arXiv subjects

B. Allen

Publications and source records attributed to B. Allen.

At least 163 records · Page 9Linked to original sources

arXiv2010

Predictions for the Rates of Compact Binary Coalescences Observable by Ground-based Gravitational-wave Detectors

We present an up-to-date, comprehensive summary of the rates for all types of compact binary coalescence sources detectable by the Initial and Advanced versions of the ground-based gravitational-wave detectors LIGO and Virgo. Astrophysical estimates for compact-binary coalescence rates depend on a number of assumptions and unknown model parameters, and are still uncertain. The most confident among these estimates are the rate predictions for coalescing binary neutron stars which are based on extrapolations from observed binary pulsars in our Galaxy. These yield a likely coalescence rate of 100 per Myr per Milky Way Equivalent Galaxy (MWEG), although the rate could plausibly range from 1 per Myr per MWEG to 1000 per Myr per MWEG. We convert coalescence rates into detection rates based on data from the LIGO S5 and Virgo VSR2 science runs and projected sensitivities for our Advanced detectors. Using the detector sensitivities derived from these data, we find a likely detection rate of 0.02 per year for Initial LIGO-Virgo interferometers, with a plausible range between 0.0002 and 0.2 per year. The likely binary neutron-star detection rate for the Advanced LIGO-Virgo network increases to 40 events per year, with a range between 0.4 and 400 per year.

astro-ph.HE↗

arXiv2010

Search for gravitational-wave inspiral signals associated with short Gamma-Ray Bursts during LIGO's fifth and Virgo's first science run

Progenitor scenarios for short gamma-ray bursts (short GRBs) include coalescenses of two neutron stars or a neutron star and black hole, which would necessarily be accompanied by the emission of strong gravitational waves. We present a search for these known gravitational-wave signatures in temporal and directional coincidence with 22 GRBs that had sufficient gravitational-wave data available in multiple instruments during LIGO's fifth science run, S5, and Virgo's first science run, VSR1. We find no statistically significant gravitational-wave candidates within a [-5, +1) s window around the trigger time of any GRB. Using the Wilcoxon-Mann-Whitney U test, we find no evidence for an excess of weak gravitational-wave signals in our sample of GRBs. We exclude neutron star-black hole progenitors to a median 90% CL exclusion distance of 6.7 Mpc.

astro-ph.HE↗

arXiv2010

Searches for gravitational waves from known pulsars with S5 LIGO data

We present a search for gravitational waves from 116 known millisecond and young pulsars using data from the fifth science run of the LIGO detectors. For this search ephemerides overlapping the run period were obtained for all pulsars using radio and X-ray observations. We demonstrate an updated search method that allows for small uncertainties in the pulsar phase parameters to be included in the search. We report no signal detection from any of the targets and therefore interpret our results as upper limits on the gravitational wave signal strength. The most interesting limits are those for young pulsars. We present updated limits on gravitational radiation from the Crab pulsar, where the measured limit is now a factor of seven below the spin-down limit. This limits the power radiated via gravitational waves to be less than ~2% of the available spin-down power. For the X-ray pulsar J0537-6910 we reach the spin-down limit under the assumption that any gravitational wave signal from it stays phase locked to the X-ray pulses over timing glitches, and for pulsars J1913+1011 and J1952+3252 we are only a factor of a few above the spin-down limit. Of the recycled millisecond pulsars several of the measured upper limits are only about an order of magnitude above their spin-down limits. For these our best (lowest) upper limit on gravitational wave amplitude is 2.3x10^-26 for J1603-7202 and our best (lowest) limit on the inferred pulsar ellipticity is 7.0x10^-8 for J2124-3358.

astro-ph.HE↗

arXiv2009

The High Energy Telescope on EXIST

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next generation multi-wavelength survey mission. The primary instrument is a High Energy telescope (HET) that conducts the deepest survey for Gamma-ray Bursts (GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients of all varieties for immediate followup studies by the two secondary instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT). EXIST will explore the early Universe using high redshift GRBs as cosmic probes and survey black holes on all scales. The HET is a coded aperture telescope employing a large array of imaging CZT detectors (4.5 m^2, 0.6 mm pixel) and a hybrid Tungsten mask. We review the current HET concept which follows an intensive design revision by the HET imaging working group and the recent engineering studies in the Instrument and Mission Design Lab at the Goddard Space Flight Center. The HET will locate GRBs and transients quickly (<10-30 sec) and accurately (< 20") for rapid (< 1-3 min) onboard followup soft X-ray and optical/IR (0.3-2.2 micron) imaging and spectroscopy. The broad energy band (5-600 keV) and the wide field of view (~90 deg x 70 deg at 10% coding fraction) are optimal for capturing GRBs, obscured AGNs and rare transients. The continuous scan of the entire sky every 3 hours will establish a finely-sampled long-term history of many X-ray sources, opening up new possibilities for variability studies.

astro-ph.IM↗

arXiv2009

Building Large Area CZT Imaging Detectors for a Wide-Field Hard X-ray Telescope - ProtoEXIST1

We have constructed a moderately large area (32 cm2), fine pixel (2.5 mm pixel, 5 mm thick) CZT imaging detector which constitutes the first section of a detector module (256 cm2) developed for a balloon-borne wide-field hard X-ray telescope, ProtoEXIST1. ProtoEXIST1 is a prototype for the High Energy Telescope (HET) in the Energetic X-ray imaging Survey Telescope (EXIST), a next generation space-borne multi-wavelength telescope. We have constructed a large (nearly gapless) detector plane through a modularization scheme by tiling of a large number of 2 cm x 2 cm CZT crystals. Our innovative packaging method is ideal for many applications such as coded-aperture imaging, where a large, continuous detector plane is desirable for the optimal performance. Currently we have been able to achieve an energy resolution of 3.2 keV (FWHM) at 59.6 keV on average, which is exceptional considering the moderate pixel size and the number of detectors in simultaneous operation. We expect to complete two modules (512 cm2) within the next few months as more CZT becomes available. We plan to test the performance of these detectors in a near space environment in a series of high altitude balloon flights, the first of which is scheduled for Fall 2009. These detector modules are the first in a series of progressively more sophisticated detector units and packaging schemes planned for ProtoEXIST2 & 3, which will demonstrate the technology required for the advanced CZT imaging detectors (0.6 mm pixel, 4.5 m2 area) required in EXIST/HET.

astro-ph.IM↗

arXiv2008

Discovery of Localized Regions of Excess 10-TeV Cosmic Rays

An analysis of 7 years of Milagro data performed on a 10-degree angular scale has found two localized regions of excess of unknown origin with greater than 12 sigma significance. Both regions are inconsistent with gamma-ray emission with high confidence. One of the regions has a different energy spectrum than the isotropic cosmic-ray flux at a level of 4.6 sigma, and it is consistent with hard spectrum protons with an exponential cutoff, with the most significant excess at ~10 TeV. Potential causes of these excesses are explored, but no compelling explanations are found.

astro-ph↗

arXiv2008

Searching for gravitational waves from Cassiopeia A with LIGO

We describe a search underway for periodic gravitational waves from the central compact object in the supernova remnant Cassiopeia A. The object is the youngest likely neutron star in the Galaxy. Its position is well known, but the object does not pulse in any electromagnetic radiation band and thus presents a challenge in searching the parameter space of frequency and frequency derivatives. We estimate that a fully coherent search can, with a reasonable amount of time on a computing cluster, achieve a sensitivity at which it is theoretically possible (though not likely) to observe a signal even with the initial LIGO noise spectrum. Cassiopeia A is only the second object after the Crab pulsar for which this is true. The search method described here can also obtain interesting results for similar objects with current LIGO sensitivity.

gr-qc↗

arXiv2008

A Measurement of the Spatial Distribution of Diffuse TeV Gamma Ray Emission from the Galactic Plane with Milagro

Diffuse $γ$-ray emission produced by the interaction of cosmic-ray particles with matter and radiation in the Galaxy can be used to probe the distribution of cosmic rays and their sources in different regions of the Galaxy. With its large field of view and long observation time, the Milagro Gamma Ray Observatory is an ideal instrument for surveying large regions of the Northern Hemisphere sky and for detecting diffuse $γ$-ray emission at very high energies. Here, the spatial distribution and the flux of the diffuse $γ$-ray emission in the TeV energy range with a median energy of 15 TeV for Galactic longitudes between 30$^\circ$ and 110$^\circ$ and between 136$^\circ$ and 216$^\circ$ and for Galactic latitudes between -10$^\circ$ and 10$^\circ$ are determined. The measured fluxes are consistent with predictions of the GALPROP model everywhere except for the Cygnus region ($l\in[65^\circ,85^\circ]$). For the Cygnus region, the flux is twice the predicted value. This excess can be explained by the presence of active cosmic ray sources accelerating hadrons which interact with the local dense interstellar medium and produce gamma rays through pion decay.

astro-ph↗

arXiv2008

Blandford's Argument: The Strongest Continuous Gravitational Wave Signal

For a uniform population of neutron stars whose spin-down is dominated by the emission of gravitational radiation, an old argument of Blandford states that the expected gravitational-wave amplitude of the nearest source is independent of the deformation and rotation frequency of the objects. Recent work has improved and extended this argument to set upper limits on the expected amplitude from neutron stars that also emit electromagnetic radiation. We restate these arguments in a more general framework, and simulate the evolution of such a population of stars in the gravitational potential of our galaxy. The simulations allow us to test the assumptions of Blandford's argument on a realistic model of our galaxy. We show that the two key assumptions of the argument (two dimensionality of the spatial distribution and a steady-state frequency distribution) are in general not fulfilled. The effective scaling dimension D of the spatial distribution of neutron stars is significantly larger than two, and for frequencies detectable by terrestrial instruments the frequency distribution is not in a steady state unless the ellipticity is unrealistically large. Thus, in the cases of most interest, the maximum expected gravitational-wave amplitude does have a strong dependence on the deformation and rotation frequency of the population. The results strengthen the previous upper limits on the expected gravitational-wave amplitude from neutron stars by a factor of 6 for realistic values of ellipticity.

gr-qc↗

arXiv2008

Beating the spin-down limit on gravitational wave emission from the Crab pulsar

We present direct upper limits on gravitational wave emission from the Crab pulsar using data from the first nine months of the fifth science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). These limits are based on two searches. In the first we assume that the gravitational wave emission follows the observed radio timing, giving an upper limit on gravitational wave emission that beats indirect limits inferred from the spin-down and braking index of the pulsar and the energetics of the nebula. In the second we allow for a small mismatch between the gravitational and radio signal frequencies and interpret our results in the context of two possible gravitational wave emission mechanisms.

astro-ph↗

arXiv2007

Wide Field Hard X-ray Survey Telescope: ProtoEXIST1

We report our progress on the development of pixellated imaging CZT detector arrays for our first-generation balloon-borne wide-field hard X-ray (20 - 600 keV) telescope, ProtoEXIST1. Our ProtoEXIST program is a pathfinder for the High Energy Telescope (HET) on the Energetic X-ray Imaging Survey telescope (EXIST), a proposed implementation of the Black Hole Finder Probe. ProtoEXIST1 consists of four independent coded-aperture telescopes with close-tiled (~0.4 mm gaps) CZT detectors that preserve their 2.5mm pixel pitch. Multiple shielding/field-of-view configurations are planned to identify optimal geometry for the HET in EXIST. The primary technical challenge in ProtoEXIST is the development of large area, close-tiled modules of imaging CZT detectors (1000 cm2 for ProtoEXIST1), with all readout and control systems for the ASIC readout vertically stacked. We describe the overall telescope configuration of ProtoEXIST1 and review the current development status of the CZT detectors, from individual detector crystal units (DCUs) to a full detector module (DM). We have built the first units of each component for the detector plane and have completed a few Rev2 DCUs (2x2 cm2), which are under a series of tests. Bare DCUs (pre-crystal bonding) show high, uniform ASIC yield (~70%) and ~30% reduction in electronics noise compared to the Rev1 equivalent. A Rev1 DCU already achieved ~1.2% FWHM at 662 keV, and preliminary analysis of the initial radiation tests on a Rev2 DCU shows ~ 4 keV FWHM at 60 keV (vs. 4.7 keV for Rev1). We therefore expect about <~1% FWHM at 662 keV with the Rev2 detectors.

astro-ph↗

arXiv2007

TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro

A survey of Galactic gamma-ray sources at a median energy of ~20 TeV has been performed using the Milagro Gamma Ray Observatory. Eight candidate sources of TeV emission are detected with pre-trials significance $>4.5σ$ in the region of Galactic longitude $l\in[30^\circ,220^\circ]$ and latitude $b\in[-10^\circ,10^\circ]$. Four of these sources, including the Crab nebula and the recently published MGRO J2019+37, are observed with significances $>4σ$ after accounting for the trials involved in searching the 3800 square degree region. All four of these sources are also coincident with EGRET sources. Two of the lower significance sources are coincident with EGRET sources and one of these sources is Geminga. The other two candidates are in the Cygnus region of the Galaxy. Several of the sources appear to be spatially extended. The fluxes of the sources at 20 TeV range from ~25% of the Crab flux to nearly as bright as the Crab.

astro-ph↗

arXiv2006

Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy

The diffuse gamma radiation arising from the interaction of cosmic ray particles with matter and radiation in the Galaxy is one of the few probes available to study the origin of the cosmic rays. Milagro is a water Cherenkov detector that continuously views the entire overhead sky. The large field-of-view combined with the long observation time makes Milagro the most sensitive instrument available for the study of large, low surface brightness sources such as the diffuse gamma radiation arising from interactions of cosmic radiation with interstellar matter. In this paper we present spatial and flux measurements of TeV gamma-ray emission from the Cygnus Region. The TeV image shows at least one new source MGRO J2019+37 as well as correlations with the matter density in the region as would be expected from cosmic-ray proton interactions. However, the TeV gamma-ray flux as measured at ~12 TeV from the Cygnus region (after excluding MGRO J2019+37) exceeds that predicted from a conventional model of cosmic ray production and propagation. This observation indicates the existence of either hard-spectrum cosmic-ray sources and/or other sources of TeV gamma rays in the region.

astro-ph↗

arXiv2003

Upper limits on the strength of periodic gravitational waves from PSR J1939+2134

The first science run of the LIGO and GEO gravitational wave detectors presented the opportunity to test methods of searching for gravitational waves from known pulsars. Here we present new direct upper limits on the strength of waves from the pulsar PSR J1939+2134 using two independent analysis methods, one in the frequency domain using frequentist statistics and one in the time domain using Bayesian inference. Both methods show that the strain amplitude at Earth from this pulsar is less than a few times $10^{-22}$.

gr-qc↗

arXiv2001

Cosmic strings, loops, and linear growth of matter perturbations

We describe the detailed study and results of high-resolution numerical simulations of string-induced structure formation in open universes and those with a non-zero cosmological constant. The effect from small loops generated from the string network has also been investigated. We provide a semi-analytical model which can reproduce these simulation results. A detailed study of cosmic string network properties regarding structure formation is also given, including the correlation time, the topological analysis of the source spectrum, the correlation between long strings and loops, and the evolution of long-string and loop energy densities. For models with $Γ=Ωh=0.1--0.2 and a cold dark matter background, we show that the linear density fluctuation power spectrum induced by cosmic strings has both an amplitude at $8 h^{-1}$Mpc, $σ_8$, and an overall shape which are consistent within uncertainties with those currently inferred from galaxy surveys. The cosmic string scenario with hot dark matter requires a strongly scale-dependent bias in order to agree with observations.

astro-ph↗

arXiv1999

Observational Limit on Gravitational Waves from Binary Neutron Stars in the Galaxy

Using optimal matched filtering, we search 25 hours of data from the LIGO 40-meter prototype laser interferometric gravitational-wave detector for gravitational-wave chirps emitted by coalescing binary systems within our Galaxy. This is the first test of this filtering technique on real interferometric data. An upper limit on the rate R of neutron star binary inspirals in our Galaxy is obtained: with 90% confidence, R< 0.5/hour. Similar experiments with LIGO interferometers will provide constraints on the population of tight binary neutron star systems in the Universe.

gr-qc↗

arXiv1998

Cosmic string loops and large-scale structure

We investigate the contribution made by small loops from a cosmic string network as seeds for large-scale structure formation. We show that cosmic string loops are highly correlated with the long-string network on large scales and therefore contribute significantly to the power spectrum of density perturbations if the average loop lifetime is comparable to or above one Hubble time. This effect further improves the large-scale bias problem previously identified in earlier studies of cosmic string models.

astro-ph↗

arXiv1998

Non-gaussian Features of Linear Cosmic String Models

We investigate the non-gaussian properties of cosmic-string-seeded linear density perturbations with cold and hot dark matter backgrounds, using high-resolution numerical simulations. We compute the one-point probability density function of the resulting density field, its skewness, kurtosis, and genus curves for different smoothing scales. A semi-analytic model is then invoked to provide a physical interpretation of our results. We conclude that on scales smaller than $1.5{(Ωh^2)}^{-1}$Mpc, perturbations seeded by cosmic strings are very non-gaussian. These scales may still be in a linear or mildly non-linear regime in an open or $Λ$-universe with $Γ=Ωh \lsim 0.2$.

astro-ph↗