  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 $M_{BH} \simeq 0.9M$ (where $M$ is
the mass of the initial star) and spin $a_{BH}/M_{BH}\simeq 0.7$. A massive,
hot, magnetized torus surrounds the remnant BH. At $\Delta t\sim
400-550M\approx 2000-2700(M/10^6M_\odot)$s following the gravitational wave
(GW) peak amplitude, an incipient jet is launched. The disk lifetime is $\Delta
t\sim 10^5(M/10^6M_\odot)$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, SWIFT
and FERMI could potentially detect these events out to large redshifts $z\sim
20$. 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 $z \sim 5-8$. GWs are copiously emitted during the collapse, and peak
at $\sim 15(10^6 M_{\odot}/M)\rm mHz$ ($\sim 0.15(10^4 M_{\odot}/M)\rm Hz$),
i.e., in the LISA (DECIGO/BBO) band; optimally oriented SMSs could be
detectable by LISA (DECIGO/BBO) at $z \lesssim 3$ ($z \lesssim 11$). Hence
$10^4 M_{\odot}$ SMSs collapsing at $z\sim 10$ are promising multimessenger
sources of coincident gravitational and electromagnetic waves.
