Supercollapsars and their X-ray Bursts
  The very first stars in the Universe can be very massive, up to
$10^3M_\odot$. If born in large numbers, such massive stars can have strong
impact on the subsequent star formation, producing strong ionising radiation
and contaminating the primordial gas with heavy elements. They would leave
behind massive black holes that could act as seeds for growing supermassive
black holes of active galactic nuclei. Given the anticipated fast rotation,
such stars would end their live as supermassive collapsars and drive powerful
magnetically-dominated jets. In this letter we investigate the possibility of
observing the bursts of high-energy emission similar to the Long Gamma Ray
Bursts associated with normal collapsars. We show that during the collapse of
supercollapsars, the Blandford-Znajek mechanism can produce jets as powerful as
few$\times10^{52}$erg/s and release up to $10^{54}$erg of the black hole
rotational energy. Due to the higher intrinsic time scale and higher redshift
the initial bright phase of the burst can last for about $10^4$ seconds,
whereas the central engine would remain active for about one day. Due to the
high redshift the burst spectrum is expected to be soft, with the spectral
energy distribution peaking at around 20-30keV. The peak total flux density is
relatively low, $10^{-7}{erg} {cm}^{-2} {s}^{-1}$, but not prohibitive. If one
supercollapsar is produced per every mini-halo of dark matter arising from the
3-$\sigma$ cosmological fluctuations then the whole sky frequency of such
bursts could reach several tens per year.
