arXiv subjects
Publications and source records attributed to Aaron Worley.
The BlueWalker 3 satellite is now fainter than during the first months after deployment. The greatest improvement is that the average maximum luminosity near zenith has been reduced from magnitude 1.0 to 2.2. However, the spacecraft is still usually bright enough to interfere with astronomical research.
Observations of Starlink V2 Mini satellites during orbit-raising suggest that SpaceX applies brightness mitigation when they reach a height of 357 km. The mean apparent magnitudes for objects below that height threshold is 2.68 while the mean for those above is 6.46. When magnitudes are adjusted to a uniform distance of 1000 km the means are 4.58 and 7.52, respectively. The difference of 2.94 between distance-adjusted magnitudes above and below threshold implies that mitigation is 93% effective in reducing the brightness of orbit-raising spacecraft. Orbit-raising Mini spacecraft have a smaller impact on astronomical observations than higher altitude on-station spacecraft because they are relatively few in number. They also spend less time traversing the sky and spend longer in the Earth's shadow. These low-altitude objects will be more out-of-focus in large telescopes such as the LSST which reduces their impact, too. However, they attract considerable public attention and airline pilots have reported them as Unidentified Aerial Phenomena.
The illumination phase functions for Starlink Mini satellites are determined for times of twilight and darkness. Those functions are then evaluated to give apparent magnitudes over a grid of points across the sky and over a range of solar angles below the horizon. Sky maps and a table of satellite magnitude distributions are presented. The largest areas of sky with satellites brighter than magnitudes 6 and 7 both occur during twilight. Brightness surges, known as flares, are also characterized.
Photometric characteristics for all models of Starlink satellites launched to date are reviewed. The Original design that lacked brightness mitigation is the most luminous. SpaceX installed a sunshade on the VisorSat model which reduced its luminosity by a factor of 3. The visor was omitted on Post-VisorSat spacecraft with laser communication which followed, but the company added a reflective layer which resulted in an intermediate brightness between Original and VisorSat. SpaceX is applying advanced brightness mitigation techniques to their Generation 2 Starlink satellites which are larger. The first of these, called Minis, are dimmer than Gen 1 Starlinks despite their greater size. Photometric observations verify that brightness mitigation efforts employed by SpaceX reduce spacecraft luminosity substantially. However, the satellites still have some negative impact on astronomical observations and the very large satellites planned for later in Gen 2 may interfere more seriously.
Starlink Generation 2 Mini satellites are fainter than Gen 1 spacecraft despite their larger size. The mean of apparent magnitudes for satellites in brightness mitigation mode is 7.06 +/- 0.10. When these magnitudes are adjusted to a uniform distance of 1,000 km that mean is 7.87 +/- 0.09. The brightness mitigation mode reduces distance-adjusted satellite luminosity by a factor of 12 relative to spacecraft that are not mitigated.
The metallic means (also known as metallic ratios) may be defined as the limiting ratio of consecutive terms of sequences connected to the Fibonacci sequence via the INVERT transform. For example, the Pell sequence (invert transform of the Fibonacci sequence) gives the so-called silver mean, and the invert transform of the Pell sequence leads to the bronze mean. The limiting ratio of the Fibonacci sequence itself is known as the golden mean or ratio. We introduce a new family of kth-degree metallic means obtained through invert transforms of the generalized kth-order Fibonacci sequence. As it is the case for k=2, each generalized metallic mean is shown to be the unique positive root of a kth-degree polynomial determined by the sequence.
Heavy-ion reactions especially those induced by radioactive beams provide useful information about the density dependence of the nuclear symmetry energy, thus the Equation of State of neutron-rich nuclear matter, relevant for many astrophysical studies. The latest developments in constraining the symmetry energy at both sub- and supra-saturation densities from analyses of the isopsin diffusion and the $π^-/π^+$ ratio in heavy-ion collisions using the IBUU04 transport model are discussed. Astrophysical ramifications of the partially constrained symmetry energy on properties of neutron star crusts, gravitational waves emitted by deformed pulsars and the w-mode oscillations of neutron stars are presented briefly.
Gravitational waves are tiny disturbances in space-time and are a fundamental, although not yet directly confirmed, prediction of General Relativity. Rapidly rotating neutron stars are one of the possible sources of gravitational radiation dependent upon pulsar's rotational frequency, details of the equation of state of stellar matter, and distance to detector. Applying an equation of state with symmetry energy constrained by recent nuclear laboratory data, we set an upper limit on the strain-amplitude of gravitational waves emitted by rapidly rotating neutron stars.
Gravitational radiation is a fundamental prediction of General Relativity. Elliptically deformed pulsars are among the possible sources emitting gravitational waves (GWs) with a strain-amplitude dependent upon the star's quadrupole moment, rotational frequency, and distance from the detector. We show that the gravitational wave strain amplitude $h_0$ depends strongly on the equation of state of neutron-rich stellar matter. Applying an equation of state with symmetry energy constrained by recent nuclear laboratory data, we set an upper limit on the strain-amplitude of GWs produced by elliptically deformed pulsars. Depending on details of the EOS, for several millisecond pulsars at distances $0.18kpc$ to $0.35kpc$ from Earth, the {\it maximal} $h_0$ is found to be in the range of $\sim[0.4-1.5]\times 10^{-24}$. This prediction serves as the first {\it direct} nuclear constraint on the gravitational radiation. Its implications are discussed.
Recent analyses of several isospin effects in heavy-ion reactions have allowed us to constrain the density dependence of nuclear symmetry energy at sub-saturation densities within a narrow range. Combined with constraints on the Equation of State (EOS) of symmetric nuclear matter obtained previously from analyzing the elliptic flow in relativistic heavy-ion collisions, the EOS of neutron-rich nuclear matter is thus partially constrained. Here we report effects of the partially constrained EOS of neutron-rich nuclear matter on the mass-radius correlation, moment of inertia, elliptical deformation and gravitational radiation of (rapidly) rotating neutron stars.
Properties and structure of neutron stars are determined by the equation of state (EOS) of neutron-rich stellar matter. While the collective flow and particle production in relativistic heavy-ion collisions have constrained tightly the EOS of symmetric nuclear matter up to about five times the normal nuclear matter density, the more recent experimental data on isospin-diffusion and isoscaling in heavy-ion collisions at intermediate energies have constrained considerably the density dependence of the nuclear symmetry energy at subsaturation densities. Although there are still many uncertainties and challenges to pin down completely the EOS of neutron-rich nuclear matter, the heavy-ion reaction experiments in terrestrial laboratories have limited the EOS of neutron-rich nuclear matter in a range much narrower than that spanned by various EOSs currently used in astrophysical studies in the literature. These nuclear physics constraints could thus provide more reliable information about properties of neutron stars. Within well established formalisms using the nuclear constrained EOSs we study the momenta of inertia of neutron stars. We put the special emphasis on the component A of the extremely relativistic double neutron star system PSR J0737-3039. Its moment of inertia is found to be between 1.30 and 1.63 $(\times10^{45}g$ $cm^2)$. Moreover, the transition density at the crust-core boundary is shown to be in the narrow range of $ρ_t=[0.091-0.093](fm^{-3})$.
Properties, structure, and thermal evolution of neutron stars are determined by the equation of state of stellar matter. Recent data on isospin-diffusion and isoscaling in heavy-ion collisions at intermediate energies as well as the size of neutron skin in $^{208}Pb$ have constrained considerably the density dependence of the nuclear symmetry energy and, in turn, the equation of state of neutron-rich nucleonic matter. These constraints could provide useful information about the global properties of rapidly rotating neutron stars. Models of rapidly rotating neutron stars are constructed applying several nucleonic equations of state. Particular emphasis is placed on configurations rotating rigidly at $716$ and $1122Hz$. The range of allowed hydrostatic equilibrium solutions is determined and tested for stability. The effect of rotation on the internal composition and thermal properties of neutron stars is also examined. At a given rotational frequency, each equation of state yields a range of possible neutron stars configurations restricted by the Keplerian (mass-shedding) limit, corresponding to the maximal circumferential radius, and the limit due to the onset of instabilities with respect to axial-symmetric perturbations, corresponding to the minimal equatorial radius of a stable neutron star model. We show that the mass of a neutron star rotating uniformly at $1122Hz$ is between $1.7$ and $2.1M_{\sun}$. Central stellar density and proton fraction decrease with increasing rotational frequency with respect to static models, and depending on the exact stellar mass and angular velocity, can drop below the Direct Urca threshold thus closing the fast cooling channel.
Properties of effective interactions in neutron-rich matter are reflected in the medium's equation of state (EOS), which is a relationship among several state variables. Spin and isospin asymmetries play an important role in the energy balance and could alter the stability conditions of the nuclear EOS. The EOS has far-reaching consequences for numerous nuclear processes in both the terrestrial laboratories and the cosmos. Presently the EOS, especially for neutron-rich matter, is still very uncertain. Heavy-ion reactions provide a unique means to constrain the EOS, particularly the density dependence of the nuclear symmetry energy. On the other hand, microscopic, self-consistent, and parameter-free approaches are ultimately needed for understanding nuclear properties in terms of the fundamental interactions among the basic constituents of nuclear systems. In this talk, after a brief review of our recent studies on spin-polarized neutron matter, we discuss constraining the changing rate of the gravitational constant $G$ and properties of (rapidly) rotating neutron stars by using a nuclear EOS partially constrained by the latest terrestrial nuclear laboratory data.