Search for gravitational-lensing signatures in the full third observing run of the LIGO-Virgo network

Gravitational lensing by massive objects along the line of sight to the source causes distortions of gravitational wave-signals; such distortions may reveal information about fundamental physics, cosmology and astrophysics. In this work, we have extended the search for lensing signatures to all binary black hole events from the third observing run of the LIGO--Virgo network. We search for repeated signals from strong lensing by 1) performing targeted searches for subthreshold signals, 2) calculating the degree of overlap amongst the intrinsic parameters and sky location of pairs of signals, 3) comparing the similarities of the spectrograms amongst pairs of signals, and 4) performing dual-signal Bayesian analysis that takes into account selection effects and astrophysical knowledge. We also search for distortions to the gravitational waveform caused by 1) frequency-independent phase shifts in strongly lensed images, and 2) frequency-dependent modulation of the amplitude and phase due to point masses. None of these searches yields significant evidence for lensing. Finally, we use the non-detection of gravitational-wave lensing to constrain the lensing rate based on the latest merger-rate estimates and the fraction of dark matter composed of compact objects

Search for eccentric black hole coalescences during the third observing run of LIGO and Virgo

Despite the growing number of confident binary black hole coalescences observed through gravitational waves so far, the astrophysical origin of these binaries remains uncertain. Orbital eccentricity is one of the clearest tracers of binary formation channels. Identifying binary eccentricity, however, remains challenging due to the limited availability of gravitational waveforms that include effects of eccentricity. Here, we present observational results for a waveform-independent search sensitive to eccentric black hole coalescences, covering the third observing run (O3) of the LIGO and Virgo detectors. We identified no new high-significance candidates beyond those that were already identified with searches focusing on quasi-circular binaries. We determine the sensitivity of our search to high-mass (total mass M>70 M⊙) binaries covering eccentricities up to 0.3 at 15 Hz orbital frequency, and use this to compare model predictions to search results. Assuming all detections are indeed quasi-circular, for our fiducial population model, we place an upper limit for the merger rate density of high-mass binaries with eccentricities 0<e≤0.3 at 0.33 Gpc−3 yr−1 at 90\% confidence level

Ultralight vector dark matter search using data from the KAGRA O3GK run

Among the various candidates for dark matter (DM), ultralight vector DM can be probed by laser interferometric gravitational wave detectors through the measurement of oscillating length changes in the arm cavities. In this context, KAGRA has a unique feature due to differing compositions of its mirrors, enhancing the signal of vector DM in the length change in the auxiliary channels. Here we present the result of a search for U(1)B−L gauge boson DM using the KAGRA data from auxiliary length channels during the first joint observation run together with GEO600. By applying our search pipeline, which takes into account the stochastic nature of ultralight DM, upper bounds on the coupling strength between the U(1)B−L gauge boson and ordinary matter are obtained for a range of DM masses. While our constraints are less stringent than those derived from previous experiments, this study demonstrates the applicability of our method to the lower-mass vector DM search, which is made difficult in this measurement by the short observation time compared to the auto-correlation time scale of DM

Study of Periodic Polarized X-ray Emission with IXPE

X-ray polarization of astronomical sources is an almost unexplored field of high energy astrophysics, with a single signicant measurement performed more than 40 years ago for the Crab Nebula by the Orbiting Solar Observatory OSO-8. The importance of this emission property is that it carries direct information on the geometry and magnetic field configuration of the source. The lack of experimental measurements is due to the difficulty in realizing conventional X-ray polarimeters (typically based on Thomson scattering and Bragg reflection) with sufficient sensitivity, along with the necessity of comparatively long observation times. In recent years, the advent of a new generation of high-sensitivity polarimeters based on the photoelectric effect, led to the selection of the Imaging X-ray Polarimetry Explorer (IXPE) mission as the next NASA SMEX (SMall EX-plorer), with a launch scheduled for 2021. IXPE will provide a fundamental opportunity to study polarization of many kind of X-ray sources, like pulsar wind nebulae, supernova remnants, X-ray binaries, active galactic nuclei, magnetars and pulsars (both isolated and accreting). The polarimetric sensitivity will be complemented by moderate spectral, imaging and timing capabilities, providing simultaneous access to all properties carried by electromagnetic radiation. The IXPE payload is composed of three identical X-ray telescopes, operating independently in the 2-8 keV energy range, each with a Mirror Module Assembly (grazing incidence X-ray optics) and a polarization-sensitive imaging detector (a Gas Pixel Detector, or GPD) at the focus. The GPD, constituting the main Italian responsibility for the mission, is designed as a proportional gas detector with an Application-Specific Integrated Circuit (ASIC) used as a pixelized collecting anode and a Gas Electron Multiplier (GEM) as amplification stage. It exploits the photoelectric effect in a gas. Since for polarized X-rays photo-electrons are preferentially emitted along the direction of the electric field, the polarization can be measured statistically by imaging the photoelectron track of each event and reconstructing its direction. Having a realistic observation-simulation framework is of crucial importance for the IXPE mission in order to assess the sensitivity of the instrument, analyze a specific science case and formulate the observation plan. In our case, being X-ray polarimetry largely a new field, the collaboration could only make a limited re-use of existing tools, and had to develop many of the technology from scratch. During my thesis work I did participate actively in the development of such a simulation and analysis framework, called ixpeobssim. This simulation framework is based on the Python programming language and the SciPy stack. Starting from an arbitrary source model (including morphological, temporal, spectral and polarimetric information), it uses the instrument response functions to produce fast and realistic observation simulations. The generated events list can be directly fed into the standard X-ray visualization and analysis tools, including XSPEC, which make it a useful tool not only for simulating observations of astronomical sources, but also to develop and test end-to-end analysis chains. The work described in this thesis is focused on the implementation of a series of simulation features and analysis tools dedicated to the study of the pulsars, with particular emphasis on transitional millisecond pulsars. Millisecond pulsars are believed to be the descendants of old, magnetized neutron stars (NSs), hosted in low mass X-ray binaries (LMXB). The transfer of angular momentum from companion to NS through accretion is considered the responsible of the spin-up of slowly rotating NSs to millisecond spinning sources. This recycling scenario switch of the NS radio pulsation, leading to an accretion phase where the X-ray pulsation and the spin frequency can be detected. In this case, the system takes on the typical features of an accretion millisecond X-ray pulsar (AMXP). The recycling scenario of MSPs has been supported by the so called missed link pulsar PSRJ1023+0038 (J1023), the first observed to alternate a radio pulsar phase to an X-ray pulsed state, powered by accretion, usually referred to us as transitional millisecond pulsar (or tMSP). The nature of tMSP pulsation due to accretion ow onto the NS (if any) is not well understood. In this thesis a simulation of J1023 observation with IXPE is presented, bringing its state of art coming from literature. The emission model assumed is the AMXP in low-luminosity state. The polarization degree is resulting from the optical polarization and the X-ray pulsed luminosity, that provide an average polarization degree equal to 8%. The analysis made to the generated events optimally reproduces the spectrum and the pulse profile of the input model for two months of observation. The analysis of the polarization degree of these events allows to resolve two phase intervals of the source emission, providing two comparable polarization degree values. Thus during two months of observation we will be able to measure significantly the mean X-ray polarization degree of J1023. Everything I developed is available from the official software package of the IXPE collaboration and it can be used to study these kind of sources. This work opens up the possibility to new perspectives, such as the possibility of simulating orbital phase modulation for binary source models (that will improve the J1023 simulation itself), simulating the brightest millisecond pulsars (like SAXJ1808) and verifying if its polarization is resolvable in phase by IXPE. So in the end, regard to J1023, I say that its low brightness does not allow it to be part of observational priorities of IXPE, but given its peculiarity, it is desirable to insert the source among the last targets of the first two years of mission or in the first renewable year

Searching for gravitational waves from binary close encounters: a deep learning analysis approach

Since 2015 Advanced LIGO and Virgo have detected 93 gravitational wave transients produced by compact object binaries in quasi-circular orbits. Dynamical capture scenarios in dense stellar environments are predicted to form eccentric orbits persisting until the merger. At each periastron passage, the close encounters between the member of the compact object pair should emit gravitational wave signals. Waveforms expected in this scenario are not modeled with the same precision as coalescences, making it a good area to explore innovative analysis methods. The thesis is focused on the application of deep learning to detect gravitational waves from close encounters in compact binary systems. We developed a classification algorithm based on convolutional neural networks capable of detecting transient signals associated with close encounters and discriminating them from noise glitches. The algorithm training is based on custom simulated data of close encounters embedded in colored noise from Advanced Virgo. To better study the contamination by noise glitches, we developed a tool to test Gaussianity based on the Rayleigh test, which has been tested on real Virgo data, and an interactive web analysis tool for the quick look of glitches. The performance of the deep learning architecture developed in this thesis has an accuracy (~ 99.9%) comparable to other deep learning-based studies on gravitational wave transients. The results open new observational scenarios for the detection of close encounters, with the final goal of making them interesting targets for future electromagnetic follow-up campaigns

Search for intermediate-mass black hole binaries in the third observing run of Advanced LIGO and Advanced Virgo

International audienceIntermediate-mass black holes (IMBHs) span the approximate mass range 100−105 M⊙, between black holes (BHs) that formed by stellar collapse and the supermassive BHs at the centers of galaxies. Mergers of IMBH binaries are the most energetic gravitational-wave sources accessible by the terrestrial detector network. Searches of the first two observing runs of Advanced LIGO and Advanced Virgo did not yield any significant IMBH binary signals. In the third observing run (O3), the increased network sensitivity enabled the detection of GW190521, a signal consistent with a binary merger of mass ∼150 M⊙ providing direct evidence of IMBH formation. Here, we report on a dedicated search of O3 data for further IMBH binary mergers, combining both modeled (matched filter) and model-independent search methods. We find some marginal candidates, but none are sufficiently significant to indicate detection of further IMBH mergers. We quantify the sensitivity of the individual search methods and of the combined search using a suite of IMBH binary signals obtained via numerical relativity, including the effects of spins misaligned with the binary orbital axis, and present the resulting upper limits on astrophysical merger rates. Our most stringent limit is for equal mass and aligned spin BH binary of total mass 200 M⊙ and effective aligned spin 0.8 at 0.056 Gpc−3 yr−1 (90% confidence), a factor of 3.5 more constraining than previous LIGO-Virgo limits. We also update the estimated rate of mergers similar to GW190521 to 0.08 Gpc−3 yr−1.Key words: gravitational waves / stars: black holes / black hole physicsCorresponding author: W. Del Pozzo, e-mail: [email protected]† Deceased, August 2020