Collapsar Model Research Articles

The host galaxies of long-duration gamma-ray bursts in a cosmological hierarchical scenario

We developed a Monte Carlo code to generate long-duration gamma-ray burst (LGRB) events within cosmological hydrodynamical simulations consistent with the concordance Λ cold dark matter model. As structure is assembled, LGRBs are generated in the substructure that formed galaxies today. We adopted the collapsar model so that LGRBs are produced by single, massive stars at the final stage of their evolution. We found that the observed properties of the LGRB host galaxies (HGs) are reproduced if LGRBs are also required to be generated by low-metallicity stars. The low-metallicity condition imposed on the progenitor stars of LGRBs selects a sample of HGs with mean gas abundances of 12 + log O/H ≈ 8.6. For z < 1 the simulated HGs of low-metallicity LGRB progenitors tend to be faint, slow rotators with high star formation efficiency, compared with the general galaxy population, in agreement with observations. At higher redshift, our results suggest that larger systems with high star formation activity could also contribute to the generation of LGRBs from low-metallicity progenitors since the fraction of low-metallicity gas available for star formation increases for all systems with look-back time. Under the hypothesis of our LGRB model, our results support the claim that LGRBs could be unbiased tracers of star formation at high redshifts.

Single star progenitors of long gamma-ray bursts

We investigate the final fate of rotating massive single stars at different metallicities. and discuss for which initial conditions long Gamma-Ray Bursts (GRBs) are expected to be produced in the frame of the collapsar model. Implications for future studies for GRB progenitors are discussed.

Collimated Jet or Expanding Outflow: Possible Origins of Gamma‐Ray Bursts and X‐Ray Flashes

We investigate the dynamics of an outflow propagating in a progenitor in the context of the collapsar model for GRBs through two-dimensional axisymmetric relativistic hydrodynamic simulations. Initially, we locally inject an outflow near the center of a progenitor. We calculate 25 models, in total, by fixing its total input energy (1051 ergs s-1) and radius of the injected outflow (7 × 107 cm) while varying its bulk Lorentz factor, Γ0 = 1.05-5, and its specific internal energy, 0/c2 = 0.1-30 (with c being speed of light). We find a smooth but dramatic transition from a collimated jet to an expanding outflow among calculated models. The half opening angle of the outflow (θsim) is sensitive to Γ0; we find θsim < 2° for Γ0 ≳ 3. The maximum Lorentz factor is, on the other hand, sensitive to both Γ0 and 0: roughly Γmax ~ Γ0(1 + 0/c2). In particular, a very high Lorentz factor of Γmax ≳ 100 is achieved in one model. A variety of opening angles can arise by changing 0, even when the maximum Lorentz factor is fixed. The jet structure totally depends on Γ0. When Γ0 is high, high-pressure progenitor gas heated by the bow shock collimates the outflow to form a narrow, relativistic jet. When Γ0 is low, on the contrary, the outflow expands soon after the injection, since the bow shock is weak and thus the pressure of the progenitor gas is not high enough to confine the flow. Our finding will explain a smooth transition between the GRBs, X-ray-rich GRBs (XRRs), and X-ray flashes (XRFs) by the same model but with different 0 values.

Single star progenitors of long gamma-ray bursts

We present grids of massive star evolution models at four different metallicities (Z=0.004, 0.002, 0.001, 0.00001). The effects of rotation on the stellar structure and the transport of angular momentum and chemical elements through the Spruit-Tayler dynamo and rotationally induced instabilities are considered. After discussing uncertainties involved with the adopted physics, we elaborate the final fate of massive stars as a function of initial mass and spin rate, at each considered metallicity. In particular, we investigate for which initial conditions long gamma-ray bursts (GRBs) are expected to be produced in the frame of the collapsar model. Then, using an empirical spin distribution of young massive metal-poor stars and a specified metallicity-dependent history of star-formation, we compute the expected GRB rate as function of metallicity and redshift based on our stellar evolution models. The GRB production in our models is limited to metallicities of Z \lsim 0.004, with the consequence that about 50 % of all GRBs are predicted to be found at redshifts above z = 4, with most supernovae occurring at redshifts below z\simeq 2.2. The average GRB/SN ratio predicted by our model is about 1/200 globally, and 1/1250 at low redshift. Future strategies for testing the considered GRB progenitor scenario are briefly discussed.

Explosive Nucleosynthesis in GRB Jets Accompanied by Hypernovae

Two-dimensional hydrodynamic simulations are performed to investigate explosive nucleosynthesis in a collapsar using the model of MacFadyen and Woosley (1999). It is shown that 56Ni is not produced in the jet of the collapsar sufficiently to explain the observed amount of a hypernova when the duration of the explosion is \sim 10 sec, which is considered to be the typical timescale of explosion in the collapsar model. Even though a considerable amount of 56Ni is synthesized if all explosion energy is deposited initially, the opening angles of the jets become too wide to realize highly relativistic outflows and gamma-ray bursts in such a case. From these results, it is concluded that the origin of 56Ni in hypernovae associated with GRBs is not the explosive nucleosynthesis in the jet. We consider that the idea that the origin is the explosive nucleosynthesis in the accretion disk is more promising. We also show that the explosion becomes bi-polar naturally due to the effect of the deformed progenitor. This fact suggests that the 56Ni synthesized in the accretion disk and conveyed as outflows are blown along to the rotation axis, which will explain the line features of SN 1998bw and double peaked line features of SN 2003jd. Some fraction of the gamma-ray lines from 56Ni decays in the jet will appear without losing their energies because the jet becomes optically thin before a considerable amount of 56Ni decays as long as the jet is a relativistic flow. We show that abundance of nuclei whose mass number \sim 40 in the ejecta depends sensitively on the energy deposition rate. So it may be determined by observations of chemical composition in metal poor stars which model is the proper one as a model of a gamma-ray burst accompanied by a hypernova.

Magnetohydrodynamic Simulations of a Rotating Massive Star Collapsing to a Black Hole

We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of the collapse of a rotating star of 40 Msun and in the light of the collapsar model of gamma-ray burst. Considering two distributions of angular momentum, up to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate the formation of an accretion disk around a black hole and the jet production near the hole. After material reaches to the black hole with the high angular momentum, the disk is formed inside a surface of weak shock. The disk becomes in a quasi-steady state for stars whose magnetic field is less than 10^{10} G before the collapse. We find that the jet can be driven by the magnetic fields even if the central core does not rotate as rapidly as previously assumed and outer layers of the star has sufficiently high angular momentum. The magnetic fields are chiefly amplified inside the disk due to the compression and the wrapping of the field. The fields inside the disk propagate to the polar region along the inner boundary near the black hole through the Alfv{\'e}n wave, and eventually drive the jet. The quasi-steady disk is not an advection-dominated disk but a neutrino cooling-dominated one. Mass accretion rates in the disks are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent for neutrinos. The dense part of the disk, which locates near the hole, emits neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The neutrino luminosity is much smaller than those from supernovae after the neutrino burst.

Cauchy-perturbative matching reexamined: Tests in spherical symmetry

During the last few years progress has been made on several fronts making it possible to revisit Cauchy-perturbative matching (CPM) in numerical relativity in a more robust and accurate way. This paper is the first in a series where we plan to analyze CPM in the light of these new results. Here we start by testing high-order summation-by-parts operators, penalty boundaries and contraint-preserving boundary conditions applied to CPM in a setting that is simple enough to study all the ingredients in great detail: Einstein's equations in spherical symmetry, describing a black hole coupled to a massless scalar field. We show that with the techniques described above, the errors introduced by Cauchy-perturbative matching are very small, and that very long term and accurate CPM evolutions can be achieved. Our tests include the accretion and ring-down phase of a Schwarzschild black hole with CPM, where we find that the discrete evolution introduces, with a low spatial resolution of \Delta r = M/10, an error of 0.3% after an evolution time of 1,000,000 M. For a black hole of solar mass, this corresponds to approximately 5 s, and is therefore at the lower end of timescales discussed e.g. in the collapsar model of gamma-ray burst engines. (abridged)

Evolution of rapidly rotating metal-poor massive stars towards gamma-ray bursts

Recent models of rotating massive stars including magnetic fields prove it difficult for the cores of single stars to retain enough angular momentum to produce a collapsar and gamma-ray burst. At low metallicity, even very massive stars may retain a massive hydrogen envelope due to the weakness of the stellar winds, posing an additional obstacle to the collapsar model. Here, we consider the evolution of massive, magnetic stars where rapid rotation induces almost chemically homogeneous evolution. We find that in this case, the requirements of the collapsar model are rather easily fulfilled if the metallicity sufficiently small: 1) Rapidly rotating helium stars are formed without the need to remove the hydrogen envelope, avoiding mass-loss induced spin-down. 2) Angular momentum transport from the helium core to hydrogen envelope by magnetic torques is insignificant. We demonstrate this by calculating evolutionary models of massive stars with various metallicities, and derive an upper metallicity limit for this scenario based on currently proposed mass loss rates. Our models also suggest the existence of a lower CO-core mass limit of about 10 Msun -- which relates to an initial mass of only about 20 Msun within our scenario -- for GRB production. We argue that the relative importance of the considered GRB progenitor channel, compared to any channel related to binary stars, may increase with decreasing metallicity, and that this channel might be the major path to GRBs from first stars.

The redshift distribution of gamma-ray bursts revisited

Abstract In this Letter, we calculate the redshift distribution of gamma-ray bursts assuming that they trace (i) the globally averaged star formation rate or (ii) the average metallicity in the Universe. While at redshifts 5 and below both the star formation rate and the metallicity are observationally determined modulo some uncertainties, at higher redshifts there are few constraints. We extrapolate the star formation rate and metallicity to higher redshifts and explore models that are broadly consistent with bounds on the optical depth from WMAP results. In addition, we also include parametric descriptions of the luminosity function, and the typical spectrum for gamma-ray bursts (GRBs). With these essential ingredients included in the modelling, we find that a substantial fraction (75 per cent) of GRBs are expected to originate at redshifts below 4, in variance with some previous estimates. Conversely, if we assume as expected for the collapsar model that gamma-ray bursts favour a low-metallicity environment, and therefore relate the GRB rate to a simple model of the average metallicity as a function of redshift, we find that a higher fraction of bursts, about 40 per cent, originate from z &amp;gt; 4. We conclude with the implications of Swift GRB detections.

Universal GRB Jets from Jet‐Cocoon Interaction in Massive Stars

We consider the time-dependent evolution of a relativistic jet following its breakout through the surface of a massive compact star, as envisaged in the collapsar model of gamma-ray bursts. At breakout, the jet is tightly collimated by the pressure of its hot cocoon, which is created as the jet traverses the star. After breakout, the cocoon pressure drops and the jet expands toward its natural opening angle. We show that the evolving opening angle of the jet produces a stratification of the total energy with the off-axis angle, resulting in a universal morphology. The angular structure is largely independent of the initial beam pattern and depends only on the luminosity of the central engine. With the minimal assumption of a constant energy release we reproduce the theta^{-2} profile required to explain observations of afterglows.

A possible bright blue supernova in the afterglow of GRB 020305

Astronomy & Astrophysics manuscript no. (will be inserted by hand later) arXiv:astro-ph/0504050v1 3 Apr 2005 A possible bright blue SN in the afterglow of GRB 020305 ⋆ J. Gorosabel 1,2 , J.P.U. Fynbo 3 , A. Fruchter 2 , A. Levan 4 , J. Hjorth 3 , P. Nugent 5 , A.J. Castro-Tirado 1 , J.M. Castro Cer´on 2,3 , J. Rhoads 2 , D. Bersier 2 , and I. Burud 2 Instituto de Astrof´isica de Andaluc´ia (IAA-CSIC), P.O. Box 03004, E-18080 Granada, Spain Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Kobenhavn O, Denmark Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK Lawrence Berkeley National Laboratory, MS 50-F, 1 Cyclotron Road, Berkeley, CA 94720, USA Received ; accepted Abstract. We report on ground-based and HST(+STIS) imaging of the afterglow and host galaxy of the Gamma-Ray Burst (GRB) of March 5 2002. The GRB occurred in a R = 25.17 ± 0.14 galaxy, which apparently is part of an interacting system. The lightcurve of the optical afterglow shows a rebrightening, or at least a plateau, 12–16 days after the gamma-ray event. U BV RIK ′ multi-band imaging of the afterglow ∼12 days after the GRB reveals a blue spectral energy distribution (SED). The SED is consistent with a power-law with a spectral index of β = −0.63 ± 0.16, but there is tentative evidence for deviations away from a power-law. Unfortunately, a spectroscopic redshift has not been secured for GRB 020305. From the SED we impose a redshift upper limit of z . 2.8, hence excluding the pseudo redshift of 4.6 reported for this burst. We discuss the possibilities for explaining the lightcurve, SED and host galaxy properties for GRB 020305. The most natural interpretation of the lightcurve and the SED is an associated supernova (SN). Our data can not precisely determine the redshift of the GRB. The most favoured explanation is a low redshift (z ∼ 0.2) SN, but a higher redshift (z & 0.5) SN can not be excluded. We also discuss less likely scenarios not based on SNe, like a burst occurring in a z = 2.5 galaxy with an extinction curve similar to that of the Milky Way. Key words. gamma rays: bursts – techniques: photometric 1. Introduction For long duration GRBs the relation with supernovae (SNe) became firmly established with the discovery of the type Ic su- pernova SN 2003dh associated with GRB 030329 (Stanek et al. 2003; Hjorth et al. 2003). This result lends strong support to the collapsar model (Woosley 1993), but a SN is also an ingredient in other models (e.g. Dado et al. 2003; Fryer & Heger 2004). However, the associated SNe follow a broad distribution of op- tical luminosities (Zeh et al. 2004). Furthermore the connection of GRBs with SNe of other types than Ic can not be excluded, motivated by the two possible associations of GRBs and II type SNe (SN 1997cy, Germany et al. 2000; SN 1999E, Rigon et al. 2003). Therefore, the afterglow lightcurves and SEDs around the SN peak are far from being described by an universal SN template. In this study we present ground and space-based opti- cal observations of GRB 020305 carried out from 11.5 to 321.2 days after the burst. Send offprint requests to: J. Gorosabel Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden. Correspondence to: jgu@iaa.es GRB 020305 was localised by the HETE-II satellite on March 5.4968 UT (Ricker et al. 2002). The high-energy emis- sion as seen by the Interplanetary network (IPN) consisted in two broad pulses, with a total GRB duration of ∼280s (Hurley et al. 2002), placing it in the long-soft burst category. Price et al. (2002) reported the presence of a transient optical source in the HETE-II/IPN error box in images taken ∼20 hours after the GRB. Further imaging confirmed the fading behaviour of the candidate (Lee et al. 2002; Ohyama et al. 2002). The paper is structured as follows: Sect. 2 details the ob- servations and the data reduction, Sect. 3 reports the results on the SED, lightcurve and host galaxy, Sect. 4 discusses several interpretations of the results, and finally Sect. 5 draws the con- clusions of this study. 2. Observations and data reduction 2.1. NOT observations We observed the field of GRB 020305 from the ground with the 2.56-m Nordic Optical Telescope (NOT) on 2002 March 16.95–22.18 UT, i.e. 11.45–16.68 days after the GRB. The in- strument used was the Andaluc´ia Faint Object Spectrograph (ALFOSC) equipped with a 2048 2 pixel Loral CCD having a

Which massive stars are gamma-ray burst progenitors?

The collapsar model for gamma-ray bursts requires three essential ingredients: a massive core, removal of the hydrogen envelope, and enough angular momentum in the core. We study current massive star evolution models of solar metallicity to determine which massive star physics is capable of producing these ingredients. In particular, we investigate the role of hydrodynamic and magnetic internal angular momentum transport and binary mass and angular momentum transfer. We follow the evolution of rotating single stars and of binary systems that include rotational processes for both stars. Neglecting magnetic fields, we show that the cores of massive single stars can maintain a high specific angular momentum when evolved with the assumption that mean molecular weight gradients suppress rotational mixing processes. In binary systems that undergo mass transfer during core hydrogen burning the mass receiving star accretes large amounts of high angular momentum material, leading to a spin-up of the core. We find, however, that this merely compensates for the tidal angular momentum loss due to spin-orbit coupling, which leads to synchronous rotation before the mass transfer event. Therefore the resulting cores do not rotate faster than in single stars. We also present models that include magnetic fields generated by differential rotation and we consider the internal angular momentum transport by magnetic torques. We investigate the capability of magnetic torques to efficiently pump angular momentum into the cores of accreting stars. Despite our finding that this mechanism works, the magnetic coupling of core and envelope after the accreting star ends core hydrogen burning leads to slower rotation than in the non-magnetic case.

Binary Merger Progenitors for Gamma‐Ray Bursts and Hypernovae

The collapsar model, the now leading model for the engine behind gamma-ray bursts and hypernovae, requires that a star collapses to form a black hole surrounded by an accretion disk of high-angular momentum material. The current best theoretical stellar models, however, do not retain enough angular momentum in the core of the star to make a centrifugally supported disk. In this paper, we present the first calculations of the helium-star/helium-star merger progenitors for the collapsar model. These progenitors invoke the merger of two helium cores during the common envelope inspiral phase of a binary system. We find that, in some cases, the merger can produce cores that are rotating 3-10 times faster than single stars. He-star/He-star gamma-ray burst progenitors have a very different redshift distribution than their single-star gamma-ray burst progenitors and we discuss how gamma-ray burst observations can constrain these progenitors.

Interpretations of gamma-ray burst spectroscopy

We analyze the spectral lags of a sample of bright gamma-ray burst pulses observed by CGRO BATSE and compare these with the results of high-resolution spectroscopical investigations. We find that pulses with hard spectra have the largest lags, and that there is a similar, but weaker correlation between hardness-intensity corre- lation index, �, and lag. We also find that the lags differ considerably between pulses within a burst. Furthermore, the peak energy mainly decreases with increasing lag. Assuming a lag-luminosity relation as suggested by Norris et al., there will thus be a positive luminosity-peak-energy correlation. We also find that the hardness ratio, of the total flux in two channels, only weakly correlates with the spectral evolution parameters. These results are consistent with those found in the analytical and numerical analysis in Paper I. Finally, we find that for these bursts, dominated by a single pulse, there is a correlation between the observed energy-flux, F, and the inverse of the lag, �t: F / �t 1 . We interpret this flux-lag relation found as a consequence of the lag-luminosity relation and that these bursts have to be relatively narrowly distributed in z. However, they still have to, mainly, lie beyond z � 0.01, since they do not coincide with the local super-cluster of galaxies. We discuss the observed correlations within the collapsar model, in which the collimation of the outflow varies. Both the thermal photospheric emission as well as non-thermal, optically-thin synchrotron emission should be important.

Evolution of a neutrino-cooled disc in gamma-ray bursts

Rapid, hyper-Eddington accretion is likely to power the central engines of gamma-ray bursts (GRBs). In the extreme conditions of densities and temperatures the accreting torus is cooled by neutrino emission rather than by radiation. Another important cooling mechanism is the advection of energy into the central black hole. We compute the time evolution of a neutrino-dominated disc that proceeds during the burst and investigate the changes in its density and temperature. The discrimination between short and long bursts is made on the basis of the different rates of material inflow to the outer parts of the disc, thus favouring the binary merger scenario for the short GRBs and the collapsar scenario for the long GRBs. Within the context of the collapsar model, we also study the evolution of the photon luminosity of the remnant disc up to times of ∼1 d, and we discuss its implications for the production of emission lines in GRB spectra.

The Host Galaxy of GRB 031203: Implications of Its Low Metallicity, Low Redshift, and Starburst Nature

We present Keck/NIRSPEC near-IR images and Magellan/IMACS optical spectroscopy of the host galaxy of GRB 031203. The host is an actively star-forming galaxy at z=0.1055 +/- 0.0001. This is the lowest redshift GRB to-date, aside from GRB 980425. From the hydrogen Balmer lines, we infer an extinction of A_V = 3.62 +/- 0.25 or a total reddening E_T(B-V) = 1.17 +/- 0.1 toward the sightline to the nebular regions. After correcting for reddening, we perform an emission-line analysis and derive an ISM temperature of T=13400+/-2000K and electron density of n_e = 300 cm^(-3). These imply a metallicity [O/H]=-0.72+/-0.15 dex and a roughly solar abundance pattern for N, Ne, S, and Ar. Integrating Ha, we infer a dust-corrected star formation rate (SFR) of > 11 Msol/yr. These observations have the following implications: (1) the galaxy has a low K'-band luminosity L ~ L^*/5, typical of GRB host galaxies; (2) the low redshift indicates GRB 031203 had an isotropic-equivalent gamma-ray energy release smaller than all previous confirmed GRB events. The burst discovery raises the likelihood of identifying many additional low z, low flux events with Swift; (3) the large SFR, low metallicity, and the inferred hard radiation field is suggestive of massive star formation, supporting the collapsar model; (4) several lines of evidence argue against the identification of GRB 031203 as an X-ray flash event.

Summary of the workshop on gamma-ray burst afterglows at the 34th COSPAR meeting

A summary is given of the presentations at the COSPAR workshop on γ-ray bursts with some personal commentary on the contributions, the SN/GRB connection, and on the role of magnetic fields in γ-ray bursts and their afterglows. Of special interest were the accumulated arguments for strong collimation and associated reduction in the total required energy for γ-ray bursts. Significant discussion was also devoted to the issues associated with iron and metal lines in X-ray spectra. It is important to note that some of the afterglows seem to require ambient densities ≪1 g cm −3, rather incompatible with a massive star environment. Of associated difficulty is the fact that few, if any, afterglows seem consistent with the r −2 wind expected for a massive star model. There are reasons to think that if γ-ray bursts are associated with supernovae they are of Type Ic. This suggests that any wind present might be rich in carbon and oxygen, not hydrogen or helium. If γ-ray bursts are narrowly collimated, then the burst is only probing a small portion of any wind, perhaps just that time-dependent and isotropic structure directly along the rotation axis. The characteristics of “hypernovae” may be the result of orientation effects in a mildly inhomogeneous set of progenitors, rather than requiring an excessive total energy or luminosity. The recent event GRB 021004 provided a rich photometric and spectroscopic record and perhaps the most direct evidence yet for the association of a specific γ-ray burst with a massive star progenitor. If the magnetic field plays a significant role in launching a relativistic γ-ray burst jet from within a collapsing star, then the magnetic field may also play a role in the propagation, collimation, and stability of that jet within and beyond the star. The magneto-rotational instability (MRI) can operate under conditions of moderate rotation. This means that the MRI will be at work generating strong fields exponentially rapidly even as the disk of material begins to form and makes a transition from a non-Keplerian to quasi-Keplerian flow in the collapsar and related models.

Presupernova Evolution of Rotating Massive Stars and the Rotation Rate of Pulsars

Rotation in massive stars has been studied on the main sequence and during helium burning for decades, but only recently have realistic numerical simulations followed the transport of angular momentum that occurs during more advanced stages of evolution. The results affect such interesting issues as whether rotation is important to the explosion mechanism, whether supernovae are strong sources of gravitational radiation, the star's nucleosynthesis, and the initial rotation rate of neutron stars and black holes. We find that when only hydrodynamic instabilities (shear, Eddington-Sweet, etc.) are included in the calculation, one obtains neutron stars spinning at close to critical rotation at their surface – or even formally in excess of critical. When recent estimates of magnetic torques (Spruit 2002) are added, however, the evolved cores spin about an order of magnitude slower. This is still more angular momentum than observed in young pulsars, but too slow for the collapsar model for gamma-ray bursts.

Axisymmetric Magnetohydrodynamic Simulations of the Collapsar Model for Gamma-Ray Bursts

We present results from axisymmetric, time-dependent magnetohydrodynamic (MHD) simulations of the collapsar model for gamma-ray bursts. We begin the simulations after the 1.7 MSUN iron core of a 25 MSUN presupernova star has collapsed and study the ensuing accretion of the 7 MSUN helium envelope onto the central black hole formed by the collapsed iron core. We consider a spherically symmetric progenitor model, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum and a weak radial magnetic field. Our MHD simulations include a realistic equation of state, neutrino cooling, photodisintegration of helium, and resistive heating. Our main conclusion is that, within the collapsar model, MHD effects alone are able to launch, accelerate and sustain a strong polar outflow. We also find that the outflow is Poynting flux-dominated, and note that this provides favorable initial conditions for the subsequent production of a baryon-poor fireball.

Explosive Nucleosynthesis Associated with Formation of Jet‐induced Gamma‐Ray Bursts in Massive Stars

We perform two-dimensional relativistic hydrodynamic simulations in the context of collapsar model. Calculations of explosive nucleosynthesis are also accomplished. We investigate the influence of the structure of the progenitor and energy deposition rate on the resulting explosive nucleosynthesis, assuming that 56Ni is mainly synthesized in the jet launched by the neutrino heating. We show that the amount of 56Ni is very sensitive to the energy deposition rate. Thus, we conclude that it is quite natural to detect no underlying supernova in some X-ray afterglows, such as GRB 010921. We also point out the possibility that the relative abundance of the elements with an intermediate mass number, such as Si and S, in the X-ray afterglow of GRB 011211 may be naturally explained if the energy deposition rate at the central engine is relatively long because little 56Ni should be synthesized under such an environment. If this discussion is true, there should be a correlation between the line features in the X-ray afterglow and the duration of the gamma-ray burst. It should be noted that the duration of GRB 011211 is 270 s, making it the longest burst ever observed by BeppoSAX, although it suffers from the effect of redshift (zhost = 2.14), and supporting our conclusion. Our results also suggest that the type I collapsar model, in which the energy deposition rate is relatively low ( ~ 1051 ergs s-1), might have difficulty in reproducing the observed amount of 56Ni in a hypernova such as SN 1998bw. This means that models of the mechanism of the central engine of a hypernova-accompanying gamma-ray burst may be constrained by the requirements of explosive nucleosynthesis.