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Upcoming seminar
Upcoming seminar
Date: Sep. 02, 2026 @ 4:00 PM (in-person)
Date: Sep. 02, 2026 @ 4:00 PM (in-person)
Venue: Multimedia Room, Physical Sciences Building, IISc
Venue: Multimedia Room, Physical Sciences Building, IISc
Speaker: Tejas Prasanna (Columbia University)
Speaker: Tejas Prasanna (Columbia University)
Title: GRMHD Simulations of Accreting Proto-Magnetars: Implications for Gamma-Ray Burst Jets, Energetic Explosions, and r-process Nucleosynthesis
Title: GRMHD Simulations of Accreting Proto-Magnetars: Implications for Gamma-Ray Burst Jets, Energetic Explosions, and r-process Nucleosynthesis
Abstract: Newly formed, rapidly rotating, strongly magnetized neutron stars are promising central engines for gamma-ray bursts (GRBs) and luminous supernovae. Although often modeled in isolation, they can be born surrounded by accretion disks in stellar collapse, neutron-star mergers, or accretion-induced collapse. We present axisymmetric GRMHD simulations of hyperaccretion onto such objects, including a physical equation of state and charged-current weak interactions. Holding the weakly magnetized accretion torus fixed, we vary the stellar dipole field strength to span crushed-magnetosphere, magnetically channeled accretion, and centrifugal-propeller regimes, and compare the results with an otherwise similar accreting black hole.
Abstract: Newly formed, rapidly rotating, strongly magnetized neutron stars are promising central engines for gamma-ray bursts (GRBs) and luminous supernovae. Although often modeled in isolation, they can be born surrounded by accretion disks in stellar collapse, neutron-star mergers, or accretion-induced collapse. We present axisymmetric GRMHD simulations of hyperaccretion onto such objects, including a physical equation of state and charged-current weak interactions. Holding the weakly magnetized accretion torus fixed, we vary the stellar dipole field strength to span crushed-magnetosphere, magnetically channeled accretion, and centrifugal-propeller regimes, and compare the results with an otherwise similar accreting black hole.
Accretion compresses the stellar magnetosphere and opens additional magnetic flux, producing relativistic jet powers that exceed isolated dipole spin-down estimates by factors of a few to ~10. Even while the magnetosphere remains compressed against the stellar surface, stronger fields increasingly impede accretion and enhance the outflows. Channeled-accretion models show strong jet variability driven by plasmoid eruptions and intermittent magnetospheric accretion, whereas the propeller model produces a steadier, more powerful jet and a rapid spin-down torque. The disk-magnetosphere interaction also regulates how efficiently the neutron star grows in mass and whether it spins up or spins down; near spin equilibrium, inefficient accretion can delay collapse to a black hole relative to estimates based on the external mass-supply rate. Accreting proto-magnetars can therefore power relativistic jets and baryon-rich outflows with energetics comparable to those inferred for long GRBs and GRB-supernovae, respectively.
Accretion compresses the stellar magnetosphere and opens additional magnetic flux, producing relativistic jet powers that exceed isolated dipole spin-down estimates by factors of a few to ~10. Even while the magnetosphere remains compressed against the stellar surface, stronger fields increasingly impede accretion and enhance the outflows. Channeled-accretion models show strong jet variability driven by plasmoid eruptions and intermittent magnetospheric accretion, whereas the propeller model produces a steadier, more powerful jet and a rapid spin-down torque. The disk-magnetosphere interaction also regulates how efficiently the neutron star grows in mass and whether it spins up or spins down; near spin equilibrium, inefficient accretion can delay collapse to a black hole relative to estimates based on the external mass-supply rate. Accreting proto-magnetars can therefore power relativistic jets and baryon-rich outflows with energetics comparable to those inferred for long GRBs and GRB-supernovae, respectively.
The presence of a magnetized neutron star qualitatively changes both the amount and composition of the ejecta. Stronger neutron star magnetic fields progressively suppress accretion and redirect a larger fraction of the inflowing material into unbound outflows, even when the magnetosphere remains strongly compressed by the disk. Once the field is strong enough to produce magnetic channeling or centrifugal acceleration, the mass loss is enhanced further. Nuclear reaction network calculations show that these magnetically driven outflows can synthesize the full range of r-process nuclei, including the heaviest elements. Moderate neutrino irradiation substantially reduces the third-peak yield, but magnetically accelerated neutron star outflows retain a heavy component more effectively than the black hole disk wind; sufficiently strong early-time irradiation suppresses it altogether. Accreting proto-magnetars may therefore be important heavy r-process sources once their neutrino emission has sufficiently declined.
The presence of a magnetized neutron star qualitatively changes both the amount and composition of the ejecta. Stronger neutron star magnetic fields progressively suppress accretion and redirect a larger fraction of the inflowing material into unbound outflows, even when the magnetosphere remains strongly compressed by the disk. Once the field is strong enough to produce magnetic channeling or centrifugal acceleration, the mass loss is enhanced further. Nuclear reaction network calculations show that these magnetically driven outflows can synthesize the full range of r-process nuclei, including the heaviest elements. Moderate neutrino irradiation substantially reduces the third-peak yield, but magnetically accelerated neutron star outflows retain a heavy component more effectively than the black hole disk wind; sufficiently strong early-time irradiation suppresses it altogether. Accreting proto-magnetars may therefore be important heavy r-process sources once their neutrino emission has sufficiently declined.
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