It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star--white dwarf (NSWD) mergers, motivated by FRB 180924-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by \cite{mar19}. By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field. If NSWD mergers can indeed create magnetars, then such objects could produce at least a subset of FRB 180924-like FRBs within the framework of flaring magnetars, since the ejecta, local environments, and host galaxies of the final remnants from NSWD mergers resemble those of BNS/BWD/AIC channels. This NSWD channel is also able to well explain both the observational properties of FRB 180924-like and FRB 180916.J0158+65-like FRBs within a large range in local environments and host galaxies.
There is growing evidence that a stable magnetar could be formed from the coalescence of double neutron stars. In previous papers, we investigated the signature of formation of stable millisecond magnetars in radio and optical/ultraviolet bands by assuming that the central rapidly rotating magnetar deposits its rotational energy in the form of a relativistic leptonized wind. We found that the optical transient PTF11agg could be the first evidence for the formation of post-merger millisecond magnetars. To enhance the probability of finding more evidence for the post-merger magnetar formation, i…
The merger of a double neutron star (NS-NS) binary may result in a rapidly rotating massive NS with an extremely strong magnetic field (i.e., a millisecond magnetar). In this case, the magnetic spin-down of the NS remnant provides an additional source of sustained energy injection, which would continuously power the merger ejecta. The thermal emission from the merger ejecta would give rise to a bright optical "magnetar-powered merger-nova". In this work, we carry out a complete search for magnetar-powered merger-nova from Swift short gamma-ray burst (SGRB) sample. We focus on short GRBs with e…
A rapidly rotating and highly magnetized remnant neutron star (NS; magnetar) could survive from a merger of double NSs and drive a powerful relativistic wind. The early interaction of this wind with the previous merger ejecta can lead to shock breakout (SBO) emission mainly in ultraviolet and soft X-ray bands, which provides an observational signature for the existence of the remnant magnetar. Here we investigate the effect of an anisotropic structure of the merger ejecta on the SBO emission. It is found that bolometric light curve of the SBO emission can be broadened, since the SBO can occur …
The study of short-duration gamma-ray bursts provides growing evidence that a good fraction of double neutron star mergers lead to the formation of stable millisecond magnetars. The launch of Poynting flux by the millisecond magnetars could leave distinct electromagnetic signatures that reveal the energy dissipation processes in the magnetar wind. In previous studies (Wang & Dai 2013b; Wang et al. 2015), we assume that the magnetar wind becomes completely lepton-dominated so that electrons/positrons in the magnetar wind are accelerated by a diffusive shock. However, theoretical modeling of pul…
GRB 080503 is a short gamma-ray burst (GRB) detected by \emph{Swift} and has been classified as a compact-star-merger-origin GRB. The soft extended emission and the simultaneous late re-brightening in both the X-ray and optical afterglow lightcurves raise interesting questions regarding its physical origin. We show that the broad-band data of GRB 080503 can be well explained within the framework of the double neutron star merger model, provided that the merger remnant is a rapidly-rotating massive neutron star with an extremely high magnetic field (i.e. a millisecond magnetar). We show that th…
Most neutron stars (NSs) and black holes (BHs) are believed to be the final remnants in the evolution of massive stars. In this study, we propose a new formation channel for the formation of BHs and peculiar NSs (specifically, magnetars and Thorne-$\dot{\rm Z}$ytkow objects [T$\dot{\rm Z}$Os]), which we refer to as the core merger-induced collapse (CMIC) model. This model involves the merger at the end of a common-envelope phase of an oxygen/neon/magnesium composition white dwarf and the core of a hydrogen-rich or helium-rich non-degenerate star, leading to the creation of peculiar new types o…
Recently born magnetars are promising candidates for the engines powering fast radio bursts (FRBs). The focus thus far has been placed on millisecond magnetars born in rare core-collapse explosions, motivated by the star forming dwarf host galaxy of the repeating FRB 121102, which is remarkably similar to the hosts of superluminous supernovae (SLSNe) and long gamma-ray bursts (LGRB). However, long-lived magnetars may also be created in binary neutron star (BNS) mergers, in the small subset of cases with a sufficiently low total mass for the remnant to avoid collapse to a black hole, or in the …
A massive millisecond magnetar may survive a merger of a neutron star (NS) binary, which would continuously power the merger ejecta. We develop a generic dynamic model for the merger ejecta with energy injection from the central magnetar. The ejecta emission (the "merger-nova") powered by the magnetar peaks in the UV band and the peak of lightcurve progressively shifts to an earlier epoch with increasing frequency. A magnetar-powered mergernova could have an optical peak brightness comparable to a supernova, which is a few tens or hundreds times brighter than the radioactive-powered merger-nov…
Magnetars, a population of isolated neutron stars with ultra-strong magnetic fields of $\sim 10^{14}-10^{15}$ G, have been increasingly accepted to explain a variety of astrophysical transients. A nascent millisecond-period magnetar can release its spin-down energy and power bright sources such as Gamma-ray Bursts (GRBs) and their subsequent X-ray plateaus, Super Luminous Supernovae (SLSNe), and the fast X-ray transients such as CDF-S XT-2. Magnetars with ages of $10^3-10^4$ years have been observed within the Milky Way Galaxy, which are found to power diverse transients with the expense of th…
Probing millisecond magnetar formation in binary neutron star mergers through X-ray follow-up of gravitational wave alerts
Université Paris-Saclay（巴黎萨克雷大学）；Université Paris Cité（巴黎西岱大学）；CEA（法国原子能和替代能源委员会）；CNRS（法国国家科学研究中心）；Max Planck Institute for Gravitational Physics (Albert Einstein Institute)（马克斯·普朗克引力物理研究所（爱因斯坦研究所））；CNRS/IN2P3, IJCLab（法国国家科学研究中心/法国国家核物理与粒子物理研究所，IJCLab实验室）；Institut d’Astrophysique de Paris（巴黎天文物理研究所）；Sorbonne Université（索邦大学）；Università di Torino（都灵大学）
Assuming that the shallow-decaying phase in the early X-ray lightcurves of gamma-ray bursts (GRBs) is attributed to the dipole radiations (DRs) of a newborn magnetar, we present a comparative analysis for the magnetars born in death of massive stars and merger of compact binaries with long and short GRB (lGRB and sGRB) data observed with the {\em Swift} mission. We show that the typical braking index ($n$) of the magnetars is $\sim 3$ in the sGRB sample, and it is $\sim 4$ for the magnetars in the lGRB sample. Selecting a sub-sample of the magnetars whose spin-down is dominated by DRs ($n\less…
The merger of a neutron star (NS) binary may result in the formation of a long-lived, or indefinitely stable, millisecond magnetar remnant surrounded by a low-mass ejecta shell. A portion of the magnetar's prodigious rotational energy is deposited behind the ejecta in a pulsar wind nebula, powering luminous optical/X-ray emission for hours to days following the merger. Ions in the pulsar wind may also be accelerated to ultra-high energies, providing a coincident source of high energy cosmic rays and neutrinos. At early times, the cosmic rays experience strong synchrotron losses; however, after…
Two bright X-ray transients were reported from the Chandra Deep Field South archival data, namely CDF-S XT1 and XT2. Whereas the nature of the former is not identified, the latter was suggested as an excellent candidate for a rapidly spinning magnetar born from a binary neutron star (BNS) merger. Here we propose a unified model to interpret both transients within the framework of the BNS merger magnetar model. According to our picture, CDF-S XT2 is observed from the "free zone" where the magnetar spindown powered X-ray emission escapes freely, whereas CDF-S XT1 originates from the "trapped zon…
There is accumulating evidence that at least a fraction of binary neutron star mergers result in rapidly spinning magnetars, with subrelativistic neutron-rich ejecta as massive as a small fraction of solar mass. The ejecta could be heated continuously by the Poynting flux emanated from the central magnetars. Such Poynting flux could become lepton-dominated so that a reverse shock develops. It was demonstrated that such a picture is capable of accounting for the optical transient PTF11agg (Wang & Dai 2013b). In this paper we investigate the X-ray and ultraviolet (UV) radiation as well as the op…
XRT 201423 is an X-ray transient with a nearly flat plateau lasting 4.1 ks followed by a steep decay. This feature indicates that it might come from a magnetar formed through a binary neutron star merger, similar to CDF-S XT2 and as predicted as a type of electromagnetic counterpart of binary neutron star mergers. We test the compliance of the data with this model and use the observed duration and flux of the X-ray signal as well as upper limits of optical emission to pose constraints on the parameters of the underlying putative magnetar. Both the free-zone and trapped-zone geometric configura…
Constraints on the central engine of merger-driven long gamma-ray bursts
Università di Tor Vergata（托尔维加塔大学）；INAF–Istituto di Radioastronomia（意大利国家天体物理研究所射电天文研究所）；The Open University of Israel（以色列开放大学）；The University of Tokyo（东京大学）；The Pennsylvania State University（宾夕法尼亚州立大学）
Fast radio bursts (FRBs) at cosmological distances still hold concealed physical origins. Previously Liu (2018) proposes a scenario that the collision between a neutron star (NS) and a white dwarf (WD) can be one of the progenitors of non-repeating FRBs and notices that the repeating FRBs can also be explained if a magnetar formed after such NS-WD merger. In this paper, we investigate this channel of magnetar formation in more detail. We propose that the NS-WD post-merger, after cooling and angular momentum redistribution, may collapse to either a black hole or a new NS or even remains as a hy…
Neutron star-neutron star mergers are known to be associated with short gamma-ray bursts. If the neutron star equation of state is sufficiently stiff, at least some of such mergers will leave behind a supramassive or even a stable neutron star that spins rapidly with a strong magnetic field (i.e., a magnetar). Such a magnetar signature may have been observed as the X-ray plateau following a good fraction (up to 50%) of short gamma-ray bursts, and it has been expected that one may observe short gamma-ray burst-less X-ray transients powered by double neutron star mergers. A fast X-ray transient …
Fast radio bursts (FRBs) are extragalactic, bright pulses of emission at radio frequency with milliseconds duration. Observationally, FRBs can be divided into two classes, repeating FRBs and non-repeating FRBs. At present, twenty repeating FRBs have been discovered with unknown physical origins. Localization of the first repeating FRB 121102 and discovery of an associated persistent radio source support that FRBs are powered by young millisecond magnetars, which could be formed by core-collapses of massive stars or binary neutron stars mergers. These two formation channels can be distinguished…
The joint detection of GW 170817 and GRB 170817A indicated that at least a fraction of short gamma ray bursts (SGRBs) originate from binary neutron star (BNS) mergers. One possible remnant of a BNS merger is a rapidly rotating, strongly magnetized neutron star, which has been discussed as one possible central engine for GRBs. For a rapidly rotating magnetar central engine, the deposition of the rotation energy into the ejecta launched from the merger could lead to bright radio emission. The brightness of radio emission years after an SGRB would provide an estimation of the kinetic energy of ej…
The coalescence of binary neutron stars (NSs) may in some cases produce a stable massive NS remnant rather than a black hole. Due to the substantial angular momentum from the binary, such a remnant is born rapidly rotating and likely acquires a strong magnetic field (a `millisecond magnetar'). Magnetic spin-down deposits a large fraction of the rotational energy from the magnetar behind the small quantity of mass ejected during the merger. This has the potential for creating a bright transient that could be useful for determining whether a NS or black hole was formed in the merger. We investig…
A 4.5-s Quasiperiodic Spectral Oscillation in GRB 230307A: Evidence for Free Precession of a Post-Merger Magnetar?
Nanjing University（南京大学）；Zhengzhou University（郑州大学）；Institute of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；The University of Hong Kong（香港大学）
Exploring the connection between Fast Radio Bursts and binary neutron star mergers
University of Pisa（比萨大学）；INFN - Pisa（意大利国家核物理研究所-比萨分部）；INAF - Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；INAF - Osservatorio Astronomico di Brera（意大利国家天体物理研究所布里埃拉天文台）；Universität Heidelberg, Zentrum für Astronomie (ZAH), Institut für Theoretische Astrophysik（海德堡大学天文学中心理论天体物理研究所）；Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen（海德堡大学跨学科科学计算中心）；University of Padova（帕多瓦大学）；INFN, Sezione di Padova（意大利国家核物理研究所帕多瓦分部）
R-process nucleosynthesis from magnetar giant flares in neutron star--white dwarf mergers: A unified picture for peculiar long gamma-ray bursts
Guangxi University of Science and Technology（广西科技大学）；Nanchang University（南昌大学）；Xiangtan University（湘潭大学）；Hunan University of Science and Technology（湖南科技大学）
As the endpoints of massive star evolution, neutron stars are enigmatic celestial objects characterized by extremely dense and exotic nuclear matter, magnetospheres with positrons (antimatter), rapid rotation and ultra-strong magnetic fields. Such an extreme environment has provided an accessible astrophysical laboratory to study physics under conditions unattainable on Earth and to tackle a range of fundamental questions related to: the aftermath of stellar evolution and the powerful explosions of massive stars, the equation of state and physics of some of the most exotic and magnetic stars i…
Millisecond magnetars can be formed via several channels: core-collapse of massive stars, accretion-induced collapse of white dwarfs (WDs), double WD mergers, double neutron star (NS) mergers, and WD-NS mergers. Because the mass of ejecta from these channels could be quite different, their light curves are also expected to be diverse. We evaluate the dynamic evolution of optical transients powered by millisecond magnetars. We find that the magnetar with short spin-down timescale converts its rotational energy mostly into the kinetic energy of the transient, while the energy of a magnetar with …
The merger of binary neutron stars (BNSs) can lead to large amplifications of the magnetic field due to the development of turbulence and instabilities in the fluid, such as the Kelvin-Helmholtz shear instability, which drive small-scale dynamo activity. In order to properly resolve such instabilities and obtain the correct magnetic field amplification, one would need to employ resolutions that are currently unfeasible in global general relativistic magnetohydrodynamic (GRMHD) simulations of BNS mergers. Here, we present a subgrid model that allows global simulations to take into account the s…
By performing fully general relativistic magnetohydrodynamic simulations of binary neutron star mergers, we investigate the possibility that the end result of the merger is a stable magnetar. In particular, we show that, for a binary composed of two equal-mass neutron stars (NSs) of gravitational mass M~1.2 Msun and equation of state similar to Shen et al. at high densities, the merger product is a stable NS. Such NS is found to be differentially rotating and ultraspinning with spin parameter J/M^2~0.86, where J is its total angular momentum, and it is surrounded by a disk of ~0.1 Msun. While …
If double neutron star mergers leave behind a massive magnetar rather than a black hole, a bright early afterglow can follow the gravitational wave burst (GWB) even if there is no short gamma-ray burst (SGRB) - GWB association or there is an association but the SGRB does not beam towards earth. Besides directly dissipating the proto-magnetar wind as suggested by Zhang, we here suggest that the magnetar wind could push the ejecta launched during the merger process, and under certain conditions, would reach a relativistic speed. Such a magnetar-powered ejecta, when interacting with the ambient m…
Magnetar bursts can be emitted by Alfvén waves growing in the outer magnetosphere to nonlinear amplitudes, $δB/B\sim 1$, and triggering magnetic reconnection. Similar magnetic flares should occur quasi-periodically in a magnetized neutron star binary nearing merger. In both cases, fast dissipation in the magnetic flare creates optically thick $e^\pm$ plasma, whose heat capacity is negligible compared with the generated radiation energy. Magnetic dissipation then involves photon viscosity and acts through Compton drag on the plasma bulk motions in the reconnection region. The effective temperat…
The contemporaneous detection of gravitational waves and gamma rays from the GW170817/GRB 170817A, followed by kilonova emission a day after, confirmed compact binary neutron-star mergers as progenitors of short-duration gamma-ray bursts (GRBs), and cosmic sources of heavy r-process nuclei. However, the nature (and lifespan) of the merger remnant and the energy reservoir powering these bright gamma-ray flashes remains debated, while the first minutes after the merger are unexplored at optical wavelengths. Here, we report the earliest discovery of bright thermal optical emission associated with…
The Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are a class of pulsars understood as neutron stars (NSs) with super strong surface magnetic fields, namely $B\gtrsim10^{14}$ G, and for that reason are known as Magnetars. However, in the last years some SGRs/AXPs with low surface magnetic fields $B\sim(10^{12}-10^{13})$ G have been detected, challenging the Magnetar description. Moreover, some fast and very magnetic white dwarfs (WDs) have also been observed, and at least one showed X-Ray energy emission as an ordinary pulsar. Following this fact, an alternative model base…
Compact binary mergers may have already been observed as they are the leading model for short gamma-ray bursts (sGRBs). Radioactive decay within the ejecta from these mergers is expected to produce an infra-red flare, dubbed macronova (or kilonova), on a time scale of a week. Recently two such macronova candidates were identified in followup observations of sGRBs, strengthening the possibility that those indeed arise from mergers. The same ejecta will also produce a long term (months to years) radio emission due to its interaction with the surrounding ISM. In search for this emission, we obser…
Magnetars younger than one century are expected to be hyper active. Besides winds powered by rotation they generate frequent magnetic flares, which launch powerful blast waves into the wind. These internal shocks act as masers producing fast (millisecond) radio bursts (FRBs) with the following properties. (1) GHz radio emission occurs at radii $r\sim 10^{14}$ cm and lasts $\lesssim 1$ ms in observer's time. (2) Induced scattering in the surrounding wind does not suppress the radio burst. (3) The emission has linear polarization set by the magnetar rotation axis. (4) The emission drifts to lowe…
As the technology of gravitational-wave and neutrino detectors becomes increasingly mature, a multi-messenger era of astronomy is ushered in. Advanced gravitational wave detectors are close to making a ground-breaking discovery of gravitational wave bursts (GWBs) associated with mergers of double neutron stars (NS-NS). It is essential to study the possible electromagnetic (EM) and neutrino emission counterparts of these GWBs. Recent observations and numerical simulations suggest that at least a fraction of NS-NS mergers may leave behind a massive millisecond magnetar as the merger product. Her…
The binary neutron star merger GW170817/GRB170817A confirmed that at least some neutron star mergers are the progenitors of short gamma-ray bursts. Many short gamma-ray bursts have long-term x-ray afterglows that have been interpreted in terms of post-merger millisecond magnetars---rapidly rotating, highly magnetised, massive neutron stars. We review our current understanding of millisecond magnetars born in short gamma-ray bursts, focusing particularly three main topics. First, whether millisecond magnetars really do provide the most plausible explain for the x-ray plateau. Second, determinin…
We review topics in searches for axion-like-particles (ALPs), covering material that is complementary to other recent reviews. The first half of our review covers ALPs in the extreme environments of neutron star cores, the magnetospheres of highly magnetized neutron stars (magnetars), and in neutron star mergers. The focus is on possible signals of ALPs in the photon spectrum of neutron stars and gravitational wave/electromagnetic signals from neutron star mergers. We then review recent developments in laboratory-produced ALP searches, focusing mainly on accelerator-based facilities including …
Mergers of two carbon-oxygen white dwarfs have long been suspected to be progenitors of Type Ia Supernovae. Here we present our modifications to the cosmological smoothed particle hydrodynamics code Gadget to apply it to stellar physics including but not limited to mergers of white dwarfs. We demonstrate a new method to map a one-dimensional profile of an object in hydrostatic equilibrium to a stable particle distribution. We use the code to study the effect of initial conditions and resolution on the properties of the merger of two white dwarfs. We compare mergers with approximate and exact b…
A magnetar formation in binary neutron star merger
Max Planck Institute for Gravitational Physics (Albert Einstein Institute)（马克斯·普朗克引力物理研究所（阿尔伯特·爱因斯坦研究所））；Yukawa Institute for Theoretical Physics, Kyoto University（京都大学汤川理论物理研究所）；Department of Physics, Toho University（东海大学物理学系）
The joint observation of GW170817 and GRB170817A proved that binary neutron star (BNS) mergers are progenitors of short Gamma-ray Bursts (SGRB): this established a direct link between the still unsettled SGRB central engine and the outcome of BNS mergers, whose nature depends on the equation of state (EOS) and on the masses of the NSs. We propose a novel method to probe the central engine of SGRBs based on this link. We produce an extended catalog of BNS mergers by combining recent theoretically predicted BNS merger rate as a function of redshift and the NS mass distribution inferred from meas…
Mergers of white dwarfs (WDs) may lead to a variety of transient astrophysical events, SNIa being one possible outcome. Lyutikov & Toonen (2017, 2019) argued that mergers of WDs result, under various parameter regimes, in unusual central engine-powered supernova and a type of short Gamma Ray Bursts that show extended emission tails. Observations by Gvaramadze et al. (2019) of the central star and the nebula J005311 match to the details the model of Lyutikov & Toonen (2017, 2019) for the immediate product of a merger of a heavy ONeMg WD with CO WD (age, luminosity, stellar size, hydrogen defici…
Following the GRB 170817A prompt emission lasting a fraction of a second, $10^8$ s of data in the X-rays, optical, and radio wavelengths have been acquired. We here present a model that fits the spectra, flux, and time variability of all these emissions, based on the thermal and synchrotron cooling of the expanding matter ejected in a binary white dwarf merger. The $10^{-3} M_\odot$ of ejecta, expanding at velocities of $10^9$ cm s$^{-1}$, are powered by the newborn massive, fast rotating, magnetized white dwarf with a mass of $1.3 M_\odot$, a rotation period of $\gtrsim 12$ s, and a dipole ma…
Newly born massive magnetars are generally considered to be produced by binary neutron star (NS) mergers, which could give rise to short gamma-ray bursts (SGRBs). The strong magnetic fields and fast rotation of these magnetars make them promising sources for gravitational wave (GW) detection using ground based GW interferometers. Based on the observed masses of Galactic NS-NS binaries, by assuming different equations of state (EOSs) of dense matter, we investigate the stochastic gravitational wave background (SGWB) produced by an ensemble of newly born massive magnetars. The massive magnetar f…
