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Register a new userFormation of Double Neutron Star Systems
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Thomas Michael Tauris
Publication date
2017
fields
Physics
and research's language is
English
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Double neutron star (DNS) systems represent extreme physical objects and the endpoint of an exotic journey of stellar evolution and binary interactions. Large numbers of DNS systems and their mergers are anticipated to be discovered using the Square-Kilometre-Array searching for radio pulsars and high-frequency gravitational wave detectors (LIGO/VIRGO), respectively. Here we discuss all key properties of DNS systems, as well as selection effects, and combine the latest observational data with new theoretical progress on various physical processes with the aim of advancing our knowledge on their formation. We examine key interactions of their progenitor systems and evaluate their accretion history during the high-mass X-ray binary stage, the common envelope phase and the subsequent Case BB mass transfer, and argue that the first-formed NSs have accreted at most $sim 0.02;M_{odot}$. We investigate DNS masses, spins and velocities, and in particular correlations between spin period, orbital period and eccentricity. Numerous Monte Carlo simulations of the second supernova (SN) events are performed to extrapolate pre-SN stellar properties and probe the explosions. All known close-orbit DNS systems are consistent with ultra-stripped exploding stars. Although their resulting NS kicks are often small, we demonstrate a large spread in kick magnitudes which may, in general, depend on the past interaction history of the exploding star and thus correlate with the NS mass. We analyze and discuss NS kick directions based on our SN simulations. Finally, we discuss the terminal evolution of close-orbit DNS systems until they merge and possibly produce a short $gamma$-ray burst.
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Using a Milky Way double neutron star (DNS) merger rate of 210 Myr$^{-1}$, as derived by the Laser Interferometer Gravitational-Wave Observatory (LIGO), we demonstrate that the Laser Interferometer Space Antenna (LISA) will detect on average 240 (330) DNSs within the Milky Way for a 4-year (8-year) mission with a signal-to-noise ratio greater than 7. Even adopting a more pessimistic rate of 42 Myr$^{-1}$, as derived by the population of Galactic DNSs, we find a significant detection of 46 (65) Milky Way DNSs. These DNSs can be leveraged to constrain formation scenarios. In particular, traditional NS-discovery methods using radio telescopes are unable to detect DNSs with $P_{rm orb}$ $lesssim$1 hour (merger times $lesssim$10 Myr). If a fast-merging channel exists that forms DNSs at these short orbital periods, LISA affords, perhaps, the only opportunity to observationally characterize these systems; we show that toy models for possible formation scenarios leave unique imprints on DNS orbital eccentricities, which may be measured by LISA for values as small as $sim$10$^{-2}$.
We report the discovery and initial follow-up of a double neutron star (DNS) system, PSR J1946$+$2052, with the Arecibo L-Band Feed Array pulsar (PALFA) survey. PSR J1946$+$2052 is a 17-ms pulsar in a 1.88-hour, eccentric ($e , =, 0.06$) orbit with a $gtrsim 1.2 , M_odot$ companion. We have used the Jansky Very Large Array to localize PSR J1946$+$2052 to a precision of 0.09 arcseconds using a new phase binning mode. We have searched multiwavelength catalogs for coincident sources but did not find any counterparts. The improved position enabled a measurement of the spin period derivative of the pulsar ($dot{P} , = , 9,pm , 2 ,times 10^{-19}$); the small inferred magnetic field strength at the surface ($B_S , = , 4 , times , 10^9 , rm G$) indicates that this pulsar has been recycled. This and the orbital eccentricity lead to the conclusion that PSR J1946$+$2052 is in a DNS system. Among all known radio pulsars in DNS systems, PSR J1946$+$2052 has the shortest orbital period and the shortest estimated merger timescale, 46 Myr; at that time it will display the largest spin effects on gravitational wave waveforms of any such system discovered to date. We have measured the advance of periastron passage for this system, $dot{omega} , = , 25.6 , pm , 0.3, deg rm yr^{-1}$, implying a total system mass of only 2.50 $pm$ 0.04 $M_odot$, so it is among the lowest mass DNS systems. This total mass measurement combined with the minimum companion mass constrains the pulsar mass to $lesssim 1.3 , M_odot$.
The discovery of a radioactively powered kilonova associated with the binary neutron star merger GW170817 was the first - and still only - confirmed electromagnetic counterpart to a gravitational-wave event. However, observations of late-time electromagnetic emission are in tension with the expectations from standard neutron-star merger models. Although the large measured ejecta mass is potentially explained by a progenitor system that is asymmetric in terms of the stellar component masses, i.e. with a mass ratio $q$ of 0.7-0.8, the known Galactic population of merging double neutron star (DNS) systems (i.e. those that will coalesce within billions of years or less) has, until now, only consisted of nearly equal-mass ($q > 0.9$) binaries. PSR J1913+1102 is a DNS system in a 5-hour, low-eccentricity ($e = 0.09$) orbit, implying an orbital separation of 1.8 solar radii, with the two neutron stars predicted to coalesce in 470 million years due to gravitational-wave emission. Here we report that the masses of the two neutron stars, as measured by a dedicated pulsar timing campaign, are $1.62 pm 0.03$ and $1.27 pm 0.03$ solar masses for the pulsar and companion neutron star, respectively; with a measured mass ratio $q = 0.78 pm 0.03$, it is the most asymmetric DNS among known merging systems. Based on this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30% (90% confidence) of the total population of merging DNS binaries. The coalescence of a member of this population offers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event.
We report here the Einstein@Home discovery of PSR J1913+1102, a 27.3-ms pulsar found in data from the ongoing Arecibo PALFA pulsar survey. The pulsar is in a 4.95-hr double neutron star (DNS) system with an eccentricity of 0.089. From radio timing with the Arecibo 305-m telescope, we measure the rate of advance of periastron to be 5.632(18) deg/yr. Assuming general relativity accurately models the orbital motion, this corresponds to a total system mass of 2.875(14) solar masses, similar to the mass of the most massive DNS known to date, B1913+16, but with a much smaller eccentricity. The small eccentricity indicates that the second-formed neutron star (the companion of PSR J1913+1102) was born in a supernova with a very small associated kick and mass loss. In that case this companion is likely, by analogy with other systems, to be a light (1.2 solar mass) neutron star; the system would then be highly asymmetric. A search for radio pulsations from the companion yielded no plausible detections, so we cant yet confirm this mass asymmetry. By the end of 2016, timing observations should permit the detection of two additional post-Keplerian parameters: the Einstein delay, which will enable precise mass measurements and a verification of the possible mass asymmetry of the system, and the orbital decay due to the emission of gravitational waves, which will allow another test of the radiative properties of gravity. The latter effect will cause the system to coalesce in ~0.5 Gyr.
In this review I briefly describe the nature of the three kinds of High-Mass X-ray Binaries (HMXBs), accreting through: (i) Be circumstellar disc, (ii) supergiant stellar wind, and (iii) Roche lobe filling supergiants. A previously unknown population of HMXBs hosting supergiant stars has been revealed in the last years, with multi-wavelength campaigns including high energy (INTEGRAL, Swift, XMM, Chandra) and optical/infrared (mainly ESO) observations. This population is divided between obscured supergiant HMXBs, and supergiant fast X-ray transients (SFXTs), characterized by short and intense X-ray flares. I discuss the characteristics of these types of supergiant HMXBs, propose a scenario describing the properties of these high-energy sources, and finally show how the observations can constrain the accretion models (e.g. clumpy winds, magneto-centrifugal barrier, transitory accretion disc, etc). Because they are the likely progenitors of Luminous Blue Variables (LBVs), and also of double neutron star systems, related to short/hard gamma-ray bursts, the knowledge of the formation and evolution of this HMXB population is of prime importance.
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