Detectability of neutron star — White dwarf coalescences by eROSITA and ART-XC

A significant fraction of the stellar population in the cusp around central\nblack holes of galaxies consists of compact remnants of evolved stars, such as\nwhite dwarfs, neutron stars and stellar mass black holes. We estimate the rate\nof capture of compact objects by massive central black holes, assuming most\nspiral galaxies have a central black hole of modest mass ($\\sim 10^6 \\msun$),\nand a cuspy spheroid. It is likely that the total capture rate is dominated by\nnucleated spirals. We estimate the flux of gravitational wave radiation from\nsuch coalescences, and the estimated detectable source count for proposed\nspace--based gravitational wave observatories such as LISA. About one event per\nyear should be detectable within $1 \\, {\\rm Gpc}$, given very conservative\nestimates of the black hole masses and central galactic densities. We expect\n$10^2$--$10^3$ detectable sources at lower frequencies ($10^{-4}$ Hz) ``en\nroute'' to capture. If stellar mass black holes are ubiquitous, the signal may\nbe dominated by stellar mass black holes coalescing with massive black holes.\nThe rate of white dwarf--white dwarf mergers in the cores of nucleated spirals\nis estimated at $\\sim10^{-6}$ per year per galaxy.\n

The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRBs). In this paper, we present three-dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed, and the energy release is negligible. Peak accretion rates are ~0.05 M☉ s-1 (less for lower mass white dwarfs) and last for approximately a minute. Many of the disk parameters can be explained by a simple analytic model that we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses that are not simulated here. Although the mergers studied here create disks with larger radii and longer accretion times than those from the merger of double neutron stars, a larger fraction of the white dwarf's mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long-duration GRB. The event rate of these mergers may be as high as 10-6 yr-1 per galaxy.

The merger of a white dwarf (WD) and a neutron star (NS) is a relatively common event that produces an observable electromagnetic signal. Furthermore, the compactness of these stellar objects makes them an interesting candidate for gravitational wave (GW) astronomy, potentially being in the frequency range of LISA and other missions. To date, three-dimensional simulations of these mergers have not fully modeled the WD disruption or have used lower resolutions and have not included magnetic fields even though they potentially shape the evolution of the merger remnant. In this work, we simulated the merger of a 1.4 M⊙ NS with a 1 M⊙ carbon oxygen WD in the magnetohydrodynamic moving mesh code AREPO. We find that the disruption of the WD forms an accretion disk around the NS, and the subsequent accretion by the NS powers the launch of strongly magnetized, mildly relativistic jets perpendicular to the orbital plane. Although the exact properties of the jets could be altered by unresolved physics around the NS, the event could result in a transient with a larger luminosity than kilonovae. We discuss possible connections to fast blue optical transients (FBOTs) and long-duration gamma-ray bursts. We find that the frequency of GWs released during the merger is too high to be detectable by the LISA mission, but suitable for deci-hertz observatories such as LGWA, BBO, or DECIGO.

Double white dwarf (WD) merger process and their post-merger evolution are important in many fields of astronomy, such as supernovae, gamma-ray bursts, gravitational waves, and so on. The evolutionary outcomes of double ultra-massive WD merger remnants are still a subject of debate, though the general consensus is that the merger remnant will collapse to form a neutron star (NS). In this work, we investigate the evolution of a $2.20\, {\rm M}_{\odot }$ merger remnant stemmed from the coalescence of double $1.10\, {\rm M}_{\odot }$ ONe WDs. We find that the remnant ignites off-centre neon burning at the position near the surface of primary WD soon after the merger, resulting in the stable inwardly propagating oxygen/neon (O/Ne) flame. The final outcomes of the merger remnant are sensitive to the effect of convective boundary mixing. If the mixing cannot stall the O/Ne flame, the flame will reach the centre within 20 yr, leading to the formation of super Chandrasekhar mass silicon core, and its final fate probably be NS through iron-core-collapse supernova. In contrast, if the convective mixing is effective enough to prevent the O/Ne flame from reaching the centre, the merger remnant will undergo electron capture supernova to form an ONeFe WD. Meanwhile, we find that the wind mass loss process may hardly alter the final fate of the remnant due to its fast evolution. Our results imply that the coalescence of double ONe WDs can form short lived giant like object, but the final outcomes (NS or ONeFe WD) are influenced by the uncertain convective mixing in O/Ne flame.

Context.The mergers of neutron stars (NSs) and white dwarfs (WDs) could give rise to explosive transients, potentially observable with current and future transient surveys. However, the expected properties and distribution of such events is not well understood.Aims.Here we characterise the rates of such events, their delay-time distributions, their progenitors, and the distribution of their properties.Methods.We use binary population synthesis models and consider a wide range of initial conditions and physical processes. In particular we consider different common-envelope evolution models and different NS natal kick distributions. We provide detailed predictions arising from each of the models considered.Results.We find that the majority of NS–WD mergers are born in systems in which mass-transfer played an important role, and the WD formed before the NS. For the majority of the mergers the WDs have a carbon-oxygen composition (60−80%) and most of the rest are with oxygen-neon WDs. The time-integrated rates of NS–WD mergers are in the range of 3−15% of the type Ia supernovae (SNe) rate. Their delay-time distribution is very similar to that of type Ia SNe, but is slightly biased towards earlier times. They typically explode in young 100 Myr <τ< 1 Gyr environments, but have a tail distribution extending to long, gigayear-timescales. Models including significant kicks give rise to relatively wide offset distribution extending to hundreds of kiloparsecs.Conclusions.The demographic and physical properties of NS–WD mergers suggest they are likely to be peculiar type Ic-like SNe, mostly exploding in late-type galaxies. Their overall properties could be related to a class of recently observed rapidly evolving SNe, while they are less likely to be related to the class of Ca-rich SNe.

Recent observational evidence has demonstrated that white dwarf (WD) mergers are a highly efficient mechanism for mass accretion onto WDs in the galaxy. In this paper, we show that WD mergers naturally produce highly magnetized, uniformly rotating WDs, including a substantial population within a narrow mass range close to the Chandrasekhar mass (M Ch). These near-M Ch WD mergers subsequently undergo rapid spin up and compression on a ∼ 102 yr timescale, either leading to central ignition and a normal SN Ia via the DDT mechanism, or alternatively to a failed detonation and SN Iax through pure deflagration. The resulting SNe Ia and SNe Iax will have spectra, light curves, polarimetry, and nucleosynthetic yields similar to those predicted to arise through the canonical near-M Ch single degenerate (SD) channel, but with a t −1 delay time distribution characteristic of the double-degenerate channel. Furthermore, in contrast to the SD channel, WD merger near-M Ch SNe Ia and SNe Iax will not produce observable companion signatures. We discuss a range of implications of these findings, from SNe Ia explosion mechanisms, to galactic nucleosynthesis of iron peak elements including manganese.

The detections of four apparently young radio pulsars in the Milky Way globular clusters are difficult to reconcile with standard neutron star formation scenarios associated with massive star evolution. Here, we discuss formation of these young pulsars through white dwarf mergers in dynamically old clusters that have undergone core collapse. Based on observed properties of magnetic white dwarfs, we argue neutron stars formed via white dwarf merger are born with spin periods of roughly $10{\!-\!}100\,$ ms and magnetic fields of roughly $10^{11}{\!-\!}10^{13}\,$ G. As these neutron stars spin down via magnetic dipole radiation, they naturally reproduce the four observed young pulsars in the Milky Way clusters. Rates inferred from N-body cluster simulations as well as the binarity, host cluster properties, and cluster offsets observed for these young pulsars hint further at a white dwarf merger origin. These young pulsars may be descendants of neutron stars capable of powering fast radio bursts analogous to the bursts observed recently in a globular cluster in M81.

Advanced LIGO and Advanced Virgo are detecting a large number of binary stellar origin black hole (BH) mergers. A promising channel for accelerated BH merger lies in active galactic nucleus (AGN) discs of gas around supermasssive BHs. Here, we investigate the relative number of compact object (CO) mergers in AGN disc models, including BH, neutron stars (NS), and white dwarfs, via Monte Carlo simulations. We find the number of all merger types in the bulk disc grows ∝ t1/3 which is driven by the Hill sphere of the more massive merger component. Median mass ratios of NS–BH mergers in AGN discs are $\tilde{q}=0.07\pm 0.06(0.14\pm 0.07)$ for mass functions (MF) M−1(− 2). If a fraction fAGN of the observed rate of BH–BH mergers (RBH–BH) come from AGN, the rate of NS–BH (NS–NS) mergers in the AGN channel is ${R}_{\mathrm{ BH}\!-\!\mathrm{ NS}} \sim f_{\mathrm{ AGN}}[10,300]\, \rm {Gpc}^{-3}\, \rm {yr}^{-1},({\mathit{ R}}_{NS\!-\!NS} \le \mathit{ f}_{AGN}400\, \rm {Gpc}^{-3}\, \rm {yr}^{-1}$). Given the ratio of NS–NS/BH–BH LIGO search volumes, from preliminary O3 results the AGN channel is not the dominant contribution to observed NS–NS mergers. The number of lower mass gap events expected is a strong function of the nuclear MF and mass segregation efficiency. CO merger ratios derived from LIGO can restrict models of MF, mass segregation, and populations embedded in AGN discs. The expected number of electromagnetic (EM) counterparts to NS–BH mergers in AGN discs at z < 1 is $\sim [30,900]\, {\rm {yr}}^{-1}(f_{\mathrm{ AGN}}/0.1)$. EM searches for flaring events in large AGN surveys will complement LIGO constraints on AGN models and the embedded populations that must live in them.

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 Margalit et al. (2019). 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.

The "Scenario Machine" (a computer code designed for studies of the evolution of close binaries) was used to carry out a population synthesis for a wide range of merging astrophysical objects: main-sequence stars with main-sequence stars; white dwarfs with white dwarfs, neutron stars, and black holes; neutron stars with neutron stars and black holes; and black holes with black holes.We calculate the rates of such events, and plot the mass distributions for merging white dwarfs and main-sequence stars. It is shown that Type Ia supernovae can be used as standard candles only after approximately one billion years of evolution of galaxies. In the course of this evolution, the average energy of Type Ia supernovae should decrease by roughly 10%; the maximum and minimum energies of Type Ia supernovae may differ by no less than by a factor of 1.5. This circumstance should be taken into account in estimations of parameters of acceleration of the Universe. According to theoretical estimates, the most massive - as a rule, magnetic - white dwarfs probably originate from mergers of white dwarfs of lower mass. At least some magnetic Ap and Bp stars may form in mergers of low-mass main sequence stars (<1.5 mass of the Sun) with convective envelopes.

Context. Fast X-ray transients (FXTs) are bright X-ray flashes with durations of a few minutes to hours, peak isotropic luminosities of LX, peak ∼ 1042 − 1047 erg s−1, and total isotropic energies of E ∼ 1047 − 1050 erg. They have been detected with space-based telescopes such as Chandra, XMM-Newton, Swift-XRT, and Einstein Probe in the soft X-ray band. Einstein Probe detected &gt; 50 in its first year of operation. While several models have been proposed, the nature of many FXTs is currently unknown. One model predicts that FXTs are powered by the spin-down energy of newly formed millisecond magnetars. In this context, they are usually thought to form in a binary neutron star (BNS) merger. However, the rates seem to be in tension: the BNS volumetric rate is estimated to be ∼102 Gpc−3 yr−1, which barely overlaps with the estimated FXT volumetric rate of 103 − 104 Gpc−3 yr−1; thus, even in the small range of overlap, BNS mergers would need to produce FXTs with nearly 100% efficiency. Aims. We explore the maximum volumetric formation rate of millisecond spin period magnetars, including several possibilities beyond the BNS channel, comparing it with the volumetric rate of FXTs to determine what fraction of FXTs could have a millisecond magnetar origin. Methods. We compiled the estimated rate densities for several different suggested formation channels of rapidly spinning magnetars, including the accretion-induced collapse of white dwarfs, binary white dwarf mergers, neutron star–white dwarf mergers, and the collapse of massive stars. We converted the Milky Way event rates to volumetric rates, wherever necessary, by considering either the star formation rate or the stellar mass density distributions as a function of redshift. Results. We find that the highest possible rates among these possibilities come from binary white dwarf mergers and the collapse of massive stars. However, both scenarios may be unfavourable for FXT production due to uncertainties in the resultant spin and magnetic field distributions of the newly formed neutron stars and several observational constraints. Moreover, in all the scenarios, we find that the fraction of neutron stars that meet both criteria of rapid rotation and a strong magnetic field is either very low or highly uncertain. We conclude that millisecond magnetars are not the most viable progenitors of FXTs and can account for at most 10% of the entire FXT population.

The merger of two white dwarfs (WDs) creates a differentially rotating remnant which is unstable to magnetohydrodynamic instabilities. These instabilities can lead to viscous evolution on a time-scale short compared to the thermal evolution of the remnant. We present multi-dimensional hydrodynamic simulations of the evolution of WD merger remnants under the action of an $\alpha$-viscosity. We initialize our calculations using the output of eight WD merger simulations from Dan et al. (2011), which span a range of mass ratios and total masses. We generically find that the merger remnants evolve towards spherical states on time-scales of hours, even though a significant fraction of the mass is initially rotationally supported. The viscous evolution unbinds only a very small amount of mass $(< 10^{-5} M_\odot)$. Viscous heating causes some of the systems we study with He WD secondaries to reach conditions of nearly dynamical burning. It is thus possible that the post-merger viscous phase triggers detonation of the He envelope in some WD mergers, potentially producing a Type Ia supernova via a double detonation scenario. Our calculations provide the proper initial conditions for studying the long-term thermal evolution of WD merger remnants. This is important for understanding WD mergers as progenitors of Type Ia supernovae, neutron stars, R Coronae Borealis stars and other phenomena.

The merging of double white dwarfs (WDs) may produce the events of accretion-induced collapse (AIC) and form single neutron stars (NSs). Meanwhile, it is also notable that the recently proposed WD+He subgiant scenario has a significant contribution to the production of massive double WDs, in which the primary WD grows in mass by accreting He-rich material from a He subgiant companion. In this work, we aim to study the binary population synthesis (BPS) properties of AIC events from the double WD mergers by considering the classical scenarios and also the contribution of the WD+He subgiant scenario to the formation of double WDs. First, we provided a dense and large model grid of WD+He star systems for producing AIC events through the double WD merger scenario. Secondly, we performed several sets of BPS calculations to obtain the rates and single NS number in our Galaxy. We found that the rates of AIC events from the double WD mergers in the Galaxy are in the range of $1.4{-}8.9\times 10^{\rm -3}\, \rm yr^{\rm -1}$ for all ONe/CO WD+ONe/CO WD mergers, and in the range of $0.3{-}3.8\times 10^{\rm -3}\, \rm yr^{\rm -1}$ when double CO WD mergers are not considered. We also found that the number of single NSs from AIC events in our Galaxy may range from 0.328 × 107 to 1.072 × 108. The chirp mass of double WDs for producing AIC events distribute in the range of $0.55{-}1.25\, \rm M_{\odot }$. We estimated that more than half of double WDs for producing AIC events are capable to be observed by the future space-based gravitational wave detectors.

Studying compact object binary mergers in star clusters is crucial for understanding stellar evolution and dynamical interactions in galaxies. Open clusters in particular are more abundant over cosmic time than globular clusters. However, previous research on low-mass clusters with ≲103 M ⊙ has focused on binary black holes (BBHs) or black hole–neutron star (BH–NS) binaries. Binary mergers of other compact objects, such as white dwarfs (WDs), are also crucial as progenitors of transient phenomena such as Type Ia supernovae and fast radio bursts (FRBs). We present simulations of three types of open clusters with masses of 102, 103, and 104 M ⊙. In massive clusters with ≳104 M ⊙, BBHs are dynamically formed; however, less massive compact binaries such as WD–WDs and WD–NSs are perturbed inside the star clusters, causing them to evolve into other objects. We further find BH–NS mergers only in 103 M ⊙ clusters. Considering star clusters with a typical open cluster mass, we observe that WD–WD merger rates slightly increase for 103 M ⊙clusters but decrease for 102 M ⊙ clusters. Since the host clusters are tidally disrupted, most of them merge outside of the clusters. Our WD–WD merger results have further implications for two classes of transients. Super-Chandrasekhar WD–WD mergers are present in our simulations, demonstrating potential sources of FRBs at a rate of 70–780 Gpc−3 yr−1, higher than the rate estimated for globular clusters. Additionally, we find that carbon–oxygen WD–WD mergers in our open clusters (34–640 Gpc−3 yr−1) only account for 0.14%–2.6% of the observed Type Ia supernova rate in our local Universe.

Recent progress on the nature of short-duration gamma-ray bursts has shown that a fraction of them originate in the local Universe. These systems may well be the result of giant flares from soft gamma-repeaters (highly magnetized neutron stars commonly known as magnetars). However, if these neutron stars are formed via the core collapse of massive stars then it would be expected that the bursts should originate from predominantly young stellar populations, while correlating the positions of BATSE short bursts with structure in the local Universe reveals a correlation with all galaxy types, including those with little or no ongoing star formation. This is a natural outcome if, in addition to magnetars formed via the core collapse of massive stars, they also form via accretion-induced collapse following the merger of two white dwarfs, one of which is magnetic. We investigate this possibility and find that the rate of magnetar production via white dwarf–white dwarf (WD–WD) mergers in the Milky Way is comparable to the rate of production via core collapse. However, while the rate of production of magnetars by core collapse is proportional to the star formation rate, the rate of production via WD–WD mergers (which have long lifetimes) is proportional to the stellar mass density, which is concentrated in early-type systems. Therefore magnetars produced via WD–WD mergers may produce soft gamma-repeater giant flares which can be identified with early-type galaxies. We also comment on the possibility that this mechanism could produce a fraction of the observed short-duration burst population at higher redshift.