Central Engine of Late-Time X-Ray Flares With Internal Origin

Abstract

This work focuses on a sample of seven extremely late-time X-ray flares with peak time t p > 10 4 s, among which two flares can be confirmed as the late-time activity of central engine. The main purpose is to investigate the mechanism of such late-time flares based on the internal origin assumption. In the hyper-accreting black hole (BH) scenario, we study the possibility of two well-known mechanisms as the central engine to power such X-ray flares, i.e., the neutrino-antineutrino annihilation and the Blandford-Znajek (BZ) process. Our results show that the annihilation luminosity is far below the observational data. Thus, the annihilation mechanism cannot account for such late-time flares. For the BZ process, if the role of outflows is taken into consideration, the inflow mass rate near the horizon will be quite low such that the magnetic field will probably be too weak to power the observed X-ray flares. We therefore argue that, for the late-time flares with internal origin, the central engine is unlikely to be associated with BHs. On the contrary, a fast rotating neutron star with strong bipolar magnetic fields may be responsible for such flares.

Black Hole Hyperaccretion Inflow–Outflow Model. I. Long and Ultra-long Gamma-Ray Bursts

“…We consider that most of ShLGRBs have an intrinsic duration much longer than a few seconds, due to the so-called "tip-of-iceberg" effect . This is consistent with the observations of early X-ray afterglows that show extended central engine activities that define an effective burst duration peaking at a few hundred seconds (e.g., Burrows et al 2005;Liang et al 2006;Zhang et al 2006;Luo et al 2013;Zhang et al 2014;Mu et al 2016).…”

Comparison of Gravitational Waves from Central Engines of Gamma-Ray Bursts: Neutrino-dominated Accretion Flows, Blandford–Znajek Mechanisms, and Millisecond Magnetars

“…Those are summarized in the left panel of Figure 3 (adapted from Figure 20 in Liu et al (2017)): First, almost all SGRBs might be described in the NDAF model, with reasonable disk masses derived from the remanent of the compact objects mergers (Liu et al 2015c); Second, only about half of LGRBs might satisfy the NDAF model, while it might be necessary for the left half to introduce massive disks, extreme Kerr BHs, and high conversion efficiency for neutrino annihilation (Song et al 2016); Third, for LGRBs and ULGRBs, BZ mechanism is especially more efficient than NDAFs. Actually, the deviation between BZ mechanism and NDAFs is more significant if X-ray flares are included 1 (e.g., Liu et al 2015b;Luo et al 2013;Mu et al 2016); Fourth, the millisecond magnetar model could cover almost all types of GRBs as said before. Cutler & Thorne (2002) reviewed the estimations for the event rates and the GW strengths of the well-known GW sources including NSs.…”

Neutrino-dominated accretion flows as the central engine of gamma-ray bursts

“…Several mechanisms have been proposed to generally explain the episodic X-ray flares, including gravitational instability in the hyperaccretion disk (Perna et al 2006;Liu et al 2014b), fragmentation of a rapidly rotating core (King et al 2005), a magnetic switch of the accretion flow (Proga & Zhang 2006;Liu et al 2012b;Cao et al 2014), differential rotation in a postmerger millisecond pulsar (Dai et al 2006), transition from a thin to a thick disk (Lazzati et al 2008), He-synthesis-driven wind (Lee et al 2009), instability in the jet (Lazzati et al 2011), outflow caused by the maximal and minimal possible mass accretion rates at each radius of NDAFs (Liu et al 2008), episodic jets produced by the magnetohydrodynamic mechanism from the disk (Yuan & Zhang 2012), and MC-NDAFs . Mu et al (2016) investigated UL flares with duration 10 4 s, and argued that the corresponding central engine may not be associated with BHs, but a fast rotating NS with strong dipolar magnetic fields as shown in Figure 20.…”