arXiv subjects

Diego Lopez-Camara

Publications and source records attributed to Diego Lopez-Camara.

18 recordsLinked to original sources

arXiv2025

Effects of polar structure and moving ejecta on the dynamics of SGRB jets

At least some short gamma-ray bursts (SGRBs) originate from neutron star mergers, systems that release both a relativistic collimated jet and slower, wider ejecta. These jets evolve through a dense, anisotropic, and expanding medium produced during the merger process, resulting in interactions that affect their morphology and observable signatures. We investigate the propagation of SGRB jets through funnel-like structures that can be static or expanding with mildly relativistic speed using 2D axisymmetric relativistic hydrodynamic simulations. Our initial conditions are inspired from radial and angular distributions of density and pressure from general-relativistic magnetohydrodynamic simulations of binary neutron star mergers. We explore different values of the funnel opening angle and density contrast. We find that the polar structure of the ejecta mainly affects the jet evolution in the early stages, whereas the effect of expanding ejecta dominates in later stages. Jets propagating through a low-density polar funnel move initially faster, while the presence of mildly relativistic ejecta maintains the outflow more collimated after the breakout. Despite differences in the external medium, the energy dissipation within the jet and cocoon remains similar across models, while the shocked ambient material shows distinct signatures that could be observationally distinguishable. Our results highlight the importance of the structure and dynamical properties of the ejecta in shaping SGRB jets.

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arXiv2025

Three-dimensional simulations of accretion disks in pre-CE systems

Before a binary system enters into a common envelope (CE) phase, accretion from the primary star onto the companion star through Roche Lobe overflow (RLOF) will lead to the formation of an accretion disk, which may generate jets. Accretion before and during the CE may alter the outcome of the interaction. Previous studies have considered different aspects of this physical mechanism. Here we study the properties of an accretion disk formed via 3D hydrodynamic simulations of the RLOF mass transfer between a 7 M$_\odot$, red supergiant star and a 1.4 M$_\odot$, neutron star companion. We simulate only the volume around the companion for improved resolution. We use a 1D implicit MESA simulation of the evolution of the system during 30,000 years between the on-set of the RLOF and the CE to guide the binary parameters and the mass-transfer rate, while we simulate only 21 years of the last part of the RLOF in 3D using an ideal gas isothermal equation of state. We expect that a pre-CE disk under these parameters will have a mass of $\sim 5\times 10^{-3}$ M$_\odot$ and a radius of $\sim$40 R$_\odot$ with a scale height of $\sim$5 R$_\odot$. The temperature profile of the disk is shallower than that predicted by the formalism of Shakura and Sunyaev, but more reasonable cooling physics would need to be included. We stress test these results with respect to a number of physical and numerical parameters, as well as simulation choices, and we expect them to be reasonable within a factor of a few for the mass and 15% for the radius. We also contextualize our results within those presented in the literature, in particular with respect to the dimensionality of simulations and the adiabatic index. We discuss the measured accretion rate in the context of the Shakura and Sunyaev formalism and debate the viscous mechanisms at play, finishing with a list of prospects for future work.

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arXiv2024

A semi-analytical model for the propagation of a structured jet in a magnetized medium

The merger of two magnetized compact objects, such as neutron stars, forms a compact object which may launch a relativistic and collimated jet. Numerical simulations of the process show that a dense and highly magnetized medium surrounds the system. This study presents a semi-analytical model that models the effects that a static magnetized medium with a tangled field produces in relativistic, collimated, and non-magnetized jets. The model is a first approximation that addresses the magnetic field present in the medium and is based on pressure equilibrium principles between the jet, cocoon, and external medium. A fraction of the ambient medium field is allowed to be entrained in the cocoon. We find that the jet and cocoon properties may be affected by high magnetic fields ($\gtrsim 10^{15}$~G) and mixing. The evolution of the system may vary up to $\sim10\%$ (compared to the non-magnetized case). Low-mixing may produce a slower-broader jet with a broader and more energetic cocoon would be produced. On the other hand, high-mixing could produce a faster-narrower jet with a narrow and less-energetic cocoon. Two-dimensional hydrodynamical simulations are used to validate the model and to constrain the mixing parameter. Although the magnetic field and mixing have a limited effect, our semi-analytic model captures the general trend consistent with numerical results. For high magnetization, the results were found to be more consistent with the low mixing case in our semi-analytic model.

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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

Jets in Common Envelopes: a low mass main sequence star in a red giant

We present small-scale three-dimensional hydrodynamical simulations of the evolution of a 0.3Msun main sequence star which launches two perpendicular jets within the envelope of a 0.88Msun red giant. Based on previous large-scale simulations, we study the dynamics of the jets either when the secondary star is grazing, when it has plunged-in, or when it is well-within the envelope of the red giant (in each stage for ~11 days). The dynamics of the jets through the common envelope (CE) depend on the conditions of the environment as well as on their powering. In the grazing stage and the commencement of the plunge self-regulated jets need higher efficiencies to break out of the envelope of the RG. Deep inside the CE, on the timescales simulated, jets are choked independently of whether they are self-regulated or constantly powered. Jets able to break out of the envelope of the RG in large-scale simulations, are choked in our small-scale simulations. The accreted angular momentum onto the secondary star is not large enough to form a disk. The mass accretion onto the MS star is 1-10% of the Bondi-Hoyle-Littleton rate (~1e-3 - 1e-1 Msun yr/s). High luminosity emission, from X-rays to UV and optical, is expected if the jets break out of the CE. Our simulations illustrate the need for inclusion of more realistic accretion and jet models in the dynamical evolution of the CEs.

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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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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

Disk formation and jet inclination effects in Common Envelopes

The evolution and physics of the common envelope (CE) phase are still not well understood. Jets launched from a compact object during this stage may define the evolutionary outcome of the binary system. We focus on the case in which jets are launched from a neutron star (NS) engulfed in the outer layers of a red giant (RG). We run a set of three-dimensional hydrodynamical simulations of jets with different luminosities and inclinations. The luminosity of the jet is self-regulated by the mass accretion rate and an efficiency $η$. Depending on the value of $η$ the jet can break out of the BHL bulge ("successful jet") and aligns against the incoming wind, in turn, it will realign in favour of the direction of the wind. The jet varies in size and orientation and may present quiescent and active epochs. The inclination of the jet and the Coriolis and centrifugal forces, only slightly affect the global evolution. As the accretion is hypercritical, and the specific angular momentum is above the critical value for the formation of a disk, we infer the formation of a disk and launching of jets. The disks' mass and size would be $\sim$10$^{-2}$~M$_\odot$ and $\gtrsim 10^{10}$ cm, and it may have rings with different rotation directions. In order to have a successful jet from a white dwarf, the ejection process needs to be very efficient ($η\sim$0.5). For main sequence stars, there is not enough energy reservoir to launch a successful jet.

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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

Self-regulating jets during the Common Envelope phase

Jets launched from a compact object (CO) during a common envelope (CE) may play a key role in the evolution of the system, and may also be an efficient removal channel for its material. In this work we study, through a large set of three-dimensional hydrodynamic simulations, the effects that jets launched from either a black hole (BH) or a neutron star (NS) have during a CE phase. The jet is self-consistently powered by a fraction ($η$) of the mass accretion rate that reaches the CO. For low mass accretion efficiencies ($η$<0.1\%), the jet is not able to drill through the material accreting onto the CO and forms a bulge around it. For higher efficiencies, the jet is able to drill through the bulge and in intermediate efficiencies ($η\sim$1-5\%) may produce an oscillating mass accretion rate behavior. We find that the jet may deposit enough energy to unbind the outer layers of the CE. The self-regulated jets present variability in size and orientation while their cocoons expand smoothly over the CE. The mass accretion rate initially decreases due to the ram pressure of the cocoon. If the launched jet is not able to drill through the bulge, the accretion is such that it may convert a NS into a BH within a decade or double the mass of a BH in a few years. If a jet is present, it deposits enough energy to unbind the outer layers of the CE and the system evolves into a grazing envelope, configuration.

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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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arXiv2016

The Astropy Problem

The Astropy Project (http://astropy.org) is, in its own words, "a community effort to develop a single core package for Astronomy in Python and foster interoperability between Python astronomy packages." For five years this project has been managed, written, and operated as a grassroots, self-organized, almost entirely volunteer effort while the software is used by the majority of the astronomical community. Despite this, the project has always been and remains to this day effectively unfunded. Further, contributors receive little or no formal recognition for creating and supporting what is now critical software. This paper explores the problem in detail, outlines possible solutions to correct this, and presents a few suggestions on how to address the sustainability of general purpose astronomical software.

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arXiv2016

Three-dimensional simulations of long duration gamma-ray burst jets: time scales from variable engines

Gamma-ray burst light curves are characterized by marked variability, each showing unique properties. The origin of such variability, at least for a fraction of long GRBs, may be the result of an unsteady central engine. It is thus important to study the effects that an episodic central engine has on the jet propagation and, eventually, on the prompt emission within the collapsar scenario. Thus, in this study we follow the interaction of pulsed outflows with their progenitor stars with hydrodynamic numerical simulations in both two and three dimensions. We show that the propagation of unsteady jets is affected by the interaction with the progenitor material well after the break-out time, especially for jets with long quiescent times, comparable to or larger than a second. We also show that this interaction can lead to an asymmetric behavior in which pulse durations and quiescent periods are systematically different. After the pulsed jets drill through the progenitor and the interstellar medium, we find that, on average, the quiescent epochs last longer than the pulses (even in simulations with symmetrical active and quiescent engine times). This could explain the asymmetry detected in the light curves of long-quiescent times duration gamma-ray bursts.

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arXiv2013

On the observed duration distribution of gamma-ray bursts from collapsars

The duration of the prompt emission of long gamma-ray bursts is generally considered to be fairly similar to the duration of the activity of the engine in the center of the progenitor star. Here, we investigate the relation between the duration of the engine activity and that of the observed light curve, using inputs from both numerical simulations and observations. We find that the observed burst duration is a good proxy for the engine duration after the time necessary for the jet to break out the star's surface is subtracted. However, the observed duration is a function of the viewing angle and can be significantly shorter than the duration of the engine activity. We also show that the observed, redshift-corrected burst duration evolves only moderately with redshift for both observations and synthetic light curves. We conclude that the broad distribution of the observed duration of long BATSE gamma-ray bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being due to viewing and redshift effects. Our results do not rule out the existence of engines with very long duration. However, we find that they are constrained to be a small minority of the BATSE detected bursts.

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arXiv2011

Gamma-Ray Burst Dynamics and Afterglow Radiation from Adaptive Mesh Refinement, Special Relativistic Hydrodynamic Simulations

We report on the development of Mezcal-SRHD, a new adaptive mesh refinement, special relativistic hydrodynamics (SRHD) code, developed with the aim of studying the highly relativistic flows in Gamma-Ray Burst sources. The SRHD equations are solved using finite volume conservative solvers. The correct implementation of the algorithms is verified by one-dimensional (1D) shock tube and multidimensional tests. The code is then applied to study the propagation of 1D spherical impulsive blast waves expanding in a stratified medium with $ρ\propto r^{-k}$, bridging between the relativistic and Newtonian phases, as well as to a two-dimensional (2D) cylindrically symmetric impulsive jet propagating in a constant density medium. It is shown that the deceleration to non-relativistic speeds in one-dimension occurs on scales significantly larger than the Sedov length. This transition is further delayed with respect to the Sedov length as the degree of stratification of the ambient medium is increased. This result, together with the scaling of position, Lorentz factor and the shock velocity as a function of time and shock radius, is explained here using a simple analytical model based on energy conservation. The method used for calculating the afterglow radiation by post-processing the results of the simulations is described in detail. The light curves computed using the results of 1D numerical simulations during the relativistic stage correctly reproduce those calculated assuming the self-similar Blandford-McKee solution for the evolution of the flow. The jet dynamics from our 2D simulations and the resulting afterglow lightcurves, including the jet break, are in good agreement with those presented in previous works. Finally, we show how the details of the dynamics critically depend on properly resolving the structure of the relativistic flow.

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arXiv2011

Simulations of GRB Jets in a Stratified External Medium: Dynamics, Afterglow Lightcurves, Jet Breaks and Radio Calorimetry

The dynamics of GRB jets during the afterglow phase is most reliably and accurately modelled using hydrodynamic simulations. All published simulations, however, have considered only a uniform external medium, while a stratified external medium is expected around long duration GRB progenitors. Here we present simulations of the dynamics of GRB jets and the resulting afterglow emission for both uniform and stratified external media with $ρ\propto r^{-k}$ for k = 0, 1, 2. The simulations are performed in 2D using the special relativistic version of the Mezcal code. The dynamics for stratified external media are broadly similar to those derived for expansion into a uniform external medium. The jet half-opening angle start increasing logarithmically with time once the Lorentz factor drops below 1/theta_0. For larger k values the lateral expansion is faster at early times and slower at late times with the jet expansion becoming Newtonian and slowly approaching spherical symmetry over progressively longer timescales. We find that contrary to analytic expectations, there is a reasonably sharp jet break in the lightcurve for k = 2 although the shape of the break is affected more by the viewing angle than by the slope of the external density profile. Steeper density profiles are found to produce more gradual jet breaks while larger viewing angles cause smoother and later appearing jet breaks. The counter-jet becomes visible as it becomes sub-relativistic, and for k=0 this results in a clear bump-like feature in the light curve. However, for larger k values the jet decelerates more gradually, causing only a mild flattening in the radio light curve that might be hard to discern when k=2. Late time radio calorimetry is likely to consistently over-estimate the true energy by up to a factor of a few for k=2, and either over-predict or under-predict it by a smaller factor for k = 0,1.

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arXiv2009

Phase transitions and He-synthesis driven winds in neutrino cooled accretion disks: prospects for late flares in short gamma-ray bursts

We consider the long term evolution of debris following the tidal disruption of compact stars in the context of short gamma ray bursts (SGRBs). The initial encounter impulsively creates a hot, dense, neutrino-cooled disk capable of powering the prompt emission. After a long delay, we find that powerful winds are launched from the surface of the disk, driven by the recombination of free nucleons into alpha particles. The associated energy release depletes the mass supply and eventually shuts off activity of the central engine. As a result, the luminosity and mass accretion rate deviate from the earlier self-similar behavior expected for an isolated ring with efficient cooling. This then enables a secondary episode of delayed activity to become prominent as an observable signature, when material in the tidal tails produced by the initial encounter returns to the vicinity of the central object. The time scale of the new accretion event can reach tens of seconds to minutes, depending on the details of the system. The associated energies and time scales are consistent with those occurring in X-ray flares.

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arXiv2009

Gamma Ray Burst Production and Supernova Signatures in Slowly Rotating Collapsars

We consider accretion onto newborn black holes following the collapse of rotating massive stellar cores, at the threshold where a centrifugally supported disk gives way to nearly radial inflow for low angular momentum. For realistic initial conditions taken from pre-supernova (pre-SN) evolution calculations, the densities and temperatures involved require the use of a detailed equation of state and neutrino cooling processes, as well as a qualitative consideration of the effects of general relativity. Through two-dimensional dynamical calculations we show how the energy release is affected by the rotation rate and the strength of angular momentum transport, giving rise to qualitatively different solutions in limits of high and low angular momentum, each being capable of powering a gamma-ray burst (GRB). We explore the likelihood of producing Fe-group elements in the two regimes and suggest that while large and massive centrifugally supported disks are capable of driving strong outflows with a possible SN-like signature, quasi-radial flows lack such a feature and may produce a GRB without such an accompanying feature, as seen in GRB060505.

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