Maximum baryon masses for static neutron stars in f(R) gravity

We investigate the upper mass limit predictions of the baryonic mass for static neutron stars in the context of f(R) gravity. We use the most popular f(R) gravity model, namely the R2gravity, and calculate the maximum baryon mass of static neutron stars adopting several realistic equations of state and one ideal equation of state, namely that of causal limit. Our motivation is based on the fact that neutron stars with baryon masses larger than the maximum mass for static neutron star configurations inevitably collapse to black holes. Thus with our analysis, we want further to enlighten the predictions for the maximum baryon masses of static neutron stars in R2gravity, which, in turn, further strengthens our understanding of the mysterious mass-gap region. As we show, the baryon masses of most of the equations of states studied in this paper, lie in the lower limits of the mass-gap region M ∼ 2.5 − 5M⊙, but intriguingly enough, the highest value of the maximum baryon masses we found is of the order of M ∼ 3M⊙. This upper mass limit also appears as a maximum static neutron star gravitational mass limit in other contexts. Combining the two results which refer to baryon and gravitational masses, we point out that the gravitational mass of static neutron stars cannot be larger than three solar masses, while based on maximum baryon masses results of the present work, we can conspicuously state that it is highly likely the lower mass limits of astrophysical black holes in the range of M ∼ 2.5 − 3M⊙. This, in turn, implies that maximum neutron star masses in the context of R2gravity are likely to be in the lower limits of the range of M ∼ 2.4 − 3M⊙.

Reconstruction of baryon fraction in intergalactic medium through dispersion measurements of fast radio bursts

ABSTRACT Fast radio bursts (FRBs) probe the total column density of free electrons in the intergalactic medium (IGM) along the path of propagation through the dispersion measures (DMs) that depend on the baryon mass fraction in the IGM, i.e. fIGM. In this letter, we investigate the large-scale clustering information of DMs to study the evolution of fIGM. When combining with the Planck 2018 measurements, we could give tight constraints on the evolution of fIGM(z) from about 104 FRBs with the intrinsic DM scatter of $30(1+z) \rm pc\, cm^{-3}$ spanning 80 per cent of the sky and redshift range z = 0–3. First, we consider the Taylor expansion of fIGM(z) up to second order, and find that the mean relative standard deviation σ(fIGM) ≡ 〈σ[fIGM(z)]/fIGM(z)〉 is about 6.7 per cent. In order to alleviate the dependence on fiducial model, we also adopt non-parametric methods in this work, the local principle component analysis. We obtain the consistent, but weaker constraints on the evolution of fIGM(z), namely the mean relative standard deviation σ(fIGM) is 21.4 per cent. With the forthcoming surveys, this could be a complimentary method to investigate the baryon mass fraction in the IGM.

Exploring the hot gaseous halo around an extremely massive and relativistic jet launching spiral galaxy with XMM−Newton

ABSTRACT We present a deep XMM−Newton observation of the extremely massive, rapidly rotating, relativistic-jet-launching spiral galaxy 2MASX J23453268−0449256. Diffuse X-ray emission from the hot gaseous halo around the galaxy is robustly detected out to a radius of 160 kpc, corresponding roughly to 35 per cent of the virial radius (≈450 kpc). We fit the X-ray emission with the standard isothermal β model, and it is found that the enclosed gas mass within 160 kpc is $1.15_{-0.24}^{+0.22} \times 10^{11} \, \rm {M}_{\odot }$. Extrapolating the gas mass profile out to the virial radius, the estimated gas mass is $8.25_{-1.77}^{+1.62} \times 10^{11} \, \rm {M}_{\odot }$, which makes up roughly 65 per cent of the total baryon mass content of the galaxy. When the stellar mass is considered and accounting for the statistical and systematic uncertainties, the baryon mass fraction within the virial radius is $0.121_{-0.043}^{+0.043}$, in agreement with the universal baryon fraction. The baryon mass fraction is consistent with all baryons falling within r200, or with only half of the baryons falling within r200. Similar to the massive spiral galaxies NGC 1961 and NGC 6753, we find a low value for the metal abundance of ≈ 0.1 Z⊙, which appears uniform with radius. We also detect diffuse X-ray emission associated with the northern and southern lobes, possibly attributed to inverse Compton scattering of cosmic microwave background photons. The estimated energy densities of the electrons and magnetic field in these radio lobes suggest that they are electron-dominated by a factor of 10−200, depending on the choice of the lower cut-off energy of the electron spectrum.

Fossils: Microwave Radiation and Light Elements

This chapter focuses on the informative fossils left from a time when the universe was very different from now, dense and hot enough to produce the light elements and the sea of thermal radiation that nearly uniformly fills space. It begins by reviewing the behavior of a sea of microwave radiation in an expanding universe. The chapter then considers how George Gamow and his colleagues, Ralph Alpher and Robert Herman, hit on the main elements of the hot big bang cosmology, including the sea of microwave radiation and the large helium abundance, but failed to capture the interest of the community. It assesses how it came to be seen that the abundance of helium is much larger than expected from production in stars but is readily understood as the result of thermonuclear reactions in the hot big bang cosmology. This attracted little attention prior to the recognition of a second fossil: the sea of microwave radiation. The chapter concludes with the steps to a persuasive measurement of the primeval abundance of deuterium and the implied baryon mass density.

Cosmology-insensitive estimate of IGM baryon mass fraction from five localized fast radio bursts

ABSTRACT Five fast radio bursts (FRBs), including three apparently non-repeating ones, FRB 180924, FRB 181112, and FRB 190523, and two repeaters, FRB 121102 and FRB 180916.J0158+65, have already been localized so far. We apply a method developed recently by us to these five localized FRBs to give a cosmology-insensitive estimate of the fraction of baryon mass in the intergalactic medium, fIGM. Using the measured dispersion measure (DM) and luminosity distance dL data (inferred from the FRB redshifts and dL of Type Ia supernovae at the same redshifts) of the five FRBs, we constrain the local $f_{\rm IGM} = 0.84^{+0.16}_{-0.22}$ with no evidence of redshift dependence. This cosmology-insensitive estimate of fIGM from FRB observations is in excellent agreement with previous constraints using other probes. Moreover, using the three apparently non-repeating FRBs only we get a little looser but consistent result: $f_{\rm IGM} = 0.74^{+0.24}_{-0.18}$. In these two cases, reasonable estimations for the host galaxy DM contribution (DMhost) can be achieved by modelling it as a function of star formation rate. The constraints on both fIGM and DMhost are expected to be significantly improved with the rapid progress in localizing FRBs.

Magnetized hybrid stars: effects of slow and rapid phase transitions at the quark–hadron interface

ABSTRACT We study the influence of strong magnetic fields in hybrid stars, composed by hadrons and a pure quark matter core, and analyse their structure and stability as well as some possible evolution channels due to the magnetic field decay. Using an ad hoc parametrization of the magnetic field strength and taking into account Landau-quantization effects in matter, we calculate hybrid magnetized equations of state and some associated quantities, such as particle abundances and matter magnetization, for different sets of parameters and different magnetic field strengths. Moreover, we compute the magnetized stable stellar configurations, the mass versus radius and the gravitational mass versus central energy density relationships, the gravitational mass versus baryon mass diagram, and the tidal deformability. Our results are in agreement with both, the $\sim 2\, \mathrm{M}_\odot$ pulsars and the data obtained from GW170817. In addition, we study the stability of stellar configurations assuming that slow and rapid phase transitions occur at the sharp hadron–quark interface. We find that, unlike in the rapid transition scenario, where ∂M/∂ϵc < 0 is a sufficient condition for instability, in the slow transition scenario there exists a connected extended stable branch beyond the maximum mass star, for which ∂M/∂ϵc < 0. Finally, analysing the gravitational mass versus baryon mass relationship, we have calculated the energy released in transitions between stable stellar configurations. We find that the inclusion of the magnetic field and the existence of new stable branches allows the possibility of new channels of transitions that fulfil the energy requirements to explain gamma-ray bursts.

Baryons in the Cosmic Web of IllustrisTNG – I: gas in knots, filaments, sheets, and voids

ABSTRACT We analyse the IllustrisTNG simulations to study the mass, volume fraction, and phase distribution of gaseous baryons embedded in the knots, filaments, sheets, and voids of the Cosmic Web from redshift z = 8 to redshift z = 0. We find that filaments host more star-forming gas than knots, and that filaments also have a higher relative mass fraction of gas in this phase than knots. We also show that the cool, diffuse intergalactic medium [IGM; $T\lt 10^5 \, {\rm K}$, $n_{\rm H}\lt 10^{-4}(1+z) \, {\rm cm^{-3}}$] and the warm-hot intergalactic medium [WHIM; $10^5 \lt T\lt 10^7 \, {\rm K}$, $n_{\rm H} \lt 10^{-4}(1+z)\, {\rm cm^{-3}}$] constitute ${\sim } 39$ and ${\sim } 46{{\ \rm per\ cent}}$ of the baryons at redshift z = 0, respectively. Our results indicate that the WHIM may constitute the largest reservoir of missing baryons at redshift z = 0. Using our Cosmic Web classification, we predict the WHIM to be the dominant baryon mass contribution in filaments and knots at redshift z = 0, but not in sheets and voids where the cool, diffuse IGM dominates. We also characterize the evolution of WHIM and IGM from redshift z = 4 to redshift z = 0, and find that the mass fraction of WHIM in filaments and knots evolves only by a factor of ∼2 from redshift z = 0 to 1, but declines faster at higher redshift. The WHIM only occupies $4\!-\!11{{\ \rm per\ cent}}$ of the volume at redshift 0 ≤ z ≤ 1. We predict the existence of a significant number of currently undetected O vii and Ne ix absorption systems in cosmic filaments, which could be detected by future X-ray telescopes like Athena.