Effects of Fallback Accretion on Protomagnetar Outflows in Gamma-Ray Bursts and Superluminous Supernovae

Abstract

Rapidly spinning, strongly magnetized proto-neutron stars ("millisecond proto-magnetars") are candidate central engines of long-duration gamma-ray bursts (GRB), superluminous supernovae (SLSNe), and binary neutron star mergers. Magnetar birth may be accompanied by the fall-back of stellar debris, lasting for seconds or longer following the explosion. Accretion alters the magnetar evolution by (1) providing an additional source of rotational energy (or a potential sink, if the propeller mechanism operates); (2) enhancing the spin-down luminosity above the dipole rate by compressing the magnetosphere and expanding the polar cap region of open magnetic field lines; (3) supplying an additional accretion-powered neutrino luminosity that sustains the wind baryon-loading, even after the magnetar's internal neutrino luminosity has subsided. The more complex evolution of the jet power and magnetization of an accreting magnetar more readily accounts for the high 56 Ni yields GRB SNe and irregular time evolution of some GRB light curves (e.g. bursts with precursors followed by a long quiescent interval before the main emission episode). Additional baryon-loading from accretion-powered neurino irradiation of the polar cap lengthens the timeframe over which the jet magnetization is in the requisite range σ 10 3 for efficient gamma-ray emission, thereby accommodating GRBs with ultra-long durations. Though accretion does not significantly raise the maximum energy budget from the limit of f ew × 10 52 ergs for an isolated magnetar, it greatly expands the range of magnetic field strengths and birth spin periods capable of powering GRB jets, reducing the differences between the magnetar properties normally invoked to explain GRBs versus SLSNe.

Formation rates and evolution histories of magnetars

“…The initial magnetic field of the magnetars at birth can be constrained to the range B 0 = 3 × 10 14 − 10 15 G (see also Gullón et al 2015). This is consistent with magnetic field values that are required for millisecond magnetars to power superluminous supernovae (SLSNe; 5 https://swift.gsfc.nasa.gov/archive/grb_table/ Metzger et al 2015Metzger et al , 2018 and gamma-ray bursts (GRBs; Beniamini et al 2017;Metzger et al 2018). The other important ingredient for powering those energetic explosions is the initial spin of the magnetar, Ω 0 .…”

On the Properties of a Newborn Magnetar Powering the X-Ray Transient CDF-S XT2

“…Since the deduced braking index n < 3, besides the magnetic dipole torque, another braking mechanism should play an important role. For instance, fall-back accretion onto the magnetar could lead to n < 3 (Metzger et al 2018). This braking index is not surprising as a systematic study of a large sample of GRBs (long and short) with X-ray plateaus also suggested n significantly smaller than 3 (Stratta et al 2018).…”

Magnetars from Neutron Star–White Dwarf Mergers: Application to Fast Radio Bursts

“…where Ω K = GM/R 3 1/2 ; (2) N dip is the magnetic dipole radiation torque for accreting NSs with r m < r lc , enhanced over the standard dipole torque by a factor of (r lc /r m ) 2 > 1 due to the enhanced open magnetic field lines via the compression of the magnetosphere (Parfrey et al 2016;Metzger et al 2018).…”