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Register a new userSuper-Massive Neutron Stars and Compact Binary Millisecond Pulsars
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Authors
Manuel Linares
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The maximum mass of a neutron star has important implications across multiple research fields, including astrophysics, nuclear physics and gravitational wave astronomy. Compact binary millisecond pulsars (with orbital periods shorter than about a day) are a rapidly-growing pulsar population, and provide a good opportunity to search for the most massive neutron stars. Applying a new method to measure the velocity of both sides of the companion star, we previously found that the compact binary millisecond pulsar PSR J2215+5135 hosts one of the most massive neutron stars known to date, with a mass of 2.27$pm$0.16 M$_odot$ (Linares, Shahbaz & Casares, 2018). We reexamine the properties of the 0.33 M$_odot$ companion star, heated by the pulsar, and argue that irradiation in this redback binary is extreme yet stable, symmetric and not necessarily produced by an extended source. We also review the neutron star mass distribution in light of this and more recent discoveries. We compile a list of all (nine) systems with published evidence for super-massive neutron stars, with masses above 2 M$_odot$. We find that four of them are compact binary millisecond pulsars (one black widow, two redbacks and one redback candidate). This shows that compact binary millisecond pulsars are key to constraining the maximum mass of a neutron star.
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A new population of neutron stars has emerged during the last decade: compact binary millisecond pulsars (CBMSPs). Because these pulsars and their companion stars are in tight orbits with typical separations of $10^{11}$ cm, their winds interact strongly forming an intrabinary shock. Electron-positron pairs reaccelerated at the shock can reach energies of about 10 TeV, which makes this new population a potential source of GeV-TeV cosmic ray positrons. We present an analytical model for the fluxes and spectra of positrons from intrabinary shocks of CBMSPs. We find that the minimum energy $E_{min}$ of the pairs that enter the shock is critical to quantify the energy spectrum with which positrons are injected into the interstellar medium. We measure for the first time the Galactic scale height of CBMSPs, $z_e=0.4pm0.1$ kpc, after correcting for an observational bias against finding them close to the Galactic plane. From this, we estimate a local density of 5-9 kpc$^{-3}$ and an extrapolated total of 2-7 thousand CBMSPs in the Galaxy. We then propagate the pairs in the isotropic diffusion approximation and find that the positron flux from the total population is about two times higher than that from the 52 currently known systems. For $E_{min}$ between 1 and 50 GeV, our model predicts only a minor contribution from CBMSPs to the diffuse positron flux at 100 GeV observed at Earth. We also quantify the effects of anisotropic transport due to the ordered Galactic magnetic field, which can change the diffuse flux from nearby sources drastically. Finally, we find that a single hidden CBMSP close to the Galactic plane can yield a positron flux comparable to the AMS-02 measurements at 600 GeV if its line-of-sight to Earth is along the ordered Galactic field lines, while its combined electron and positron flux at higher energies would be close to the measurements of CALET, DAMPE and Fermi-LAT.
Measuring the spin of Accreting Neutron Stars is important because it can provide constraints on the Equation of State of ultra-dense matter. Particularly crucial to our physical understanding is the discovery of sub-millisecond pulsars, because this will immediately rule out many proposed models for the ground state of dense matter. So far, it has been impossible to accomplish this because, for still unknown reasons, only a small amount of Accreting Neutron Stars exhibit coherent pulsations. An intriguing explanation for the lack of pulsations is that they form only on neutron stars accreting with a very low average mass accretion rate. I have searched pulsations in the faintest persistent X-ray source known to date and I found no evidence for pulsations. The implications for accretion theory are very stringent, clearly showing that our understanding of the pulse formation process is not complete. I discuss which sources are optimal to continue the search of sub-ms pulsars and which are the new constraints that theoretical models need to explain to provide a complete description of these systems
It is usually thought that a single equation of state (EoS) model correctly represents cores of all compact stars. Here we emphasize that two families of compact stars, viz., neutron stars and strange stars, can coexist in nature, and that neutron stars can get converted to strange stars through the nucleation process of quark matter in the stellar center. From our fully general relativistic numerical computations of the structures of fast-spinning compact stars, known as millisecond pulsars, we find that such a stellar conversion causes a simultaneous spin-up and decrease in gravitational mass of these stars. This is a new type of millisecond pulsar evolution through a new mechanism, which gives rise to relatively lower mass compact stars with higher spin rates. This could have implication for the observed mass and spin distributions of millisecond pulsars. Such a stellar conversion can also rescue some massive, spin-supported millisecond pulsars from collapsing into black holes. Besides, we extend the concept of critical mass $M_{rm cr}$ for the neutron star sequence (Berezhiani et al. 2003; Bombaci et al. 2004) to the case of fast-spinning neutron stars, and point out that neutron star EoS models cannot be ruled out by the stellar mass measurement alone. Finally, we emphasize the additional complexity for constraining EoS models, for example, by stellar radius measurements using X-ray observations, if two families of compact stars coexist.
We present observations of fields containing eight recently discovered binary millisecond pulsars using the telescopes at MDM Observatory. Optical counterparts to four of these systems are detected, one of which, PSR J2214+3000, is a novel detection. Additionally, we present the fully phase-resolved B, V, and R light curves of the optical counterparts to two objects, PSR J1810+1744 and PSR J2215+5135 for which we employ model fitting using the ELC model of Orosz & Hauschildt (2000) to measure the unknown system parameters. For PSR J1810+1744 we find that the system parameters cannot be fit even assuming that 100% of the spin-down luminosity of the pulsar is irradiating the secondary, and so radial velocity measurements of this object will be required for the complete solution. However, PSR J2215+5135 exhibits light curves that are extremely well constrained using the ELC model and we find that the mass of the neutron star is constrained by these and the radio observations to be greater than 1.75 solar masses at the 3-sigma level. We also find a discrepancy between the model temperature and the measured colors of this object which we interpret as possible evidence for an additional high-temperature source such as a quiescent disk. Given this and the fact that PSR J2214+5135 contains a relatively high mass companion (greater than 0.1 solar masses), we propose that similar to the binary pulsar systems PSR J1023+0038 and IGR J18245-2452, the pulsar may transition between accretion- and rotation-powered modes.
An understanding of spin frequency ($ u$) evolution of neutron stars in the low-mass X-ray binary (LMXB) phase is essential to explain the observed $ u$-distribution of millisecond pulsars (MSPs), and to probe the stellar and binary physics, including the possibility of continuous gravitational wave emission. Here, using numerical computations we conclude that $ u$ can evolve in two distinctly different modes, as $ u$ may approach a lower spin equilibrium value ($ u_{rm eq,per}$) for persistent accretion for a long-term average accretion rate ($dot{M}_{rm av}$) greater than a critical limit ($dot{M}_{rm av,crit}$), and may approach a higher effective spin equilibrium value ($ u_{rm eq,eff}$) for transient accretion for $dot{M}_{rm av} < dot{M}_{rm av,crit}$. For example, when $dot{M}_{rm av}$ falls below $dot{M}_{rm av,crit}$ for an initially persistent source, $ u$ increases considerably due to transient accretion, which is counterintuitive. We also find that, contrary to what was suggested, a fast or sudden decrease of $dot{M}_{rm av}$ to zero in the last part of the LMXB phase is not essential for the genesis of spin-powered MSPs, and neutron stars could spin up in this $dot{M}_{rm av}$-decreasing phase. Our findings imply that the traditional way of $ u$-evolution computation is inadequate in most cases, even for initially persistent sources, and may not even correctly estimate whether $ u$ increases or decreases.
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