Close Binary Progenitors of Long Gamma Ray Bursts

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the LGRBs with duration in excess of 100 seconds as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 10,000 seconds. These data suggest that some other, most likely magnetic mechanisms have to be considered. The magnetic models do not require the development of accretion disk within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion disks in the collapsar model and investigate the available range of mass accretion rates, black hole masses, and spins. A particularly interesting version of the binary progenitor involves merger of a WR star with an ultra-compact companion, neutron star or black hole. In this case we expect the formation of very long-lived accretion disks, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation.

💡 Research Summary

The paper revisits the central‑engine problem of long‑duration gamma‑ray bursts (LGRBs) in light of recent Swift observations that reveal activity lasting up to 10⁴ s, shallow‑decay phases, and X‑ray flares separated from the main prompt emission by hundreds of seconds. The authors argue that the traditional neutrino‑annihilation (ν ν̄) mechanism, which powers jets only when the mass‑accretion rate (\dot M) exceeds ∼0.1 M⊙ s⁻¹, cannot accommodate such long‑lasting or delayed emission because the required (\dot M) would imply unrealistically massive progenitors (≫10² M⊙). Moreover, the ν ν̄ efficiency drops sharply with decreasing (\dot M) and with lower black‑hole spin, making it unsuitable for bursts with precursors separated by hundreds of seconds.

Consequently, the authors turn to magnetic mechanisms, especially the Blandford‑Znajek (BZ) process, which extracts rotational energy from a spinning black hole via magnetic fields anchored in an accretion disk. Unlike ν ν̄ annihilation, the BZ power scales as (P_{\rm BZ}\propto a^{2}\Phi_{\rm BH}^{2}) and is far less sensitive to (\dot M). This property makes it a natural candidate for powering both the prompt emission and the prolonged central‑engine activity observed by Swift.

A central theme of the paper is the exploration of close high‑mass binary systems as progenitors. The authors focus on a scenario where a Wolf‑Rayet (WR) star is in a tight orbit with an ultra‑compact companion—either a neutron star (NS) or a black hole (BH). Tidal synchronization forces the WR star to rotate with the orbital angular velocity (\Omega_{s}=\sqrt{G M_{s}(1+q)/L^{3}}), where (q) is the companion‑to‑star mass ratio and (L) the orbital separation. By requiring that the specific angular momentum at the stellar equator exceed that of the marginally bound circular orbit around the nascent black hole, they derive a disk‑formation condition that translates into a maximum orbital period of roughly 48 h for typical WR parameters (mass ∼10 M⊙, radius ∼1 R⊙). This limit is about five times larger than earlier estimates that demanded an immediate disk after iron‑core collapse, thereby allowing the disk to form later when (\dot M) has already declined.

The authors then estimate the black‑hole mass and spin at the moment the disk appears. Because a substantial fraction of the WR star’s mass collapses directly, the black hole typically exceeds 2–3 M⊙ and can be as massive as half the pre‑supernova star. The spin parameter (a) is moderate (0.4–0.8), sufficient for an efficient BZ engine provided the magnetic flux threading the horizon exceeds ∼10²⁸ G cm². Their semi‑analytic treatment, supplemented by numerical simulations, confirms that such fluxes are required for magnetically driven explosions, though the origin of such strong fields in stellar interiors remains uncertain.

Two distinct emission episodes naturally arise in this framework. The first, a “precursor,” can be produced by the supernova shock powered by a magnetar‑like wind or a magnetically driven explosion that clears a low‑density channel. The second, the main prompt pulse, follows the fallback of material onto the newly formed black hole, where the BZ mechanism launches a relativistic jet. Because the fallback accretion rate ((10^{-2}–10^{-3},M_{\odot},{\rm s}^{-1})) is too low for ν ν̄ annihilation, the main pulse must be magnetic in origin, consistent with observations of long delays between precursor and main emission.

The paper also examines the extreme case of a direct merger between a WR star and its compact companion. In such a merger, a massive, long‑lived accretion disk can form, persisting for thousands of seconds. This prolonged disk supplies the magnetic flux needed for a sustained BZ jet, offering a natural explanation for the shallow‑decay phase and late‑time X‑ray flares seen in many Swift LGRBs.

Finally, the authors discuss single‑star models that invoke chemically homogeneous evolution at low metallicity, which can retain rapid core rotation without binary interaction. While these models also produce fast‑spinning black holes, the binary channel presented here relaxes the stringent requirement of an initially rapidly rotating core, expanding the viable progenitor population.

In summary, the study proposes that close binary systems—particularly WR + compact‑object configurations—can generate long‑lived, magnetically powered central engines capable of reproducing the full temporal complexity of LGRBs observed by Swift. By allowing accretion disks to form later in the collapse, the model accommodates lower mass‑accretion rates while still delivering sufficient magnetic flux for an efficient Blandford‑Znajek jet. This work broadens the landscape of plausible LGRB progenitors beyond the classic collapsar paradigm and highlights the importance of magnetic processes in shaping gamma‑ray burst phenomenology.