arXiv subjects

B. Allen

Publications and source records attributed to B. Allen.

At least 145 records · Page 8Linked to original sources

arXiv2012

Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts

Aims. A transient astrophysical event observed in both gravitational wave (GW) and electromagnetic (EM) channels would yield rich scientific rewards. A first program initiating EM follow-ups to possible transient GW events has been developed and exercised by the LIGO and Virgo community in association with several partners. In this paper, we describe and evaluate the methods used to promptly identify and localize GW event candidates and to request images of targeted sky locations. Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to Oct 20 2010), a low-latency analysis pipeline was used to identify GW event candidates and to reconstruct maps of possible sky locations. A catalog of nearby galaxies and Milky Way globular clusters was used to select the most promising sky positions to be imaged, and this directional information was delivered to EM observatories with time lags of about thirty minutes. A Monte Carlo simulation has been used to evaluate the low-latency GW pipeline's ability to reconstruct source positions correctly. Results. For signals near the detection threshold, our low-latency algorithms often localized simulated GW burst signals to tens of square degrees, while neutron star/neutron star inspirals and neutron star/black hole inspirals were localized to a few hundred square degrees. Localization precision improves for moderately stronger signals. The correct sky location of signals well above threshold and originating from nearby galaxies may be observed with ~50% or better probability with a few pointings of wide-field telescopes.

astro-ph.IM↗

arXiv2011

Spectroscopic Follow-up of X-ray Sources in the ChaMPlane Survey: Identification of a New Cataclysmic Variable

We present a multi-object optical spectroscopy follow-up study of X-ray sources in a field along the Galactic Plane (l=327.42 deg, b=2.26 deg) which is part of the Chandra Multi-wavelength Plane survey (ChaMPlane). We obtained spectra for 46 stars, including 15 likely counterparts to X-ray sources, and sources showing an H-alpha color excess. This has led to the identification of a new cataclysmic variable (CV), CXOPS J154305.5-522709, also named ChaMPlane Bright Source 7 (CBS~7), and we identified 8 X-ray sources in the field as active late-type stars. CBS~7 was previously studied in X-rays and showed a hard spectrum and two periods: 1.22+/-0.08 h and 2.43+/-0.26 h. We present here clear evidence that the source is a CV through the detection of H, HeI and HeII emission lines in its optical spectrum. The hard X-ray spectrum and the presence of the HeII 4686 in emission with a large equivalent width suggest a magnetic CV. The near-infrared counterpart is significantly variable, and we found a period consistent with the longest X-ray period at 2.39+/-0.05 h but not the shortest X-ray period. If this period is the orbital period, this would place the system in the CV period gap. The possible orbital period suggests a dM4+/-1 companion star. The distance is then estimated to be ~1 kpc. The system could be a relatively hard and X-ray luminous polar or an intermediate polar, possibly nearly-synchronous.

astro-ph.HE↗

arXiv2011

Discovery of Nine Gamma-Ray Pulsars in Fermi-LAT Data Using a New Blind Search Method

We report the discovery of nine previously unknown gamma-ray pulsars in a blind search of data from the Fermi Large Area Telescope (LAT). The pulsars were found with a novel hierarchical search method originally developed for detecting continuous gravitational waves from rapidly rotating neutron stars. Designed to find isolated pulsars spinning at up to kHz frequencies, the new method is computationally efficient, and incorporates several advances, including a metric-based gridding of the search parameter space (frequency, frequency derivative and sky location) and the use of photon probability weights. The nine pulsars have spin frequencies between 3 and 12 Hz, and characteristic ages ranging from 17 kyr to 3 Myr. Two of them, PSRs J1803-2149 and J2111+4606, are young and energetic Galactic-plane pulsars (spin-down power above 6e35 erg/s and ages below 100 kyr). The seven remaining pulsars, PSRs J0106+4855, J0622+3749, J1620-4927, J1746-3239, J2028+3332, J2030+4415, J2139+4716, are older and less energetic; two of them are located at higher Galactic latitudes (|b| > 10 deg). PSR J0106+4855 has the largest characteristic age (3 Myr) and the smallest surface magnetic field (2e11 G) of all LAT blind-search pulsars. PSR J2139+4716 has the lowest spin-down power (3e33 erg/s) among all non-recycled gamma-ray pulsars ever found. Despite extensive multi-frequency observations, only PSR J0106+4855 has detectable pulsations in the radio band. The other eight pulsars belong to the increasing population of radio-quiet gamma-ray pulsars.

astro-ph.HE↗

arXiv2011

All-sky Search for Periodic Gravitational Waves in the Full S5 LIGO Data

We report on an all-sky search for periodic gravitational waves in the frequency band 50-800 Hz and with the frequency time derivative in the range of 0 through -6e-9 Hz/s. Such a signal could be produced by a nearby spinning and slightly non-axisymmetric isolated neutron star in our galaxy. After recent improvements in the search program that yielded a 10x increase in computational efficiency, we have searched in two years of data collected during LIGO's fifth science run and have obtained the most sensitive all-sky upper limits on gravitational wave strain to date. Near 150 Hz our upper limit on worst-case linearly polarized strain amplitude $h_0$ is 1e-24, while at the high end of our frequency range we achieve a worst-case upper limit of 3.8e-24 for all polarizations and sky locations. These results constitute a factor of two improvement upon previously published data. A new detection pipeline utilizing a Loosely Coherent algorithm was able to follow up weaker outliers, increasing the volume of space where signals can be detected by a factor of 10, but has not revealed any gravitational wave signals. The pipeline has been tested for robustness with respect to deviations from the model of an isolated neutron star, such as caused by a low-mass or long-period binary companion.

gr-qc↗

arXiv2011

Directional limits on persistent gravitational waves using LIGO S5 science data

The gravitational-wave (GW) sky may include nearby pointlike sources as well as astrophysical and cosmological stochastic backgrounds. Since the relative strength and angular distribution of the many possible sources of GWs are not well constrained, searches for GW signals must be performed in a model-independent way. To that end we perform two directional searches for persistent GWs using data from the LIGO S5 science run: one optimized for pointlike sources and one for arbitrary extended sources. The latter result is the first of its kind. Finding no evidence to support the detection of GWs, we present 90% confidence level (CL) upper-limit maps of GW strain power with typical values between 2-20x10^-50 strain^2 Hz^-1 and 5-35x10^-49 strain^2 Hz^-1 sr^-1 for pointlike and extended sources respectively. The limits on pointlike sources constitute a factor of 30 improvement over the previous best limits. We also set 90% CL limits on the narrow-band root-mean-square GW strain from interesting targets including Sco X-1, SN1987A and the Galactic Center as low as ~7x10^-25 in the most sensitive frequency range near 160 Hz. These limits are the most constraining to date and constitute a factor of 5 improvement over the previous best limits.

astro-ph.CO↗

arXiv2011

Flight Performance of an advanced CZT Imaging Detector in a Balloon-borne Wide-Field Hard X-ray Telescope - ProtoEXIST1

We successfully carried out the first high-altitude balloon flight of a wide-field hard X-ray coded-aperture telescope ProtoEXIST1, which was launched from the Columbia Scientific Balloon Facility at Ft. Sumner, New Mexico on October 9, 2009. ProtoEXIST1 is the first implementation of an advanced CdZnTe (CZT) imaging detector in our ongoing program to establish the technology required for next generation wide-field hard X-ray telescopes. The CZT detector plane in ProtoEXIST1 consists of an 8 x 8 array of closely tiled 2 cm x 2 cm x 0.5 cm thick pixellated CZT crystals, each with 8 x 8 pixels, covering a 256 cm^2 active area with 2.5 mm pixels. A tungsten mask, mounted at 90 cm above the detector provides shadowgrams of X-ray sources in the 30 - 600 keV band for imaging, allowing a fully coded field of view of 9 Deg x 9 Deg with an angular resolution of 20 arcmin. To reduce the background radiation, the detector is surrounded by semi-graded (Pb/Sn/Cu) passive shields on the four sides all the way to the mask. On the back side, a 26 cm x 26 cm x 2 cm CsI(Na) active shield provides signals to tag charged particle induced events as well as >~ 100 keV background photons from below. The flight duration was only about 7.5 hours due to strong winds (60 knots) at float altitude (38-39 km). Throughout the flight, the CZT detector performed excellently. The telescope observed Cyg X-1 for ~ 1 hour at the end of the flight. Despite a few problems with the pointing and aspect systems that caused the telescope to track about 6.4 deg off the target, the analysis of the Cyg X-1 data revealed an X-ray source at 7.2 sigma in the 30-100 keV energy band at the expected location from the optical images taken by the onboard daytime star camera. The success of this first flight is very encouraging for the future development of the advanced CZT imaging detectors (ProtoEXIST2, with 0.6 mm pixels).

astro-ph.IM↗

arXiv2011

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

We present direct upper limits on continuous gravitational wave emission from the Vela pulsar using data from the Virgo detector's second science run. These upper limits have been obtained using three independent methods that assume the gravitational wave emission follows the radio timing. Two of the methods produce frequentist upper limits for an assumed known orientation of the star's spin axis and value of the wave polarization angle of, respectively, $1.9\ee{-24}$ and $2.2\ee{-24}$, with 95% confidence. The third method, under the same hypothesis, produces a Bayesian upper limit of $2.1\ee{-24}$, with 95% degree of belief. These limits are below the indirect {\it spin-down limit} of $3.3\ee{-24}$ for the Vela pulsar, defined by the energy loss rate inferred from observed decrease in Vela's spin frequency, and correspond to a limit on the star ellipticity of $\sim 10^{-3}$. Slightly less stringent results, but still well below the spin-down limit, are obtained assuming the star's spin axis inclination and the wave polarization angles are unknown.

astro-ph.HE↗

arXiv2011

Search for Gravitational Wave Bursts from Six Magnetars

Soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) are thought to be magnetars: neutron stars powered by extreme magnetic fields. These rare objects are characterized by repeated and sometimes spectacular gamma-ray bursts. The burst mechanism might involve crustal fractures and excitation of non-radial modes which would emit gravitational waves (GWs). We present the results of a search for GW bursts from six galactic magnetars that is sensitive to neutron star f-modes, thought to be the most efficient GW emitting oscillatory modes in compact stars. One of them, SGR 0501+4516, is likely ~1 kpc from Earth, an order of magnitude closer than magnetars targeted in previous GW searches. A second, AXP 1E 1547.0-5408, gave a burst with an estimated isotropic energy >10^{44} erg which is comparable to the giant flares. We find no evidence of GWs associated with a sample of 1279 electromagnetic triggers from six magnetars occurring between November 2006 and June 2009, in GW data from the LIGO, Virgo, and GEO600 detectors. Our lowest model-dependent GW emission energy upper limits for band- and time-limited white noise bursts in the detector sensitive band, and for f-mode ringdowns (at 1090 Hz), are 3.0x10^{44} d_1^2 erg and 1.4x10^{47} d_1^2 erg respectively, where d_1 = d_{0501} / 1 kpc and d_{0501} is the distance to SGR 0501+4516. These limits on GW emission from f-modes are an order of magnitude lower than any previous, and approach the range of electromagnetic energies seen in SGR giant flares for the first time.

astro-ph.HE↗

arXiv2011

Arecibo PALFA Survey and Einstein@Home: Binary Pulsar Discovery by Volunteer Computing

We report the discovery of the 20.7 ms binary pulsar J1952+2630, made using the distributed computing project Einstein@Home in Pulsar ALFA survey observations with the Arecibo telescope. Follow-up observations with the Arecibo telescope confirm the binary nature of the system. We obtain a circular orbital solution with an orbital period of 9.4 hr, a projected orbital radius of 2.8 lt-s, and a mass function of f = 0.15 solar masses by analysis of spin period measurements. No evidence of orbital eccentricity is apparent; we set a 2-sigma upper limit e < 1.7e-3. The orbital parameters suggest a massive white dwarf companion with a minimum mass of 0.95 solar masses, assuming a pulsar mass of 1.4 solar masses. Most likely, this pulsar belongs to the rare class of intermediate mass binary pulsars. Future timing observations will aim to determine the parameters of this system further, measure relativistic effects, and elucidate the nature of the companion star.

astro-ph.GA↗

arXiv2011

Search for gravitational waves from binary black hole inspiral, merger and ringdown

We present the first modeled search for gravitational waves using the complete binary black hole gravitational waveform from inspiral through the merger and ringdown for binaries with negligible component spin. We searched approximately 2 years of LIGO data taken between November 2005 and September 2007 for systems with component masses of 1-99 solar masses and total masses of 25-100 solar masses. We did not detect any plausible gravitational-wave signals but we do place upper limits on the merger rate of binary black holes as a function of the component masses in this range. We constrain the rate of mergers for binary black hole systems with component masses between 19 and 28 solar masses and negligible spin to be no more than 2.0 per Mpc^3 per Myr at 90% confidence.

gr-qc↗

arXiv2010

A search for gravitational waves associated with the August 2006 timing glitch of the Vela pulsar

The physical mechanisms responsible for pulsar timing glitches are thought to excite quasi-normal mode oscillations in their parent neutron star that couple to gravitational wave emission. In August 2006, a timing glitch was observed in the radio emission of PSR B0833-45, the Vela pulsar. At the time of the glitch, the two co-located Hanford gravitational wave detectors of the Laser Interferometer Gravitational-wave observatory (LIGO) were operational and taking data as part of the fifth LIGO science run (S5). We present the first direct search for the gravitational wave emission associated with oscillations of the fundamental quadrupole mode excited by a pulsar timing glitch. No gravitational wave detection candidate was found. We place Bayesian 90% confidence upper limits of 6.3e-21 to 1.4e-20 on the peak intrinsic strain amplitude of gravitational wave ring-down signals, depending on which spherical harmonic mode is excited. The corresponding range of energy upper limits is 5.0e44 to 1.3e45 erg.

gr-qc↗

arXiv2010

The Proposed High Energy Telescope (HET) for EXIST

The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR is rich with energetic phenomena and highly variable non-thermal phenomena on a broad range of timescales. The High Energy Telescope (HET) on the proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey the full sky for rare and luminous hard X-ray phenomena at unprecedented sensitivities. It will detect and localize (<20", at 5 sigma threshold) X-ray sources quickly for immediate followup identification by two other onboard telescopes - the Soft X-ray imager (SXI) and Optical/Infrared Telescope (IRT). The large array (4.5 m^2) of imaging (0.6 mm pixel) CZT detectors in the HET, a coded-aperture telescope, will provide unprecedented high sensitivity (~0.06 mCrab Full Sky in a 2 year continuous scanning survey) in the 5 - 600 keV band. The large field of view (90 deg x 70 deg) and zenith scanning with alternating-orbital nodding motion planned for the first 2 years of the mission will enable nearly continuous monitoring of the full sky. A 3y followup pointed mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and spectroscopy for thousands of sources discovered in the Survey. We review the HET design concept and report the recent progress of the CZT detector development, which is underway through a series of balloon-borne wide-field hard X-ray telescope experiments, ProtoEXIST. We carried out a successful flight of the first generation of fine pixel large area CZT detectors (ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 & 3) for the technology development needed for the HET.

astro-ph.IM↗

arXiv2010

First search for gravitational waves from the youngest known neutron star

We present a search for periodic gravitational waves from the neutron star in the supernova remnant Cassiopeia A. The search coherently analyzes data in a 12-day interval taken from the fifth science run of the Laser Interferometer Gravitational-Wave Observatory. It searches gravitational wave frequencies from 100 to 300 Hz, and covers a wide range of first and second frequency derivatives appropriate for the age of the remnant and for different spin-down mechanisms. No gravitational wave signal was detected. Within the range of search frequencies, we set 95% confidence upper limits of 0.7--1.2e-24 on the intrinsic gravitational wave strain, 0.4--4e-4 on the equatorial ellipticity of the neutron star, and 0.005--0.14 on the amplitude of r-mode oscillations of the neutron star. These direct upper limits beat indirect limits derived from energy conservation and enter the range of theoretical predictions involving crystalline exotic matter or runaway r-modes. This is the first gravitational wave search to present upper limits on r-modes.

gr-qc↗

arXiv2010

Overview of EXIST mission science and implementation

The Energetic X-ray Imaging Survey Telescope (EXIST) is designed to i) use the birth of stellar mass black holes, as revealed by cosmic Gamma-Ray Bursts (GRBs), as probes of the very first stars and galaxies to exist in the Universe. Both their extreme luminosity (~104 times larger than the most luminous quasars) and their hard X-ray detectability over the full sky with wide-field imaging make them ideal "back-lights" to measure cosmic structure with X-ray, optical and near-IR (nIR) spectra over many sight lines to high redshift. The full-sky imaging detection and rapid followup narrow-field imaging and spectroscopy allow two additional primary science objectives: ii) novel surveys of supermassive black holes (SMBHs) accreting as very luminous but rare quasars, which can trace the birth and growth of the first SMBHs as well as quiescent SMBHs (non-accreting) which reveal their presence by X-ray flares from the tidal disruption of passing field stars; and iii) a multiwavelength Time Domain Astrophysics (TDA) survey to measure the temporal variability and physics of a wide range of objects, from birth to death of stars and from the thermal to non-thermal Universe. These science objectives are achieved with the telescopes and mission as proposed for EXIST described here.

astro-ph.CO↗

arXiv2010

Pulsar Discovery by Global Volunteer Computing

Einstein@Home aggregates the computer power of hundreds of thousands of volunteers from 192 countries to "mine" large data sets. It has now found a 40.8 Hz isolated pulsar in radio survey data from the Arecibo Observatory taken in February 2007. Additional timing observations indicate that this pulsar is likely a disrupted recycled pulsar. PSR J2007+2722's pulse profile is remarkably wide with emission over almost the entire spin period; the pulsar likely has closely aligned magnetic and spin axes. The massive computing power provided by volunteers should enable many more such discoveries.

astro-ph.GA↗

arXiv2010

Calibration of the LIGO Gravitational Wave Detectors in the Fifth Science Run

The Laser Interferometer Gravitational Wave Observatory (LIGO) is a network of three detectors built to detect local perturbations in the space-time metric from astrophysical sources. These detectors, two in Hanford, WA and one in Livingston, LA, are power-recycled Fabry-Perot Michelson interferometers. In their fifth science run (S5), between November 2005 and October 2007, these detectors accumulated one year of triple coincident data while operating at their designed sensitivity. In this paper, we describe the calibration of the instruments in the S5 data set, including measurement techniques and uncertainty estimation.

gr-qc↗

arXiv2010

Sensitivity to Gravitational Waves from Compact Binary Coalescences Achieved during LIGO's Fifth and Virgo's First Science Run

We summarize the sensitivity achieved by the LIGO and Virgo gravitational wave detectors for compact binary coalescence (CBC) searches during LIGO's fifth science run and Virgo's first science run. We present noise spectral density curves for each of the four detectors that operated during these science runs which are representative of the typical performance achieved by the detectors for CBC searches. These spectra are intended for release to the public as a summary of detector performance for CBC searches during these science runs.

gr-qc↗

arXiv2010

Search for gravitational-wave bursts associated with gamma-ray bursts using data from LIGO Science Run 5 and Virgo Science Run 1

We present the results of a search for gravitational-wave bursts associated with 137 gamma-ray bursts (GRBs) that were detected by satellite-based gamma-ray experiments during the fifth LIGO science run and first Virgo science run. The data used in this analysis were collected from 2005 November 4 to 2007 October 1, and most of the GRB triggers were from the Swift satellite. The search uses a coherent network analysis method that takes into account the different locations and orientations of the interferometers at the three LIGO-Virgo sites. We find no evidence for gravitational-wave burst signals associated with this sample of GRBs. Using simulated short-duration (<1 s) waveforms, we set upper limits on the amplitude of gravitational waves associated with each GRB. We also place lower bounds on the distance to each GRB under the assumption of a fixed energy emission in gravitational waves, with typical limits of D ~ 15 Mpc (E_GW^iso / 0.01 M_o c^2)^1/2 for emission at frequencies around 150 Hz, where the LIGO-Virgo detector network has best sensitivity. We present astrophysical interpretations and implications of these results, and prospects for corresponding searches during future LIGO-Virgo runs.

astro-ph.HE↗