A quark-nova is the theorized violent explosion resulting from the conversion of a neutron star to a quark star. Analogous to a supernova heralding the birth of a neutron star, a quark nova signals the creation of a quark star. The concept of quark-novae was suggested by Dr. Rachid Ouyed (University of Calgary, Canada) and Drs. Dey and Dey (Calcutta University, India). An animation describing the idea behind the quark-nova can be found here.
When a neutron star spins down, it may convert to a quark star through a process known as quark deconfinement. The resultant star would have quark matter in its interior. The process would release immense amounts of energy, perhaps explaining the most energetic explosions in the universe; calculations have estimated that as much as 1047 J could be released from the phase transition inside a neutron star. Quark-novae may be one cause of gamma ray bursts. According to Jaikumar et al., they may also be involved in producing heavy elements such as platinum through r-process nucleosynthesis.
Rapidly spinning neutron stars with masses between 1.5 and 1.8 solar masses are theoretically the best candidates for conversion due to spin down of the star within a Hubble time. This amounts to a small fraction of the projected neutron star population. A conservative estimate based on this indicates that up to two quark-novae may occur in the observable universe each day.