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 naturally account for the broad-band (from radio to X-rays) spectral energy distribution of the system.
The binary system AR Scorpii hosts an M-type main sequence cool star orbiting around a magnetic white dwarf in the Milky Way Galaxy. The broadband non-thermal emission over radio, optical and X-ray wavebands observed from AR Scorpii indicates strong modulations on the spin frequency of the white dwarf as well as the spin-orbit beat frequency of the system. Therefore, AR Scorpii is also referred to as a white dwarf pulsar wherein a fast spinning white dwarf star plays very crucial role in the broadband non-thermal emission. Several interpretations for the observed features of AR Scorpii appear …
We present high-speed optical photometric observations, spanning ~2 years, of the recently-discovered white dwarf pulsar AR Scorpii. The amplitudes of the orbital, spin and beat modulations appear to be remarkably stable and repeatable over the time span of our observations. It has been suggested that the polarized and non-polarized emission from AR Scorpii is powered by the spin-down of the white dwarf. However, we find that our new data is inconsistent with the published spin-down ephemeris. Whilst our data is consistent with a constant spin period further observations over an extended time-…
We present detailed optical photo-polarimetric observations of the recently-discovered white dwarf pulsar AR Scorpii. Our extensive dataset reveals that the polarized emission is remarkably stable and repeatable with spin, beat and orbital modulations. This has enabled us to construct a simple geometrical model which assumes that all of the optically polarized emission emanates from two diametrically opposed synchrotron emission regions on the white dwarf magnetosphere. We suggest that the observed polarimetric modulations occur as a result of an enhanced injection of relativistic electrons in…
We report a study of the X-ray emission from the white dwarf/M-type star binary system AR Scorpii using archival data taken in 2016-2020. It has been known that the X-ray emission is dominated by the optically thin thermal plasma emission, and its flux level varies significantly over the orbital phase. The X-ray emission also contains a component that modulates with the beat frequency between the white dwarf's spin frequency and orbital frequency. In this new analysis, the 2020 data taken by NICER shows that the X-ray emission is modulating with the spin frequency as well as the beat frequency…
AR~Scorpii is an intermediate polar system composed of a magnetic white dwarf (WD) and an M-type star, and shows non-thermal, pulsed, and highly linearly polarized emission. The radio/optical emission modulates with the WD's spin and show the double peak structure in the light curves. In this paper, we discuss a possible scenario for the radiation mechanism of AR~Scorpii. The magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. The accelerated…
In this work, we have analyzed the $γ$-ray data in the energy range 100 MeV to 500 GeV from the \emph{Fermi}-Large Area Telescope (LAT) observations for the period August 4, 2008 to March 31, 2019. The $γ$-ray emission from AR Scorpii over the last decade is not statistically significant and therefore 2$σ$ upper limit on the integral flux above 100 MeV has been estimated. We reproduce the non-thermal broadband spectral energy distribution of AR Scorpii using an emission model having two synchrotron components due to the relativistic electrons in very high magnetic fields. The first component (…
The discovery of the white dwarf binary system AR Scorpii (AR Sco) with its fascinating non-thermal dominated multi-frequency emission has sparked renewed interest in potential high energy gamma-ray emission from white dwarf pulsars. The Spectral Energy Distribution (SED) below and above optical shows evidence of non-thermal synchrotron emission, with pulsed emission in optical and X-ray bands at the white dwarf spin period ($P_{*} = 117 \, \mbox{s}$) as well as a beat period ($P_{\rm b} = 118.1 \, \mbox{s}$) with the binary period. From an energy perspective, the highly magnetic rotating whit…
AR Scorpii is a close binary system containing a rotation powered white dwarf and a low mass M type companion star. This system shows non-thermal emission extending up to the X-ray energy range. We consider hybrid (lepto-hadronic) and pure hadronic models for the high energy non-thermal processes in this binary system. Relativistic electrons and hadrons are assumed to be accelerated in a strongly magnetised, turbulent region formed in collision of a rotating white dwarf magnetosphere and a magnetosphere/dense atmosphere of the M dwarf star. We propose that the non-thermal X-ray emission is pro…
AR Scorpii (AR Sco), the only-known radio-pulsing white dwarf binary, shows unusual pulsating emission at the radio, infrared, optical and ultraviolet bands. To determine its astrometric parameters at the radio band independently, we conducted multi-epoch Very Long Baseline Interferometry (VLBI) phase-referencing observations with the European VLBI Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) plus the Warkworth 30-metre telescope (New Zealand) at 8.6 GHz. By using the differential VLBI astrometry, we provide high-precision astrometric measurements on the parallax ($π=8.52_{-0.07}^…
We analyze rapid-cadence, multiwavelength photometry of AR Scorpii from three observatories, covering five observing seasons. We measure the arrival times of the system's beat pulses and use them to compute an updated ephemeris. The white dwarf spin-down rate is estimated with an uncertainty of only 4%. These results confirm, beyond any doubt, that the white dwarf's spin period is increasing at the rate consistent with by that of Stiller et al. (2018). We study the evolution of the beat pulse's color index across the orbit. The color of the primary pulse maxima varies significantly across the …
We report the analysis result of UV/X-ray emission from AR~Scorpii, which is an intermediate polar (IP) composed of a magnetic white dwarf and a M-type star, with the XMM-Newton data. The X-ray/UV emission clearly shows a large variation over the orbit, and their intensity maximum (or minimum) is located at the superior conjunction (or inferior conjunction) of the M-type star orbit. The hardness ratio of the X-ray emission shows a small variation over the orbital phase, and shows no indication of the absorption by an accretion column. These properties are naturally explained by the emission fr…
Orbital Period Changes of Recurrent Nova U Scorpii Demonstrate that M$_{\rm ejecta}$=26$\times$M$_{\rm accreted}$ and Is Not a Type Ia Supernova Progenitor
Constraining the Bulk Properties of Dense Matter by Measuring Millisecond Pulsar Masses - A White Paper for the Astronomy and Astrophysics Decadal Survey, CFP Panel
This paper investigates the progenitors of Millisecond Pulsars (MSPs) with a distribution of long orbital periods (P_orb > 2 d), to show the link between white dwarf (WD) binaries and long orbits for some binary MSPs through the Accretion Induced Collapse (AIC) of a WD. For this purpose, a model is presented to turn binary MSPs into wide binaries and highly circular orbits (e < 0.1) through the asymmetric kick imparted to the pulsar during the AIC process, which may indicate a sizeable kick velocity along the rotation of the proto-neutron star. The results show the effects of shock wave, bindi…
Recent observational and theoretical results have suggested that some of ultraluminous X-ray (ULX) sources may contain neutron star (NS) accretors. However, the formation channel and properties of donor stars of NS ULXs remain uncertain. By adopting the non-conservative and rotation-dependent mass transfer model in the primordial binary evolution, we investigate the way to form pulsar ULXs like observed pulsar ULXs in a systematic way. Our simulation results indicate that pulsar ULXs with Be stars and intermediate or/and high mass donors match observed apparent luminosities, orbital periods an…
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…
Recently, extra motivation has been given to the investigations of an unresolved problem of millisecond pulsars (MSPs) produced by the recycling process, as an apparent role of the accretion-induced collapse (AIC) in white dwarfs (WDs) was suggested to this concern. I have found that the distribution of the orbital periods of binary MSPs in the Galactic disk closely follows an exponential distribution. I have also determined the best-fit mean value of Nobs by fitting our data with an exponential distribution for the MSP population. As a result, it can be stated that reaching the Chandrasekhar …
Redbacks (RBs) and black widows (BWs) are two peculiar classes of eclipsing millisecond pulsars (MSPs). The accretion-induced collapse (AIC) of an oxygen/neon/magnesium composition white dwarf to a neutron star has been suggested as one possible formation pathway for those two classes of MSPs. However, it is difficult to produce all known MSPs with the traditional AIC scenario. In this study by using the MESA stellar evolution code, we investigate the detailed pre-AIC evolution of magnetized white dwarf binaries with the magnetic confinement model where the high magnetic field strength of the …
Binary millisecond pulsars (MSPs) are believed to have descended from low-mass X-ray binaries (LMXBs), which have experienced substantial mass transfer and tidal circularization. Therefore, they should have very circular orbits. However, the discovery of several eccentric binary MSPs (with eccentricity $e\sim 0.01-0.1$) challenges this standard picture. Three models have been proposed thus far based on accretion-induced collapse of massive white dwarfs (WDs), neutron star-strange star transition, and formation of circumbinary disks. All of them are subject to various uncertainties, and are not…
We have discovered a new X-ray emitting compact binary that is the likely counterpart to the unassociated Fermi-LAT GeV $γ$-ray source 4FGL J1120.0-2204, the second brightest Fermi source that still remains formally unidentified. Using optical spectroscopy with the SOAR telescope, we have identified a warm ($T_{\textrm{eff}}\sim8500$ K) companion in a 15.1-hr orbit around an unseen primary, which is likely a yet-undiscovered millisecond pulsar. A precise Gaia parallax shows the binary is nearby, at a distance of only $\sim 820$ pc. Unlike the typical "spider" or white dwarf secondaries in shor…
We report on a search for pulsars at the positions of eight low-mass white dwarfs and one higher-mass white dwarf with the 100-m Effelsberg Radio Telescope. These systems have orbital parameters suggesting that their unseen companions are either massive white dwarfs or neutron stars. Our observations were performed at 1.36 GHz, reaching sensitivities of 0.1-0.2 mJy. We searched our data accounting for the possible acceleration and jerk of the pulsar signals due to orbital motion, but found no significant pulsar signals. Considering our result jointly with 20 non-detections of similar systems w…
The standard formation theory of binary millisecond pulsars (BMSPs) predicts efficient orbital circularization due to tidal interaction during the previous mass transfer phase. Therefore, BMSPs are expected to have a circular orbit. However, the discovery of several eccentric BMSPs (eBMSPs) with a white dwarf (WD) companion has challenged this picture. In particular, recent observation reveals that the spin angular momentum of the eBMSP J0955$-$6150 is tilted at an angle $>4.8^{\rm \degree}$ from the orbital angular momentum. This is the first time that a tilt angle is deduced for eBMSPs, whic…
We found that ZTF J185139.81+171430.3 = ZTF18abnbzvx shows a very short [0.00858995(3) d = 12.37 min] and large-amplitude (0.8 mag) coherent variations using Public Data Release of Zwicky Transient Facility observations. The only known object that shows similar very short period, large-amplitude and coherent variations is the unique white dwarf pulsar AR Sco. The variations in ZTF J185139.81+171430.3 may arise from the mechanism as in AR Sco and should deserve attention.
Binaries harbouring millisecond pulsars enable a unique path to determine neutron star masses: radio pulsations reveal the motion of the neutron star, while that of the companion can be characterised through studies in the optical range. PSR J1012+5307 is a millisecond pulsar in a 14.5-h orbit with a helium-core white dwarf companion. In this work we present the analysis of an optical spectroscopic campaign, where the companion star absorption features reveal one of the lightest known white dwarfs. We determine a white dwarf radial velocity semi-amplitude of K_2 = 218.9 +- 2.2 km/s, which comb…
The one and the only: the pulsar - white dwarf system in NGC 6749
Max-Planck-Institut für Radioastronomie（马克斯·普朗克射电天文研究所）；Beijing Normal University（北京师范大学）；Alma Mater Studiorum Università di Bologna（博洛尼亚大学）；INAF - Osservatorio di Astrofisica e Scienze dello Spazio di Bologna（意大利国家天体物理研究所博洛尼亚天文与空间科学台）；National Time Service Center, Chinese Academy of Sciences（中国科学院国家授时中心）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Guizhou University（贵州大学）；Guizhou Radio Astronomical Observatory（贵州无线电天文观测站）；Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台射电天文与技术重点实验室）
The low frequency radio source GLEAM-X J162759.5$-$523504.3 emits pulsed coherent polarized emission like that of radio pulsars. Yet its period of 18.18 min (1091 s) is hundreds of times longer than those of confirmed radio pulsars, and if it is a neutron star its mean radiated power exceeds the upper limit on its spin-down power by more than an order of magnitude. This may be explained if it is a white dwarf pulsar, with a moment of inertia several orders of magnitude greater than those of neutron stars. The Lorentz factor of the emitting charge bunches may be bounded from below by the widths…
I found that ASASSN-V J205543.90+240033.5 shows large-amplitude (1.2-1.4 mag) nearly sinusoidal variations with a period of 10.803(2) d and very short period variations with a period of 0.0068 d using Public Data Release of Zwicky Transient Facility observations. The only known object that shows a similar combination of nearly sinusoidal reflection variations and very short period, large-amplitude variations is the unique white dwarf pulsar AR Sco. ASASSN-V J205543.90+240033.5 appears to be very similar to AR Sco and observations at various wavelengths are desired.
Formation of millisecond pulsars with CO white dwarf companions - II. Accretion, spin-up, true ages and comparison to MSPs with He white dwarf companions
Using the SDSS and Pan-STARRS1 survey data, we found a likely companion of the recently discovered binary $γ$-ray radio-loud millisecond pulsar J0621+2514. Its visual brightness is about 22 mag. The broad band magnitudes and colours suggest that this is a white dwarf. Comparing the data with various white dwarfs evolutionary tracks, we found that it likely belongs to a class of He-core white dwarfs with a temperature of about 10 000 K and a mass of $\lesssim 0.5$M$_\odot$. For a thin hydrogen envelope of the white dwarf its cooling age is $\lesssim0.5$ Gyr which is smaller than the pulsar char…
This paper uses population synthesis to investigate the possible origin of isolated millisecond pulsars (IMSPs) as born from the coalescence of a neutron star (NS) and a white dwarf (WD). Results show that the galactic birth - rate of IMSPs is likely to lie between $5.8\times10^{-5}$ $\rm yr^{-1}$ and $2.0\times10^{-4}$ $\rm yr^{-1}$, depending on critical variables, such as the stability of mass transfer via the Roche lobe and the value of kick velocity. In addition to this, this paper estimates that the solar mass of IMSPs can range from $1.5$ and 2.0 $M_{\odot}$, making them more massive th…
We report the discovery of a previously unidentified pulsar as part of a radio campaign to identify neutron star companions to low-mass white dwarfs (LMWDs) using the Robert C.\ Byrd Green Bank Telescope (GBT). PSR J0802-0955, which is coincident with the position of a WD with a mass of 0.2 solar masses, has a pulse period of 571 ms. Because of its relatively long pulse period, the lack of radial velocity (RV) variations in the radio data, and GBT's large beam size at the observing frequency of 340 MHz, we conclude that PSR J0802-0955 is unassociated with the LMWD at roughly the same position …
We apply the standard radio pulsar rotating vector model to the white dwarf pulsar AR Sco's optical polarization position angle swings folded at the white dwarf's spin period as obtained by Buckley et al. (2017). Owing to the long duty cycle of spin pulsations with a good signal-to-noise ratio over the entire spin phase, in contrast to neutron star radio pulsars, we find well-constrained values for the magnetic obliquity $α$ and observer viewing direction $ζ$ with respect to the spin axis. We find $\cosα=0.060^{+0.050}_{-0.053}$ and $\cosζ=0.49^{+0.09}_{-0.08}$, implying an orthogonal rotator …
