Supercollapsars and their X-ray bursts

The very first stars in the Universe can be very massive, up to 103 M(circle dot). If born in large numbers, such massive stars can have a strong impact on the subsequent star formation, producing strong ionizing 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 life 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 x1052 erg s-1 and release up to 1054 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 104 s, whereas the central engine would remain active for about 1 d. Due to the high redshift the burst spectrum is expected to be soft, with the spectral energy distribution peaking at around 20-30 keV. 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 minihalo of dark matter arising from the 3 Sigma cosmological fluctuations, then the whole sky frequency of such bursts could reach several tens per year.