Bogomol'nyi equations for Dirac-Born-Infeld cosmic string
We revisit the question of whether Dirac-Born-Infeld (DBI) cosmic strings can admit Bogomol'nyi-Prasad-Sommerfield (BPS) configurations. Earlier work by Babichev et al. arXiv:0809.2013 concluded that DBI strings with the standard Mexican-hat potential possess no BPS limit, implying an unavoidable nonzero binding energy. In contrast, using the BPS Lagrangian method, we show that DBI strings do admit BPS solutions, provided the potential is chosen self-consistently. Imposing the existence of Bogomol'nyi equations uniquely determines the admissible potential and yields exact first-order BPS equations for DBI vortices. We independently verify the consistency of these equations using the stressless (vanishing-pressure) condition on the energy-momentum tensor. The resulting solutions saturate the Bogomol'nyi bound, exhibit zero binding energy, and smoothly recover the Nielsen-Olesen string in the limit $α\to 0$. Regularity of the gauge-field equation requires $α< π^2$. A notable outcome of the construction is that the BPS-compatible potential takes a trigonometric form closely related to the sine-Gordon potential, revealing a natural correspondence between the sine-Gordon string and the BPS DBI string. The BPS tension scales linearly with the winding number $n$ but acquires an $α$-dependent deformation.