Elemental abundances associated with gamma-ray bursts: nucleosynthesis in outflows

Abstract

Aims. Outflows from accretion disks have been suggested to be suitable sites for producing heavy elements. The objective of this work is to investigate nucleosynthesis in the outflows launched from the inner accretion disks of collapsars that are associated with gamma-ray bursts (GRBs). Methods. By using a collapsar model of advection-dominated accretion and an adiabatic expanding outflow model, we simulated chemical yields inside the outflows associated with GRBs. As a result of the extremely high temperature (above 10 10 K), the dominated species existing in the inner region of the disk are proton, neutron, 2 H, and 4 He, which are photodisintegrated from heavy nuclei. Assuming that an explosive nucleosynthesis could operate in outflows from collapsar accretions associated with GRBs, heavier species including the iron-group will be synthesized significantly. Although various outflow models have been investigated carefully in the past, the uncertainties in predicting the outflow birthplace (ejected-radius r j ) in the accretion disk are still quite large. Unlike recent researches on this topic that located the production of heavy elements in GRBs in the outer parts of the disk, in this study we focus our attention mainly on the outflows triggered from the inner region of collapsar accretion. Results. We find 56 Ni can be abundantly produced in all the outflow cases considered here from the inner collapsar disk. The highest 56 Ni's mass fraction in the outflows reaches 0.463, which can imply that the 56 Ni-rich outflows will lead to an observable violent explosion of a GRB event. The mass fractions of 57 Ni, 59 Cu (decays to 59 Ni), and 60 Zn (decays to 60 Ni) could be 10 −2 , and the abundance of 58 Ni reaches 10 −3 . Our simulations support that the effective production of heavy elements can occur in the outflows from the inner regions of the collapsar accretion disks. Our results also show that the amount of heavy species produced in outflows depends on the eject-radius r j of the accretion disk where the outflows take place; outflows launched at larger radii can be helpful to increase the quantities of heavy nuclei by means of the lower disk temperature, which agrees with the conclusions of recent studies. Furthermore, a recently published article reported that some nuclei, such as 28 Si, 32 S, 36 Ar, 40 Ca, 44 Ti, 48 Cr, 54 Fe, and 55−56 Co, experience a dip and then rise in their mass fraction while evolving in the outflow, and this characteristic behavior has occurred in many more outflow cases, and this is reported for the first time in the literature. This characteristic phenomenon also occurs in our nucleosynthetic simulations.

Nuclear Burning in Collapsar Accretion Disks

“…Several of these models find considerable production of IME and IGE, and in some cases comparable to our findings (e.g. Hu (2015) up to a factor of a few), however, these models and the regimes in these various works are considerably different than those we explore, and are thus not directly comparable.…”

Neutrino-dominated accretion flows as the central engine of gamma-ray bursts

“…Surman et al (2011) focused on nucleosynthesis, particularly for the progenitor of 56 Ni in the hot outflows from GRB accretion disks. Metzger et al (2011) suggested that the composition of ultrahigh energy cosmic rays becomes dominated by heavy nuclei at high energies forming GRB jets or outflows (also see e.g., Sigl et al 1995;Horiuchi et al 2012;Hu 2015).…”

Black Hole Hyperaccretion Inflow–Outflow Model. II. Long-duration Gamma-Ray Bursts and Supernova 56Ni Bumps

“…Statistical analysis of the bolometric properties of SNe shows that the average 56 Ni mass is 0.4 ± 0.2 M ⊙ in an explosion (e.g., Cano et al 2017). Additionally, during the explosive burning of the collapsar, 56 Ni synthesis also occurs in the hyperaccretion disk (e.g., Chakrabarti et al 1987;Surman et al 2008;Liu et al 2013) or the winds/outflows from the accretion disk (e.g., Pruet et al 2004;Kohri et al 2005;Surman & McLaughlin 2005;Surman et al 2006Surman et al , 2011Hu & Peng 2008;Liu et al 2013;Hu 2015;Wu et al 2016). This disk is widely considered the main factory of 56 Ni in these studies.…”