* Astronomy

Blobrana · L

RE: AE Aquarii

Blobrana · L

Title: AE Aquarii represents a new subclass of Cataclysmic Variables
Authors: N.R. Ikhsanov, N.G. Beskrovnaya

We analyse properties of the unique nova-like star AE Aquarii identified with a close binary system containing a red dwarf and a very fast rotating magnetised white dwarf. It cannot be assigned to any of the three commonly adopted sub-classes of Cataclysmic Variables: Polars, Intermediate Polars, and Accreting non-magnetised 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 x 10^{34} 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.

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Blobrana · L

Title: Multiwavelength Campaign of Observations of AE Aqr
Authors: C. W. Mauche, M. Abada-Simon, J.-F. Desmurs, M. J. Dulude, Z. Ioannou, J. D. Neill, A. Price, N. Sidro, W. F. Welsh, the Center for Backyard Astrophysics, the American Association of Variable Star Observers

We provide a summary of results, obtained from a multiwavelength (TeV gamma-ray, X-ray, UV, optical, and radio) campaign of observations of AE Aqr conducted in 2005 August 28-September 2, on the nature and correlation of the flux variations in the various wavebands, the white dwarf spin evolution, the properties of the X-ray emission region, and the very low upper limits on the TeV gamma-ray flux.

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Blobrana · L

Title: AE Aquarii: The first white dwarf in the family of spin-powered pulsars
Authors: N.R. Ikhsanov, N.G. Beskrovnaya

Simulation of Doppler H-alpha tomogram of the nova-like star AE Aquarii suggests that the dipole magnetic moment of the white dwarf is close to 1.5E+34 G cm³. This is consistent with the lower limit to the magnetic field strength of the white dwarf derived from observations of circularly polarized optical emission of the system. The rapid braking of the white dwarf and the nature of pulsing hard X-ray emission recently detected with SUZAKU space telescope under these conditions can be explained in terms of spin-powered pulsar mechanism. A question about the origin of strongly magnetized white dwarf in the system remains, however, open. Possible evolutionary tracks of AE Aquarii are briefly discussed.

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Blobrana · L

Title: High Dispersion Absorption-line Spectroscopy of AE Aqr
Authors: J. Echevarria (1), R. Connon Smith (2), R. Costero, (1), S. Zharikov, (1), R. Michel (1) ((1) IA UNAM, Mexico, (2) University of Sussex, UK)

High-dispersion time-resolved spectroscopy of the unique magnetic cataclysmic variable AE Aqr is presented. A radial velocity analysis of the absorption lines yields K_2 = 168.7 ± 1 km/s. Substantial deviations of the radial velocity curve from a sinusoid are interpreted in terms of intensity variations over the secondary star's surface. A complex rotational velocity curve as a function of orbital phase is detected which has a modulation frequency of twice the orbital frequency, leading to an estimate of the binary inclination angle that is close to 70^o. The minimum and maximum rotational velocities are used to indirectly derive a mass ratio of q= 0.6 and a radial velocity semi-amplitude of the white dwarf of K_1 = 101 ±3 km/s. We present an atmospheric temperature indicator, based on the absorption line ratio of Fe I and Cr I lines, whose variation indicates that the secondary star varies from K0 to K4 as a function of orbital phase. The ephemeris of the system has been revised, using more than one thousand radial velocity measurements, published over nearly five decades. From the derived radial velocity semi-amplitudes and the estimated inclination angle, we calculate that the masses of the stars are M_1 = 0.63 ±0.05M_sun; M_2 = 0.37 ±0.04 M_sun, and their separation is a = 2.33 ±0.02R_sun. Our analysis indicates the presence of a late-type star whose radius is larger, by a factor of nearly two, than the radius of a normal main sequence star of its mass. Finally we discuss the possibility that the measured variations in the rotational velocity, temperature, and spectral type of the secondary star as functions of orbital phase may, like the radial velocity variations, be attributable to regions of enhanced absorption on the star's surface.

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Blobrana · L

AE Aquarii.kmz
Google Sky file (2kb, kmz)

-- Edited by Blobrana at 14:20, 2008-01-12

Blobrana · L

New observations from Suzaku, a joint Japanese Aerospace Exploration Agency (JAXA) and NASA X-ray observatory, have challenged scientists conventional understanding of white dwarfs. Observers had believed white dwarfs were inert stellar corpses that slowly cool and fade away, but the new data tell a completely different story.
At least one white dwarf, known as AE Aquarii, emits pulses of high-energy (hard) X-rays as it whirls around on its axis.

"Were seeing behaviour like the pulsar in the Crab Nebula, but were seeing it in a white dwarf. This is the first time such pulsar-like behaviour has ever been observed in a white dwarf" - Koji Mukai of NASA Goddard Space Flight Centre in Greenbelt, Md.

The Crab Nebula is the shattered remnant of a massive star that ended its life in a supernova explosion.
Mukai is co-author of a paper presented at a Suzaku science conference in San Diego, California, in December.

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Blobrana · L

Title: Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aqr
Authors: G. Dubus, R. E. Taam, C. Hull, D. M. Watson, J. C. Mauerhan

The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white dwarf which is thought to expel most of the material streaming onto it. Observations of AE Aqr have been obtained in the wavelength range of 5 - 70 microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space Telescope. The spectral energy distribution reveals a significant excess above the K4V spectrum of the donor star with the flux increasing with wavelength above 12.5 microns. Superposed on the energy distribution are several hydrogen emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared spectrum above 12.5 microns can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding clouds with an initial evolution timescale in seconds. However, too many components must then be superposed to explain satisfactorily both the mid-infrared continuum and the observed radio variability. Thermal emission from cold circumbinary material can contribute, but it requires a disk temperature profile intermediate between that produced by local viscous dissipation in the disk and that characteristic of a passively irradiated disk. Future high-time resolution observations spanning the optical to radio regime could shed light on the acceleration process and the subsequent particle evolution.

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Blobrana · L

On August 30-August 31, 2005 two space-based and four professional ground-based observatories are scheduled to observe the cataclysmic variable star AE Aquarii Each of the observatories covers a different wavelength of light.
Amateur astronomers have been asked to cover the visible-light portion and help unravel the mystery of the puzzling binary star system.

"This observing campaign will take place over nearly a full day, and since no single ground-based observatory can observe AE Aquarii for that long due to Earth's rotation, amateur astronomers can make a unique and invaluable contribution to this campaign" - Dr. Christopher Mauche of Lawrence Livermore National Laboratory, the principal investigator of the project.

Because they are spaced all across the globe, amateur astronomers can observe this star and other celestial objects unhindered by nightfall or weather.
The Chandra and GALEX space telescopes will be working with the HESS, MAGIC, VLT, and VLA ground-based telescopes. Combined, they will provide coverage of AE Aqr from high-energy gamma-rays to low-energy radio waves.
Such simultaneous multiwavelength coverage is required to provide the clearest picture of the locations, mass motions, energies, and inter-relationships of the various emission regions in the star.

Position(2000): RA = 20h40m09.07s, Dec = -00d52m16.3s

Chart:

AE Aquarii is an intermediate polar, a type of cataclysmic variable star. It actually consists of two stars - a red dwarf and rapidly spinning magnetic white dwarf.
It has a variable brightness that ranges from about magnitude 12 (quiescence) to magnitude 10 (flare).

And a orbital period of 9.88 hours.

Material drawn off the red dwarf falls toward the white dwarf, but instead of landing on the white dwarf surface, it is flung out of the system by the white dwarf's rapidly spinning magnetic field. This mechanism, which is uncommon but not unique to AE Aquarii, is referred to as a magnetic propeller.

"Amateurs astronomers have been observing AE Aquarii since 1944. Since then, they have recorded over 28,815 measurements of the star, most of them made with just a telescope and their eyes. This type of historical data is immensely valuable in studying variable stars and only amateurs can provide it" - Dr. Arne Henden, Director of the American Association of Variable Star Observers (AAVSO).