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The very first stars in the Universe can be very massive, up to . If born in large numbers, such massive stars can have a strong impact on the subsequent star formation, producing strong ionizing radiation and contaminating the primordial gas with heavy elements. They would leave behind massive black holes that could act as seeds for growing supermassive black holes of active galactic nuclei. Given the anticipated fast rotation, such stars would end their life as supermassive collapsars and drive powerful magnetically dominated jets. In this Letter, we investigate the possibility of observing the bursts of high-energy emission similar to the long gamma-ray bursts associated with normal collapsars. We show that during the collapse of supercollapsars, the Blandford–Znajek mechanism can produce jets as powerful as few erg s−1 and release up to 1054 erg of the black hole rotational energy. Due to the higher intrinsic time-scale and higher redshift, the initial bright phase of the burst can last for about 104 s, whereas the central engine would remain active for about 1 d. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20–30 keV. The peak total flux density is relatively low, , but not prohibitive. If one supercollapsar is produced per every minihalo of dark matter arising from the 3σ cosmological fluctuations, then the whole sky frequency of such bursts could reach several tens per year. [ABSTRACT FROM AUTHOR]

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the long-duration gamma-ray bursts (LGRBs) with duration in excess of 100 s as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 104 s. These data suggest that some other, most likely magnetic mechanisms have to be considered. Since the efficiency of magnetic mechanisms does not depend that much on the mass accretion rate, the magnetic models do not require the development of accretion disc within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova (SN) state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion discs in the collapsar model and investigate the available range of mass accretion rates, black hole (BH) masses and spins. We find that the BH mass can be much higher than , usually assumed in the collapsar model, and normally exceeds half of the pre-SN mass. The BH spin is rather moderate, , but still high enough for the Blandford–Znajek mechanism to remain efficient provided the magnetic field is sufficiently strong. Our numerical simulations confirm the possibility of magnetically driven stellar explosions, in agreement with previous studies, but point towards the required magnetic flux on the BH horizon in excess of . At present, we cannot answer with certainty whether such a strong magnetic field can be generated in the stellar interior. Perhaps, the SN explosions associated with LGRBs are still neutrino-driven and their gamma-ray signature is the precursors. The SN blast clears up escape channels for the magnetically driven gamma-ray burst (GRB) jets, which may produce the main pulse. In this scenario, the requirements on the magnetic field strength can be lowered. A particularly interesting version of the binary progenitor involves merger of a Wolf–Rayet star with an ultracompact companion, neutron star or BH. In this case, we expect the formation of very long-lived accretion discs, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation. [ABSTRACT FROM AUTHOR]

The collapse of massive stars may result in the formation of accreting black holes in their interiors. The accreting stellar matter may advect substantial magnetic flux on to the black hole and promote the release of its rotational energy via magnetic stresses (the Blandford–Znajek mechanism). In this paper we explore whether this process can explain the stellar explosions and relativistic jets associated with long gamma-ray bursts. In particular, we show that the Blandford–Znajek mechanism is activated when the rest mass–energy density of matter drops below the energy density of the magnetic field in the near vicinity of the black hole (within its ergosphere). We also discuss whether such a strong magnetic field is in conflict with the rapid rotation of the stellar core required in the collapsar model, and suggest that the conflict can be avoided if the progenitor star is a component of a close binary. In this case the stellar rotation can be sustained via spin-orbital interaction. In an alternative scenario the magnetic field is generated in the accretion disc, but in this case the magnetic flux through the black hole ergosphere is not expected to be sufficiently high to explain the energetics of hypernovae by the BZ mechanism alone. However, this energy deficit can be recovered via the additional power provided by the disc. [ABSTRACT FROM AUTHOR]

We present numerical simulations of axisymmetric, magnetically driven outflows that reproduce the inferred properties of ultrarelativistic gamma-ray burst (GRB) jets. These results extend our previous simulations of outflows accelerated to moderately relativistic speeds, which are applicable to jets of active galactic nuclei. In contrast to several recent investigations, which have employed the magnetodynamics approximation, our numerical scheme solves the full set of equations of special relativistic, ideal magnetohydrodynamics, which enables us to explicitly calculate the jet velocity and magnetic-to-kinetic energy conversion efficiency – key parameters of interest for astrophysical applications. We confirm that the magnetic acceleration scheme remains robust into the ultrarelativistic regime, as previously indicated by semi-analytic self-similar solutions. We find that all current-carrying outflows exhibit self-collimation and consequent acceleration near the rotation axis, but that unconfined outflows lose causal connectivity across the jet and therefore do not collimate or accelerate efficiently in their outer regions. We show that magnetically accelerated jets confined by an external pressure that varies as assume a paraboloidal shape (where are cylindrical coordinates and ), and we obtain analytic expressions for the one-to-one correspondence between the pressure distribution and the asymptotic jet shape. We demonstrate that the acceleration efficiency of jets with paraboloidal streamlines is ≳50 per cent, with the numerical value being higher the lower the initial magnetization. We derive asymptotic analytic expressions for the acceleration of initially cold outflows along paraboloidal streamlines and verify that they provide good descriptions of the simulated flows. Our modelled jets (corresponding to ) attain Lorentz factors on scales , consistent with the possibility that long/soft GRB jets are accelerated within envelopes of collapsing massive stars, and on scales , consistent with the possibility that short/hard GRB jets are accelerated on scales where they can be confined by moderately relativistic winds from accretion discs. We also find that for outflows that undergo an efficient magnetic-to-kinetic energy conversion, where is the opening half-angle of the poloidal streamlines. This relation implies that the γ-ray emitting components of GRB outflows accelerated in this way are very narrow, with in regions where , and that the afterglow light curves of these components would either exhibit a very early jet break or show no jet break at all. [ABSTRACT FROM AUTHOR]

In this Letter, we briefly describe the first results of our numerical study on the possibility of magnetic origin of relativistic jets of long-duration gamma-ray bursters within the collapsar scenario. We track the collapse of massive rotating stars on to a rotating central black hole using axisymmetric general relativistic magnetohydrodynamic code that utilizes a realistic equation of state of stellar matter, and takes into account the cooling associated with emission of neutrinos and the energy losses due to dissociation of nuclei. The neutrino heating is not included. We describe the solution for one particular model where the progenitor star has magnetic field . The solution exhibits strong explosion driven by the Poynting-dominated jets whose power exceeds . The jets originate mainly from the black hole, and they are powered via the Blandford–Znajek mechanism. [ABSTRACT FROM AUTHOR]

In this paper, we report on the early evolution of a core-collapse supernova explosion following the birth of a magnetar with the dipolar magnetic field of and the rotational period of 2 ms, which was studied by means of axisymmetric general relativistic magnetohydrodynamic simulations. In this study, we use realistic equation of state and take into account the cooling and heating associated with emission, absorption, annihilation, and scattering of neutrinos; the neutrino transport is treated in the optically-thin regime. The supernova explosion is initiated via introducing into the initial solution the ‘radiation bubble’, whose total thermal energy is comparable with the typical energy of supernova ejecta. The numerical models exhibit highly collimated magnetically driven jets very early on. The jets are super-Alfvénic but remain subfast until the end of the simulations . The power released in the jets is about which implies the spin-down time of ≃37 s. The total rotational energy of the magnetar, , is sufficient to drive a hypernova but it is not clear as to how large a fraction of this energy can be transferred to the stellar ejecta. Given the observed propagation speed of the jets, , they are expected to traverse the progenitor in few seconds and after this most of the released rotational energy would be simply carried away by these jets into the surrounding space. Three-dimensional effects such as the kink mode instability may reduce the jet propagation speed and increase the amount of energy transferred by the jets to the supernova ejecta. Our results provide the first more or less self-consistent numerical model of a central engine capable of producing, in the supernova setting and on a long-term basis, collimated jets with sufficient power to explain long-duration gamma-ray bursts (GRBs) and their afterglows. Although the flow speed of our jets is relatively low, only , the cooling of protoneutron star will eventually result in much higher magnetization of its magnetosphere and ultrarelativistic asymptotic speeds of the jets. Given the relatively long cooling time-scale, we still expect the jets to be only weakly relativistic by the time of break-out. This leads to a model of GRB jets with a systematic longitudinal variation in the Lorentz factor which may have specific observational signatures both in the prompt and in the afterglow emission. The simulations also reveal quasi-periodic ejection of plasma clouds into the jet on a time-scale of 20 ms related to the large-scale global oscillation of magnetar's magnetosphere caused by the opening–closing of the dead zone field lines. These kinds of central engine variability may be partly responsible for the internal shocks of GRB jets and the short-time variability of their gamma-ray emission. [ABSTRACT FROM AUTHOR]