Collisions and jets in a Large Hadron Merger

... for numerical relativity and magnetohydrodynamics programmers' delight

The video above shows magnetohydrodynamic simulations in full general relativity (GRMHD) of the time evolution of the rest-mass density and magnetic fields for a binary black hole - neutron star on a quasicircular orbit that undergoes merger. The binary mass ratio is 3:1, the black hole has initial spin parameter a/m=0.75 (m is the black hole Christodoulou mass) aligned with the orbital angular momentum. The rest-mass density is normalized to the maximum density at t=0. Time is measured in M=2.5×10−2ms. The  initial neutron star is an irrotational Γ=2 polytrope (a Γ-law equation of state is adopted).  

About two orbits prior to merger (at time t=647M), the neutron star is seeded with a dynamically weak dipolar magnetic field B that extends into the stellar exterior. The current and radius of the loop generating B such that in the interior the maximum value of the ratio of magnetic to gas pressure β≡P_gas/P_mag is β−¹=0.05, which is dynamically weak and does not affect the evolution. The field in the exterior magnetosphere is strong, with β−¹≲100, and mimics the force-free conditions likely characterizing such a region.  

The neutron star spirals in towards the black hole until it is tidally disrupted, and then it forms a disk of matter that orbits the black hole. Varying β at t_B in the exterior from 0.1 to 0.01, one finds that at time ∼4000M∼100(M_NS/1.4M_⊙)ms following the onset of accretion of tidally disrupted debris, magnetic winding above the remnant black hole poles builds up the magnetic field sufficiently to launch a mildly relativistic collimated outflow - an incipient jet. The field lines above the poles form a helical wall which drives and confines the matter (speed displayed on frozen image at time t =5862M) in the jet. The luminosity of the source is ∼10⁵¹ergs/s and the lifetime of the jet is Δt∼0.5(M_NS/1.4M_⊙)s, values that are consistent with observations of short gamma ray bursts. The opening angle of the jet is 20 degrees. These simulations furnish the first explicit examples in GRMHD which show that a jet can emerge following a black hole - neutron star merger. 

Adapted from Relativistic simulations of black hole-neutron star coalescence: the jet emerges  Vasileios Paschalidis, Milton Ruiz, Stuart L. Shapiro 2014

... for hadron and astro-physicists happiness too!

The finding of Gravitational Waves (GW) by the aLIGO scientific and VIRGO collaborations opens opportunities to better test and understand strong interactions, both nuclear-hadronic and gravitational. Assuming General Relativity (GR) holds, one can constrain hadron physics at a neutron star. But precise knowledge of the Equation of State (EOS) and transport properties in hadron matter can also be used to constrain the theory of gravity itself. I review a couple of these opportunities in the context of modified f(R) gravity, the maximum mass of neutron stars, and progress in the Equation of State of neutron matter from the chiral effective field theory of QCD...

As long as one deals with black hole-black hole (BH-BH) collisions, the tests of GR do not require feedback from the Quantum Chromodynamics (QCD) community. But once neutron star-black hole (NS-BH) or NS-NS mergers are detected, this will change... one can extract the tidal deformability Λ ..., defined as the coefficient of proportionality, in linear response, of the induced quadrupole of a neutron star to the induced tidal stress due to its binary companion ... If the tidal deformability of a neutron star becomes known, it will be a challenge to theorists to calculate it from first principles and will serve as one more constraint on the neutron matter therein. Taking as reference an aLIGO detection rate of some 10 NS-BH mergers per year [14], the tidal deformability Λ may be constrained in order of magnitude from a single observed NS-BH merger [15] or to O(10%) from 25-50 observations combined, by studying the ratio of gravitational wave signals h_NS/BH / h_BH-BH, whose magnitude and phase can be simulated.

... generically thinking, the community has been constraining hadron EOS from neutron star data assuming the validity of General Relativity. The EOS is known in conventional nuclei, and the extrapolation (in density) needed for neutron star interiors is a factor of 2-5. However, the gravitational acceleration outside the star (where binary measurements have constrained GR directly), is g ∼300m/s²... and inside a neutron star, g ∼10¹²m/s². Going from the former to the latter requires extrapolating gravity over 10 ... orders of magnitude...; it seems more sensible to use computations in reverse gear, and put to use everything that is known about hadron physics (a much smaller extrapolation) for constraining gravity.

Constraining gravity with hadron physics: neutron stars, modified gravity and gravitational waves

Felipe J. Llanes-Estrada (Univ. Complutense de Madrid)

(Submitted on 11 Nov 2016 (v1), last revised 30 Nov 2016 (this version, v2))

 ... for high energy physicists' solace at last(?)

Fermi sees from where gamma ray bursts come

BlackGEM will look at their golden afterglows

LIGO and Virgo are three of a kind

They're looking for whatever spacetime vibes they can find

Gravity accelerates merging neutron stars

And all the waves they radiate will blow astronomers' sox off

New contingency lyrics improvised to a familiar tune (for an "LHCM* rap";-)

Thanks to the rise of multimessenger astronomy one can expect tremendous progress in astrophysics in the near future such that spacetime on large scales may become the new matter under high energy physics' scrutiny with standard theory fields as the probe of course...

* It is interesting to notice that the size of the LHC is the same (in order of magnitude) as a typical LHM !