Showing papers by "G. M. Guidi published in 2009"

An upper limit on the stochastic gravitational-wave background of cosmological origin

Abstract: A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations. Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 times 10-6 at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models. This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100 Hz.

Searches for gravitational waves from known pulsars with S5 LIGO data

Abstract: 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.

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

Abstract: 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.

Gravitational wave burst search in the Virgo C7 data

Abstract: A search for gravitational wave burst events has been performed with the Virgo C7 commissioning run data that have been acquired in September 2005 over 5 days. It focused on unmodeled short duration signals in the frequency range 150 Hz to 2 kHz. A search aimed at detecting the GW emission from the merger and ring-down phases of binary black hole coalescences was also carried out. An extensive understanding of the data was required to be able to handle a burst search using the output of only one detector. A 90% confidence level upper limit on the number of expected events given the Virgo C7 sensitivity curve has been derived as a function of the signal strength, for unmodeled gravitational wave searches. The sensitivity of the analysis presented is, in terms of the root sum square strain amplitude, h_rss sime 10−20 Hz−1/2. This can be interpreted in terms of a frequentist upper limit on the rate {\cal{R}}_{90\%} of detectable gravitational wave bursts at the level of 1.1 events per day at a 90% confidence level. From the binary black hole search, we obtained the distance reach at 50% and 90% efficiency as a function of the total mass of the final black hole. The maximal detection distance for non-spinning high and equal mass black hole binary system obtained by this analysis in C7 data is sime2.9 ± 0.1 Mpc for a detection efficiency of 50% for a binary of total mass 80 Modot.

Laser with an in-loop relative frequency stability of 1.0x10(-21) on a 100-ms time scale for gravitational-wave detection

Abstract: We report on the stabilization of the laser frequency for the Virgo gravitational-wave detector. We have obtained a frequency noise level, measured in loop, of 1.9×10−7 Hz/sqrt(Hz) at 10 Hz for the 1064 nm laser; this value is limited by shot noise. The Allan standard deviation for relative frequency noise is 1.0×10−21 on a 100-ms time scale. The spectral density of the laser frequency noise is negligible in the channel where gravitational waves ought to appear and meets the specifications for the target spectral resolution of the Virgo interferometer in the 10 Hz-10 kHz detection bandwidth.

Cleaning the Virgo sampled data for the search of periodic sources of gravitational waves

Abstract: The cleaning procedure used to produce the data that we analyze for the search of periodic sources of gravitational waves is based on different steps, which are applied to both time and frequency domain data. We have recently improved the procedure, which now consists of different steps. The use of a cleaned procedure is in principle important, since it is aimed to recover at best the observation time from the data by vetoing only times where disturbances act and not entire data chunks. Clearly, the effect of the procedure depends on the nature of the data, and is thus highly related to the detector characteristics in a particular run. We will here describe the whole cleaning chain, by giving details and examples based on the C7 and WSR10 Virgo runs.

Control of the laser frequency of the Virgo gravitational wave interferometer with an in-loop relative frequency stability of 1.0 × 10 −21 on a 100 ms time scale

Abstract: The measurement of the space-time structure variations induced by strong cosmic events (supernovae, coalescing binaries of neutron stars, etc.) requires an oscillator with a relative stability of 10−21 on time scales typically ≈100 ms. We demonstrate that the Virgo interferometer with a wavelength of 1.064 °m has a laser frequency with an in-loop stability of 1.0 × 10−21 on a 100 ms time scale, and an in-loop frequency noise of 2 × 10−7 Hz/√Hz at 10 Hz. We show that this fits the specifications. Two references successively stabilize the laser frequency. The first one is a 144 m long suspended cavity; the second one is the common mode of two perpendicular 3 km long Fabry-Perot cavities. The differential mode of the relative length variations of these two optical cavities is the port where we expect the signal for the gravitational waves; this out-of-loop measurement, less sensitive to laser frequency noise, does not show up correlations with the in-loop error signal. This is the best ever performance of short term laser frequency stabilization reported.