Search for the Signatures of a New-Born Black Hole from the Collapse of a Supra-massive Millisecond Magnetar in Short GRB Light Curves

'Internal plateau' followed by a sharp decay is commonly seen in short gamma-ray burst (GRB) light curves. The plateau component is usually interpreted as the dipole emission from a supra-massive magnetar, and the sharp decay may imply the collapse of the magnetar to a black hole (BH). Fall-back accretion on to the new-born BH could produce long-lasting activities via the Blandford-Znajek (BZ) process. The magnetic flux accumulated near the BH would be confined by the accretion discs for a period of time. As the accretion rate decreases, the magnetic flux is strong enough to obstruct gas infall, leading to a magnetically arrested disc. Within this scenario, we show that the BZ process could produce two types of typical X-ray light curves: type I exhibits a long-lasting plateau, followed by a power-law (PL) decay with slopes ranging from 5/3 to 40/9; type II shows roughly a single PL decay with a slope of 5/3. The former requires low magnetic field strength, while the latter corresponds to relatively high values. We search for such signatures of the new-born BH from a sample of short GRBs with an internal plateau, and find two candidates: GRB 101219A and GRB 160821B, corresponding to type II and type I light curves, respectively. It is shown that our model can explain the data very well.©2017 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
We acknowledge the anonymous referee for helpful comments and suggestions. We also thank Bing Zhang and He Gao for helpful discussions. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB23040400). SLX was also supported by the Hundred Talents Program of the Chinese Academy of Sciences (Grant No. Y629113). WHL and WC acknowledge support from the National Natural Science Foundation of China (Grant U1431124). BBZ acknowledges support from the Spanish Ministry Projects AYA2012-39727-C03-01 and AYA201571718-R. LMS acknowledges support from the National Program on Key Research and Development Project (Grant No. 2016YFA0400801) and the National Basic Research Program of China (Grant No. 2014CB845802).