Phase transitions in neutron star and magnetars and their connection with high energetic bursts in astrophysics

The phase transition from normal hadronic matter to quark matter in neutron stars (NS) could give rise to several interesting phenomena. Compact stars can have such exotic states up to their surface (called strange stars (SS)) or they can have quark core surrounded by hadronic matter, known as hybrid stars (HS). As the state of matter of the resultant SS/HS is different from the initial hadronic matter, their masses also differ. Therefore, such conversion leads to huge energy release, sometimes of the order of $10^{53}$ ergs. In the present work we study the qualitative energy released by such conversion. Recent observations reveal huge surface magnetic field in certain stars, termed magnetars. Such huge magnetic fields can modify the equations of state (EOS) of the matter describing the star. Therefore, the mass of magnetars are different from normal NS. The energy released during the conversion process from neutron magnetar (NM) to strange magnetar/hybrid magnetar (SS/HS) is different from normal NS to SS/HS conversion. In this work we calculate the energy release during the phase transition in magnetars. The energy released during NS to SS/HS conversion exceeds the energy released during NM to SM/HM conversion. The energy released during the conversion of NS to SS is always of the order of $10^{53}$ ergs. The amount of energy released during such conversion can only be compared to the energy observed during the gamma ray bursts (GRB). The energy liberated during NM to HM conversion is few times lesser, and is not likely to power GRB at cosmological distances. However, the magnetars are more likely to lose their energy from the magnetic poles and can produce giant flares, which are usually associated with magnetars.

💡 Research Summary

The paper investigates the energetic consequences of a phase transition from normal hadronic matter to deconfined quark matter inside compact stars, focusing on both ordinary neutron stars (NS) and highly magnetized neutron stars (magnetars, NM). Two possible end‑states are considered: a completely converted strange star (SS) and a hybrid star (HS) that retains a hadronic mantle surrounding a quark core. The authors adopt representative equations of state (EOS) for hadronic and quark phases, and they estimate the released energy by calculating the difference in gravitational mass before and after conversion, ΔE ≈ ΔM c². For a non‑magnetized NS converting to an SS, the mass loss is of order 0.1 M⊙, yielding an energy release around 10⁵³ erg—comparable to the isotropic energy of cosmological gamma‑ray bursts (GRBs). The conversion to an HS releases somewhat less energy but still remains in the 10⁵²–10⁵³ erg range.

The study then incorporates the effect of ultra‑strong magnetic fields (10¹⁴–10¹⁵ G) typical of magnetars. Such fields modify the EOS through magnetic pressure and anisotropic stresses, effectively stiffening the matter and altering the mass–radius relation. When a magnetar undergoes a transition to a strange magnetar (SM) or hybrid magnetar (HM), the mass loss is reduced to roughly 0.03–0.05 M⊙, and the corresponding energy release drops to a few ×10⁵² erg. This is an order of magnitude lower than the NS→SS case, making it unlikely to power a distant GRB on its own.

However, the authors argue that magnetars possess an additional channel for energy release: magnetic reconnection and re‑configuration at the poles during the transition. The sudden rearrangement of the magnetic field can trigger giant flares with energies of 10⁴⁴–10⁴⁶ erg, consistent with observed magnetar bursts. Moreover, the structural readjustment changes the star’s moment of inertia, potentially producing observable glitches or rapid spin‑down in the pulsar timing data immediately after the transition. The paper also mentions that copious neutrino and neutron emission during the conversion could interact with surrounding material, possibly accelerating particles to ultra‑high energies.

In summary, the conversion of an ordinary NS to an SS or HS can liberate ∼10⁵³ erg, sufficient to explain the most energetic GRBs, whereas the analogous conversion of a magnetar yields somewhat less energy, insufficient for cosmological GRBs but compatible with magnetar giant flares when magnetic reconnection is taken into account. The work highlights the importance of including magnetic‑field effects in EOS modeling and suggests that timing irregularities, neutrino signals, and flare observations could serve as indirect probes of such phase transitions in compact stars.