It is generally believed that long-duration gamma-ray bursts (GRBs) are associated with massive star core-collapse 1 , whereas short-duration GRBs are associated with mergers of compact star binaries 2 . However, growing observations [3][4][5][6] have suggested that oddball GRBs do exist, and multiple criteria (prompt emission properties, supernova/kilonova associations, and host galaxy properties) rather than burst duration only are needed to classify GRBs physically 7 . A previously reported long-duration burst, GRB 060614 3 , could be viewed as a short GRB with extended emission if it were observed at a larger distance 8 and was associated with a kilonova-like feature 9 . As a result, it belongs to the Type-I (compact star merger) GRB category and is likely of the binary neutron star merger origin. Here we report a peculiar long-duration gamma-ray burst, GRB 211211A, whose prompt emission properties in many aspects differ from all known Type-I GRBs, yet its multi-band observations suggest a non-massive-star origin. In particular, significant excess emission in both optical and nearinfrared wavelengths has been discovered (see also Ref. 10 ), which resembles kilonova emission as observed in some Type-I GRBs. These observations point towards a new progenitor type of GRBs. A scenario invoking a white dwarf-neutron star merger with a post-merger magnetar engine provides a self-consistent interpretation for all the observations, including prompt gamma-rays, early X-ray afterglow, as well as the engine-fed 11, 12 kilonova emission.
Limited by the sensitivities of the current gravitational wave (GW) detectors, the central remnant of the binary neutron star (NS) merger associated with GW170817 remains an open question. Considering the relatively large total mass, it is generally proposed that the merger of GW170817 would lead to a shortly lived hypermassive NS or directly produce a black hole (BH). There is no clear evidence to support or rule out a long-lived NS as the merger remnant. Here we utilize the GW and electromagnetic (EM) signals to comprehensively investigate the parameter space that allows a long-lived NS to survive as the merger remnant of GW170817. We find that for some stiff equations of state, the merger of GW170817 could, in principle, lead to a massive NS, which has a millisecond spin period. The post-merger GW signal could hardly constrain the ellipticity of the NS. If the ellipticity reaches 10 −3 , in order to be compatible with the multi-band EM observations, the dipole magnetic field of the NS (B p ) is constrained to the magnetar level of ∼ 10 14 G. If the ellipticity is smaller than 10 −4 , B p is constrained to the level of ∼ 10 10 − 10 12 G. These conclusions weakly depend on the adoption of equations of state.
The post-merger product of the first binary neutron star merger event detected in gravitational waves, GW170817, depends on neutron star equation of state (EoS) and is not well determined. We generally discuss the constraints one may pose on the maximum mass of a non-spinning neutron star, M TOV , based on the observations and some EoS-independent universal relations of rapidly-spinning neutron stars. If the merger product is a black hole after a brief hypermassive neutron star (HMNS) phase, we derive M TOV < 2.09 +0.06 −0.04 M ⊙ (2.09 +0.11 −0.09 M ⊙ ) at the 1σ (2σ) level. The cases for a massive neutron star (MNS), either a supra-massive neutron star (SMNS) or even a stable neutron star (SNS), are also allowed by the data. We derive 2.09 +0.06 −0.04 M ⊙ (2.09 +0.11 −0.09 M ⊙ ) ≤ M TOV < 2.43 +0.06 −0.04 M ⊙ (2.43 +0.10 −0.08 M ⊙ ) for the SMNS case and M TOV ≥ 2.43 +0.06 −0.04 M ⊙ (2.43 +0.10 −0.08 M ⊙ ) for the SNS case, at the 1σ (2σ) confidence level. In the MNS cases, we also discuss the constraints on the neutron star parameters (the dipolar magnetic field strength at the surface B p and the ellipticity ǫ) that affect the spindown history, by considering different MNS survival times, e.g. 300 s, 1 d, and 155 d after the merger, as suggested by various observational arguments. We find that once an SMNS is formed, without violating the EM observational constraints, there always exist a set of (B p , ǫ) parameters that allow the SMNS to survive for 300s, 1 d, 155 d, or even longer.
Some recent findings have shown that the duration of gamma-ray burst (GRB), although crucially related to the GRB central engine time scale, is not determinative in inferring the GRB origins in terms of their progenitors. In this paper, we report a peculiarly long-duration gamma-ray burst, GRB 211211A, that is associated with a kilonova in optical and near-infrared bands and is therefore likely the result of a binary neutron star merger. The burst broadly resembles the properties of GRB 060614 but with a much higher brightness in its light curve and harder spectra in both the main and extended emission phases, making it difficult to be explained as a short GRB with soft extended emission. Such a genuinely long-duration GRB suggests that merger product is likely a magnetar, which powers up the burst through magnetic and rotation energy for at least ∼ 70 seconds.
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