Dear Colleagues,

Neutron stars (NSs), stellar corpses born from the core collapse of massive stars (~10 M_sun) which underwent supernova (SN) explosion, are probably the stars with the most unique properties in the Universe, letting black holes (BHs) aside, where physics laws can be tested at their extreme. Overall, NSs are endowed with the highest magnetic fields (up to 10^13-14 G), with a mass of about 1.4 M_sun compressed in a radius of about 12 km they have supra-nuclear densities of 10¹⁴ g cm^-3, they have the highest surface temperatures, up to a few million degrees, and they have the highest spatial velocity, ~400 km s^-1 on average, resulting from the natal kick imparted by the SN explosion. Therefore, NSs are ideal cosmic laboratories to perform a number of studies in fundamental physics unattainable on Earth, e.g., the radiation emission processes under extreme magnetic fields, the behavior of matter at supra-nuclear densities and its equation of state (EoS), the cooling of this hyper-dense matter and its composition, general relativity (GR) effects in extreme gravitational fields, gravitational wave (GW) emission in hyper-dense, fast-rotating (periods as low as few ms), objects, and the confinement of relativistic charged particles injected in the interstellar medium (ISM) from a source moving at supersonic velocities.

In the last 20 years, important progress has been made in the study of NSs. An energy range was discovered using radio over 50 years ago; most NSs today are still detected as radio pulsars (~3000). Observations made using space and ground-based telescopes have consolidated the view of NSs as multi-wavelength emitters, from the sub mm to very-high-energy gamma rays, with, e.g., about 300 of them identified as gamma-ray pulsars by the Fermi Gamma-ray Space Telescope. This harvest of data now paves the way to unprecedented studies of the emission from the NS magnetosphere, possibly yielding to a unified picture, and of the thermal emission from the NS surface, exploiting the synergy between X-ray and ultraviolet (UV) observations. At the same time, radio/optical observations of pulsars in binary systems led to very accurate measurements of the NS masses, breaking the paradigm of the assumed value of 1.4 M_sun and indicating that NSs as massive as ~2 M_sun indeed exist, suggesting that pulsars in binary systems which might have undergone an accretion phase and spin-up from the companion star, hence dubbed ms-pulsars, span a different mass range wrt. Isolated NSs. Wherever this is due to accretion alone or to a different evolutionary path of the pulsar progenitor, it has outstanding implications on the determination of the NS EoS.

The aim of this Special Issue is to set the state of the art of neutron star astrophysics, with particular emphasis on the progress accomplished in the last 20 years, and to discuss future challenges in this field. Review articles and research articles are equally welcome.

Selected topics include (but not only):

• Emission from the NS magnetosphere;
• NS cooling;
• NS polarimetry and QED;
• The NS variety and the NS census (isolated, binary, magnetars, etc.);
• NS masses and radii;
• NS EoS;
• NS and general relativity;
• Future challenges and goals;
• Observations with future facilities.

Prof. Dr. Roberto Mignani
Dr. Massimiliano Razzano
Prof. Dr. Sergei B. Popov
Guest Editors

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• emission from the NS magnetosphere
• NS cooling
• NS polarimetry and QED
• the NS variety and the NS census (isolated, binary, magnetars, etc.)
• NS masses and radii
• NS EoS
• NS and general relativity
• future challenges and goals
• observations with future facilities.