We perform MHD simulations in full GR of uniformly rotating stars that aremarginally unstable to collapse. Our simulations model the direct collapse ofsupermassive stars (SMSs) to seed black holes (BHs) that can grow to become thesupermassive BHs at the centers of quasars and AGNs. They also crudely modelthe collapse of massive Pop III stars to BHs, which could power a fraction ofdistant, long gamma-ray bursts (GRBs). The initial stellar models we adopt are$\Gamma = 4/3$ polytropes seeded with a dynamically unimportant dipole magneticfield (B field). We treat initial B-field configurations either confined to thestellar interior or extending out from the interior into the stellar exterior.The BH formed following collapse has mass $M_{BH} \simeq 0.9M$ (where $M$ isthe mass of the initial star) and spin $a_{BH}/M_{BH}\simeq 0.7$. A massive,hot, magnetized torus surrounds the remnant BH. At $\Delta t\sim400-550M\approx 2000-2700(M/10^6M_\odot)$s following the gravitational wave(GW) peak amplitude, an incipient jet is launched. The disk lifetime is $\Deltat\sim 10^5(M/10^6M_\odot)$s, and the jet luminosity is $L_{EM}\sim 10^{51-52}$ergs/s. If $\gtrsim 1-10\%$ of this power is converted into gamma rays, SWIFTand FERMI could potentially detect these events out to large redshifts $z\sim20$. Thus, SMSs could be sources of ultra-long GRBs and massive Pop III starscould be the progenitors that power a fraction of the long GRBs observed atredshift $z \sim 5-8$. GWs are copiously emitted during the collapse, and peakat $\sim 15(10^6 M_{\odot}/M)\rm mHz$ ($\sim 0.15(10^4 M_{\odot}/M)\rm Hz$),i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could bedetectable by LISA (DECIGO/BBO) at $z \lesssim 3$ ($z \lesssim 11$). Hence$10^4 M_{\odot}$ SMSs collapsing at $z\sim 10$ are promising multimessengersources of coincident gravitational and electromagnetic waves.
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