Lensing bias on cosmological parameters from bright standard sirens

Next generation gravitational waves (GWs) observatories are expected to measure GW signals with unprecedented sensitivity, opening new, independent avenues to learn about our Universe. The distance-redshift relation is a fulcrum for cosmology and can be tested with GWs emitted by merging binaries of compact objects, called standard sirens, thanks to the fact that they provide the absolute distance from the source. On the other hand, fluctuations of the intervening matter density field induce modifications on the measurement of luminosity distance compared to that of a homogeneous universe. Assuming that the redshift information is obtained through the detection of an electromagnetic counterpart, we investigate the impact that lensing of GWs might have in the inference of cosmological parameters. We treat lensing as a systematic error and check for residual bias on the values of the cosmological parameters. We do so by means of mock catalogues of bright sirens events in different scenarios relevant to Einstein Telescope. For our fiducial scenario, the lensing bias can be comparable to or greater than the expected statistical uncertainty of the cosmological parameters, although non-negligible fluctuations in the bias values are observed for different realisations of the mock catalogue. We also discuss some mitigation strategies that can be adopted in the data analysis. Overall, our work highlights the need to model lensing effects when using standard sirens as probes of the distance-redshift relation.Comment: 15 pages, 14 figure

Sensitivity of a Compton nanosatellite for Gamma-Ray Burst observation. Sensitività di un nanosatellite Compton per osservazioni di Gamma-ray Burst

Gamma-Ray Bursts (GRB) are of particular interest for multi-messenger astrophysics because they are associated with the production of gravitational waves (GW). The intrinsically unpredictable, short nature of the electromagnetic events make the joint detection of GWs and GRBs still rare. The solution that we explore is the use of one or more nanosatellites to augment the sky coverage for GRB events. In particular, we focus on a CubeSat nanosatellite with a small detector on board working in the MeV regime, from few keV to few MeV. The nanosatellites could play a key role in confirming and localizing the GRBs in response to alerts sent by detectors working with signals of different type. To evaluate the CubeSat sensitivity, we simulate the performance of the detector to GRBs with parameters taken from the 3th Fermi GBM catalog. Several assumptions were made in formulating the problem: the presence of an alert coming to the CubeSat, the need of confirming it at 3-sigma significance and localizing it, a fast procedure that could run unassisted. The short hard GRB 090510 and the faint GRB 170817 were studied in more detail to validate the procedure. I Gamma-ray Bursts (GRB) sono di particolare interesse per l'astrofisica multi-messenger in quanto sono associati alla produzione di onde gravitazionali. La natura imprevedibile, di breve durata e non localizzata dell'evento rende la misurazione simultanea di GRB e onde gravitazionali ad oggi ancora rara. La soluzione che consideriamo in questa tesi è l'uso di uno o più nanosatelliti per aumentare la copertura del cielo per osservazioni di GRB. In particolare, il nanosatellite CubeSat che studiamo lavorerà nel regime MeV, cioè ad energie che vanno da pochi keV a pochi MeV. Il nanosatellite potrebbe avere un ruolo chiave nel confermare e localizzare i GRB a seguito di segnali di allerta forniti da altri strumenti operanti con segnali di vario tipo. Al fine di valutare la sensitività di CubeSat abbiamo simulato GRBs con parametri tratti dal 3th Fermi GBM Catalog. Diverse assunzioni sono state utilizzate nel formulare il problema: la presenza di un segnale di allerta proveniente da altri strumenti (o generato internamente al nanosatellite), il bisogno di confermare il segnale con un livello di confidenza pari a 3-sigma e di localizzare la sorgente, una procedura veloce e automatizzabile. Il GRB 090510, intenso e hard, e il debole GRB170817 sono stati infine studiati in maggior dettaglio per validare la procedur

Sensitivity to Gamma-Ray Bursts of a Nanosatellite MeV Telescope with a Silicon Tracker

We propose a nanosatellite Compton telescope based on a silicon tracker, both as a fast and low-cost mission for astrophysics in the MeV regime and as a pathfinder for future large-scale Compton orbital observatories. In this paper we assess the sensitivity of such an instrument for the observation of gamma-ray bursts

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