Resonant production of gamma rays in jolted cold neutron stars

Acoustic shock waves passing through colliding cold neutron stars can cause repetitive superconducting phase transitions in which the proton condensate relaxes to its equilibrium value via coherent oscillations. As a result, a resonant non-thermal production of gamma rays in the MeV energy range with power up to 10 erg/s can take place during the short period of time before the nuclear matter is heated by the shock waves. CERN-TH/98-125 April, 1998 ∗ email address: [email protected] The recent discovery of the afterglow associated with the gamma ray bursts (GRB) [1] has provided convincing evidence that the GRB originate at redshift z ∼ 1 from an event that releases of order 10 erg in photons (assuming the spherical symmetry of emission). A number of theoretical attempts to explain the phenomenon invoked a small energetic fireball [2]. Although many astrophysical events, for instance, neutron star collisions, can release the required amount of energy, it is extremely difficult to reconcile the optical thickness of a fireball with the short duration and the high energy of the gamma ray burst it is supposed to produce. In this Letter we describe a new mechanism for the gamma-ray production by the colliding cold neutron stars prior to the eruption of a fireball. We comment on the possible connection of this phenomenon with the observed GRB. We will show that powerful acoustic shock waves passing through a neutron star can cause repetitive superconducting phase transitions during which the relaxation of the order parameter is accompanied by a resonant non-thermal production of photons with energies of order MeV and the total power up to 10 erg s. These gamma-rays are copiously produced due to a coherent motion of the superconducting proton condensate. Such oscillations can be powered by the density waves generated, for example, in the collision of two neutron stars. The production of gamma rays continues until the medium is heated to the temperature comparable to Tc, the superconducting phase transition temperature. When two neutron stars collide, the kinetic energy is transfered into the shock waves that propagate through the nuclear matter and eventually dissipate their energy into heat. The density variations in a relativistic shock wave are much slower than the time scale of nuclear interactions. The interior of the neutron star is a superconductor because it contains the superfluid proton condensate [3]. An acoustic wave excited by the initial impact of a collision, or by some precursory tidal effects, causes large variations of density in some macroscopic domains of the size of order the wavelength. The proton energy gap depends on the density [4] as shown in Fig. 1; the shock wave can drive it to zero in some formerly superconducting regions, or vice versa. Suppose a surge (or an ebb) of density