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Register a new userNeutron stars in $f(mathtt{R,L_m})$ gravity with realistic equations of state: joint-constrains with GW170817, massive pulsars, and the PSR J0030+0451 mass-radius from ${it NICER}$ data
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Added by
Ronaldo Vieira Lobato
Publication date
2021
fields
Physics
and research's language is
English
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In this work we investigate neutron stars (NS) in $f(mathtt{R,L_m})$ theory of gravity for the case $f(mathtt{R,L_m}) = mathtt{R} + mathtt{L_m} + sigmamathtt{R}mathtt{L_m}$, where $mathtt{R}$ is the Ricci scalar and $mathtt{L_m}$ the Lagrangian matter density. In the term $sigmamathtt{R}mathtt{L_m}$, $sigma$ represents the coupling between the gravitational and particles fields. For the first time the hydrostatic equilibrium equations in the theory are solved considering realistic equations of state and NS masses and radii obtained are subject to joint constrains from massive pulsars, the gravitational wave event GW170817 and from the PSR J0030+0451 mass-radius from NASAs Neutron Star Interior Composition Explorer (${it NICER}$) data. We show that in this theory of gravity, the mass-radius results can accommodate massive pulsars, while the general theory of relativity can hardly do it. The theory also can explain the observed NS within the radius region constrained by the GW170817 and PSR J0030+0451 observations for masses around $1.4~M_{odot}$.
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In this work we investigate neutron stars (NS) in $f(mathcal{R,T})$ gravity for the case $R+2lambdamathcal{T}$, $mathcal{R}$ is the Ricci scalar and $mathcal{T}$ the trace of the energy-momentum tensor. The hydrostatic equilibrium equations are solved considering realistic equations of state (EsoS). The NS masses and radii obtained are subject to a joint constrain from massive pulsars and the event GW170817. The parameter $lambda$ needs to be negative as in previous NS studies, however we found a minimum value for it. The value should be $|lambda|lesssim0.02$ and the reason for so small value in comparison with previous ones obtained with simpler EsoS is due to the existence of the NS crust. The pressure in theory of gravity depends on the inverse of the sound velocity $v_s$. Since, $v_s$ is low in the crust, $|lambda|$ need to be very small. We found that the increment in the star mass is less than $1%$, much smaller than previous ones obtained not considering the realistic stellar structure, and the star radius cannot become larger, its changes compared to GR is less than $3.6%$ in all cases. The finding that using several relativistic and non-relativistic models the variation on the NS mass and radius are almost the same for all the EsoS, manifests that our results are insensitive to the high density part of the EsoS. It confirms that stellar mass and radii changes depend only on crust, where the EoS is essentially the same for all the models. The NS crust effect implying very small values of $|lambda|$ does not depend on the theorys function chosen, since for any other one the hydrostatic equilibrium equation would always have the dependence $1/v_s$. Finally, we highlight that our results indicate that conclusions obtained from NS studies done in modified theories of gravity without using realistic EsoS that describe correctly the NS interior can be unreliable.
Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists, because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state of this dense matter is to measure both a stars equatorial circumferential radius $R_e$ and its gravitational mass $M$. Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Explorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are $R_e = 13.02^{+1.24}_{-1.06}$ km and $M = 1.44^{+0.15}_{-0.14} M_odot$ (68%). The independent analysis reported in the companion paper by Riley et al. (2019) explores different emitting spot models, but finds spot shapes and locations and estimates of $R_e$ and $M$ that are consistent with those found in this work. We show that our measurements of $R_e$ and $M$ for PSR J0030$+$0451 improve the astrophysical constraints on the equation of state of cold, catalyzed matter above nuclear saturation density.
Very recently the NICER collaboration has published the first-ever accurate measurement of mass and radius together for PSR J0030+0451, a nearby isolated quickly-rotating neutron star (NS). In this work we set the joint constraints on the equation of state (EoS) and some bulk properties of NSs with the data of PSR J0030+0451, GW170817 and some nuclear experiments. The piecewise polytropic expansion method and the spectral decomposition method have been adopted to parameterize the EoS. The resulting constraints are consistent with each other. Assuming the maximal gravitational mass of non-rotating NS $M_{rm TOV}$ lies between $2.04 rm M_{odot}$ and $2.4 rm M_{odot}$, with the piecewise method the pressure at twice nuclear saturation density is measured to be $3.38^{+2.43}_{-1.50}times 10^{34}~{rm dyn~cm^{-2}}$ at the $90%$ level. For a NS with canonical mass of $1.4 rm M_odot$, we have the moment of inertia $I_{1.4} = {1.43}^{+0.28}_{-0.13} times 10^{38}~{rm kg cdot m^2}$, tidal deformability $Lambda_{1.4} = 390_{-140}^{+320}$, radius $R_{1.4} = 12.2_{-0.9}^{+1.0}~{rm km}$, and binding energy $BE_{1.4} = {0.16}^{+0.01}_{-0.02} rm M_{odot}$ at the $90%$ level, which are improved in comparison to the constraints with the sole data of GW170817.
We investigate the nonrotating neutron stars in $f(T)$ gravity with $f(T)=T+alpha T^2$, where $T$ is the torsion scalar in the teleparallel formalism of gravity. In particular, we utilize the SLy and BSk family of equations of state for perfect fluid to describe the neutron stellar matter and search for the effects of the $f(T)$ modification on the models of neutron stars. For positive $alpha$, the modification results in a stronger gravitation exerted on the stellar matter, leading to a smaller stellar mass in comparison to general relativity. Moreover, there seems to be an upper limit for the central density of the neutron stars with $alpha>0$, beyond which the effective $f(T)$ fluid would have a steplike phase transition in density and pressure profiles, collapsing the numerical system. For negative $alpha$, the $f(T)$ modification provides additional support for neutron stars to contain larger amount of matter. We obtain the mass-radius relations of the realistic models of neutron stars and subject them to the joint constraints from the observed massive pulsars PSR J0030+0451, PSR J0740+6620, and PSR J2215+5135, and gravitational wave events GW170817 and GW190814. For BSk19 equation of state, the neutron star model in $f(T)$ gravity can accommodate all the mentioned data when $alphale 3.5 G^2M_odot^2/c^4$. For BSk20, BSk21 and SLy equations of state, the observational data constrain the model parameter $alpha$ to be negative. If one considers the unknown compact object in the event GW190814 not to be a neutron star and hence excludes this dataset, the constraints for BSk20 and BSk21 models can be loosened to $alphale 0.4 G^2M_odot^2/c^4$ and $alphale 1.9 G^2M_odot^2/c^4$, respectively.
Both the mass and radius of the millisecond pulsar PSR J0030+0451 have been inferred via pulse-profile modeling of X-ray data obtained by NASAs NICER mission. In this Letter we study the implications of the mass-radius inference reported for this source by Riley et al. (2019) for the dense matter equation of state (EOS), in the context of prior information from nuclear physics at low densities. Using a Bayesian framework we infer central densities and EOS properties for two choices of high-density extensions: a piecewise-polytropic model and a model based on assumptions of the speed of sound in dense matter. Around nuclear saturation density these extensions are matched to an EOS uncertainty band obtained from calculations based on chiral effective field theory interactions, which provide a realistic description of atomic nuclei as well as empirical nuclear matter properties within uncertainties. We further constrain EOS expectations with input from the current highest measured pulsar mass; together, these constraints offer a narrow Bayesian prior informed by theory as well as laboratory and astrophysical measurements. The NICER mass-radius likelihood function derived by Riley et al. (2019) using pulse-profile modeling is consistent with the highest-density region of this prior. The present relatively large uncertainties on mass and radius for PSR J0030+0451 offer, however, only a weak posterior information gain over the prior. We explore the sensitivity to the inferred geometry of the heated regions that give rise to the pulsed emission, and find a small increase in posterior gain for an alternative (but less preferred) model. Lastly, we investigate the hypothetical scenario of increasing the NICER exposure time for PSR J0030+0451.
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