Neutron Star Research Articles

Viscous hydrodynamic evolution of neutron star merger accretion disks: A code comparison

Viscous hydrodynamic evolution of neutron star merger accretion disks: A code comparison

Long term variability of Cygnus X-1

The Neutron Star Interior Composition Explorer (NICER) monitoring campaign of Cyg X-1 allows us to study its spectral-timing behavior at energies < 1 keV across all states. The hard state power spectrum can be decomposed into two main broad Lorentzians with a transition at around 1 Hz. The lower-frequency Lorentzian is the dominant component at low energies. The higher-frequency Lorentzian begins to contribute significantly to the variability above 1.5 keV and dominates at high energies. We show that the low- and high-frequency Lorentzians likely represent individual physical processes. The lower-frequency Lorentzian can be associated with a (possibly Comptonized) disk component, while the higher-frequency Lorentzian is clearly associated with the Comptonizing plasma. At the transition of these components, we discover a low-energy timing phenomenon characterized by an abrupt lag change of hard (≳2 keV) with respect to soft (≲1.5 keV) photons, accompanied by a drop in coherence, and a reduction in amplitude of the second broad Lorentzian. The frequency of the phenomenon increases with the frequencies of the Lorentzians as the source softens and cannot be seen when the power spectrum is single-humped. A comparison to transient low-mass X-ray binaries shows that this feature does not only appear in Cyg X-1, but that it is a general property of accreting black hole binaries. In Cyg X-1, we find that the variability at low and high energies is overall highly coherent in the hard and intermediate states. The high coherence shows that there is a process at work which links the variability, suggesting a physical connection between the accretion disk and Comptonizing plasma. This process fundamentally changes in the soft state, where strong red noise at high energies is incoherent to the variability at low energies.

Jetsimpy: A Highly Efficient Hydrodynamic Code for Gamma-Ray Burst Afterglow

Gamma-ray burst (GRB) afterglows are emissions from ultrarelativistic blast waves produced by a narrow jet interacting with surrounding matter. Since the first multimessenger observation of a neutron star merger, hydrodynamic modeling of GRB afterglows for structured jets with smoothly varying angular energy distributions has gained increased interest. While the evolution of a jet is well described by self-similar solutions in both ultrarelativistic and Newtonian limits, modeling the transitional phase remains challenging. This is due to the nonlinear spreading of a narrow jet to a spherical configuration and the breakdown of self-similar solutions. Analytical models are limited in capturing these nonlinear effects, while relativistic hydrodynamic simulations are computationally expensive, which restricts the exploration of various initial conditions. In this work, we introduce a reduced hydrodynamic model that approximates the blast wave as an infinitely thin two-dimensional surface. Further assuming axial symmetry, this model simplifies the simulation to one dimension and drastically reduces the computational costs. We have compared our modeling to relativistic hydrodynamic simulations and semianalytic methods, and applied it to fit the light curve and flux centroid motion of GRB 170817A. These comparisons demonstrate good agreement and validate our approach. We have developed this method into a numerical tool, jetsimpy, which models the synchrotron GRB afterglow emission from a blast wave with arbitrary angular energy and Lorentz factor distribution. Although the code is built with GRB afterglow in mind, it applies to any relativistic jet. This tool is particularly useful in Markov Chain Monte Carlo studies and is provided to the community.

Multi-wavelength pulse profiles from the force-free neutron star magnetosphere

Context. The last two decades have witnessed dramatic progress in our understanding of neutron star magnetospheres thanks to force-free and particle-in-cell simulations. However, the associated particle dynamics and its emission mechanisms and locations have not been fully constrained, notably in X-rays. Aims. In this paper, we compute a full atlas of radio, X-ray, and γ-ray pulse profiles, relying on the force-free magnetosphere model. Our goal is to use such a data bank of multi-wavelength profiles to fit a substantial number of radio-loud γ-ray pulsars that have also been detected in non-thermal X-rays to decipher the X-ray radiation mechanism and sites. Using results from the third γ-ray pulsar catalogue (3PC), we investigate the statistical properties of this population. Methods. We assume that radio emission emanates from field lines rooted to the polar caps, at varying height above the surface, close to the surface, at an altitude about 5–10% of the light cylinder radius, r L. The X-ray photons are produced in the separatrix region within the magnetosphere; that is, the current sheet formed by the jump from closed to open magnetic field lines. We allow for substantial variations in emission height. The γ-rays are produced within the current sheet of the striped wind, outside the light cylinder. Results. A comprehensive set of radio, X-ray, and γ-ray light curves was computed. Based on only geometric considerations about magnetic obliquity, line-of-sight inclination, and the radio beam cone opening angle, pulsars can be classified as radio-loud or quiet and as γ-ray-loud or quiet. We found that the 3PC sample is compatible with an isotropic distribution of obliquity and line of sight. Conclusions. The atlases constructed in this work are the fundamental tools with which to explore individual pulsars and fit their multi-wavelength pulse profiles in order to constrain their magnetic topology, the emission sites, and the observer’s line of sight.

Tidal Disruption Event AT2020ocn: Early Time X-Ray Flares Caused by a Possible Disk Alignment Process

A tidal disruption event (TDE) may occur when a star is torn apart by the tidal force of a black hole (BH). Eventually, an accretion disk is thought to form out of stellar debris falling back toward the BH. If the star’s orbital angular momentum vector prior to disruption is not aligned with the BH spin angular momentum vector, the disk will be tilted with respect to the BH equatorial plane. The disk will eventually be drawn into the BH equatorial plane due to a combination of the Bardeen–Petterson effect and internal torques. Here, we analyze the X-ray and UV observations of the TDE AT2020ocn obtained by Swift, XMM-Newton, and Neutron star Interior Composition ExploreR. The X-ray light curve shows strong flares during the first ≈100 days, while, over the same period, the UV emission decays gradually. We find that the X-ray flares can be explained by a model that also explains the spectral evolution. This model includes a slim disk viewed under a variable inclination plus an inverse-Comptonization component processing the slim disk emission. A scenario where the ongoing Lense–Thirring precession during the disk alignment process is responsible for the observed inclination variations is consistent with the data. In later observations, we find that the X-ray spectrum of AT2020ocn becomes harder, while the mass accretion rate remains at super-Eddington levels, suggesting the formation of a corona in line with accretion onto other compact objects. We constrain the BH mass to be (7−3+13)×105 M ⊙ at the 1σ (68%) confidence level.

The Nuclear EOS of PNM and sensitivity of Neutron Star Properties to modern NN Potentials

The Nuclear EOS of PNM and sensitivity of Neutron Star Properties to modern NN Potentials

GRB 180128A: A second magnetar giant flare candidate from the Sculptor Galaxy

Magnetars are slowly rotating neutron stars that possess the strongest magnetic fields known in the cosmos (1014 − 1015 G). They display a range of transient high-energy electromagnetic activity. The brightest and most energetic of these events are the gamma-ray bursts (GRBs) known as magnetar giant flares (MGFs), with isotropic energies Eiso ≈ 1044 − 1046 erg. Only seven MGF detections have been made to date: three unambiguous events occurred in our Galaxy and the Magellanic Clouds, and the other four MGF candidates are associated with nearby star-forming galaxies. As all seven identified MGFs are bright at Earth, additional weaker events likely remain unidentified in archival data. We conducted a search of the Fermi Gamma-ray Burst Monitor database for candidate extragalactic MGFs and, when possible, collected localization data from the Interplanetary Network (IPN) satellites. Our search yielded one convincing event, GRB 180128A. IPN localizes this burst within NGC 253, commonly known as the Sculptor Galaxy. The event is the second MGF in modern astronomy to be associated with this galaxy and the first time two bursts have been associated with a single galaxy outside our own. Here we detail the archival search criteria that uncovered this event and its spectral and temporal properties, which are consistent with expectations for a MGF. We also discuss the theoretical implications and finer burst structures resolved from various binning methods. Our analysis provides observational evidence of an eighth identified MGF.

GRB 231115A: A Nearby Magnetar Giant Flare or a Cosmic Short Gamma-Ray Burst?

There are two classes of gamma-ray transients with a duration shorter than 2 s. One consists of cosmic short gamma-ray bursts (GRBs) taking place in the deep Universe via the neutron star mergers, and the other is the magnetar giant flares (GFs) with energies of ∼1044 − 1046 erg from “nearby” galaxies. Though the magnetar GFs and the short GRBs have rather similar temporal and spectral properties, their energies (E γ,iso) are different by quite a few orders of magnitude and hence can be distinguished supposing the host galaxies have been robustly identified. The newly observed GRB 231115A has been widely discussed as a new GF event for its high probability of being associated with M82. Here we conduct a detailed analysis of its prompt emission observed by Fermi-GBM and compare the parameters with existing observations. The prompt gamma-ray emission properties of GRB 231115A, if associated with M82, nicely follow the E p,z–E γ,iso relation of the GFs, where E p,z is the peak energy of the gamma-ray spectrum after the redshift (z) correction. To be a short GRB, the redshift needs to be ∼1. Though such a chance is low, the available X-ray/GeV observation upper limits are not stringent enough to further rule out this possibility. We have also discussed the prospect of convincingly establishing the magnetar origin of GRB 231115A-like events in the future.

VLBA Astrometry of the Galactic Double Neutron Stars PSR J0509+3801 and PSR J1930–1852: A Preliminary Transverse Velocity Distribution of Double Neutron Stars and its Implications

The mergers of double neutron star (DNS) systems are believed to drive the majority of short γ-ray bursts (SGRBs), while also serving as production sites of heavy r-process elements. Despite being key to (i) confirming the nature of the extragalactic SGRBs, (ii) addressing the poorly understood r-process enrichment in the ultrafaint dwarf galaxies (UFDGs), and (iii) probing the formation process of DNS systems, the space velocity distribution of DNSs is still poorly constrained, due to the small number of DNSs with well-determined astrometry. In this work, we determine new proper motions and parallaxes of two Galactic DNSs, PSR J0509+3801 and PSR J1930−1852, using the Very Long Baseline Array, and we estimate the transverse velocities v ⊥ of all 11 isolated Galactic DNSs having proper-motion measurements in a consistent manner. Our correlation analysis reveals that the DNS v ⊥ is tentatively correlated with three parameters: spin period, orbital eccentricity, and companion mass. With the preliminary v ⊥ distribution, we obtain the following findings. First, the refined v ⊥ distribution is confirmed to agree with the observed displacements of the localized SGRBs from their host galaxy birth sites. Second, we estimate that around 11% and 25% of DNSs remain gravitationally bound to UFDGs with escape velocities of 15 and 25 km s−1, respectively. Hence, the retained DNSs might indeed be responsible for the r-process enrichment confirmed so far in a few UFDGs. Finally, we discuss how a future ensemble of astrometrically determined DNSs may probe the multimodality of the v ⊥ distribution.

Spectrum and Polarization of the Galactic Center Radio Transient ASKAP J173608.2–321635 from THOR-GC and VLITE

The radio transient ASKAP J173608.2–321635, at the position (ℓ, b) = (356.°0872, −0.°0390), was serendipitously observed by The H i/OH/Recombination line survey of the Galactic center at three epochs in 2020 March, 2020 April, and 2021 February. The source was detected only on 2020 April 11 with a flux density of 20.6 ± 1.1 mJy at 1.23 GHz and in-band spectral index of α = −3.1 ± 0.2. The commensal Very Large Array Low-band Ionsophere and Transient Experiment simultaneously detected the source at 339 MHz with a flux density of 122.6 ± 20.4 mJy, indicating a spectral break below 1 GHz. The rotation measure (RM) in 2020 April was 63.9 ± 0.3 rad m−2, which almost triples the range of the variable RM observed by Wang et al. to ∼130 rad m−2. The polarization angle, corrected for Faraday rotation, was 97° ± 6°. The 1.23 GHz linear polarization was 76.7% ± 3.9% with wavelength-dependent depolarization, indicating a Faraday depth dispersion of σϕ=4.8−0.7+0.5radm−2 . We find an upper limit to the circular polarization of ∣V∣/I < 10.1%. Interpretation of the data in terms of diffractive scattering of radio waves by a plasma near the source indicates an electron density and a line-of-sight magnetic field strength within a factor of 3 of n e ∼ 2 cm−3 and B ∥ ∼ 2 × 105 μ G. Combined with causality limits to the size of the source, these parameters are consistent with the low-frequency spectral break resulting from synchrotron self-absorption, not free–free absorption. A possible interpretation of the source is a highly supersonic neutron star interacting with a changing environment.

Constraining Possible γ-Ray Burst Emission from GW230529 Using Swift-BAT and Fermi-GBM

GW230529 is the first compact binary coalescence detected by the LIGO–Virgo–KAGRA collaboration with at least one component mass confidently in the lower mass gap, corresponding to the range 3–5 M ⊙. If interpreted as a neutron star–black hole merger, this event has the most symmetric mass ratio detected so far and therefore has a relatively high probability of producing electromagnetic (EM) emission. However, no EM counterpart has been reported. At the merger time t 0, Swift-BAT and Fermi-GBM together covered 100% of the sky. Performing a targeted search in a time window [t 0 − 20 s, t 0 + 20 s], we report no detection by the Swift-BAT and Fermi-GBM instruments. Combining the position-dependent γ-ray flux upper limits and the gravitational-wave posterior distribution of luminosity distance, sky localization, and inclination angle of the binary, we derive constraints on the characteristic luminosity and structure of the jet possibly launched during the merger. Assuming a top-hat jet structure, we exclude at 90% credibility the presence of a jet that has at the same time an on-axis isotropic luminosity ≳1048 erg s−1 in the bolometric band 1 keV–10 MeV and a jet opening angle ≳15°. Similar constraints are derived by testing other assumptions about the jet structure profile. Excluding GRB 170817A, the luminosity upper limits derived here are below the luminosity of any GRB observed so far.

Calculation of the TOV Limit Based on Neutron Degeneracy Pressure

Original theory on the mass limit beyond which a cold, non-rotating neutron star cannot be formed, instead only stellar black holes will be created, was stipulated by J.R. Oppenheimer and G.M. Volkoff based on R.C. Tolman’s work in 1939. The limit calculated from the equation established by them is known as the TOV limit which is analogous to the Chandrasekhar limit for White Dwarfs. But the results obtained using the formula was found to be not valid today. Subsequent theoretical works place the limit in the range 1.5 to 3 solar masses. There are several basic theories and related formulae for calculating the TOV limit. In this article a different and novel approach has been adopted to calculate the TOV limit using the theory on neutron degeneracy pressure. As per present calculations, the TOV limit is around 2.928 times the solar mass. These calculations also highlight two aspects which are conceptually new; first, a black hole having mass higher than the TOV limit can also become a neutron star and both can coexist concurrently up to a certain limit; and second, that upper limit of star mass beyond which a black hole will explode in supernova before becoming a neutron star is 7.15 times the solar mass as at that stage the gravitational energy of the black hole will be equal or exceed to its nuclear binding energy.

Neutron star accretion events in AGN discs: mutimessenger implications

ABSTRACT This paper investigates the accretion of neutron stars (NSs) in active galactic nucleus (AGN) accretion discs. We classify potential accretion modes of NSs in AGN discs, proposing a hierarchical model of NS accretion: accretion flow from the Bondi sphere to accretion columns. The accretion of NSs in AGN discs differs from that of BHs, especially within the scale of the NS’s magnetosphere due to its hard surface and magnetic field. As the accretion flow approaches the magnetosphere, the magnetic fields guide the accretion flow to form accretion columns, primarily dominated by neutrinos. While neutrinos generated from single NS accretion may not have observable effects, considering the all-sky background, they contribute to the neutrino background in the sub-MeV energy range comparable to that of supernova explosions. NS accretion may also lead to the generation of mass quadrupole moments, consequently generating gravitational waves (GWs). The GWs, which exhibit characteristic effects like periodic modulations and echoes, could be observed by third-generation GW detectors. The emission of neutrinos and GWs carries away energy and angular momentum brought by accretion, reducing the feedback effect on the AGN disc. This results in an exceptionally high NS accretion rate, leading to a collapse time-scale shorter than the migration-merge time-scale, making it less likely that binary NS mergers originate from AGN discs.

Relativistic Brueckner–Hartree–Fock Calculations for Cold and Hot Neutron Stars

This study investigates the properties of symmetric and asymmetric nuclear matter using the relativistic Brueckner–Hartree–Fock formalism, examining both zero and finite temperatures up to 70 MeV. Employing the full Dirac space, we incorporate three Bonn potentials (A, B, and C), which account for meson masses, coupling strengths, cutoff parameters, and form factors. The calculated properties of asymmetric nuclear matter form the basis for constructing equation-of-state (EOS) models tailored for neutron stars. These models, in turn, enable the computation of bulk properties for nonrotating, uniformly rotating, and differentially rotating neutron stars. Notably, the EOS models studied in this paper are sufficiently versatile to accommodate the mass of the most massive neutron star ever detected, PSR J0952–0607, estimated to be 2.35 ± 0.17 M ⊙. Furthermore, they yield masses and radii for PSR J0030+451 that align with the confidence intervals established for this pulsar.

Pulsars with the masses 2.14M⊙ and 2.27M⊙ as strange star candidates

Strange stars with maximal masses, more than the values of recently precisely measured masses of two radio pulsars: SRJ0740+6620, with the mass 2.14М⊙, and PSRJ2215+5135, with the mass 2.27М⊙, have been studied. For the examination of strange quark matter (SQM), the bag model, developed in Massachusetts Technological Institute was chosen. The observed bag model depends on vacuum pressure B, quark-gluon interaction constant αc, and strange quark mass ms. Since the transition to SQM state takes place at the energy density, not exceeding double density in atomic nuclei, neutron stars with small mass and configuration, consisting of SQM, form one family in the dependence curve of the mass M, of the equilibrium super-dense configurations, on the energy central density ρc (curve M(ρc)). The state equation of strange quark matter was studied when the vacuum pressure was constant. Groups of the values of these parameters were determined; their application in the state equation of SQM leads to the maximal mass of the equilibrium quark configurations Mmax, which are heavier than the aforementioned radio pulsars. For such configurations, the values of mass, radius, the entire number of baryons, red shift from the strange star surface were calculated, depending on the energy central density ρc. For each series with Mmax>2.14M⊙ and Mmax>2.27M⊙ the values of the mentioned integral parameters were calculated as well for super-dense configurations with 2.27and2.14 solar masses that were precisely determined from observations. According to the obtained state equations, these two radio pulsars can be the possible candidates for the strange stars.

The “Magnificent Seven” X-Ray Isolated Neutron Stars Revisited. I. Improved Timing Solutions and Pulse Profile Analysis

We present the first systematic X-ray pulse timing analysis of the six members of the so-called “Magnificent Seven” nearby thermally emitting isolated neutron stars (XINS) with detected pulsations. Using the extensive collection of archival XMM-Newton, Chandra, and NICER observations spanning over two decades, we obtain the first firm measurement of the spin-down rate for RX J2143.0+0654, while for the rest we improve upon previously published spin ephemerides and extend them by up to an additional decade. Five of the XINS follow steady spin-down with no indication of major anomalies in their long-term timing behavior; the notable exception is RX J0720.4−3125, for which, in addition to confirming the previously identified glitch, we detect a second spin derivative. The high-quality folded X-ray pulse profiles produced with the updated timing solutions exhibit diverse and complex morphologies, as well as striking energy dependence. These peculiarities cannot be readily explained by blackbody-like isotropic emission and simple hot-spot configurations, hinting at the presence of complex multitemperature surface heat distributions and highly anisotropic radiation patterns, such as may arise from a strongly magnetized atmospheric layer.

Exploring reheated sub-40000 Kelvin neutron stars with JWST, ELT, and TMT

Exploring reheated sub-40000 Kelvin neutron stars with JWST, ELT, and TMT

Circular motion and collisions of particles with magnetic dipole moment and electric charge in dipolar magnetosphere around Schwarzschild black holes

No-hair theorem indicates that black holes cannot have their own magnetic dipole moment. They can be weakly magnetized in binary systems with a neutron star companion and an accretion disc of charged particles. A simple model suggested by Petterson states that a current loop accreting a Schwarzschild black hole generates dipole-like magnetic fields in the outer region of the loop that are uniform in the inner region. This study considers circular motion and collisions of charged test particles with magnetic dipole moments in the inner and outer regions. First, we derive the effective potential taking into account the magnetic interactions between external magnetic fields with electric charge and the magnetic dipole moment of the particle. We investigate the possible innermost stable circular orbits (ISCOs) of the charged and magnetized particles orbiting the magnetized Schwarzschild black hole inside and outside the current loop. Finally, we explore the collisional processes of these particles near the black hole horizons, examining the effects of magnetic interactions on the critical angular momentum of particles that may collide and the center of mass energy of the colliding particles. We discuss astrophysical relevant objects with magnetic dipole moment and electric charge: magnetized neutron stars, white dwarfs, rotating stellar-mass black holes, electrons, and protons, and also estimate the interaction parameters for them.

On the lack of X-ray pulsation in most neutron star low-mass X-ray binaries

On the lack of X-ray pulsation in most neutron star low-mass X-ray binaries

Supernova 1987A’s Keyhole: A Long-lived Jet-pair in the Final Explosion Phase of Core-collapse Supernovae

I further study the manner by which a pair of opposite jets shape the “keyhole” morphological structure of the core-collapse supernova (CCSN) SN 1997A, now the CCSN remnant (CCSNR) 1987A. By doing so, I strengthen the claim that the jittering-jet explosion mechanism accounts for most, likely all, CCSNe. The “keyhole” structure comprises a northern low-intensity zone closed with a bright rim on its front and an elongated low-intensity nozzle in the south. This rim-nozzle asymmetry is observed in some cooling flow clusters and planetary nebulae that are observed to be shaped by jets. I build a toy model that uses the planar jittering jets pattern, where consecutive pairs of jets tend to jitter in a common plane, implying that the accreted gas onto the newly born neutron star at the late explosion phase flows perpendicular to that plane. This allows for a long-lived jet-launching episode. This long-lasting jet-launching episode launches more mass into the jets that can inflate larger pairs of ears or bubbles, forming the main jets’ axis of the CCSNR that is not necessarily related to a possible pre-collapse core rotation. I discuss the relation of the main jets’ axis to the neutron star’s natal kick velocity.