arXiv subjects

Davide Lazzati

Publications and source records attributed to Davide Lazzati.

At least 55 records · Page 3Linked to original sources

arXiv2019

Astro2020 Science White Paper: Radio Counterparts of Compact Object Mergers in the Era of Gravitational-Wave Astronomy

GHz radio astronomy has played a fundamental role in the recent dazzling discovery of GW170817, a neutron star (NS)-NS merger observed in both gravitational waves (GWs) and light at all wavelengths. Here we show how the expected progress in sensitivity of ground-based GW detectors over the next decade calls for U.S.-based GHz radio arrays to be improved beyond current levels. We discuss specifically how several new scientific opportunities would emerge in multi-messenger time-domain astrophysics if a next generation GHz radio facility with sensitivity and resolution $10\times$ better than the current Jansky Very Large Array (VLA) were to work in tandem with ground-based GW detectors. These opportunities include probing the properties, structure, and size of relativistic jets and wide-angle ejecta from NS-NS mergers, as well as unraveling the physics of their progenitors via host galaxy studies.

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arXiv2019

Limits on electromagnetic counterparts of gravitational wave-detected binary black hole mergers

Unlike mergers of two compact objects containing a neutron star (NS), binary black hole (BBH) mergers are not accompanied by the production of tidally disrupted material, and hence lack the most direct source of accretion to power a jet and generate electromagnetic (EM) radiation. However, following a tentative detection by the Fermi GBM of a $γ$-ray counterpart to GW150914, several alternative ideas were proposed for driving a jet and producing EM radiation. {\em If} such jets were in fact produced, they would however lack the bright cocoon emission that makes jets from binary NSs bright also at large viewing angles. Here, via Monte Carlo simulations of a population of BBH mergers with properties consistent with those inferred from the existing LIGO/Virgo observations, and the angular emission characteristic of jets propagating into the interstellar medium, we derive limits on the allowed energetics and Lorentz factors of such jets from EM follow ups to GW-detected BBH merger events to date, and we make predictions which will help tighten these limits with broadband EM follow ups to events in future LIGO/Virgo runs. The condition that $\lesssim 1$ event out of 10 GW-detected BBH mergers be above the Fermi/GBM threshold imposes that any currently allowed emission model has to satisfy the condition $E_{\rm iso}/(10^{49} {\rm erg})(θ_{jet}/20^\circ)\lesssim 1$.

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arXiv2019

STROBE-X: X-ray Timing and Spectroscopy on Dynamical Timescales from Microseconds to Years

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

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arXiv2019

Radiative shocks create environments for dust formation in novae

Classical novae commonly show evidence of rapid dust formation within months of the outburst. However, it is unclear how molecules and grains are able to condense within the ejecta, given the potentially harsh environment created by ionizing radiation from the white dwarf. Motivated by the evidence for powerful radiative shocks within nova outflows, we propose that dust formation occurs within the cool, dense shell behind these shocks. We incorporate a simple molecular chemistry network and classical nucleation theory with a model for the thermodynamic evolution of the post-shock gas, in order to demonstrate the formation of both carbon and forsterite ($\rm Mg_2SiO_4$) grains. The high densities due to radiative shock compression ($n \sim 10^{14}$ cm$^{-3}$) result in CO saturation and rapid dust nucleation. Grains grow efficiently to large sizes $\gtrsim 0.1μ$m, in agreement with IR observations of dust-producing novae, and with total dust masses sufficient to explain massive extinction events such as V705 Cas. As in dense stellar winds, dust formation is CO-regulated, with carbon-rich flows producing carbon-rich grains and oxygen-rich flows primarily forming silicates. CO is destroyed by non-thermal particles accelerated at the shock, allowing additional grain formation at late times, but the efficiency of this process appears to be low. Given observations showing that individual novae produce both carbonaceous and silicate grains, we concur with previous works attributing this bimodality to chemical heterogeneity of the ejecta. Nova outflows are diverse and inhomogeneous, and the observed variety of dust formation events can be reconciled by different abundances, the range of shock properties, and the observer viewing angle. The latter may govern the magnitude of extinction, with the deepest extinction events occurring for observers within the binary equatorial plane.

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arXiv2018

Photospheric Emission From Variable Engine Gamma Ray Burst Simulations

By coupling radiation transfer calculations to hydrodynamic simulations, there have been major advancements in understanding the long Gamma Ray Burst (LGRB) prompt emission. Building upon these achievements, we present an analysis of photospheric emission acquired by using the Monte Carlo Radiation Transfer (MCRaT) code on hydrodynamic simulations with variable jet profiles. MCRaT propagates and Compton scatters individual photons that have been injected into the collimated outflow in order to produce synthetic light curves and spectra. These light curves and spectra allow us to compare our results to LGRB observational data. We find excellent agreement between our fitted time resolved $β$ parameters and those that are observed. Additionally, our simulations show that photospheric emission, under certain conditions, is able to create the observationally expected Band $α$ parameter. Finally, we show that the simulations are consistent with the Golenetskii correlation but exhibit some strain with the Amati and Yonetoku correlations.

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arXiv2018

Science with an ngVLA: Compact binary mergers as traced by gravitational waves

In light of the recent dazzling discovery of GW170817, we discuss several new scientific opportunities that would emerge in multi-messenger time-domain astrophysics if a facility like the next generation Very Large Array (ngVLA) were to work in tandem with ground-based gravitational wave (GW) detectors. These opportunities include probing wide-angle ejecta and off-axis afterglows of neutron star (NS)-NS mergers; enabling direct size measurements of radio ejecta from NS-NS mergers; and unraveling the physics behind the progenitors of compact binary mergers via host galaxy studies at radio wavelengths. Our results show that, thanks to its unprecedented sensitivity and resolution, the ngVLA will enable transformational results in the multi-messenger exploration of the transient radio sky.

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arXiv2018

An upper-limit on the linear polarization fraction of the GW170817 radio continuum

We present late-time radio observations of GW170817, the first binary neutron star merger discovered through gravitational waves by the advanced LIGO and Virgo detectors. Our observations, carried out with the Karl G. Jansky Very Large Array, were optimized to detect polarized radio emission, and thus to constrain the linear polarization fraction of GW170817. At an epoch of ~244 days after the merger, we rule out linearly polarized emission above a fraction of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived upper-limit on the radio continuum linear polarization fraction strongly constrains the magnetic field configuration in the shocked ejecta. We show that our results for GW170817 are compatible with the low level of linear polarization found in afterglows of cosmological long gamma-ray bursts. Finally, we discuss our findings in the context of future expectations for the study of radio counterparts of binary neutron star mergers identified by ground-based gravitational-wave detectors.

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arXiv2018

Late time afterglow observations reveal a collimated relativistic jet in the ejecta of the binary neutron star merger GW170817

The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multi-wavelength electromagnetic radiation. The almost simultaneous observation of a pulse of gamma-rays proved that BNS mergers are associated with at least some short gamma-ray bursts (GRBs). However, the gamma-ray pulse was faint, casting doubts on the association of BNS mergers with the luminous, highly relativistic outflows of canonical short GRBs. Here we show that structured jets with a relativistic, energetic core surrounded by slower and less energetic wings produce afterglow emission that brightens characteristically with time, as recently seen in the afterglow of GW170817. Initially, we only see the relatively slow material moving towards us. As time passes, larger and larger sections of the outflow become visible, increasing the luminosity of the afterglow. The late appearance and increasing brightness of the multi-wavelength afterglow of GW170817 allow us to constrain the geometry of its ejecta and thus reveal the presence of an off-axis jet pointing about 30 degrees away from Earth. Our results confirm a single origin for BNS mergers and short GRBs: GW170817 produced a structured outflow with a highly relativistic core and a canonical short GRB. We did not see the bright burst because it was beamed away from Earth. However, approximately one in 20 mergers detected in gravitational waves will be accompanied by a bright, canonical short GRB.

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arXiv2018

Ultra Long Gamma-Ray Bursts from the collapse of Blue Super Giant stars: an end-to-end simulation

Ultra-long gamma ray bursts (ULGRBs) are a distinct class of GRBs characterized by durations of several thousands of seconds, about two orders of magnitude longer than those of standard long GRBs (LGRBs). The driving engine of these events has not been uncovered yet, and ideas range from magnetars, to tidal disruption events, to extended massive stars, such as blue super giants (BSG). BSGs, a possible endpoint of stellar evolution, are attractive for the relatively long free-fall times of their envelopes, allowing accretion to power a long-lasting central engine. At the same time, their large radial extension poses a challenge to the emergence of a jet. Here we perform an end-to-end simulation aimed at assessing the viability of BSGs as ULGRB progenitors. The evolution to core collapse of a BSG star model is calculated with the MESA code. We then compute the accretion rate for the fraction of envelope material with enough angular momentum to circularize and form an accretion disk, and input the corresponding power into a jet which we evolve through the star envelope with the FLASH code. Our simulation shows that the jet can emerge, and the resulting light curves resemble those observed in ULGRBs, with durations $T_{90}$ ranging from $\approx$4000~s to $\approx10^4$ s depending on the viewing angle.

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arXiv2018

The formation of astrophysical Mg-rich silicate dust

We present new results for ground-state candidate energies of Mg-rich olivine (MRO) clusters and use the binding energies of these clusters to determine their nucleation rates in stellar outflows, with particular interest in the en- vironments of core-collapse supernovae (CCSNe). Low-lying structures of clusters (Mg2SiO4)n 2 \le n \le 13 are determined from a modified minima hopping algorithm using an empirical silicate potential in the Buckingham form. These configurations are further refined and optimized using the den- sity functional theory code Quantum Espresso. Utilizing atomistic nucleation theory, we determine the critical size and nucleation rates of these clusters. We find that configurations and binding energies in this regime are very dis- similar from those of the bulk lattice. Clusters grow with SiO_4-MgO layering and exhibit only global, rather than local, symmetries. When compared to classical nucleation theory we find suppressed nucleation rates at most temperatures and pressures, with enhanced nucleation rates at very large pressures. This implies a slower progression of silicate dust formation in stellar environments than previously assumed.

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arXiv2018

Dynamic Monte Carlo Simulations of Radiatively Accelerated GRB Fireballs

We present a novel Dynamic Monte Carlo code (DynaMo code) which self-consistently simulates the Compton scattering driven dynamic evolution of a plasma. We use the DynaMo code to investigate the time-dependent expansion and acceleration of dissipationless GRB fireballs by varying their initial opacities and baryonic content. We study the opacity and energy density evolution of an initially optically thick, radiation-dominated fireball across its entire phase space - in particular during the $R_{\rm ph} < R_{\rm sat}$ regime. Our results reveal new phases of fireball evolution: a transition phase with a radial extent of several orders of magnitude - the fireball transitions from $Γ\propto R$ to $Γ\propto R^0$, a post-photopsheric acceleration phase - where fireballs accelerate beyond the photosphere, and a Thomson-dominated acceleration phase - characterized by slow acceleration of optically thick, matter-dominated fireballs due to Thomson scattering. We quantify the new phases by providing analytical expressions of Lorentz factor evolution, which will be useful for deriving jet parameters.

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arXiv2017

A Monte Carlo Radiation Transfer Study of Photospheric Emission in Gamma Ray Bursts

We present the analysis of photospheric emission for a set of hydrodynamic simulations of long duration gamma-ray burst jets from massive compact stars. The results are obtained by using the Monte Carlo Radiation Transfer code (MCRaT) to simulate thermal photons scattering through the collimated outflows. MCRaT allows us to study explicitly the time evolution of the photosphere within the photospheric region, as well as the gradual decoupling of the photon and matter counterparts of the jet. The results of the radiation transfer simulations are also used to construct light curves and time resolved spectra at various viewing angles, which are then used to make comparisons with observed data, and outline the agreement and strain points between the photospheric model and long duration gamma-ray burst observations. We find that our fitted time resolved spectral Band $β$ parameters are in agreement with observations, even though we do not consider the effects of non-thermal particles. Finally, the results are found to be consistent with the Yonetoku correlation, but bear some strain with the Amati correlation.

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arXiv2017

Off-axis prompt X-ray transients from the cocoon of short gamma-ray bursts

We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15<theta<45 degrees and gives origin to X-ray transients with a duration of a few to ~10 seconds, delayed by a few seconds from the time of the merger. We also discuss the distance at which such transients can be detected, finding that it depends sensitively on the assumptions that are made about the radiation spectrum. Purely thermal cocoon transients are detectable only out to a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out to several tens of Mpc.

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arXiv2017

Off-axis emission of short gamma-ray bursts and the detectability of electromagnetic counterparts of gravitational wave detected binary mergers

We present calculations of the wide angle emission of short-duration gamma-ray bursts from compact binary merger progenitors. Such events are expected to be localized by their gravitational wave emission, fairly irrespective of the orientation of the angular momentum vector of the system, along which the gamma-ray burst outflow is expected to propagate. We show that both the prompt and afterglow emission are dim and challenging to detect for observers lying outside of the cone within which the relativistic outflow is propagating. If the jet initially propagates through a baryon contaminated region surrounding the merger site, however, a hot cocoon forms around it. The cocoon subsequently expands quasi-isotropically producing its own prompt emission and external shock powered afterglow. We show that the cocoon prompt emission is detectable by Swift BAT and Fermi GBM, We also show that the cocoon afterglow peaks a few hours to a few days after the burst and is detectable for up to a few weeks at all wavelengths. The timing and brightness of the transient are however uncertain due to their dependence on unknown quantities such as the density of the ambient medium surrounding the merger site, the cocoon energy, and the cocoon Lorentz factor. For a significant fraction of the gravitationally-detected neutron-star-binary mergers, the cocoon afterglow could possibly be the only identifiable electromagnetic counterpart, at least at radio and X-ray frequencies.

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arXiv2016

Formation and properties of astrophysical carbonaceous dust

The classical theory of grain nucleation suffers from both theoretical and predictive deficiencies. We strive to alleviate these deficiencies in our understanding of dust formation and growth by utilizing an atomistic model of nucleation. Carbon cluster geometries are determined with a set of global minimization algorithms. Using density functional theory, the binding energies of carbon clusters from $n=2$ to $n=99$ are then calculated. These energies are used to calculate the critical size and nucleation rate of carbon clusters. We find that the critical cluster size is largely determined by the changes in geometry of the clusters. Clusters with size $n=27$ and $n=8$, roughly corresponding to the transition from ring-to-fullerene geometry and chain-to-ring geometry respectively, are the critical sizes across the range of temperature and saturation where nucleation is significant. In contrast to the classical theory, nucleation is enhanced at low-temperatures, and suppressed at high temperatures. These results will be applied to a modified chemical evolution code using results from supernova simulations.

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arXiv2016

On radiative acceleration in spine-sheath structured blazar jets

It has been proposed that blazar jets are structured, with a fast spine surrounded by a slower sheath or layer. This structured jet model explains some properties of their emission and morphology. Because of their relative motion, the radiation produced by one component is seen amplified by the other, thus enhancing the inverse Compton emission of both. Radiation is emitted anisotropically in the comoving frames, and causes the emitting plasma to recoil. As seen in the observer frame, this corresponds to a deceleration of the fastest component (the spine) and an acceleration of the slower one (the layer). While the deceleration of the spine has already been investigated, here we study for the first time the acceleration of the sheath and find self-consistent velocity profile solutions for both the spine and the sheath while accounting for radiative cooling. We find that the sheath can be accelerated to the velocities required by the observations if its leptons remain energetic in the acceleration region, assumed to be of the order of 100 Schwarzschild radii, demanding continuous injection of energetic particles in that region.

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arXiv2016

Monte Carlo radiation transfer simulations of photospheric emission in long-duration Gamma-Ray Bursts

We present MCRaT, a Monte Carlo Radiation Transfer code for self-consistently computing the light curves and spectra of the photospheric emission from relativistic, unmagnetized jets. We apply MCRaT to a relativistic hydrodynamic simulation of a long duration gamma-ray burst jet, and present the resulting light-curves and time-dependent spectra for observers at various angles from the jet axis. We compare our results to observational results and find that photospheric emission is a viable model to explain the prompt phase of long-duration gamma-ray bursts at the peak frequency and above, but faces challenges in reproducing the flat spectrum below the peak frequency. We finally discuss possible limitations of these results both in terms of the hydrodynamics and the radiation transfer and how these limitations could affect the conclusions that we present.

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arXiv2016

Short Gamma-Ray Bursts from the Merger of Two Black Holes

Short Gamma-Ray Bursts (GRBs) are explosions of cosmic origin believed to be associated with the merger of two compact objects, either two neutron stars, or a neutron star and a black hole. The presence of at least one neutron star has long been thought to be an essential element of the model: its tidal disruption provides the needed baryonic material whose rapid accretion onto the post-merger black hole powers the burst. The recent tentative detection by the Fermi satellite of a short GRB in association with the gravitational wave signal GW150914 produced by the merger of two black holes has challenged this standard paradigm. Here we show that the evolution of two high-mass, low-metallicity stars with main sequence rotational speeds a few tens of percent of the critical speed eventually undergoing a weak supernova explosion {\em can} produce a short gamma-ray burst. The outer layers of the envelope of the last exploding star remain bound and circularize at large radii. With time, the disk cools and becomes neutral, suppressing the magneto-rotational instability, and hence the viscosity. The disk remains 'long-lived dead' until tidal torques and shocks during the pre-merger phase heat it up and re-ignite accretion, rapidly consuming the disk and powering the short gamma-ray burst.

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