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Register a new userThe lowest-mass stellar black holes: catastrophic death of neutron stars in gamma-ray bursts
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Added by
Krzysztof Belczynski
Publication date
2008
fields
Physics
and research's language is
English
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Mergers of double neutron stars are considered the most likely progenitors for short gamma-ray bursts. Indeed such a merger can produce a black hole with a transient accreting torus of nuclear matter (Lee & Ramirez-Ruiz 2007, Oechslin & Janka 2006), and the conversion of a fraction of the torus mass-energy to radiation can power a gamma-ray burst (Nakar 2006). Using available binary pulsar observations supported by our extensive evolutionary calculations of double neutron star formation, we demonstrate that the fraction of mergers that can form a black hole -- torus system depends very sensitively on the (largely unknown) maximum neutron star mass. We show that the available observations and models put a very stringent constraint on this maximum mass under the assumption that a black hole formation is required to produce a short gamma-ray burst in a double neutron star merger. Specifically, we find that the maximum neutron star mass must be within 2 - 2.5 Msun. Moreover, a single unambiguous measurement of a neutron star mass above 2.5 Msun would exclude a black hole -- torus central engine model of short gamma-ray bursts in double neutron star mergers. Such an observation would also indicate that if in fact short gamma-ray bursts are connected to neutron star mergers, the gamma-ray burst engine is best explained by the lesser known model invoking a highly magnetized massive neutron star (e.g., Usov 1992; Kluzniak & Ruderman 1998; Dai et al. 2006; Metzger, Quataert & Thompson 2007).
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Gamma-ray bursts are associated with catastrophic cosmic events. They appear when a new black hole, created after the explosion of a massive star or the merger of two compact stars, quickly accretes the matter around it and ejects a transient relativistic jet in our direction. This review discusses the various types of gamma-ray bursts, their progenitors, their beaming and their rate in the local universe. We emphasize the broad astrophysical interest of GRB studies, and the crucial role of high-energy satellites as exclusive suppliers of GRB alerts and initial locations.
By means of three-dimensional hydrodynamic simulations with a Eulerian PPM code we investigate the formation and the properties of the accretion torus around the stellar mass black hole which originates from the merging of two neutron stars. The simulations are performed with four nested cartesian grids which allow for both a good resolution near the central black hole and a large computational volume. They include the use of a physical equation of state as well as the neutrino emission from the hot matter of the torus. The gravity of the black hole is described with a Newtonian and alternatively with a Paczynski-Wiita potential. In a post-processing step, we evaluate our models for the energy deposition by nu-nubar annihilation around the accretion torus. Our models show that nu-nubar annihilation can yield the energy to account for weak, short gamma-ray bursts, if moderate beaming is involved. In fact, the barrier of the dense baryonic gas of the torus suggests that the low-density pair-photon-plasma is beamed as axial jets into a fraction 2 delta Omega/ (4 pi) between 1/100 and 1/10 of the sky, corresponding to opening half-angles of roughly ten to several tens of degrees. Thus gamma-burst energies of 10^{50}--10^{51} erg seem within the reach of our models (if the source is interpreted as radiating isotropically), corresponding to luminosities around 10^{51} erg/s for typical burst durations of 0.1--1 s. Gravitational capture of radiation by the black hole, redshift and ray bending do not reduce the jet energy significantly. Effects associated with the Kerr character of the rapidly rotating black hole, however, could increase the gamma-burst energy considerably, and effects due to magnetic fields might even be required to get the energies for long complex gamma-ray bursts.
LOFAR, the Low Frequency Array, is an innovative new radio telescope currently under construction in the Netherlands. With its continuous monitoring of the radio sky we expect LOFAR will detect many new transient events, including GRB afterglows and pulsating/single-burst neutron stars. We here describe all-sky surveys ranging from a time resolution of microseconds to a cadence span of years.
Neutron stars can be destroyed by black holes at their center accreting material and eventually swallowing the entire star. Here we note that the accretion model adopted in the literature, based on Bondi accretion or variations thereof, is inadequate for small black holes -- black holes whose Schwarzschild radius is comparable to, or smaller than, the neutrons de Broglie wavelength. In this case, quantum mechanical aspects of the accretion process cannot be neglected, and give rise to a completely different accretion rate. We show that for the case of black holes seeded by the collapse of bosonic dark matter, this is the case for electroweak-scale dark matter particles. In the case of fermionic dark matter, typically the black holes that would form at the center of a neutron star are more massive, unless the dark matter particle mass is very large, larger than about 10$^{10}$ GeV. We calculate the lifetime of neutron stars harboring a small black hole, and find that black holes lighter than $sim 10^{11}$ kg quickly evaporate, leaving no trace. More massive black holes destroy neutron stars via quantum accretion on time-scales much shorter than the age of observed neutron stars.
If primordial black holes with masses of $10^{25},mbox{g}gtrsim m gtrsim 10^{17},mbox{g}$ constitute a non-negligible fraction of the galactic dark-matter haloes, their existence should have observable consequences: they necessarily collide with galactic neutron stars, nest in their centers and accrete the dense matter, eventually converting them to neutron-star mass black holes while releasing the neutron-star magnetic field energy. Such processes may explain the fast radio bursts phenomenology, in particular their millisecond durations, large luminosities ${sim}10^{43}$ erg/s, high rate of occurrence $gtrsim 1000/mbox{day}$, as well as high brightness temperatures, polarized emission and Faraday rotation. Longer than the dynamical timescale of the Bondi-like accretion for light primordial black holes allows for the repeating fast radio bursts. This explanation follows naturally from (assumed) existence of the dark matter primordial black holes and requires no additional unusual phenomena, in particular no unacceptably large magnetic fields of neutron stars. In our model, the observed rate of fast radio bursts throughout the Universe follows from the presently known number of neutron stars in the Galaxy.
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