arXiv subjects

Linqing Wen

Publications and source records attributed to Linqing Wen.

At least 55 records · Page 3Linked to original sources

arXiv2008

Astrodynamical Space Test of Relativity using Optical Devices I (ASTROD I) - A class-M fundamental physics mission proposal for Cosmic Vision 2015-2025

ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is an international project, with major contributions from Europe and China and is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals. A second mission, ASTROD II is envisaged as a three-spacecraft mission which would test General Relativity to one part per billion, enable detection of solar g-modes, measure the solar Lense-Thirring effect to 10 parts per million, and probe gravitational waves at frequencies below the LISA bandwidth. In the third phase (ASTROD III or Super-ASTROD), larger orbits could be implemented to map the outer solar system and to probe primordial gravitational-waves at frequencies below the ASTROD II bandwidth.

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arXiv2008

Orbital and superorbital variability and their coupling in X-ray binaries

We review X-ray flux modulation from X-ray binaries on time scales corresponding to the orbital period and those at longer time scales (so called superorbital). Those modulations provide a powerful tool to constrain geometry of the accretion flow. The most common cause of the superorbital variability appears to be precession. We then discuss two specific examples of discoveries of a coupling between the two types of variability and their physical interpretation. One is Cyg X-1, a black-hole binary with a high-mass companion, in which case we find the presence of an accretion bulge formed by collision of the stellar wind with the outer edge of the precessing accretion disc. The other is 4U 1820-303, a neutron star accreting from a low-mass white dwarf, in which case we interpret the superorbital variability as accretion rate modulation induced by interactions in a triple stellar system. Then, the varying accretion rate leads to changes of the size of the accretion bulge in that system, obscuring the centrally-emitted X-rays.

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arXiv2008

Geometrical Expression of the Angular Resolution of a Network of Gravitational-Wave Detectors and Improved Localization Methods

We report for the first time a method-independent geometrical expression for the angular resolution of an arbitrary network of interferometric gravitational wave (GW) detectors when the arrival-time of a GW is unknown. We discuss the implications of our results on how to improve angular resolutions of a GW network and on improvements of localization methods. An example of an improvement to the null-stream localization method for GWs of unknown waveforms is demonstrated.

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arXiv2008

Improved time-frequency analysis of extreme-mass-ratio inspiral signals in mock LISA data

The planned Laser Interferometer Space Antenna (LISA) is expected to detect gravitational wave signals from ~100 extreme-mass-ratio inspirals (EMRIs) of stellar-mass compact objects into massive black holes. The long duration and large parameter space of EMRI signals makes data analysis for these signals a challenging problem. One approach to EMRI data analysis is to use time-frequency methods. This consists of two steps: (i) searching for tracks from EMRI sources in a time-frequency spectrogram, and (ii) extracting parameter estimates from the tracks. In this paper we discuss the results of applying these techniques to the latest round of the Mock LISA Data Challenge, Round 1B. This analysis included three new techniques not used in previous analyses: (i) a new Chirp-based Algorithm for Track Search for track detection; (ii) estimation of the inclination of the source to the line of sight; (iii) a Metropolis-Hastings Monte Carlo over the parameter space in order to find the best fit to the tracks.

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arXiv2007

Report on the second Mock LISA Data Challenge

The Mock LISA Data Challenges are a program to demonstrate LISA data-analysis capabilities and to encourage their development. Each round of challenges consists of several data sets containing simulated instrument noise and gravitational-wave sources of undisclosed parameters. Participants are asked to analyze the data sets and report the maximum information about source parameters. The challenges are being released in rounds of increasing complexity and realism: in this proceeding we present the results of Challenge 2, issued in January 2007, which successfully demonstrated the recovery of signals from supermassive black-hole binaries, from ~20,000 overlapping Galactic white-dwarf binaries, and from the extreme-mass-ratio inspirals of compact objects into central galactic black holes.

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arXiv2007

Time-frequency analysis of extreme-mass-ratio inspiral signals in mock LISA data

Extreme-mass-ratio inspirals (EMRIs) of ~ 1-10 solar-mass compact objects into ~ million solar-mass massive black holes can serve as excellent probes of strong-field general relativity. The Laser Interferometer Space Antenna (LISA) is expected to detect gravitational wave signals from apprxomiately one hundred EMRIs per year, but the data analysis of EMRI signals poses a unique set of challenges due to their long duration and the extensive parameter space of possible signals. One possible approach is to carry out a search for EMRI tracks in the time-frequency domain. We have applied a time-frequency search to the data from the Mock LISA Data Challenge (MLDC) with promising results. Our analysis used the Hierarchical Algorithm for Clusters and Ridges to identify tracks in the time-frequency spectrogram corresponding to EMRI sources. We then estimated the EMRI source parameters from these tracks. In these proceedings, we discuss the results of this analysis of the MLDC round 1.3 data.

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arXiv2007

Data Analysis of Gravitational Waves Using A Network of Detectors

Several large-scale gravitational wave (GW) interferometers have achieved long term operation at design sensitivity. Questions arise on how to best combine all available data from detectors of different sensitivities for detection, consistency check or veto, localization and waveform extraction. We show that these problems can be formulated using the singular value decomposition (SVD) method. We present techniques based on the SVD method for (1) detection statistic, (2) stable solutions to waveforms, (3) null-stream construction for an arbitrary number of detectors, and (4) source localization for GWs of unknown waveforms.

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arXiv2006

Extracting Information about EMRIs using Time-Frequency Methods

The inspirals of stellar-mass compact objects into supermassive black holes are some of the most exciting sources of gravitational waves for LISA. Detection of these sources using fully coherent matched filtering is computationally intractable, so alternative approaches are required. In Wen & Gair (2005), we proposed a detection method based on searching for significant deviation of power density from noise in a time-frequency spectrogram of the LISA data. The performance of the algorithm was assessed in Gair & Wen (2005) using Monte-Carlo simulations on several trial waveforms and approximations to the noise statistics. We found that typical extreme mass ratio inspirals (EMRIs) could be detected at distances of up to 1-3 Gpc, depending on the source parameters. In this paper, we first give an overview of our previous work in Wen & Gair (2005) and Gair & Wen (2005), and discuss the performance of the method in a broad sense. We then introduce a decomposition method for LISA data that decodes LISA's directional sensitivity. This decomposition method could be used to improve the detection efficiency, to extract the source waveform, and to help solve the source confusion problem. Our approach to constraining EMRI parameters using the output from the time-frequency method will be outlined.

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arXiv2006

Probing Spacetime Foam with Photons: Suppression of Observable Effects due to Uncertainty in Optical Paths

It was recently proposed to use extra-galactic point sources to constrain space-time quantum fluctuations in the universe. In these proposals, the fundamental "fuzziness" of distance caused by space-time quantum fluctuations have been directly identified with fluctuations in optical paths. Phase-front corrugations deduced from these optical-path fluctuations are then applied to light from extragalactic point sources, and used to constrain various models of quantum gravity. In particular, the so-called random-walk model has been claimed to be ruled out by existing astrophysical observations from the Hubble Space Telescope. However, when a photon propagates in three spatial dimensions, it does not follow a specific ray, but would rather sample a finite, three-dimensional region around that ray -- thereby averaging over space-time quantum fluctuations all through that region. We use a simple, random-walk type model to demonstrate that, once the appropriate wave optics is applied, the averaging of neighboring space-time fluctuations will cause much less distortions on the phase front. In our model, the extra suppression factor due to diffraction is (Planck Length)/(Wavelength), which is at least 19 orders of magnitude for astronomical observations.

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arXiv2005

A Systematic Search for Periodicities in RXTE/ASM Data

We present the results of a systematic search in 8.5 years of Rossi X-ray Timing Explorer All-Sky Monitor data for evidence of periodicities. The search was conducted by application of the Lomb-Scargle periodogram to the light curves of each of 458 actually or potentially detected sources in each of four energy bands (1.5-3 keV, 3-5 keV, 5-12 keV, and 1.5-12 keV). A whitening technique was applied to the periodograms before evaluation of the statistical significance of the powers. We discuss individual detections with focus on relatively new findings.

astro-ph↗

arXiv2005

Coherent Network Detection of Gravitational Waves: The Redundancy Veto

A network of gravitational wave detectors is called redundant if, given the direction to a source, the strain induced by a gravitational wave in one or more of the detectors can be fully expressed in terms of the strain induced in others in the network. Because gravitational waves have only two polarizations, any network of three or more differently oriented interferometers with similar observing bands is redundant. For LISA, this ``null'' output is known as the Sagnac mode, and its use in discriminating between detector noise and a cosmological gravitational wave background is well understood [1][2]. But the usefulness of the null veto for ground-based detector networks has been ignored until now. We show that it should make it possible to discriminate in a model-independent way between real gravitational waves and accidentally coincident non-Gaussian noise ``events'' in redundant networks of two or more broadband detectors (e.g, three LIGO detectors and GEO). It has been shown that with three detectors, the null output can be used to locate the direction to the source, and then two other linear combinations of detector outputs give the optimal ``coherent'' reconstruction of the two polarization components of the signal. We discuss briefly the implementation of such a detection strategy in realistic networks.

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arXiv2005

Detecting extreme mass ratio inspirals with LISA using time-frequency methods II: search characterization

The inspirals of stellar-mass compact objects into supermassive black holes constitute some of the most important sources for LISA. Detection of these sources using fully coherent matched filtering is computationally intractable, so alternative approaches are required. In a previous paper (Wen and Gair 2005, gr-qc/0502100), we outlined a detection method based on looking for excess power in a time-frequency spectrogram of the LISA data. The performance of the algorithm was assessed using a single `typical' trial waveform and approximations to the noise statistics. In this paper we present results of Monte Carlo simulations of the search noise statistics and examine its performance in detecting a wider range of trial waveforms. We show that typical extreme mass ratio inspirals (EMRIs) can be detected at distances of up to 1--3 Gpc, depending on the source parameters. We also discuss some remaining issues with the technique and possible ways in which the algorithm can be improved.

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arXiv2005

Detecting extreme mass ratio inspirals with LISA using time-frequency methods

The inspirals of stellar-mass compact objects into supermassive black holes are some of the most important sources for LISA. Detection techniques based on fully coherent matched filtering have been shown to be computationally intractable. We describe an efficient and robust detection method that utilizes the time-frequency evolution of such systems. We show that a typical extreme mass ratio inspiral (EMRI) source could possibly be detected at distances of up to ~2 Gpc, which would mean ~10s of EMRI sources can be detected per year using this technique. We discuss the feasibility of using this method as a first step in a hierarchical search.

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arXiv2003

Constraining the properties of the proposed supermassive black hole system in 3c66b: Limits from pulsar timing

Data from long term timing observations of the radio pulsar PSR B1855+09 have been searched for the signature of Gravitational waves (G-waves) emitted by the proposed supermassive binary black hole system in 3C66B. For the case of a circular orbit, the emitted G-waves would generate detectable fluctuations in the pulse arrival times of PSR B1855+09. General expressions for the expected timing residuals induced by G-wave emission from a slowly evolving, eccentric, binary black hole system are derived here for the first time. These waveforms are used in a Monte-Carlo analysis in order to place limits on the mass and eccentricity of the proposed black hole system. The reported analysis also demonstrates several interesting features of a gravitational wave detector based on pulsar timing.

astro-ph↗

arXiv2002

On the Eccentricity Distribution of Coalescing Black Hole Binaries Driven by the Kozai Mechanism in Globular Clusters

In a globular cluster, hierarchical triple black hole systems can be produced through binary-binary interaction. It has been proposed recently that the Kozai mechanism could drive the inner binary of the triple system to merge before it is interrupted by interactions with other field stars. We investigate qualitatively and numerically the evolution of the eccentricities in these binaries under gravitational radiation (GR) reaction. We predict that ~ 30 % of the systems will possess eccentricities >0.1 when their emitted gravitational waves pass through 10 Hz frequency. The implications for gravitational wave detection, especially the relevance to data analyses for broad-band laser-interferometer gravitational wave detectors, are discussed.

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arXiv2002

Understanding the long-term spectral variability of Cygnus X-1 with BATSE and ASM observations

We present an analysis of all observations of Cygnus X-1 by the BATSE (20-300 keV) and by ASM (1.5-12 keV) until 2002 June, including 1200 days of simultaneous data. We find a number of correlations between fluxes and hardnesses in different energy bands. In the hard state, the variability can be explained by softening the overall spectrum with a pivot at 50 keV and another, independent variability pattern where the spectral shape does not change when the luminosity changes. In the soft state, the variability is caused by a variable hard tail of a constant shape superimposed on a constant soft component. These variability patterns are in agreement with the energy-dependent rms variability in the two states. We also study in detail recent soft states in 2000-02. The last of them has lasted so far for >200 days. Their spectra are harder in the 1.5-5 keV band but similar or in the 3-12 keV band than those of the 1996 soft state whereas the rms variability is stronger in all the ASM bands. On the other hand, the 1994 soft state transition observed by BATSE appears very similar to the 1996 one. We interpret the variability patterns by theoretical Comptonization models. In the hard state, the variability appears to be driven mostly by changing flux in seed photons Comptonized in a hot thermal plasma cloud with an approximately constant power supply. In the soft state, the variability is consistent with flares of hybrid, thermal/nonthermal, plasma with variable power above a stable cold disk. The spectral and timing differences between the 1996 and 2000-02 soft states are explained by a decrease of the color disk temperature. Also, based on broad-band pointed observations, we find the intrinsic bolometric luminosity increases by a factor of 3-4 from the hard state to the soft one.

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arXiv2000

X-ray intensity-hardness correlation in Cyg X-1

Using data from the All-Sky Monitor aboard the Rossi X-ray Timing Explorer (RXTE), we found that the 1.5-12 keV X-ray count rate of Cyg X-1 is, on time scales from 90 seconds to at least 10 days, strongly correlated with the spectral hardness of the source in the soft state, but is weakly anti-correlated with the latter in the hard state. The correlation shows an interesting evolution during the 1996 spectral state transition. The entire episode can be roughly divided into three distinct phases: (1) a 20-d transition phase from the hard state to the soft state, during which the correlation changes from being negative to positive, (2) a 50-d soft state with a steady positive correlation, and (3) a 20-d transition back to the hard state. The pointed RXTE observations confirmed the ASM results but revealed new behaviors of the source at energies beyond the ASM pass band. We discuss the implications of our findings.

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arXiv1999

X 1908+075: An X-ray Binary with a 4.4 day Period

X 1908+075 is an optically unidentified and highly absorbed X-ray source that appears in early surveys such as Uhuru, OSO-7, Ariel V, HEAO-1, and the EXOSAT Galactic Plane Survey. These surveys measured a source intensity in the range of 2-12 mCrab at 2-10 keV, and the position was localized to ~ 0.5 degrees. We use the Rossi X-ray Timing Explorer (RXTE) All Sky Monitor (ASM) to confirm our expectation that a particular Einstein IPC detection (1E 1908.4+0730) provides the correct position for X 1908+075. The analysis of the coded mask shadows from the ASM for the position of 1E 1908.4+0730 yields a persistent intensity ~ 8 mCrab (1.5-12 keV) over a 3 year interval beginning in 1996 February. Furthermore, we detect a period of 4.400 +- 0.001 days with a false alarm probability < 1.0e-7 . The folded light curve is roughly sinusoidal, with an amplitude that is 22 % of the mean flux. The X-ray period may be attributed to the scattering and absorption of X-rays through a stellar wind combined with the orbital motion in a binary system. We suggest that X 1908+075 is an X-ray binary with a high mass companion star.

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