We propose a compact binary model with an eccentric orbit to explain periodically active fast radio burst (FRB) sources, where the system consists of a neutron star (NS) with strong dipolar magnetic fields and a magnetic white dwarf (WD). In our model, the WD fills its Roche lobe at periastron, and mass transfer occurs from the WD to the NS around this point. The accreted material may be fragmented into a number of parts, which arrive at the NS at different times. The fragmented magnetized material may trigger magnetic reconnection near the NS surface. The electrons can be accelerated to an ultra-relativistic speed, and therefore the curvature radiation of the electrons can account for the burst activity. In this scenario, the duty cycle of burst activity is related to the orbital period of the binary. We show that such a model may work for duty cycles roughly from ten minutes to two days. For the recently reported 16.35-day periodicity of FRB 180916.J0158+65, our model does not naturally explain such a long duty cycle, since an extremely high eccentricity ($e>0.95$) is required.
Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs. It has been predicted that localised thermonuclear bursts on white dwarfs can also take place, similar to Type I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately-strong magnetised white dwarf from a low-mass companion, have been observed on several occasions in the past $\approx40$ years. During these bursts the optical/UV luminosity increases by a factor of $>3$ in less t…
FRB 180916.J0158+65 has been found to repeatedly emit fast radio bursts with the period in roughly 16 days. We propose that such periodicity comes from orbit-induced, spin precession of the emitter, which is possibly a neutron star. Depending on the mass of the companion, the binary period ranges from several hundred to thousands of seconds. Such tight binaries have a relatively short lifetime, which does not likely come from the gravitational decay of a wide binary. We comment on the relation to GW190425 and the possibility in LISA and LIGO detections.
Rapid bursts at optical wavelengths have been reported for several accreting white dwarfs, where the optical luminosity can increase by up to a factor 30 in less than an hour fading on timescales of several hours, and where the energy release can reach $\approx10^{39}$ erg ("micronovae"). Several systems have also shown these bursts to be semi-recurrent on timescales of days to months and the temporal profiles of these bursts strongly resemble those observed in Type-I X-ray bursts in accreting neutron stars. It has been suggested that the observed micronovae may be the result of localised ther…
Recently, repeating fast radio bursts (FRBs) with a period of $P_{\rm FRB}=16.35\pm0.18$ days from FRB 180916.J0158+65 had been reported. It still remains controversial how to give rise to such a periodicity of this FRB. In this Letter, based on an assumption of a young pulsar surrounding by a debris disk, we attempt to diagnose whether the Lense-Thirring precession of the disk on the emitter can produce the observed periodicity. Our calculations indicate that the Lense-Thirring effect of a tilted disk can result in a precession period of 16 days for a mass inflow rate of $0.5-1.5\times10^{18}…
We estimate the size of a primordial black hole exploding today via a white hole transition, and the power in the resulting explosion, using a simple model. We point out that Fast Radio Bursts, strong signals with millisecond duration, probably extragalactic and having unknown source, have wavelength not far from the expected size of the exploding hole. We also discuss the possible higher energy components of the signal.
Gamma-ray bursts (GRBs) are luminous and violent phenomena in the universe. Traditionally, long GRBs are expected to be produced by the collapse of massive stars and associated with supernovae. However, some low-redshift long GRBs have no detection of supernova association, such as GRBs 060505, 060614 and 111005A. It is hard to classify these events convincingly according to usual classifications, and the lack of the supernova implies a non-massive star origin. We propose a new path to produce long GRBs without supernova association, the unstable and extremely violent accretion in a contact bi…
We propose a compact binary model for the fast radio burst (FRB) repeaters, where the system consists of a magnetic white dwarf (WD) and a neutron star (NS) with strong bipolar magnetic fields. When the WD fills its Roche lobe, mass transfer will occur from the WD to the NS through the inner Lagrange point. The accreted magnetized materials may trigger magnetic reconnection when they approach the NS surface, and therefore the electrons can be accelerated to an ultra-relativistic speed. In this scenario, the curvature radiation of the electrons moving along the NS magnetic field lines can accou…
Black holes are extremely dense and compact objects from which light cannot escape. There is an overall consensus that black holes exist and many astronomical objects are identified with black holes. White holes were understood as the exact time reversal of black holes, therefore they should continuously throw away material. It is accepted, however, that a persistent ejection of mass leads to gravitational pressure, the formation of a black hole and thus to the "death of while holes". So far, no astronomical source has been successfully tagged a white hole. The only known white hole is the Big…
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 fiel…
Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of $\sim 10^{4}$ s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy …
Fast radio bursts (FRBs) with unknown origin emit a huge luminosity (about 1 Jy at 1 GHz) with a duration of milliseconds or less at extragalactic distances estimated from their large dispersion measure (DM). We propose herein a scenario for a collision between a neutron star (NS) and a white dwarf (WD) as the progenitor of the FRBs by considering the burst duration scaling to the collision time and the radio luminosity proportional to the kinetic energy of the collision. The relations among the observed flux density, pulse width, and the DM are derived from the model and compared with the sta…
Repeating Fast Radio Bursts show temporal symmetry breaking on millisecond time scales (the "sad trombone"). On a time scale of days the repetitions of FRB 180916B occur at frequency-dependent phases of its 16.3 d period. Some models predict that all such periodic repeating FRB have the same sign of temporal asymmetry, while others predict that sources with both signs are equally abundant. Future observations of other periodically modulated repeating FRB may distinguish among models on this basis.
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…
Recently, Thornton et al. reported the detection of four fast radio bursts (FRBs). The dispersion measures indicate that the sources of these FRBs are at cosmological distance. Given the large full sky event rate ~ 10^4 sky^-1 day^-1, the FRBs are a promising target for multi-messenger astronomy. Here we propose double degenerate, binary white-dwarf (WD) mergers as the source of FRBs, which are produced by coherent emission from the polar region of a rapidly rotating, magnetized massive WD formed after the merger. The basic characteristics of the FRBs, such as the energetics, emission duration…
Periodic Radio and X-ray Emission from an Accreting White Dwarf Binary
The University of Sydney（悉尼大学）；Australia Telescope National Facility, CSIRO（澳大利亚望远镜国家设施，CSIRO）；ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)（引力波发现卓越研究中心（OzGrav））；University of Wisconsin-Milwaukee（威斯康星大学密尔沃基分校）；Curtin University（科廷大学）；University of North Carolina, Chapel Hill（北卡罗来纳大学教堂山分校）
White dwarfs are often found in binary systems with orbital periods ranging from tens of minutes to hours in which they can accrete gas from their companion stars. In about 15% of these binaries, the magnetic field of the white dwarf is strong enough ($\geq 10^6$ Gauss) to channel the accreted matter along field lines onto the magnetic poles. The remaining systems are referred to as "non-magnetic", since to date there has been no evidence that they have a dynamically significant magnetic field. Here we report an analysis of archival optical observations of the "non-magnetic" accreting white dw…
Fast radio bursts (FRBs) are transient intense radio pulses with duration of milliseconds. Although the first FRB was detected more than a decade ago, the progenitors of these energetic events are not yet known. The currently preferred formation channel involves the formation of a neutron star (NS)/magnetar. While these objects are often the end product of the core-collapse (CC) explosion of massive stars, they could also be the outcome of the merging of two massive white dwarfs. In the merger scenario the ejected material interacts with a constant-density circumbinary medium and creates super…
We revisit the eccentric neutron star (NS)-white dwarf (WD) binary model for the periodic activity of fast radio burst (FRB) sources, by including the effects of gravitational wave (GW) radiation. In this model, the WD fills its Roche lobe at the periastron and mass transfer occurs from the WD to the NS. The accreted materials can be fragmented and arrive at the NS episodically, resulting in multiple bursts through curvature radiation. Consequently, the WD may be kicked away owing to the conservation of angular momentum. To initiate the next mass transfer, the WD has to refill its Roche lobe t…
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 activity of the repeating FRB 20180916B is periodically modulated with a period of 16.3 days, and FRB 121102 may be similarly modulated with a period of about 160 days. In some models of this modulation the period derivative is insensitive to the uncertain parameters; these models can be tested by measurement of or bounds on the derivative. In other models values of the uncertain parameters can be constrained. Periodic modulation of aperiodic bursting activity may result from emission by a narrow beam wandering within a cone or funnel along the axis of a precessing disc, such as the accret…
We discuss a scenario of short Gamma-Ray Bursts (GRBs) following a merger of a massive ONeMg white dwarf (WD) with a CO WD, and an ensuing accretion induced collapse (AIC). An initial system with the primary mass $M_1 \sim 6-10 \, M_\odot$ and the secondary mass $M_2 \sim 3-6\, M_\odot$ forms, via two distinct evolutionary channels, a double degenerate CO-ONeMg WD system. For sufficiently large mass ratio $q \equiv M_2/M_1 > q_{crit} \sim 0.25$ the ensuing gravitational wave-driven mass transfer is unstable, whereby the less massive CO WD is disrupted and transfers its mass to the primary ONeM…
The Galactic hard X-ray transient SWIFT J195509+261406 was first observed as a gamma-ray burst GRB 070610. Within 3 days after the burst, more than forty optical flares had been observed. Here we propose that this peculiar event should be associated with a white dwarf. The hard X-ray burst itself may be triggered by the collision between two planets orbiting the white dwarf. Some cracked frag-ments produced in the collision then fall onto the surface of the white dwarf in several days, giving birth to the observed optical flares via cyclotron radiation. Our model can satisfactorily explain the…
We investigate observational properties of accretion-induced collapse (AIC) of white dwarfs in radio frequencies. If AIC is triggered by accretion from a companion star, a dense circumstellar medium can be formed around the progenitor system. Then, the ejecta from AIC collide to the dense circumstellar medium, making a strong shock. The strong shock can produce synchrotron emission which can be observed in radio frequencies. Even if AIC occurs as a result of white dwarf mergers, we argue that AIC may cause fast radio bursts if a certain condition is satisfied. If AIC forms neutron stars which …
The variable star AR Sco was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. The system is composed of a cool, low-mass star in a tight, 3.55 hr orbit with a more massive white dwarf. Here we report new optical observations of AR Sco that show strong linear polarization (up to 40%) which varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low level (< a few %) circular polarization. These observations support the notion that, similar to neutr…
The two radio pulses (TRPs) from SGR 1935+2154 detected by \cite{BKRMHB} and \cite{2020arXiv200510324T} have similar features to that of cosmological fast radio bursts (FRBs). Many authors directly call the TRPs as FRB 200428 without consider two questions carefully. (1) Are the TRPs just two brighter subpulses of a normal radio pulse liking the normal radio pulses seen in other magnetars during their outburst? (2) If the TRPs are two bursts of an FRB, does this make other FRBs difficult to understand? In this paper, we try to clarify these two questions. First, we compare these TRPs with prev…
Gamma-ray bursts (GRBs) and supernovae (SNe) bring new perspectives to the study of neutron stars and white dwarfs, as well as opening new branches of theoretical physics and astrophysics.
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）
The repeating fast radio burst (FRB) localized to a globular cluster in M81 challenges our understanding of FRB models. In this Letter, we explore dynamical formation scenarios for objects in old globular clusters that may plausibly power FRBs. Using N-body simulations, we demonstrate that young neutron stars may form in globular clusters at a rate of up to $\sim50\,\rm{Gpc}^{-3}\,\rm{yr}^{-1}$ through a combination of binary white dwarf mergers, white dwarf--neutron star mergers, binary neutron star mergers, and accretion induced collapse of massive white dwarfs in binary systems. We consider…
We show that the 216.8$\pm$0.1 ms periodicity reported for the fast radio burst (FRB) 20191221A is very constraining for burst models. The high accuracy of burst periodicity (better than one part in 10$^3$), and the 2\% duty cycle (ratio of burst-duration and inter-burst interval), suggest a pulsar-like rotating beam model for the observed activity; the radio waves are produced along open field lines within $\sim 10^7$ cm of the neutron star surface, and the beam periodically sweeps across the observer as the star spins. According to this picture, FRB 20191221A is a factor $\sim 10^{12}$ scale…
The hydrogen and helium accreted by X-ray bursting neutron stars is periodically consumed in runaway thermonuclear reactions that cause the entire surface to glow brightly in X-rays for a few seconds. With models of the emission, the mass and radius of the neutron star can be inferred from the observations. By simultaneously probing neutron star masses and radii, X-ray bursts are one of the strongest diagnostics of the nature of matter at extremely high densities. Accurate determinations of these parameters are difficult, however, due to the highly non-ideal nature of the atmospheres where X-r…
We analyze temporal evolution of the number of accreting white dwarfs with shell hydrogen burning in semidetached and detached binaries. We consider a stellar system in which star formation lasts for 10 Gyr with a constant rate, as well as a system in which the same amount of stars is formed in a single burst lasting for 1 Gyr. Evolution of the number of white dwarfs is confronted to the evolution of occurrence rate of events that usually are identified with SN Ia or accretion-induced collapses, i.e. with accumulation of Chandrasekhar mass by a white dwarf or a merger of a pair of CO white dwa…
A 3.56-hour white dwarf (WD) - M dwarf (MD) close binary system, AR Scorpii, was recently reported to show pulsating emission in radio, IR, optical, and UV, with a 1.97-minute period, which suggests the existence of a WD with a rotation period of 1.95 minutes. We propose a model to explain the temporal and spectral characteristics of the system. The WD is a nearly perpendicular rotator, with both open field line beams sweeping the MD stellar wind periodically. A bow shock propagating into the stellar wind accelerates electrons in the wind. Synchrotron radiation of these shocked electrons can n…
Two X-ray sources were recently discovered by Irwin et al. in compact companions to elliptical galaxies to show ultra-luminous flares with fast rise (~ minute) and decay (~ hour), and with a peak luminosity ~10^{40-41} erg/s. Together with two other sources found earlier, they constitute a new type of fast transients which cannot be attributed to neutron stars but might be due to intermediate-mass black holes (IMBHs; 10^{2-4} M_sun). The flaring behavior is recurrent for at least two sources. If the flare represents a short period of accretion onto an IMBH during the periastron passage of a do…
We study the final stages of the evolution of a binary system consisted of a black hole and a white dwarf star. We implement the quantum hydrodynamic equations and carry out numerical simulations. As a model of a white dwarf star, we consider a zero temperature droplet of attractively interacting degenerate atomic bosons and spin-polarized atomic fermions. Such mixtures are investigated experimentally nowadays. We find that the white dwarf star is stripped off its mass while passing the periastron. Due to nonlinear effects, the accretion disk originated from the white dwarf becomes fragmented …
White dwarfs inspiraling into black holes of mass $\MBH\simgt 10^5M_\odot$ are detectable sources of gravitational waves in the LISA band. In many of these events, the white dwarf begins to lose mass during the main observational phase of the inspiral. The mass loss starts gently and can last for thousands of orbits. The white dwarf matter overflows the Roche lobe through the $L_1$ point at each pericenter passage and the mass loss repeats periodically. The process occurs very close to the black hole and the released gas can accrete, creating a bright source of radiation with luminosity close …
Long Period Transients (LPTs): a comprehensive review
Institute of Space Sciences (ICE), CSIC（空间科学研究所）；Institut d’Estudis Espacials de Catalunya (IEEC)（加泰罗尼亚太空研究学院）；International Centre for Radio Astronomy Research, Curtin University（西澳大学国际射电天文研究中心）；Sydney Institute for Astronomy, School of Physics, The University of Sydney（悉尼大学物理学院天体物理学研究所）；ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)（引力波发现卓越中心）
The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint gamma-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model in…
Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on time-scales of ~days or longer. Here, we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on time-scales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches betwee…
The aim of this white paper is to discuss the observing strategies for the LSST Wide-Fast-Deep that would improve the study of blazars (emission variability, census, environment) and Fast Radio Bursts (FRBs). For blazars, these include the adoption of: i) a reference filter to allow reconstruction of a well-sampled light curve not affected by colour changes effects; ii) two snapshots/visit with different exposure times to avoid saturation during flaring states; iii) a rolling cadence to get better-sampled light curves at least in some time intervals. We also address the potential importance of…
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…
A Falsifiable Timing Test for the Double-White-Dwarf Model of Long-Period Transients
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天文学与天体物理重点实验室（南京大学））
It has been argued that the long gamma-ray burst (GRB) of GRB 060614 without an associated supernova (SN) has challenged the current classification and fuel model for long GRBs, and thus a tidal disruption model has been proposed to account for such an event. Since it is difficult to detect SNe for long GRBs at high redshift, the absence of an SN association cannot be regarded as the solid criterion for a new classification of long GRBs similar to GRB 060614, called GRB 060614-type bursts. Fortunately, we now know that there is an obvious periodic substructure observed in the prompt light curv…
The recent detection of a repeating fast radio burst (FRB) in an old globular cluster in M81 challenges traditional FRB formation mechanisms based on magnetic activity in young neutron stars formed recently in core-collapse supernovae. Furthermore, the detection of this repeater in such a nearby galaxy implies a high local universe rate of similar events in globular clusters. Building off the properties inferred from the M81 FRB, we predict the number of FRB sources in nearby ($d \lesssim 20\,$Mpc) galaxies with large globular cluster systems known. Incorporating the uncertain burst energy dis…
Nonlinear effects are crucial for the propagation of Fast Radio Bursts (FRBs) near the source. We study the filamentation of FRBs in the relativistic winds of magnetars, which are commonly invoked as the most natural FRB progenitors. As a result of filamentation, the particle number density and the radiation intensity develop strong gradients along the direction of the wind magnetic field. A steady state is reached when the plasma pressure balances the ponderomotive force. In such a steady state, particles are confined into periodically spaced thin sheets, and electromagnetic waves propagate b…
Marsh et al. detected radio and optical pulsations from the binary system AR Scorpii (AR Sco). This system, with an orbital period of 3.56 h, is composed of a cool, low-mass star and a white dwarf with a spin period of 1.95 min. Optical observations by Buckley et al. showed that the polarimetric emission from the white dwarf is strongly linearly polarised ( up to $\sim40\%$) with periodically changing intensities. This periodic non-thermal emission is thought to be powered by the highly magnetised ($ 5 \times 10^{8} $ G) white dwarf that is spinning down. The morphology of the polarisation sig…
Fast radio bursts (FRBs) are mysterious radio bursts with a time scale of approximately milliseconds. Two populations of FRB, namely repeating and non-repeating FRBs, are observationally identified. However, the differences between these two and their origins are still cloaked in mystery. Here we show the time-integrated luminosity-duration ($L_ν$-$w_{\rm int,rest}$) relations and luminosity functions (LFs) of repeating and non-repeating FRBs in the FRB Catalogue project. These two populations are obviously separated in the $L_ν$-$w_{\rm int,rest}$ plane with distinct LFs, i.e., repeating FRBs…
Astrophysical explosions result from the release of magnetic, gravitational, or thermonuclear energy on dynamical timescales, typically the sound-crossing time for the system. These explosions include solar and stellar flares, eruptive phenomena in accretion disks, thermonuclear combustion on the surfaces of white dwarfs and neutron stars, violent magnetic reconnection in neutron stars, thermonuclear and gravitational collapse supernovae and cosmic gamma-ray bursts, each representing a different type and amount of energy release. This paper summarizes the properties of these explosions and des…
