Small radii of neutron stars as an indication of novel in-medium effects

Abstract

At present, neutron star radii from both observations and model predictions remain very uncertain. Whereas different models can predict a wide range of neutron star radii, it is not possible for most models to predict radii that are smaller than about 10 km, thus if such small radii are established in the future they will be very difficult to reconcile with model estimates. By invoking a new term in the equation of state that enhances the energy density, but leaves the pressure unchanged we simulate the current uncertainty in the neutron star radii. This new term can be possibly due to the exchange of the weakly interacting light U-boson with appropriate in-medium parameters, which does not compromise the success of the conventional nuclear models. The validity of this new scheme will be tested eventually by more precise measurements of neutron star radii. 14.70.Pw, 97.60.Jd Neutron star (NS) is a unique place to test fundamental forces at the extremes of matter density, gravity and magnetic fields. Unfortunately, uncertainties in both the Equation of State (EOS) of super-dense nuclear matter and the strong-field gravity strongly interplay with each other in determining observational properties of neutron stars, for the latest review, see, e.g., [1]. For instance, in a simple version of modified gravity where the non-Newtonian gravity exists, neutron stars could have very different structures compared to predictions using Einstein's General Relativity (GR) theory of gravity [2][3][4]. The radius of a neutron star is one of the most important observables sensitive to the underlying nuclear EOS and gravity theories used. Currently, within GR the radius of a canonical NS has been predicted to be roughly from 11 to 15 km [5-9] depending on the EOS used. Provided the third family of compact stars known as strange stars exist, their radii could be as small as 7 or 8 km [10][11][12], although these models normally predict star masses much smaller than the masses of observed massive neutron stars. Thus, the measurement of NS radii plays a very important role in resolving several issues in fundamental physics. Unfortunately, the extraction of NS radii from observations still suffers from large systematic uncertainties [13] involved in the distance measurements and theoretical analyses of the light spectrum [6,[14][15][16]. Consequently, a wide range of the radius with the mass around 1.4 M ⊙ has been reported [16][17][18][19][20][21][22][23][24]. In particular, using the thermal spectra from quiescent low-mass X-ray binaries (qLMXBs) Guillot and collaborators extracted NS radii of R NS = 9.4 ± 1.2 km [24]. Another recent comprehensive study of spectroscopic radius measurements suggest that for a 1.5 M ⊙ NS the extracted * Electronic address: wzjiang@seu.edu.cn † Electronic address: bao-an.li@tamuc.edu ‡ Electronic address: ffattoye@indiana.edu radii are 10.8 +0.5 −0.4 km [25]. It is important to note that at the moment no consensus has been reached yet on the extracted NS radii. For instance, Bogdanov found a 3-σ lowe...

Equation of state and hybrid star properties with the weakly interacting light U-boson in relativistic models

“…We note that these estimated orders of magnitude for the U-boson parameters can be compatible with parameter regions allowed by a few experimental constraints [36]. Moreover, the density-dependent parameter of the U-boson is found to be consistent with the usual predictions on the NS radius, while the density dependence would originate from the in-medium effect in the nuclear many-body system [98,100,101].…”

“…The density-dependent coupling constant, depicted in the inset of Fig. 6, is designed to little change the pressure in the low-density regime [98]. While the energy density still acquires a significant increase from the U-boson, the resulting NS radius is even less than the one obtained with the pure hadron phase EOS.…”

Equation of State for Nucleonic and Hyperonic Neutron Stars With Mass and Radius Constraints

“…Many of the current nuclear models for the EoS are able to satisfy the 2 M e constraint required by the discovery of massive neutron stars (Demorest et al 2010;Antoniadis et al 2013). However, the possible existence of neutron stars with small radii suggested by recent astrophysical analyses (Guillot et al 2013;Guver & Ozel 2013;Guillot & Rutledge 2014;Heinke et al 2014;Lattimer & Steiner 2014;Lattimer & Prakash 2016;Ozel & Freire 2016;) poses a difficult challenge to most of the nuclear models (Chen & Piekarewicz 2015a;Dexheimer et al 2015;Jiang et al 2015;Ozel & Freire 2016).…”

“…Moreover, the requirement of maximum masses of 2 M e does not allow a significant reduction of the total pressure. Indeed, very few models seem to exist that can meet both constraints (small radius and large mass) simultaneously, and fewer such models can in addition render accurate descriptions of the finite nuclei properties (Horowitz & Piekarewicz 2001a, 2001bChen & Piekarewicz 2015a;Jiang et al 2015;Sharma et al 2015).…”

Probing the low and high density nuclear matter by hadron scattering

“…Hadron modification has been one of the most debated topics in nuclear and particle physics conferences and attracted a great attention of scientists in different branches of particle, nuclear physics and astrophysics [5,6]. …”