arXiv subjects

Luciano Rezzolla

Publications and source records attributed to Luciano Rezzolla.

At least 91 records · Page 5Linked to original sources

arXiv2022

Impact of extreme spins and mass ratios on the post-merger observables of high-mass binary neutron stars

The gravitational-wave events GW170817 and GW190425 have led to a number of important insights on the equation of state of dense matter and the properties of neutron stars, such as their radii and the maximum mass. Some of these conclusions have been drawn on the basis of numerical-relativity simulations of binary neutron-star mergers with vanishing initial spins. While this may be a reasonable assumption in equal-mass systems, it may be violated in the presence of large mass asymmetries accompanied by the presence of high spins. To quantify the impact of high spins on multi-messenger gravitational-wave events, we have carried out a series of high-mass binary neutron-star mergers with a highly spinning primary star and large mass asymmetries that have been modelled self-consistently using two temperature-dependent equations of state. We show that, when compared with equal-mass, irrotational binaries, these systems can lead to significant differences in the remnant lifetime, in the dynamical ejecta, in the remnant disc masses, in the secular ejecta, and on the bulk kilonova properties. These differences could be exploited to remove the degeneracy between low- and high-spin priors in the detection of gravitational waves from binary neutron-star mergers.

astro-ph.HE↗

arXiv2022

Distinguishing gravitational and emission physics in black-hole imaging: spherical symmetry

Imaging a supermassive black hole and extracting physical information requires good knowledge of both the gravitational and the astrophysical conditions near the black hole. When the geometrical properties of the black hole are well understood, extracting information on the emission properties is possible. Similarly, when the emission properties are well understood, extracting information on the black-hole geometry is possible. At present however, uncertainties are present both in the geometry and in the emission, and this inevitably leads to degeneracies in the interpretation of the observations. We explore here the impact of varying geometry and emission coefficient when modelling the imaging of a spherically-accreting black hole. Adopting the Rezzolla-Zhidenko parametric metric to model arbitrary static black-holes, we first demonstrate how shadow-size measurements leave degeneracies in the multidimensional space of metric-deviation parameters, even in the limit of infinite-precision measurements. Then, at finite precision, we show that these degenerate regions can be constrained when multiple pieces of information, such as the shadow-size and the peak image intensity contrast, are combined. Such degeneracies can potentially be eliminated with measurements at increased angular-resolution and flux-sensitivity. While our approach is restricted to spherical symmetry and hence idealised, we expect our results to hold also when more complex geometries and emission processes are considered.

gr-qc↗

arXiv2021

State-of-the-art energetic and morphological modelling of the launching site of the M87 jet

M87 has been the target of numerous astronomical observations across the electromagnetic spectrum and Very Long Baseline Interferometry (VLBI) resolved an edge-brightened jet. However, the origin and formation of its jets remain unclear. In our current understand black holes (BH) are the driving engine of jet formation, and indeed the recent Event Horizon Telescope (EHT) observations revealed a ring-like structure in agreement with theoretical models of accretion onto a rotating Kerr BH. In addition to the spin of the BH being a potential source of energy for the launching mechanism, magnetic fields are believed to play a key role in the formation of relativistic jets. A priori, the spin, $a_\star$, of BH in M87* is unknown, however, when accounting for the estimates on the X-ray luminosity and jet power, values $\left |a_\star \right| \gtrsim 0.5$ appear favoured. Besides the properties of the accretion flow and the BH spin, the radiation microphysics including the particle distribution (thermal and non-thermal) as well as the particle acceleration mechanism play a crucial role. We show that general-relativistic magnetohydrodynamics simulations and general-relativistic radiative transfer calculations can reproduce the broadband spectrum from the radio to the near-infrared regime and simultaneously match the observed collimation profile of M87, thus allowing us to set rough constraints on the dimensionless spin of M87* to be $0.5\lesssim a_{\star}\lesssim 1.0$, with higher spins being possibly favoured.

astro-ph.HE↗

arXiv2021

Impact of non-thermal particles on the spectral and structural properties of M87

The recent 230 GHz observations of the Event Horizon Telescope (EHT) are able to image the innermost structure of the M87 and show a ring-like structure which is in agreement with thermal synchrotron emission generated in a torus surrounding a supermassive black hole. However, at lower frequencies M87 is characterised by a large-scale and edge-brightened jet with clear signatures of non-thermal emission. In order to bridge the gap between these scales and to provide a theoretical interpretation of these observations we perform general relativistic magnetohydrodynamic simulations of accretion on to black holes and jet launching. M87 has been the target for multiple observations across the entire electromagnetic spectrum. Among these VLBI observations provide unique details on the collimation profile of the jet down to several gravitational radii. In this work we aim to model the observed broad-band spectrum of M87 from the radio to the NIR regime and at the same time fit the jet structure as observed with Global mm-VLBI at 86 GHz. We use general relativistic magnetohydrodynamics and simulate the accretion of the magnetised plasma onto Kerr-black holes in 3D. The radiative signatures of these simulations are computed taking different electron distribution functions into account and a detailed parameter survey is performed in order to match the observations. The results of our simulations show that magnetically arrested disks around fast spinning black holes ($a_\star\geq0.5$) together with a mixture of thermal and non-thermal particle distributions are able to model simultaneously the broad-band spectrum and the innermost jet structure of M87

astro-ph.HE↗

arXiv2021

Quasi-universal behaviour of the threshold mass in unequal-mass, spinning binary neutron-star mergers

The lifetime of the remnant produced by the merger of two neutron stars can provide a wealth of information on the equation of state of nuclear matter and on the processes leading to the electromagnetic counterpart. Hence, it is essential to determine when this lifetime is the shortest, corresponding to when the remnant has a mass equal to the threshold mass, $M_{\rm th}$, to prompt collapse to a black hole. We report on the results of more than 360 simulations of merging neutron-star binaries covering 40 different configurations differing in mass ratio and spin of the primary. Using this data, we have derived a quasi-universal relation for $M_{\rm th}$ and expressed its dependence on the mass ratio and spin of the binary. The new expression recovers the results of Koeppel et al for equal-mass, irrotational binaries and reveals that $M_{\rm th}$ can increase (decrease) by $5\%~(10\%)$ for binaries that have spins aligned (anti-aligned) with the orbital angular momentum and provides evidence for a non-monotonic dependence of $M_{\rm th}$ on the mass asymmetry in the system. Finally, we extend to unequal-masses and spinning binaries the lower limits that can be set on the stellar radii once a neutron-star binary is detected, illustrating how the merger of an unequal-mass, rapidly spinning binary can significantly constrain the allowed values of the stellar radii.

gr-qc↗

arXiv2021

Fully general-relativistic simulations of isolated and binary strange quark stars

The hypothesis that strange quark matter is the true ground state of matter has been investigated for almost four decades, but only a few works have explored the dynamics of binary systems of quark stars. This is partly due to the numerical challenges that need to be faced when modelling the large discontinuities at the surface of these stars. We here present a novel technique in which the EOS of a quark star is suitably rescaled to produce a smooth change of the specific enthalpy across a very thin crust. The introduction of the crust has been carefully tested by considering the oscillation properties of isolated quark stars, showing that the response of the simulated quark stars matches accurately the perturbative predictions. Using this technique, we have carried out the first fully general-relativistic simulations of the merger of quark-star binaries finding several important differences between quark-star binaries and hadronic-star binaries with the same mass and comparable tidal deformability. In particular, we find that dynamical mass loss is significantly suppressed in quark-star binaries. In addition, quark-star binaries have merger and post-merger frequencies that obey the same quasi-universal relations derived from hadron stars if expressed in terms of the tidal deformability, but not when expressed in terms of the average stellar compactness. Hence, it may be difficult to distinguish the two classes of stars if no information on the stellar radius is available. Finally, differences are found in the distributions in velocity and entropy of the ejected matter, for which quark-stars have much smaller tails. Whether these differences in the ejected matter will leave an imprint in the electromagnetic counterpart and nucleosynthetic yields remains unclear, calling for the construction of an accurate model for the evaporation of the ejected quarks into nucleons.

astro-ph.HE↗

arXiv2021

A new public code for initial data of unequal-mass, spinning compact-object binaries

The construction of constraint-satisfying initial data is an essential element for the numerical exploration of the dynamics of compact-object binaries. While several codes have been developed over the years to compute generic quasi-equilibrium configurations of binaries comprising either two black holes, or two neutron stars, or a black hole and a neutron star, these codes are often not publicly available or they provide only a limited capability in terms of mass ratios and spins of the components in the binary. We here present a new open-source collection of spectral elliptic solvers that are capable of exploring the major parameter space of binary black holes (BBHs), binary neutron stars (BNSs), and mixed binaries of black holes and neutron stars (BHNSs). Particularly important is the ability of the spectral-solver library to handle neutron stars that are either irrotational or with an intrinsic spin angular momentum that is parallel to the orbital one. By supporting both analytic and tabulated equations of state at zero or finite temperature, the new infrastructure is particularly geared towards allowing for the construction of BHNS and BNS binaries. For the latter, we show that the new solvers are able to reach the most extreme corners in the physically plausible space of parameters, including extreme mass ratios and spin asymmetries, thus representing the most extreme BNS computed to date. Through a systematic series of examples, we demonstrate that the solvers are able to construct quasi-equilibrium and eccentricity-reduced initial data for BBHs, BNSs, and BHNSs, achieving spectral convergence in all cases. Furthermore, using such initial data, we have carried out evolutions of these systems from the inspiral to after the merger, obtaining evolutions with eccentricities $\lesssim 10^{-4}-10^{-3}$, and accurate gravitational waveforms.

gr-qc↗

arXiv2021

Comparison of the ion-to-electron temperature ratio prescription: GRMHD simulations with electron thermodynamics

The Event Horizon Telescope (EHT) collaboration, an Earth-size sub-millimetre radio interferometer, recently captured the first images of the central supermassive black hole in M87. These images were interpreted as gravitationally-lensed synchrotron emission from hot plasma orbiting around the black hole. In the accretion flows around low-luminosity active galactic nuclei such as M87, electrons and ions are not in thermal equilibrium. Therefore, the electron temperature, which is important for the thermal synchrotron radiation at EHT frequencies of 230 GHz, is not independently determined. In this work, we investigate the commonly used parameterised ion-to-electron temperature ratio prescription, the so-called R-$β$ model, considering images at 230 GHz by comparing with electron-heating prescriptions obtained from general-relativistic magnetohydrodynamical (GRMHD) simulations of magnetised accretion flows in a Magnetically Arrested Disc (MAD) regime with different recipes for the electron thermodynamics. When comparing images at 230 GHz, we find a very good match between images produced with the R-$β$ prescription and those produced with the turbulent- and magnetic reconnection- heating prescriptions. Indeed, this match is on average even better than that obtained when comparing the set of images built with the R-$β$ prescription with either a randomly chosen image or with a time-averaged one. From this comparative study of different physical aspects, which include the image, visibilities, broadband spectra, and light curves, we conclude that, within the context of images at 230 GHz relative to MAD accretion flows around supermassive black holes, the commonly-used and simple R-$β$ model is able to reproduce well the various and more complex electron-heating prescriptions considered here.

astro-ph.HE↗

arXiv2021

On accretion disks formed in MHD simulations of black hole-neutron star mergers with accurate microphysics

Remnant accretion disks formed in compact object mergers are an important ingredient in the understanding of electromagnetic afterglows of multi-messenger gravitational-wave events. Due to magnetically and neutrino driven winds, a significant fraction of the disk mass will eventually become unbound and undergo r-process nucleosynthesis. While this process has been studied in some detail, previous studies have typically used approximate initial conditions for the accretion disks, or started from purely hydrodynamical simulations. In this work, we analyse the properties of accretion disks formed from near equal-mass black hole-neutron star mergers simulated in general-relativistic magnetohydrodynamics in dynamical spacetimes with an accurate microphysical description. The post-merger systems were evolved until $120\, {\rm ms}$ for different finite-temperature equations of state and black-hole spins. We present a detailed analysis of the fluid properties and of the magnetic-field topology. In particular, we provide analytic fits of the magnetic-field strength and specific entropy as a function of the rest-mass density, which can be used for the construction of equilibrium disk models. Finally, we evolve one of the systems for a total of $350\, \rm ms$ after merger and study the prospect for eventual jet launching. While our simulations do not reach this stage, we find clear evidence of continued funnel magnetization and clearing, a prerequisite for any jet-launching mechanism.

astro-ph.HE↗

arXiv2021

TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper

This paper presents the ESA Voyage 2050 White Paper for a concept of TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the science case and some implementation issues of a space-borne radio interferometric system for ultra-sharp imaging of celestial radio sources at the level of angular resolution down to (sub-) microarcseconds. THEZA focuses at millimetre and sub-millimetre wavelengths (frequencies above $\sim$300~GHz), but allows for science operations at longer wavelengths too. The THEZA concept science rationale is focused on the physics of spacetime in the vicinity of supermassive black holes as the leading science driver. The main aim of the concept is to facilitate a major leap by providing researchers with orders of magnitude improvements in the resolution and dynamic range in direct imaging studies of the most exotic objects in the Universe, black holes. The concept will open up a sizeable range of hitherto unreachable parameters of observational astrophysics. It unifies two major lines of development of space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline Interferometry) and mm- and sub-mm astrophysical studies with "single dish" instruments. It also builds upon the recent success of the Earth-based Event Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the super-massive black hole in the centre of the galaxy M87. As an amalgam of these three major areas of modern observational astrophysics, THEZA aims at facilitating a breakthrough in high-resolution high image quality studies in the millimetre and sub-millimetre domain of the electromagnetic spectrum.

astro-ph.IM↗

arXiv2021

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes.

gr-qc↗

arXiv2021

Multi-Messenger Astrophysics with THESEUS in the 2030s

Multi-messenger astrophysics is becoming a major avenue to explore the Universe, with the potential to span a vast range of redshifts. The growing synergies between different probes is opening new frontiers, which promise profound insights into several aspects of fundamental physics and cosmology. In this context, THESEUS will play a central role during the 2030s in detecting and localizing the electromagnetic counterparts of gravitational wave and neutrino sources that the unprecedented sensitivity of next generation detectors will discover at much higher rates than the present. Here, we review the most important target signals from multi-messenger sources that THESEUS will be able to detect and characterize, discussing detection rate expectations and scientific impact.

astro-ph.IM↗

arXiv2021

An 86-GHz search for Pulsars in the Galactic Center with the Atacama Large Millimeter/submillimeter Array

We report on the first pulsar and transient survey of the Galactic Center (GC) with the Atacama Large Millimeter/submillimeter Array (ALMA). The observations were conducted during the Global Millimeter VLBI Array campaign in 2017 and 2018. We carry out searches using timeseries of both total intensity and other polarization components in the form of Stokes parameters. We incorporate acceleration and its derivative in the pulsar search, and also search in segments of the entire observation to compensate for potential orbital motion of the pulsar. While no new pulsar is found, our observations yield the polarization profile of the GC magnetar PSR J1745-2900 at mm-wavelength for the first time, which turns out to be nearly 100 % linearly polarized. Additionally, we estimate the survey sensitivity placed by both system and red noise, and evaluate its capability of finding pulsars in orbital motion with either Sgr A* or a binary companion. We show that the survey is sensitive to only the most luminous pulsars in the known population, and future observations with ALMA in Band-1 will deliver significantly deeper survey sensitivity on the GC pulsar population.

astro-ph.HE↗

arXiv2021

Searching for pulsars in the Galactic Centre at 3 and 2 mm

Pulsars in the Galactic centre promise to enable unparalleled tests of gravity theories and black hole physics and to serve as probes of the stellar formation history and evolution and the interstellar medium in the complex central region of the Milky Way. The community has surveyed the innermost region of the galaxy for decades without detecting a population of pulsars, which is puzzling. A strong scattering of the pulsed signals in this particular direction has been argued to be a potential reason for the non-detections. Scattering has a strong inverse dependence on observing frequency, therefore an effective way to alleviate its effect is to use higher frequencies in a survey for pulsars in the Galactic centre, in particular, close to the supermassive black hole Sagittarius A*. We present the first pulsar survey at short millimetre wavelengths, using several frequency bands between 84 and 156 GHz (3.57-1.92 mm), targeted to the Galactic centre. The observations were made with the Institut de Radioastronomie Millimetrique (IRAM) 30m Telescope in 28 epochs between 2016 December and 2018 May. This survey is the first that is essentially unaffected by scattering and therefore unbiased in population coverage, including fast-spinning pulsars that might be out of reach of lower-frequency Galactic centre surveys. We discovered no new pulsars and relate this result mainly to the decreased flux density of pulsars at high frequencies, combined with our current sensitivity. However, we demonstrate that surveys at these extremely high radio frequencies are capable of discovering new pulsars, analyse their sensitivity limits with respect to a simulated Galactic centre pulsar population, and discuss the main challenges and possible improvements for similar surveys in the future.

astro-ph.HE↗

arXiv2021

Black hole parameter estimation with synthetic Very Long Baseline Interferometry data from the ground and from space

The Event Horizon Telescope (EHT) has imaged the shadow of the supermassive black hole in M87. A library of general relativistic magnetohydrodynamics (GMRHD) models was fit to the observational data, providing constraints on black hole parameters. We investigate how much better future experiments can realistically constrain these parameters and test theories of gravity. We generate realistic synthetic 230 GHz data from representative input models taken from a GRMHD image library for M87, using the 2017, 2021, and an expanded EHT array. The synthetic data are run through a data reduction pipeline used by the EHT. Additionally, we simulate observations at 230, 557, and 690 GHz with the Event Horizon Imager (EHI) Space VLBI concept. Using one of the EHT parameter estimation pipelines, we fit the GRMHD library images to the synthetic data and investigate how the black hole parameter estimations are affected by different arrays and repeated observations. Repeated observations play an important role in constraining black hole and accretion parameters as the varying source structure is averaged out. A modest expansion of the EHT already leads to stronger parameter constraints. High-frequency observations from space rule out all but ~15% of the GRMHD models in our library, strongly constraining the magnetic flux and black hole spin. The 1$σ$ constraints on the black hole mass improve by a factor of five with repeated high-frequency space array observations as compared to observations with the current ground array. If the black hole spin, magnetization, and electron temperature distribution can be independently constrained, the shadow size for a given black hole mass can be tested to ~0.5% with the EHI, which allows tests of deviations from general relativity. High-precision tests of the Kerr metric become within reach from observations of the Galactic Center black hole Sagittarius A*.

astro-ph.HE↗

arXiv2021

3D magnetised jet break-out from neutron-star binary merger ejecta: afterglow emission from the jet and the ejecta

We perform three-dimensional (3D) general-relativistic magnetohydrodynamic simulations to model the jet break-out from the ejecta expected to be produced in a binary neutron-star merger. The structure of the relativistic outflow from the 3D simulation confirms our previous results from 2D simulations, namely, that a relativistic magnetized outflow breaking out from the merger ejecta exhibits a hollow core of $θ_{\rm core}\approx4^{\circ}$, an opening angle of $θ_{\rm jet}\gtrsim10^{\circ}$, and is accompanied by a wind of ejected matter that will contribute to the kilonova emission. We also compute the non-thermal afterglow emission of the relativistic outflow and fit it to the panchromatic afterglow from GRB170817A, together with the superluminal motion reported from VLBI observations. In this way, we deduce an observer angle of $θ_{\rm obs}= 35.7^{\circ \,\,+1.8}_{\phantom{\circ \,\,}-2.2}$. We further compute the afterglow emission from the ejected matter and constrain the parameter space for a scenario in which the matter responsible for the thermal kilonova emission will also lead to a non-thermal emission yet to be observed.

astro-ph.HE↗

arXiv2021

GW170817 and GW190814: tension on the maximum mass

The detection of the binary events GW170817 and GW190814 has provided invaluable constraints on the maximum mass of nonrotating configurations of neutron stars, $M_{_{\rm TOV}}$. However, the large differences in the neutron-star masses measured in GW170817 and GW190814 has also lead to a significant tension between the predictions for such maximum masses, with GW170817 suggesting that $M_{_{\rm TOV}} \lesssim 2.3\,M_{\odot}$, and GW190814 requiring $M_{_{\rm TOV}} \gtrsim 2.5\,M_{\odot}$ if the secondary was a (non- or slowly rotating) neutron star at merger. Using a genetic algorithm, we sample the multidimensional space of parameters spanned by gravitational-wave and astronomical observations associated with GW170817. Consistent with previous estimates, we find that all of the physical quantities are in agreement with the observations if the maximum mass is in the range $M_{_{\rm TOV}} = 2.210^{+0.116}_{-0.123} \,M_{\odot}$ within a $2\textrm{-}σ$ confidence level. By contrast, maximum masses with $M_{_{\rm TOV}} \gtrsim 2.5\,M_{\odot}$, not only require efficiencies in the gravitational-wave emission that are well above the numerical-relativity estimates, but they also lead to a significant under-production of the ejected mass. Hence, the tension can be released by assuming that the secondary in GW190814 was a black hole at merger, although it could have been a rotating neutron star before.

astro-ph.HE↗

arXiv2020

Fast ejecta as a potential way to distinguish black holes from neutron stars in high-mass gravitational-wave events

High-mass gravitational-wave events in the neutron-star mass range, such as GW190425, have recently started to be detected by the LIGO/Virgo detectors. If the masses of the two binary components fall in the neutron-star mass range, such a system is typically classified as a binary neutron-star system, although the detected gravitational-wave signal may be too noisy to clearly establish a neutron-star nature of the high-mass component in the binary and rule out a black hole--neutron star system for such an event. We here show that high-mass binary neutron-star mergers with a very massive neutron-star primary close to the maximum-mass limit, $m_1 \gtrsim 2.2 \, M_\odot$, produce fast dynamical mass ejecta from the spin-up of the primary star at merger. By simulating the merger of black hole--neutron star systems of exactly the same masses and spins, we show that these fast ejecta are entirely absent, if the primary is instead a black hole. In addition, we find that both systems leave almost identical amounts of baryon mass behind, which is not immediately accreted by the black hole. This implies that both systems will likely have comparable electromagnetic afterglow emission stemming from the remnant disk. Hence, fast ejecta at merger have the potentialto distinguish neutron stars from black holes in high-mass mergers, although these ejecta may be challenging to detect observationally.

astro-ph.HE↗