The binary neutron star (BNS) merger GW170817 was the first astrophysical source detected in gravitational waves and multiwavelength 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 doubt 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 multiwavelength 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° 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.
ABSTRACT
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 previously formed 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 disc, we infer the formation of a disc and launching of jets. The discs’ mass and size would be ∼10−2 M⊙ and ≳1010 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 ∼ 0.5). For main-sequence stars, there is not enough energy reservoir to launch a successful jet.
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 and gives origin to X-ray transients with a duration of a few to ∼10 s, 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, while Comptonized transients can instead be detected by the Fermi Gamma-ray Burst Monitor (GBM) out to several tens of Mpc.
Dynamics of jets during the common-envelope phase Moreno Méndez, Enrique; López-Cámara, Diego; De Colle, Fabio
Monthly notices of the Royal Astronomical Society,
09/2017, Letnik:
470, Številka:
3
Journal Article
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Abstract
Common envelope (CE) is an important phase in the evolution of many binary systems. Giant star/compact object interaction in binaries plays an important role in high-energy phenomena as well ...as in the evolution of their environment. Material accreted on to the compact object may form a disc and power a jet. We study analytically and through numerical simulations the interaction between the jet and the CE. We determine the conditions under which accreting material quenches the jet or allows it to propagate successfully, in which case even the envelope may be ejected. Close to the stellar core of the companion, the compact object accretes at a larger rate. A jet launched from this region needs a larger accretion-to-ejection efficiency to successfully propagate through the CE compared to a jet launched far from the stellar core, and is strongly deflected by the orbital motion. The energy deposited by the jet may be larger than the binding energy of the envelope. The jet can, thus, play a fundamental role in the CE evolution. We find that the energy dissipation of the jet into the CE may stop accretion on to the disc. We expect the jet to be intermittent, unless the energy deposited is large enough to lead to the unbinding of the outer layers of the CE. Given that the energy and duration of the jet are similar to those of ultralong gamma-ray bursts, we suggest this as a new channel to produce these events.
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 parameter. Finally, we show that the simulations are consistent with the Golenetskii correlation but exhibit some strain with the Amati and Yonetoku correlations.
ABSTRACT
After the detection of GRB 170817A, the first unambiguous off-axis gamma-ray burst (GRB), several studies tried to understand the structure of GRB jets. The initial jet structure (directly ...produced by the central engine) can be partially preserved, or can be completely modified by the interaction with the environment. In this study, we perform three-dimensional, special relativistic hydrodynamics simulations of long GRB jets evolving through a massive progenitor star. Different jet scenarios were considered: Top-hat, Gaussian jets dominated by pressure or by kinetic energy, as well as a model of a supernova (SN) plus a jet both propagating through the progenitor. We found that, while propagating inside the progenitor star, jets with different initial structures are nearly indistinguishable. Kinetic-dominated jets are faster and more collimated than pressure-dominated jets. The dynamics of jets inside the progenitor star strongly depends on the presence of an associated SN, which can substantially decelerate the jet propagation. We show that the initial structure of GRB jets is preserved, or not, mainly depending on the jet collimation. The initial structure is preserved in uncollimated jets, i.e. jets which move through low-density environments. Meanwhile, jets which move through dense environments are shaped by the interaction with the medium and remain collimated.
ABSTRACT
In this work, we revisit the steady-state, spherically symmetric gas accretion problem from the non-relativistic regime to the ultrarelativistic one. We first perform a detailed comparison ...between the Bondi and Michel models, and show how the mass accretion rate in the Michel solution approaches a constant value as the fluid temperature increases, whereas the corresponding Bondi value continually decreases, the difference between these two predicted values becoming arbitrarily large at ultrarelativistic temperatures. Additionally, we extend the Michel solution to the case of a fluid with an equation of state corresponding to a monoatomic, relativistic gas. Finally, using general relativistic hydrodynamic simulations, we study spherical accretion on to a rotating black hole, exploring the influence of the black hole spin on the mass accretion rate, the flow morphology and characteristics, and the sonic surface. The effect of the black hole spin becomes more significant as the gas temperature increases and as the adiabatic index γ stiffens. For an ideal gas in the ultrarelativistic limit (γ = 4/3), we find a reduction of 10 per cent in the mass accretion rate for a maximally rotating black hole compared to a non-rotating one, while this reduction is of up to 50 per cent for a stiff fluid (γ = 2).
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 time-resolved polarization degrees and angles of the synthetic LGRBs. While the obtained time-integrated polarization degrees ( 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 ( 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.
ABSTRACT
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. If the binary ...undergoes delayed collapse a collimated and relativistic jet, which will eventually produce a short gamma-ray burst (SGRB), may be launched. 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 2.5D 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 of 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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