Magnetorotational collapse of supermassive stars: Black hole formation, gravitational waves, and jets

We perform MHD simulations in full GR of uniformly rotating stars that are marginally unstable to collapse. Our simulations model the direct collapse of supermassive stars (SMSs) to seed black holes (BHs) that can grow to become the supermassive BHs at the centers of quasars and AGNs. They also crudely model the collapse of massive Pop III stars to BHs, which could power a fraction of distant, long gamma-ray bursts (GRBs). The initial stellar models we adopt are Gamma=4/3 polytropes seeded with a dynamically unimportant dipole magnetic field (B field). We treat initial B-field configurations either confined to the stellar interior or extending out from the interior into the stellar exterior. The BH formed following collapse has mass M-BH similar or equal to 0.9M (where M is the mass of the initial star) and spin a(BH)/M-BH similar or equal to 0.7. A massive, hot, magnetized torus surrounds the remnant BH. At Delta t similar to 400-550M approximate to 2000-2700(M/10(6)M(circle dot))s following the gravitational wave (GW) peak amplitude, an incipient jet is launched. The disk lifetime is Delta t similar to 10(5)(M/10(6)M(circle dot))s, and the jet luminosity is L-EM similar to 10(51-52) ergs/s. If greater than or similar to 1%-10% of this power is converted into gamma rays, SWIFT and FERMI could potentially detect these events out to large redshifts z similar to 20. Thus, SMSs could be sources of ultra-long GRBs and massive Pop III stars could be the progenitors that power a fraction of the long GRBs observed at redshift z similar to 5-8. GWs are copiously emitted during the collapse, and peak at similar to 15(10(6)M(circle dot)/M)mHz (similar to 0.15(10(4)M(circle dot)/M)Hz), i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could be detectable by LISA (DECIGO/BBO) at z less than or similar to 3 (z less than or similar to 11). Hence 10(4)M(circle dot) SMSs collapsing at z similar to 10 are promising multimessenger sources of coincident gravitational and electromagnetic waves.