Signature of a Newborn Black Hole from the Collapse of a Supra-massive Millisecond Magnetar

Abstract

The X-ray plateau followed by a steep decay ("internal plateau") has been observed in both long and short gamma-ray burst (GRBs), implying a millisecond magnetar operating in some GRBs. The sharp decay at the end of plateau, marking the abrupt cessation of the magnetar central engine, has been considered as the collapse of a supra-massive magnetar to a black hole (BH) when it spins down. If "internal plateau" is indeed the evidence of a magnetar central engine, a natural expectation is a signature from the new-born BH in some candidates. In this work, we find that GRB 070110 is a particular case, which shows a small X-ray bump following its "internal plateau". We interpret the plateau with a spin-down supra-massive magnetar and the X-ray bump with a fall-back BH accretion. This indicates that the new-born BH is likely active in some GRBs. Therefore, GRB 070110-like events may provide a further support to the magnetar central engine model and enable us to investigate the properties of the magnetar as well as the new-born BH.

Search for the signatures of a new-born black hole from the collapse of a supra-massive millisecond magnetar in short GRB light curves

“…For example, if we assumeṀ scales withṀ ∝ t −4/3 , the decay slope of the MAD luminosity would be different from 5/3, and the value of M fb would be an order of magnitude smaller than the results obtained in Section 3 (Kisaka, Ioka & Sakamoto 2017). Besides, we did not consider the BH evolution during the accretion and BZ processes, which can in principle affect the BH mass M • and spin parameter a • (e.g., Chen et al 2017;Lei et al 2017). However, for very low accretion rate as studied in this work, this effect might be ignored.…”

“…Besides GRB 160821B, are there other short GRBs with an internal plateau showing these X-ray signatures? Recently, Chen et al (2017) found that the X-ray bump following the internal plateau of long GRB 070110 is a possible signature of a new-born BH from the collapse of a supra-massive magnetar and interpreted the X-ray bump as the result of a fall-back accretion onto the BH. Their work further encourages us to explore the possible X-ray signatures of the new-born BH in the case of NS-NS mergers, and to search for these signatures in the sample of short GRBs that show an internal plateau.…”

The evolution effects of radius and moment of inertia for rapidly rotating neutron stars

“…If our interpretation is correct, this may explain why some GRBs that present the internal plateau feature, still show late time central engine activity, manifested through flares and second shallow plateaus (Troja et al 2007;Perley et al 2009;Margutti et al 2011;Gao et al 2015Gao et al , 2017bZhao et al 2020). In the magnetar collapsing scenario, only flares or plateaus close to the internal plateau could be interpreted with fall-back accretion model (Chen et al 2017;Zhao et al 2020).…”

How to understand high-energy gamma-ray bursts?

“…Dai等人 [22] 在2006年提出新生中子星的磁重联过程可 以解释强烈的X射线耀发现象, 其间歇性和持续时标与 观测一致, 尤其对于解释短暴的X射线耀发有显著优 势. Lü等人 [23,24] 发现一些长暴和短暴的X射线平台后 伴随着陡降, 这表明磁星(或夸克星)在旋转变慢后会塌 缩为黑洞. Chen等人 [25] [30] 、额外的幂律谱成分 [31] 等. 其中额外的幂律谱 可从keV能段一直延伸到GeV, 然后可能出现截断 [32] .…”