AE Aquarii Research Articles

Accretion-driven Spin-up of the White Dwarfs in AR Scorpii and AE Aquarii

Accretion-driven Spin-up of the White Dwarfs in AR Scorpii and AE Aquarii

VLBI detection of the AE Aqr twin, LAMOST J024048.51+195226.9

ABSTRACT LAMOST J024048.51+195226.9 (J0240+1952) was recently identified as the second AE Aquarii (AE Aqr)-type cataclysmic variable, possessing the fastest known rotating white dwarf. We performed a Very Long Baseline Interferometry (VLBI) observation of J0240+1952 utilizing the European VLBI Network at 1.7 GHz, to obtain the first view of the radio morphology on mas scale. Our high-resolution VLBI image clearly shows that the radio emission is compact on mas scale (≲2 AU), with no evidence for a radio jet or extended emission. The compact radio source has an average flux density of ∼0.37 mJy, and its brightness temperature is given at ≳2.3 × 107 K, confirming a non-thermal origin. The emission exhibits irregular variations on a time-scale of tens of minutes, similar to the radio flares seen in AE Aqr. The measured VLBI position of J0240+1952 is consistent with that derived from Gaia. Our results favour the model in which the radio emission is attributed to a superposition of synchrotron radiation from expanding magnetized blobs of this system.

Spun-Up Rotation-Powered Magnetized White Dwarfs in Close Binaries as Possible Gamma-ray Sources: Signatures of Pulsed Modulation from AE Aquarii and AR Scorpii in Fermi-LAT Data

In this paper, the possibility of periodic pulsar-like gamma-ray emission from the white dwarfs in AE Aquarii and AR Scorpii is investigated. We show that the white dwarf magnetospheres in AE Aquarii and AR Scorpii can possibly induce potentials to accelerate charged particles to energies in excess of one tera electronvolt (TeV) with associated gamma-ray emission through processes such as curvature radiation, inverse Compton, and hadronic processes such as neutral pion decay. We report here pulsed gamma-ray signatures at or close to the spin period of white dwarfs in both AE Aquarii and AR Scorpii in the Fermi-LAT dataset. This may indicate that both these white dwarfs possibly contain a particle accelerator that can produce relativistic electrons and ions and associated high energy radiation. The possibility of pair production is also investigated, which could provide a source for relativistic e± pairs in the magnetosphere. This could possibly be a driver for other forms of lepton-induced multi-wavelength pulsar-like emission from these two systems as well, for example, to explain the recently detected pulsed radio emission from AE Aquarii and R Scorpii in MeerKAT observations at the spin period of the white dwarf. The possibility of future detection of AE Aquarii and AR Scorpii with the Cherenkov Telescope Array (CTA) is also discussed. The future Vera Rubin Observatory will make a revolutionary contribution to time-domain astrophysics, which may lead to the discovery of thousands of new transient sources, possibly also many more close binaries with highly spun-up magnetized white dwarfs such as AE Aquarii and AR Scorpii for future investigation.

Found: a rapidly spinning white dwarf in LAMOST J024048.51+195226.9

ABSTRACT We present optical photometry of the cataclysmic variable LAMOST J024048.51+195226.9 taken with the high-speed, five-band CCD camera HiPERCAM on the 10.4 m Gran Telescopio Canarias. We detect pulsations originating from the spin of its white dwarf, finding a spin period of 24.9328(38) s. The pulse amplitude is of the order of 0.2 per cent in the g band, below the detection limits of previous searches. This detection establishes LAMOST J024048.51+195226.9 as only the second white dwarf magnetic propeller system, a twin of its long-known predecessor, AE Aquarii. At 24.93 s, the white dwarf in LAMOST J024048.51+195226.9 has the shortest known spin period of any cataclysmic variable star. The white dwarf must have a mass of at least 0.7 M⊙ to sustain so short a period. The observed faintest u-band magnitude sets an upper limit on the white dwarf’s temperature of $\sim\!25\, 000$ K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of $\sim\!30\, 000\,$K covering ∼2 per cent of the white dwarf’s visible area, although spots that are hot and smaller, or cooler and larger cannot be ruled out.

High-dispersion spectroscopy of AE Aquarii – II. Evidence of material orbiting the primary star

ABSTRACT We present a second paper related to the analyses of high-dispersion spectroscopic observations of the magnetic cataclysmic variable AE Aquarii. We focus our efforts on the study of the emission lines and their radial velocities. We detect a sinusoidal behaviour, in several of the observing runs, with variable amplitudes. Of those runs presented, the velocity curve of 2000 August shows less instability in the emission material. In this case, we obtain K1 = 114 ± 8 km s−1, which we take as our best value for the radial velocity of the primary. This result is consistent within 2σ with previously published values obtained using indirect methods. We interpret this consistency as observational evidence of material orbiting the rapidly rotating primary star. We present a Doppler tomography study, which shows that the H α emission is primarily concentrated within a blob in the lower left quadrant, a structure similar to that predicted by the propeller model. However, for 2000 August, we find the emission centred around the position of the white dwarf, which supports the possibility of the K1 value of this run of being a valid approximation of the orbital motion of the white dwarf.

Confirmation of a Second Propeller: A High-inclination Twin of AE Aquarii

Abstract For decades, AE Aquarii (AE Aqr) has been the only cataclysmic variable star known to contain a magnetic propeller: a persistent outflow whose expulsion from the binary is powered by the spin-down of the rapidly rotating, magnetized white dwarf. In 2020, LAMOST J024048.51+195226.9 (J0240) was identified as a candidate eclipsing AE Aqr object, and we present three epochs of time-series spectroscopy that strongly support this hypothesis. We show that, during the photometric flares noted by Thorstensen, the Balmer and He i emission lines reach velocities of ∼3000 km s−1, well in excess of what is observed in normal cataclysmic variables. This is, however, consistent with the high-velocity emission seen in flares from AE Aqr. Additionally, we confirm beyond doubt that J0240 is a deeply eclipsing system. The flaring continuum, He i and much of the Balmer emission likely originate close to the WD because they disappear during the eclipse that is centered on inferior conjunction of the secondary star. The fraction of the Balmer emission remaining visible during eclipse is likely produced in the extended outflow. Most enticingly of all, this outflow produces a narrow P Cygni absorption component for nearly half of the orbit, and we demonstrate that this scenario closely matches the outflow kinematics predicted by Wynn et al. While an important piece of evidence for the magnetic-propeller hypothesis—a rapid WD spin period—remains elusive, our spectra provide compelling support for the existence of a propeller-driven outflow viewed nearly edge-on, enabling a new means of rigorously testing theories of the propeller phenomenon.

Gravitational waves from fast-spinning white dwarfs

ABSTRACT Two mechanisms of gravitational waves (GWs) emission in fast-spinning white dwarfs (WDs) are investigated: accretion of matter and magnetic deformation. In both cases, the GW emission is generated by an asymmetry around the rotation axis of the star. However, in the first case, the asymmetry is due to the amount of accreted matter on the magnetic poles, while in the second case it is due to the intense magnetic field. We have estimated the GW amplitude and luminosity for three binary systems that have a fast-spinning magnetized WD, namely, AE Aquarii, AR Scorpii, and RX J0648.0−4418. We find that, for the first mechanism, the systems AE Aquarii and RX J0648.0−4418 can be observed by the space detectors BBO and DECIGO if they have an amount of accreted mass of δm ≥ 10−5 M⊙. For the second mechanism, the three systems studied require that the WD have a magnetic field above ∼109 G to emit GWs that can be detected by BBO. We also verified that, in both mechanisms, the gravitational luminosity has an irrelevant contribution to the spin-down luminosity of these three systems. Therefore, other mechanisms of energy emission are needed to explain the spin-down of these objects.

On the origin of pulsing X-ray emission of AE Aqr

The cataclysmic variable AE Aquarii is a low-mass close binary system containing a red dwarf and a 33 s rotation period magnetic white dwarf which operates as a rotation-powered pulsar. The 33 s pulsations are detected in the optical, UV and X-rays. The 16.5 s harmonic is also present in the optical and UV. This pulsing emission comes from two hot spots (Tp ∼ 26000 K) located in the regions of magnetic poles on the white dwarf surface. The nature of the X-ray pulsations of AE Aqr is still under discussion. No 16.5 s harmonic in X-ray is observed and the luminosity of the pulsing component in X-rays is significantly smaller than the luminosity of the pulsing component in the optical and UV. We suggest that the source of pulsing X-ray emission is also located in the magnetic pole region at the surface of the white dwarf and can be associated with a hot spot (T ∼ 106 − 107 K). This spot is heated by the backflowing charged particles. However, the source of particles responsible for the heating the X-ray spot differs from the source of particles responsible for the heating the area emitting in the optical an UV.

Ultra-high energy cosmic rays from white dwarf pulsars and the Hillas criterion

The origins of ultra-high-energy cosmic rays (E ≳ 1019 eV) are a mystery and still under debate in astroparticle physics. In recent years some efforts were made to understand their nature. In this contribution we consider the possibility of Some Soft Gamma Repeaters (SGRs) and Anomalous X-ray Pulsars (AXPs) beeing white dwarf pulsars, and show that these sources can achieve large electromagnetic potentials on their surface that accelerate particle almost at the speed of light, with energies E ~ 1020-21 eV. The sources SGRs/AXPs considered as highly magnetized white dwarfs are well described in the Hillas diagram, lying close to the AR Sorpii and AE Aquarii which are understood as white dwarf pulsars.

The cataclysmic variable AE Aquarii: B–V color of the flares

We report simultaneous observations of the flaring behavior of the cataclysmic variable star AE Aquarii (AE Aqr). The observations are in Johnson B and V bands. The color–magnitude diagrams (B–Vvs. V and B–Vvs. B) show that the star becomes more blue as it becomes brighter. In our model, AE Aqr's behavior can be explained with flares (fireballs) with 0.03⩽B–V⩽0.30, and temperature in the interval 8000⩽T⩽12000.

Timing analysis of AE Aquarii X-ray observations

We reanalysed the archival X-ray observations of double system AE Aquarii, obtained using orbital observatories «XMM-Newton» and «Chandra» in 2001 and 2005 respectively. We made an independent timing analysis with two numerical methods. Our result confirmed the presence of 33 s rotational period of white dwarf in the system. In addition, we confirmed that X-ray pulsations with a period of 16.5 s, which were detected in optical and UV ranges, absent in AE Aquarii spectrum. This may mean that the X-ray emission comes from one of the poles of white dwarf surface.

Features of the matter flows in the peculiar cataclysmic variable AE Aquarii

The structure of plasma flows in close binary systems in which one of the components is a rapidly rotating magnetic white dwarf is studied. The main example considered is the AE Aquarii system; the spin period of the white dwarf is about a factor of 1000 shorter than the orbital period, and the magnetic field on the white dwarf surface is of order of 50 MG. The mass transfer in this system was analyzed via numerical solution of the system of MHD equations. These computations show that the magnetic field of the white dwarf does not significantly influence the velocity field of the material in its Roche lobe in the case of laminar flow regime, so that the field does not hinder the formation of a transient disk (ring) surrounding the magnetosphere. However, the efficiency of the energy and angular momentum exchange between the white dwarf and the surrounding material increases considerably with the development of turbulent motions in the matter, resulting in its acceleration at the magnetospheric boundary and further escape from the system at a high rate. The time scales of the transition of the system between the laminar and turbulent modes are close to those of the AE\,Aqr transition between its quiet and active phases.

AE Aquarii: A Short Review

The nova-like variable AE Aquarii has been continuously studied since its discovery on photographic plates in 1934. In this short review the peculiar multi-wavelength properties of AE Aquarii will be reviewed and explained in context of its evolution from a high mass transfer phase, during which period it could have been a supersoft X-ray source (SSS).

MAGIC search for VHEγ-ray emission from AE Aquarii in a multiwavelength context

It has been claimed that the nova-like cataclysmic variable (CV) AE Aquarii (AE Aqr) is a very-high-energy (VHE, $E>$100 GeV) source both on observational and theoretical grounds. We aim to search for VHE gamma-ray emission from AE Aqr during different states of the source at several wavelengths to confirm or rule out previous claims of detection of gamma-ray emission from this object. We report on observations of AE Aqr performed by MAGIC. The source was observed during 12 hours as part of a multiwavelength campaign carried out between May and June 2012 covering the optical, X-ray, and gamma-ray ranges. Besides MAGIC, the other facilities involved were the KVA, Skinakas, and Vidojevica telescopes in the optical and Swift in X-rays. We calculated integral upper limits coincident with different states of the source in the optical. We computed upper limits to the pulsed emission limiting the signal region to 30% of the phaseogram and we also searched for pulsed emission at different frequencies applying the Rayleigh test. AE Aqr was not detected at VHE energies during the multiwavelength campaign. We establish integral upper limits at the 95\% confidence level for the steady emission assuming the differential flux proportional to a power-law function d\phi/dE \propto E^{-Gamma}, with a Crab-like photon spectral index of Gamma=2.6. The upper limit above 200 GeV is 6.4\times10^{-12} cm^{-2}s^{-1} and above 1 TeV is 7.4\times10^{-13} cm^{-2}s^{-1}. We obtained an upper limit for the pulsed emission of 2.6\times10^{-12} cm^{-2}s^{-1} for energies above 200 GeV. Applying the Rayleigh test for pulsed emission at different frequencies we did not find any significant signal. Our results indicate that AE Aqr is not a VHE gamma-ray emitter at the level of emission previously claimed. We have established the most constraining upper limits for the VHE gamma-ray emission of AE Aqr.

Optimising the time series analysis methods for changeable astrophysic sources: AE Aquarii

Optimising the time series analysis methods for changeable astrophysic sources: AE Aquarii. Time series analysis of photometric data of the nova-like star AE Aquarii was done. The photometry was done using the 1.5 m Boyden telescope. The aim was to study the spin ephemeris of the white dwarf component of the system. Time series analysis methods were then applied to the data with the aim on finding the most optimised method for the system available. The frequency spectrum analysis methods include AA, FFT and Lomb-Scargle. The results of the different methods will be compared and discussed, after which a conclusion will follow.

NuSTAR results and future plans for magnetar and rotation‐powered pulsar observations

AbstractThe Nuclear Spectroscopic Telescope Array (NuSTAR) is the first focusing hard X‐ray mission in orbit and operates in the 3–79 keV range. NuSTAR's sensitivity is roughly two orders of magnitude better than previous missions in this energy band thanks to its superb angular resolution. Since its launch in 2012 June, NuSTAR has performed excellently and observed many interesting sources including four magnetars, two rotation‐powered pulsars and the cataclysmic variable AE Aquarii. NuSTAR also discovered 3.76‐s pulsations from the transient source SGR J1745–29 recently found by Swift very close to the Galactic center, clearly identifying the source as a transient magnetar. For magnetar 1E 1841–045, we show that the spectrum is well fit by an absorbed blackbody plus broken power‐law model with a hard power‐law photon index of ∼ 1.3. This is consistent with previous results by INTEGRAL and RXTE. We also find an interesting double‐peaked pulse profile in the 25–35 keV band. For AE Aquarii, we show that the spectrum can be described by a multi‐temperature thermal model or a thermal plus non‐thermal model; a multi‐temperature thermal model without a non‐thermal component cannot be ruled out. Furthermore, we do not see a spiky pulse profile in the hard X‐ray band, as previously reported based on Suzaku observations. For other magnetars and rotation‐powered pulsars observed with NuSTAR, data analysis results will be soon available. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

NuSTARANDSWIFTOBSERVATIONS OF THE FAST ROTATING MAGNETIZED WHITE DWARF AE AQUARII

AE Aquarii is a cataclysmic variable with the fastest known rotating magnetized white dwarf (P_spin = 33.08 s). Compared to many intermediate polars, AE Aquarii shows a soft X-ray spectrum with a very low luminosity (L_X ~ 10^{31} erg/s). We have analyzed overlapping observations of this system with the NuSTAR and the Swift X-ray observatories in September of 2012. We find the 0.5-30 keV spectra to be well fitted by either an optically thin thermal plasma model with three temperatures of 0.75 +0.18 -0.45, 2.29 +0.96 -0.82, and 9.33 +6.07 -2.18 keV, or an optically thin thermal plasma model with two temperatures of 1.00 +0.34 -0.23 and 4.64 +1.58 -0.84 keV plus a power-law component with photon index of 2.50 +0.17 -0.23. The pulse profile in the 3-20 keV band is broad and approximately sinusoidal, with a pulsed fraction of 16.6 +/- 2.3%. We do not find any evidence for a previously reported sharp feature in the pulse profile.

The Peculiar Binary System AE Aquarii from its Characteristic Multi-wavelength Emission

The multi-wavelength properties of the novalike variable system AE Aquarii are discussed in terms of the interaction between the accretion inflow from a late-type main sequence star and the magnetosphere of a fast rotating white dwarf. This results in an efficient magnetospheric propeller process and particle acceleration. The spin-down of the white dwarf at a period rate of 5.64×10 −14 s s −1 results in a huge spin-down luminosity of L s−d ≃ 6 10× 33 erg s −1 . Hence, the observed non-thermal hard X-ray emission and VHE and TeV gamma-ray emission may suggest that AE Aquarii can be placed in the category of spin-powered pulsars. Besides, observed hard X-ray luminosity of L X,hard ≤ 5 × 10 30 erg s −1 constitutes 0.1 % of the total spin-down luminosity of the white dwarf. This paper will discuss some recent theoretical studies and data analysis of the system.

Search for Non-Thermal Emissions from an Isolated Magnetic White Dwarf, EUVE J0317 855, with Suzaku

Abstract To search for possible non-thermal emissions from magnetic white dwarfs (WDs), we selected the most suitable candidates from among magnetic cataclysmic variables and isolated WDs. After the magnetic cataclysmic variable AE Aquarii, which emits hard X-ray pulses, the isolated WD EUVE J0317$-$855 is expected to emit the second most powerful magnetic dipole radiation with energies reaching $\sim$1.1 $\times$10$^{29}$ erg s$^{-1}$. EUVE J0317$-$855 was observed with Suzaku from 2009 July 16 to 2009 July 17 for about 60 ks. No significant emissions were detected from the object in the 0.5–10 keV band, and the upper limit of the X-ray flux was estimated to be 1.7 $\times$10$^{-13}$ erg cm$^{-2}$ s$^{-1}$ in the 2–10 keV band. This value corresponds to less than 0.05% of the calculated spin-down energy. It excludes the possibility of explaining soft gamma-ray repeaters and anomalous X-ray pulsars as WDs, but may indicate that the activity in the magnetosphere of WDs is not so different from that occurring in radio pulsars. Curvature radiation, which is one of the suggested mechanism for emissions from the magnetosphere of compact objects, may explain the observed X-ray flux of AE Aquarii, and is consistent with the non-detection of EUVE J0317$-$855.

AE Aquarii represents a new subclass of Cataclysmic Variables

We analyze properties of the unique nova-like star AE Aquarii identified with a close binary system containing a red dwarf and a very fast rotating magnetized white dwarf. It cannot be assigned to any of the three commonly adopted sub-classes of Cataclysmic Variables: Polars, Intermediate Polars, and Accreting non-magnetized White Dwarfs. Our study has shown that the white dwarf in AE Aqr is in the ejector state and its dipole magnetic moment is µ≃1.5 × 1034 G cm3. It switched into this state due to intensive mass exchange between the system components during a previous epoch. A high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state had occurred at a final stage of the spin-up epoch as its magnetic field emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar.