discoveries. These discoveries, which to a large extent define much of modern astrophysical research were the result of the right people being in the right place at the right time using powerful new instruments, which in many cases they had designed and built. They were not the result of trying to test any particular theoretical model or trying to answer previously posed questions, but they opened up whole new areas of exploration and discovery. Rather many important discoveries came from military or communications research; others while looking for something else; and yet others from just looking. Traditionally, the designers of big telescopes invariably did not predict what the telescopes would ultimately be known for. The place in history of the next generation of telescopes will not likely be found in the science case created to justify their construction, but in the unexpected new phenomena, new theories, and new ideas which will emerge from these discoveries. It is important that those who are in a position to filter research proposals and plans not dismiss as butterfly collecting, investigations which explore new areas without having predefined the result they are looking for. Progress must also allow for new discoveries, as well as for the explanation of old discoveries. New telescopes need to be designed with the flexibility to make new discoveries which will invariably raise new questions and new problems.

astro-ph.IM↗

arXiv2009

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 (~10^-9 to 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.

astro-ph.SR↗

arXiv2009

The North American Nanohertz Observatory for Gravitational Waves

The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers whose goal is the creation of a galactic scale gravitational wave observatory sensitive to gravitational waves in the nHz-microHz band. It is just one component of an international collaboration involving similar organizations of European and Australian astronomers who share the same goal. Gravitational waves, a prediction of Einstein's general theory of relativity, are a phenomenon of dynamical space-time generated by the bulk motion of matter, and the dynamics of space-time itself. They are detectable by the small disturbance they cause in the light travel time between some light source and an observer. NANOGrav exploits radio pulsars as both the light (radio) source and the clock against which the light travel time is measured. In an array of radio pulsars gravitational waves manifest themselves as correlated disturbances in the pulse arrival times. The timing precision of today's best measured pulsars is less than 100 ns. With improved instrumentation and signal-to-noise it is widely believed that the next decade could see a pulsar timing network of 100 pulsars each with better than 100 ns timing precision. Such a pulsar timing array (PTA), observed with a regular cadence of days to weeks, would be capable of observing supermassive black hole binaries following galactic mergers, relic radiation from early universe phenomena such as cosmic strings, cosmic superstrings, or inflation, and more generally providing a vantage on the universe whose revolutionary potential has not been seen in the 400 years since Galileo first turned a telescope to the heavens.

astro-ph.IM↗

arXiv2009

The Dynamic Radio Sky: An Opportunity for Discovery

The time domain of the sky has been only sparsely explored. Nevertheless, recent discoveries from limited surveys and serendipitous discoveries indicate that there is much to be found on timescales from nanoseconds to years and at wavelengths from meters to millimeters. These observations have revealed unexpected phenomena such as rotating radio transients and coherent pulses from brown dwarfs. Additionally, archival studies have found not-yet identified radio transients without optical or high-energy hosts. In addition to the known classes of radio transients, possible other classes of objects include extrapolations from known classes and exotica such as orphan gamma-ray burst afterglows, radio supernovae, tidally-disrupted stars, flare stars, magnetars, and transmissions from extraterrestrial civilizations. Over the next decade, meter- and centimeter-wave radio telescopes with improved sensitivity, wider fields of view, and flexible digital signal processing will be able to explore radio transient parameter space more comprehensively and systematically.

astro-ph.IM↗

arXiv2009

Clusters and Large-Scale Structure: the Synchrotron Keys

For over four decades, synchrotron-radiating sources have played a series of pathfinding roles in the study of galaxy clusters and large scale structure. Such sources are uniquely sensitive to the turbulence and shock structures of large-scale environments, and their cosmic rays and magnetic fields often play important dynamic and thermodynamic roles. They provide essential complements to studies at other wavebands. Over the next decade, they will fill essential gaps in both cluster astrophysics and the cosmological growth of structure in the universe, especially where the signatures of shocks and turbulence, or even the underlying thermal plasma itself, are otherwise undetectable. Simultaneously, synchrotron studies offer a unique tool for exploring the fundamental question of the origins of cosmic magnetic fields. This work will be based on the new generation of m/cm-wave radio telescopes now in construction, as well as major advances in the sophistication of 3-D MHD simulations.

astro-ph.CO↗

arXiv2009

Magnetospheric Emission from Extrasolar Planets

The magnetospheric emissions from extrasolar planets represent a science frontier for the next decade. All of the solar system giant planets and the Earth produce radio emissions as a result of interactions between their magnetic fields and the solar wind. In the case of the Earth, its magnetic field may contribute to its habitability by protecting its atmosphere from solar wind erosion and by preventing energetic particles from reaching its surface. Indirect evidence for at least some extrasolar giant planets also having magnetic fields includes the modulation of emission lines of their host stars phased with the planetary orbits, likely due to interactions between the stellar and planetary magnetic fields. If magnetic fields are a generic property of giant planets, then extrasolar giant planets should emit at radio wavelengths allowing for their direct detection. Existing observations place limits comparable to the flux densities expected from the strongest emissions. Additional sensitivity at low radio frequencies coupled with algorithmic improvements likely will enable a new means of detection and characterization of extrasolar planets within the next decade.

astro-ph.EP↗

arXiv2009

How do Galaxies Accrete Gas and Form Stars?

Great strides have been made in the last two decades in determining how galaxies evolve from their initial dark matter seeds to the complex structures we observe at z=0. The role of mergers has been documented through both observations and simulations, numerous satellites that may represent these initial dark matter seeds have been discovered in the Local Group, high redshift galaxies have been revealed with monstrous star formation rates, and the gaseous cosmic web has been mapped through absorption line experiments. Despite these efforts, the dark matter simulations that include baryons are still unable to accurately reproduce galaxies. One of the major problems is our incomplete understanding of how a galaxy accretes its baryons and subsequently forms stars. Galaxy formation simulations have been unable to accurately represent the required gas physics on cosmological timescales, and observations have only just begun to detect the star formation fuel over a range of redshifts and environments. How galaxies obtain gas and subsequently form stars is a major unsolved, yet tractable problem in contemporary extragalactic astrophysics. In this paper we outline how progress can be made in this area in the next decade.

astro-ph.CO↗

arXiv2009

Strong Gravitational Lensing as a Probe of Gravity, Dark-Matter and Super-Massive Black Holes

Whereas considerable effort has been afforded in understanding the properties of galaxies, a full physical picture, connecting their baryonic and dark-matter content, super-massive black holes, and (metric) theories of gravity, is still ill-defined. Strong gravitational lensing furnishes a powerful method to probe gravity in the central regions of galaxies. It can (1) provide a unique detection-channel of dark-matter substructure beyond the local galaxy group, (2) constrain dark-matter physics, complementary to direct-detection experiments, as well as metric theories of gravity, (3) probe central super-massive black holes, and (4) provide crucial insight into galaxy formation processes from the dark matter point of view, independently of the nature and state of dark matter. To seriously address the above questions, a considerable increase in the number of strong gravitational-lens systems is required. In the timeframe 2010-2020, a staged approach with radio (e.g. EVLA, e-MERLIN, LOFAR, SKA phase-I) and optical (e.g. LSST and JDEM) instruments can provide 10^(2-4) new lenses, and up to 10^(4-6) new lens systems from SKA/LSST/JDEM all-sky surveys around ~2020. Follow-up imaging of (radio) lenses is necessary with moderate ground/space-based optical-IR telescopes and with 30-50m telescopes for spectroscopy (e.g. TMT, GMT, ELT). To answer these fundamental questions through strong gravitational lensing, a strong investment in large radio and optical-IR facilities is therefore critical in the coming decade. In particular, only large-scale radio lens surveys (e.g. with SKA) provide the large numbers of high-resolution and high-fidelity images of lenses needed for SMBH and flux-ratio anomaly studies.

astro-ph.CO↗

arXiv2009

Gravitational Wave Astronomy Using Pulsars: Massive Black Hole Mergers & the Early Universe

Gravitational waves (GWs) are fluctuations in the fabric of spacetime predicted by Einstein's theory of general relativity. Using a collection of millisecond pulsars as high-precision clocks, the nanohertz band of this radiation is likely to be directly detected within the next decade. Nanohertz-frequency GWs are expected to be emitted by mergers of massive black hole binary systems, and potentially also by cosmic strings or superstrings formed in the early Universe. Direct detection of GWs will open a new window to the Universe, and provide astrophysical information inaccessible via electromagnetic observations. In this paper, we describe the potential sources of low-frequency GWs and the current status and key advances needed for the detection and exploitation of GWs through pulsar timing.

astro-ph.CO↗

arXiv2008

An Eccentric Binary Millisecond Pulsar in the Galactic Plane

Binary pulsar systems are superb probes of stellar and binary evolution and the physics of extreme environments. In a survey with the Arecibo telescope, we have found PSR J1903+0327, a radio pulsar with a rotational period of 2.15 ms in a highly eccentric (e = 0.44) 95-day orbit around a solar mass companion. Infrared observations identify a possible main-sequence companion star. Conventional binary stellar evolution models predict neither large orbital eccentricities nor main-sequence companions around millisecond pulsars. Alternative formation scenarios involve recycling a neutron star in a globular cluster then ejecting it into the Galactic disk or membership in a hierarchical triple system. A relativistic analysis of timing observations of the pulsar finds its mass to be 1.74+/-0.04 Msun, an unusually high value.

astro-ph↗

arXiv2008

Radio Wavelength Transients: Current and Emerging Prospects

Known classes of radio wavelength transients range from the nearby--stellar flares and radio pulsars--to the distant Universe--γ-ray burst afterglows. Hypothesized classes of radio transients include analogs of known objects, e.g., extrasolar planets emitting Jovian-like radio bursts and giant-pulse emitting pulsars in other galaxies, to the exotic, prompt emission from γ-ray bursts, evaporating black holes, and transmitters from other civilizations. A number of instruments and facilities are either under construction or in early observational stages and are slated to become available in the next few years. With a combination of wide fields of view and wavelength agility, the detection and study of radio transients will improve immensely.

astro-ph↗

arXiv2007

Radio Wavelength Observatories within the Exploration Architecture

Observations at radio wavelengths address key problems in astrophysics, astrobiology, and lunar structure including the first light in the Universe (the Epoch of Reionization), the presence of magnetic fields around extrasolar planets, particle acceleration mechanisms, and the structure of the lunar ionosphere. Moreover, achieving the performance needed to address these scientific questions demands observations at wavelengths longer than those that penetrate the Earth's ionosphere, observations in extremely "radio quiet" locations such as the Moon's far side, or both. We describe a series of lunar-based radio wavelength interferometers of increasing capability. The Radio Observatory for Lunar Sortie Science (ROLSS) is an array designed to be deployed during the first lunar sorties (or even before via robotic rovers) and addressing particle acceleration and the lunar ionosphere. Future arrays would be larger, more capable, and deployed as experience is gained in working on the lunar surface.

astro-ph↗

arXiv1999

The Interaction of two Prominent Galactic Center Sources: Sgr A East and the Molecular Ring

We present a synthesis of a number of recent observations in the near-IR H_2 and [Fe II] lines, OH (1720 MHz) maser line and various radio continuum measurements using the NICMOS of the HST, UNSWIRF on the AAT and the VLA. These observations suggest that the outer edge of the CND is collisionally excited whereas the inner edge is likely to be heated predominantly by the IRS 16 cluster. The velocity and spatial correlation of H_2 and OH (1720 MHz) as well as the spatial distribution of radio continuum emission at 90cm suggest that Sgr A East is responsible for shocking the gas and interacting with the circumnuclear ring at the Galactic center.

astro-ph↗