arXiv subjects

Davide Lazzati

Publications and source records attributed to Davide Lazzati.

At least 37 records · Page 2Linked to original sources

arXiv2022

Dynamics of a relativistic jet through magnetized media

The merger of two neutron stars (NSs) produces the emission of gravitational waves, the formation of a compact object surrounded by a dense and magnetized environment, and the launching of a collimated and relativistic jet, which will eventually produce a short gamma-ray burst (SGRB). The interaction of the jet with the environment has been shown to play a major role in shaping the structure of the outflow that eventually powers the gamma-ray emission. In this paper, we present a set of 2.5 dimensional RMHD simulations that follow the evolution of a relativistic non-magnetized jet through a medium with different magnetization levels, as produced after the merger of two NSs. We find that the predominant consequence of a magnetized ambient medium is that of suppressing instabilities within the jet, and preventing the formation of a series of collimation shocks. One implication of this is that internal shocks lose efficiency, causing bursts with low-luminosity prompt emission. On the other hand, the jet-head velocity and the induced magnetization within the jet are fairly independent from the magnetization of the ambient medium. Future numerical studies with a larger domain are necessary to obtain light curves and spectra in order to better understand the role of magnetized media.

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arXiv2022

GW170817 4.5 years after merger: Dynamical ejecta afterglow constraints

GW170817 is the first binary neutron star (NS) merger detected in gravitational waves (GWs) and photons, and so far remains the only GW event of its class with a definitive electromagnetic (EM) counterpart. Radio emission from the structured jet associated with GW170817 has faded below the sensitivity achievable via deep radio observations with the most sensitive radio arrays currently in operation. Hence, we now have the opportunity to probe the radio re-brightening that some models predict, should emerge at late times from the interaction of the dynamically-stripped merger ejecta with the interstellar medium. Here we present the latest results from our deep radio observations of the GW170817 field with the Karl G. Jansky Very Large Array (VLA), 4.5 years after the merger. Our new data at $3\,$GHz do not show any compelling evidence for emission in excess to the tail of the jet afterglow ($<3.3\,μ$Jy), confirming our previous results. We thus set new constraints on the dynamical ejecta afterglow models. These constraints favor single-speed ejecta with energy $\lesssim 10^{50}\,$erg (for an ejecta speed of $β_0=0.5$), or steeper energy-speed distributions of the kilonova ejecta. Our results also suggest larger values of the cold, non-rotating maximum NS mass in equal mass scenarios. However, without a detection of the dynamical ejecta afterglow, obtaining precise constraints on the NS equation of state remains challenging.

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arXiv2022

Prompt Emission of Gamma-Ray Bursts in the High-density Environment of Active Galactic Nuclei Accretion Disks

Long and short gamma-ray bursts are traditionally associated with galactic environments, where circumburst densities are small or moderate (few to hundreds of protons per cubic cm). However, both are also expected to occur in the disks of Active Galactic Nuclei, where the ambient medium density can be much larger. In this work we study, via semi-analytical methods, the propagation of the GRB outflow, its interaction with the external material, and the ensuing prompt radiation. In particular, we focus on the case in which the external shock develops early in the evolution, at a radius that is smaller than the internal shock one. We find that bursts in such high density environments are likely characterized by a single, long emission episode that is due to the superposition of individual pulses, with a characteristic hard to soft evolution irrespective of the light curve luminosity. While multi-pulse light curves are not impossible, they would require the central engine to go dormant for a long time before re-igniting. In addition, short GRB engines would produce bursts with prompt duration that would exceed the canonical 2 s separation threshold and would likely be incorrectly classified as long events, even though they would not be accompanied by a simultaneous supernova. Finally, these events have a large dynamical efficiency which would produce a bright prompt emission followed by a somewhat dim afterglow.

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arXiv2022

The emergence of diffused Gamma-Ray Burst afterglows from the disks of Active Galactic Nuclei

The disks of Active Galactic Nuclei (AGNs) have emerged as rich environments for the production and capture of stars and the compact objects that they leave behind. These stars produce long Gamma-Ray Bursts (LGRBs) at their deaths, while frequent interactions among compact objects form binary neutron stars and neutron star-black hole binaries, leading to short GRBs (SGRBs) upon their merger. Predicting the properties of these transients as they emerge from the dense environments of AGN disks is key to their proper identification and to better constrain the star and compact object population in AGN disks. Some of these transients would appear unusual because they take place in much higher densities than the interstellar medium. Others, which are the subject of this paper, would additionally be modified by radiation diffusion since they are generated within optically thick regions of the accretion disks. Here we compute the GRB afterglow light curves for diffused GRB sources for a representative variety of central black-hole masses and disk locations. We find that the radiation from radio to UV and soft X-rays can be strongly suppressed by synchrotron self-absorption in the dense medium of the AGN disk. In addition, photon diffusion can significantly delay the emergence of the emission peak, turning a beamed, fast transient into a slow, isotropic, and dimmer one. These would appear as broadband-correlated AGN variability with dominance at the higher frequencies. Their properties can constrain both the stellar populations within AGN disks as well as the disk structure.

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arXiv2021

Host galaxies and electromagnetic counterparts to binary neutron star mergers across the cosmic time: Detectability of GW170817-like events

The detection of electromagnetic radiation (EM) accompanying the gravitational wave (GW) signal from the binary neutron star (BNS) merger GW170817 has revealed that these systems constitute at least a fraction of the progenitors of short gamma-ray bursts (SGRBs). As gravitational wave detectors keep pushing their detection horizons, it is important to assess coupled GW/EM probabilities, and how to maximize observational prospects. Here we perform population synthesis calculations of BNS evolution with the code MOBSE, and seed the binaries in galaxies at three representative redshifts (z=0.01,0.1,1) of the Illustris TNG50 simulation. The binaries are evolved and their locations numerically tracked in the host galactic potentials until merger. Adopting the astrophysical parameters of GRB170817A as a prototype, we numerically compute the broadband lightcurves of jets from BNS mergers, with the afterglow brightness depending on the local medium density at the merger sites. We perform Monte Carlo simulations of the resulting EM population assuming either a random viewing angle with respect to the jet, or a jet aligned with the orbital angular momentum of the binary, which biases the viewing angle probability for GW-triggered events. We find that ~70-80% of BNSs from z=0.01 should be detectable in gamma-rays. The afterglow detection probabilities of GW-triggered BNS mergers vary between ~0.3-0.7%, with higher values for jets aligned with the BNS angular momentum, and are comparable across the high and low-energy bands, unlike gamma-ray-triggered events (cosmological SGRBs) which are significantly brighter at higher energies. We further quantify observational biases with respect to host galaxy masses.

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arXiv2021

Photospheric Prompt Emission From Long Gamma Ray Burst Simulations -- II. Spectropolarimetry

Although Gamma Ray Bursts (GRBs) have been detected for many decades, the lack of knowledge regarding the radiation mechanism that produces the energetic flash of radiation, or prompt emission, from these events has prevented the full use of GRBs as probes of high energy astrophysical processes. While there are multiple models that attempt to describe the prompt emission, each model can be tuned to account for observed GRB characteristics in the gamma and X-ray energy bands. One energy range that has not been fully explored for the purpose of prompt emission model comparison is that of the optical band, especially with regards to polarization. Here, we use an improved MCRaT code to calculate the expected photospheric optical and gamma-ray polarization signatures ($Π_\mathrm{opt}$ and $Π_γ$, respectively) from a set of two relativistic hydrodynamic long GRB simulations, which emulate a constant and variable jet. We find that time resolved $Π_\mathrm{opt}$ can be large ($\sim 75\%$) while time-integrated $Π_\mathrm{opt}$ can be smaller due to integration over the asymmetries in the GRB jet where optical photons originate; $Π_γ$ follows a similar evolution as $Π_\mathrm{opt}$ with smaller polarization degrees. We also show that $Π_\mathrm{opt}$ and $Π_γ$ agree well with observations in each energy range. Additionally, we make predictions for the expected polarization of GRBs based on their location within the Yonetoku relationship. While improvements can be made to our analyses and predictions, they exhibit the insight that global radiative transfer simulations of GRB jets can provide with respect to current and future observations.

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arXiv2021

Monte Carlo Simulations of Photospheric Emission in Gamma Ray Bursts

The study of Gamma Ray Bursts (GRBs) has the potential to improve our understanding of high energy astrophysical phenomena. In order to reliably use GRBs to this end, we first need to have a well-developed grasp of the mechanism that produces the radiation within GRB jets and how that relates to their structure. One model for the emission mechanism of GRBs invokes radiation produced deep in the jet which eventually escapes the jet at its photosphere. While this model has been able to explain a number of observed GRB characteristics, it is currently lacking in predictive power and in ability to fully reproduce GRB spectra. In order to address these shortcomings of the model, we have expanded the capabilities of the MCRaT code, a state of the art radiative transfer code that can now simulate optical to gamma ray radiation propagating in a hydrodynamically simulated GRB jet. Using the MCRaT code, we have constructed mock observed light curves, spectra, and polarization from optical to gamma ray energies for the simulated GRBs. Using these mock observables, we have compared our simulations of photospheric emission to observations and found much agreement between the two. Furthermore, the MCRaT calculations combined with the hydrodynamical simulations allow us to connect the mock observables to the structure of the simulated GRB jet in a way that was not previously possible. While there are a number of improvements that can be made to the analyses, the steps taken here begin to pave the way for us to fully understand the connection between the structure of a given GRB jet and the radiation that would be expected from it.

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arXiv2021

The Panchromatic Afterglow of GW170817: The full uniform dataset, modeling, comparison with previous results and implications

We present the full panchromatic afterglow light curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data, and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups, and attempted to mitigate these differences to provide a more uniform dataset. Simple power-law fits to the uniform afterglow light curve indicate a $t^{0.86\pm0.04}$ rise, a $t^{-1.92\pm0.12}$ decline, and a peak occurring at $155\pm4$ days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index ($-0.584\pm0.002$) across all epochs. This gives a precise and updated estimate of the electron power-law index, $p=2.168\pm0.004$. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized ISM density, $<$0.01 cm$^{-3}$, consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having magnetic field strength between 10$^{10.4}$ G and 10$^{16}$ G. Our fits to the afterglow data using an analytical model that includes VLBI proper motion from Mooley et al. (2018), and a structured jet model that ignores the proper motion, indicates that the proper motion measurement needs to be considered while seeking an accurate estimate of the viewing angle.

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arXiv2021

Photospheric Prompt Emission From Long Gamma Ray Burst Simulations -- I. Optical Emission

A complete understanding of Gamma Ray Bursts (GRBs) has been difficult to achieve due to our incomplete knowledge of the radiation mechanism that is responsible for producing the prompt emission. This emission, which is detected in the first tens of seconds of the GRB, is typically dominated by hard X-ray and gamma ray photons although, there have also been a few dozen prompt optical detections. These optical detections have the potential to discriminate between plausible prompt emission models, such as the photospheric and synchrotron shock models. In this work we use an improved MCRaT code, which includes cyclo-synchrotron emission and absorption, to conduct radiative transfer calculations from optical to gamma ray energies under the photospheric model. The calculations are conducted using a set of two dimensional relativistic hydrodynamic long GRB jet simulations, consisting of a constant and variable jet. We predict the correlations between the optical and gamma ray light curves as functions of observer angle and jet variability and find that there should be extremely dim optical prompt precursors for large viewing angles. Additionally, the detected optical emission originates from dense regions of the outflow such as shock interfaces and the jet-cocoon interface. Our results also show that the photospheric model is not able to account for the current set of optical prompt detections that have been made and additional radiative mechanisms are needed to explain these prompt optical observations. These findings show the importance of conducting global radiative transfer simulations using hydrodynamically calculated jet structures.

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arXiv2021

Two steps forward and one step sideways: the propagation of relativistic jets in realistic binary neutron star merger ejecta

The association of GRB170817A with GW170817 has confirmed the long-standing hypothesis that binary neutron star (BNS) mergers are the progenitors of at least some short gamma-ray bursts (SGRBs). This connection has ushered in an era in which broadband observations of SGRBs, together with measurements of the time delay between the gravitational waves and the electromagnetic radiation, allow to probe the properties of the emitting outflow and its engine to an unprecedented detail. Since the structure of the radiating outflow is molded by the interaction of a relativistic jet with the binary ejecta, it is of paramount importance to study the system in a realistic setting. Here we present a three-dimensional hydrodynamic simulation of a relativistic jet propagating in the ejecta of a BNS merger, which were computed with a general relativistic magnetohydrodynamic simulation. We find that the jet's centroid oscillates around the axis of the system, due to inhomogeneities encountered in the propagation. These oscillations allow the jet to find the path of least resistance and travel faster than an identical jet in smooth ejecta. In our setup the breakout time is ~0.6 sec, comparable to the expected central engine duration in SGRBs and possibly a non-negligible fraction of the total delay between the gravitational and gamma-ray signals. Our simulation also shows that energy is carried in roughly equal amounts by the jet and by the cocoon, and that about 20 per cent of the injected energy is transferred to the ejecta via mechanical work.

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arXiv2021

Continued radio observations of GW170817 3.5 years post-merger

We present new radio observations of the binary neutron star merger GW170817 carried out with the Karl G. Jansky Very large Array (VLA) more than 3\,yrs after the merger. Our combined dataset is derived by co-adding more than $\approx32$\,hours of VLA time on-source, and as such provides the deepest combined observation (rms sensitivity $\approx 0.99\,μ$Jy) of the GW170817 field obtained to date at 3\,GHz. We find no evidence for a late-time radio re-brightening at a mean epoch of $t\approx 1200$\,d since merger, in contrast to a $\approx 2.1\,σ$ excess observed at X-ray wavelengths at the same mean epoch. Our measurements agree with expectations from the post-peak decay of the radio afterglow of the GW170817 structured jet. Using these results, we constrain the parameter space of models that predict a late-time radio re-brightening possibly arising from the high-velocity tail of the GW170817 kilonova ejecta, which would dominate the radio and X-ray emission years after the merger (once the structured jet afterglow fades below detection level). Our results point to a steep energy-speed distribution of the kilonova ejecta (with energy-velocity power law index $α\gtrsim 5$). We suggest possible implications of our radio analysis, when combined with the recent tentative evidence for a late-time re-brightening in the X-rays, and highlight the need for continued radio-to-X-ray monitoring to test different scenarios.

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arXiv2021

The variable absorption in the X-ray spectrum of GRB 190114C

GRB 190114C was a bright burst that occurred in the local Universe (z=0.425). It was the first gamma-ray burst (GRB) ever detected at TeV energies, thanks to MAGIC. We characterize the ambient medium properties of the host galaxy through the study of the absorbing X-ray column density. Joining Swift, XMM-Newton, and NuSTAR observations, we find that the GRB X-ray spectrum is characterized by a high column density that is well in excess of the expected Milky Way value and decreases, by a factor of ~2, around ~$10^5$ s. Such a variability is not common in GRBs. The most straightforward interpretation of the variability in terms of photoionization of the ambient medium is not able to account for the decrease at such late times, when the source flux is less intense. Instead, we interpret the decrease as due to a clumped absorber, denser along the line of sight and surrounded by lower-density gas. After the detection at TeV energies of GRB 190114C, two other GRBs were promptly detected. They share a high value of the intrinsic column density and there are hints for a decrease of the column density, too. We speculate that a high local column density might be a common ingredient for TeV-detected GRBs.

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arXiv2020

Short duration gamma-ray bursts and their outflows in light of GW170817

The detection of GW170817, it's extensive multi-wavelength follow-up campaign, and the large amount of theoretical development and interpretation that followed, have resulted in a significant step forward in the understanding of the binary neutron star merger phenomenon as a whole. One of its aspects is seeing the merger as a progenitor of short gamma-ray bursts (SGRB), which will be the subject of this review. On the one hand, GW170817 observations have confirmed some theoretical expectations, exemplified by the confirmation that binary neutron star mergers are the progenitors of SGRBs. In addition, the multimessenger nature of GW170817 has allowed for gathering of unprecedented data, such as the trigger time of the merger, the delay with which the gamma-ray photons were detected, and the brightening afterglow of an off-axis event. All together, the incomparable richness of the data from GW170817 has allowed us to paint a fairly detailed picture of at least one SGRB. I will detail what we learned, what new questions have arisen, and the perspectives for answering them when a sample of GW170817-comparable events have been studied.

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arXiv2020

Electromagnetic Signatures of Relativistic Explosions in AGN Disks

The disks of Active Galactic Nuclei (AGNs), traditionally studied as the feeders of the supermassive black holes (SMBHs) at their centers, have recently triggered a lot of interest also as hosts to massive stars and hence their neutron star (NS) and black hole (BH) remnants. Migration traps and gas torques in these disks favor binary formation and enhance the rate of compact object mergers. In these environments both long gamma-ray bursts (GRBs) from the death of massive stars and short GRBs from NS-NS and NS-BH mergers are expected. However, their properties in the unique environments of AGN disks have never been studied. Here we show that GRBs in AGNs can display unique features, owing to the unusual relative position of the shocks that characterize the burst evolution and the Thomson photosphere of the AGN disk. In dense environments, for example, the external shock develops before the internal shocks, leading to prompt emission powered by a relativistic reverse shock. The transient's time evolution is also compressed, yielding afterglow emission that is much brighter and peaks much earlier than for GRBs in the interstellar medium. Additionally, in regions of the disk that are sufficiently dense and extended, the light curves are dominated by diffusion, since the fireball is trapped inside the disc photosphere. These diffusion-dominated transients emerge on timescales of days in disks around SMBHs of $\sim 10^6 M_\odot$ to years for SMBHs of $\sim 10^8 M_\odot$. Finally, a large fraction of events, especially in AGNs with SMBHs $\lesssim 10^7 M_\odot$, display time-variable absorption in the X-ray band.

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arXiv2020

Intrinsic properties of the engine and jet that powered the short gamma-ray burst associated with GW170817

GRB 170817A was a subluminous short gamma-ray burst detected about 1.74 s after the gravitational wave signal GW170817 from a binary neutron star (BNS) merger. It is now understood as an off-axis event powered by the cocoon of a relativistic jet pointing 15 to 30 degrees away from the direction of observation. The cocoon was energized by the interaction of the incipient jet with the non-relativistic baryon wind from the merger remnant, resulting in a structured outflow with a narrow core and broad wings. In this paper, we couple the observational constraints on the structured outflow with a model for the jet-wind interaction to constrain the intrinsic properties with which the jet was launched by the central engine, including its time delay from the merger event. Using wind prescriptions inspired by magnetized BNS merger simulations, we find that the jet was launched within about 0.4 s from the merger, implying that the 1.74 s observed delay was dominated by the fireball propagation up to the photospheric radius. We also constrain, for the first time for any gamma-ray burst, the jet opening angle at injection and set a lower limit to its asymptotic Lorentz factor. These findings suggest an initially Poynting-flux dominated jet, launched via electromagnetic processes. If the jet was powered by an accreting black hole, they also provide a significant constraint on the survival time of the metastable neutron star remnant.

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arXiv2020

Photospheric Polarization Signatures From Long Gamma Ray Burst Simulations

A comprehensive understanding of Gamma Ray Bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carlo Radiation Transport (MCRaT) code. MCRaT has been enhanced by the inclusion of polarization; it has been first verified by reproducing a variety of results in the literature and then used to obtain the time integrated and resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees {($\lesssim 1$\%)} are consistent with the constraints from the POLAR experiment, they are lower than other theoretical studies due to the lack of strong gradients in the model jet profiles that we use. The time resolved results suggests that GRBs with wide jets observed on axis will have small polarization degrees ($\lesssim 2\%$) and {constant polarization angles}, during the brightest portion of the light curve. GRBs observed off axis will have larger polarization degrees and polarization angles that change with the temporal structure of radiating shells in the outflow. We then place our results in the context of GRB prompt emission models and future LEAP and POLAR-2 GRB polarimetry detections.

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arXiv2020

Polarization constraints on the geometry of the magnetic field in the external shock of gamma-ray bursts

We study the ensemble of linear polarization measurement in the optical afterglows of long-duration gamma-ray bursts. We assume a non sideways-expanding top-hat jet geometry and use the relatively large number of measurements under the assumption that they represent a statistically unbiased sample. This allows us to constrain the ratio between the maximum predicted polarization and the measured one, which is an indicator of the geometry of the magnetic field in the downstream region of the external shock. We find that the measured polarization is substantially suppressed with respect to the maximum possible for either a completely ordered magnetic field parallel to the shock normal or to a field that is entirely contained in the shock plane. The measured polarization is limited, on average, to between 25 and 30% of the maximum theoretically possible value. This reduction requires the perpendicular component of the magnetic field to be dominant in energy with respect to the component parallel to the shock front, as expected for a shock generated and/or shock compressed field. We find, however, that the data only marginally support the assumption of a simple top-hat jet, pointing towards a more complex geometry for the outflow.

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arXiv2019

Jet-cocoon outflows from neutron star mergers: structure, light curves, and fundamental physics

The discovery of GW170817, the merger of a binary neutron star (NS) triggered by a gravitational wave detection by LIGO and Virgo, has opened a new window of exploration in the physics of NSs and their cosmological role. Among the important quantities to measure are the mass and velocity of the ejecta produced by the tidally disrupted NSs and the delay - if any - between the merger and the launching of a relativistic jet. These encode information on the equation of state of the NS, the nature of the merger remnant, and the jet launching mechanism, as well as yielding an estimate of the mass available for r-process nucleosynthesis. Here we derive analytic estimates for the structure of jets expanding in environments with different density, velocity, and radial extent. We compute the jet-cocoon structure and the properties of the broadband afterglow emission as a function of the ejecta mass, velocity, and time delay between merger and launch of the jet. We show that modeling of the afterglow light curve can constrain the ejecta properties and, in turn, the physics of neutron density matter. Our results increase the interpretative power of electromagnetic observations by allowing for a direct connection with the merger physics.

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