G. A. Wynn

The Spin Periods and Magnetic Moments of White Dwarfs in Magnetic Cataclysmic Variables

We have used a model of magnetic accretion to investigate the rotational equilibria of magnetic cataclysmic variables (mCVs). The results of our numerical simulations demonstrate that there is a range of parameter space in the Pspin=Porb versus � 1 plane at which rotational equilibrium occurs. This has allowed us to calculate the theoretical histogram describing the distribution of mCVs as a function of Pspin=Porb. We show that this agrees with the observed distribution, assuming that the number of systems as a function of white dwarf magnetic moment is distributed approximately according to N � 1 ðÞ d� 1 / � � 1 1 d� 1 . The rotational equilibria also allow us to infer approximate values for the magnetic moments of all known intermediate polars. We predict that intermediate polars with � 1 k5 ; 10 33 Gc m 3 and Porb > 3 hr will evolve into polars, while those with � 1 P5 ; 10 33 Gc m 3 and Porb > 3 hr will either evolve into low field strength polars that are (presumably) unobservable, and possibly EUV emitters, or, if their fields are buried by high accretion rates, evolve into conventional polars, once their magnetic fields resurface when the mass accretion rate reduces. We speculate that EX Hya‐like systems may have low magnetic field strength secondaries and so avoid synchronization. Finally, we note that the equilibria we have investigated correspond to a variety of different types of accretion flow, including disklike accretion at small Pspin=Porb values, streamlike accretion at intermediate Pspin=Porb values, and accretion fed from a ring at the outer edge of the white dwarf Roche lobe at higher Pspin=Porb values.

Accretion dynamics in neutron star-black hole binaries

We perform three-dimensional, Newtonian hydrodynamic simulations with a nuclear equation of state to investigate the accretion dynamics in neutron star black hole systems. We find as a general result that non-spinning donor stars yield larger circularization radii than corotating donors. Therefore, the matter from a neutron star without spin will more likely settle into an accretion disk outside the Schwarzschild radius. With the used stiff equation of state we find it hard to form an accretion disk that is promising to launch a gamma-ray burst. In all relevant cases the core of the neutron star survives and keeps orbiting the black hole as a mini neutron star for the rest of the simulation time (up to several hundred dynamical neutron star times scales). The existence of this mini neutron star leaves a clear imprint on the gravitational wave signal which thus can be used to probe the physics at supra-nuclear densities.

The spin period of EX Hydrae

We show that the spin period of the white dwarf in the magnetic CV EX Hydrae represents an equilibrium state in which the corotation radius is comparable with the distance from the white dwarf to the inner Lagrange point. We also show that a continuum of spin equilibria exists at which Pspin is significantly longer than� 0.1Porb. Most systems occupying these equilibrium states should have orbital periods below the CV period gap, as observed.

AE Aquarii: how cataclysmic variables descend from supersoft binaries

AE Aquarii (AE Aqr) is a propeller system. It has the shortest spin period among cataclysmic variables (CVs), and this is increasing on a 107 yr time-scale. Its ultraviolet spectrum shows very strong carbon depletion versus nitrogen, and its secondary mass indicates a star far from the zero-age main sequence. We show that these properties strongly suggest that AE Aqr has descended from a supersoft X-ray binary. We calculate the evolution of systems descending through this channel, and show that many of them end as AM CVn systems. The short spin-down time-scale of AE Aqr requires a high birth rate for such systems, implying that a substantial fraction of cataclysmic variables must have formed in this way. A simple estimate suggests that this fraction could be of the order of one-third of current CVs. We emphasize the importance of measurements of the C/N abundance ratio in CVs, particularly via the C iv 1550/N v 1238 ratio, in determining how large the observed fraction is.

Fe K emission and absorption features in XMM-Newton spectra of Markarian 766: evidence for reprocessing in flare ejecta

We report on the analysis of a long XMM–Newton European Photon Imaging Camera (EPIC) observation in 2001 May of the narrow-line Seyfert 1 galaxy Markarian 766 (Mrk 766). The 3–11 keV spectrum exhibits a moderately steep power-law continuum, with a broad emission line at ∼6.7 keV, probably blended with a narrow line at ∼6.4 keV, and a broad absorption trough above ∼8.7 keV. We identify both broad spectral features with reprocessing in He-like Fe. An earlier XMM–Newton observation of Mrk 766 in 2000 May, when the source was a factor ∼2 fainter, shows a similar broad emission line, but with a slightly flatter power law and absorption at a lower energy. In neither observation do we find a requirement for the previously reported broad ‘red wing’ to the line and hence of reflection from the innermost accretion disc. More detailed examination of the longer XMM–Newton observation reveals evidence for rapid spectral variability in the Fe K band, apparently linked with the occurrence of X-ray ‘flares’. A reduction in the emission line strength and increased high-energy absorption during the X-ray flaring suggests that these transient effects are due to highly ionized ejecta associated with the flares. Simple scaling from the flare avalanche model proposed for the luminous quasi-stellar object PDS 456 confirms the feasibility of coherent flaring being the cause of the strong peaks seen in the X-ray light curve of Mrk 766.

Can magnetar spin-down power extended emission in some short GRBs?

Extended emission gamma-ray bursts are a subset of the ‘short’ class of burst which exhibit an early time rebrightening of gamma emission in their light curves. This extended emission arises just after the initial emission spike, and can persist for up to hundreds of seconds after trigger. When their light curves are overlaid, our sample of 14 extended emission bursts show a remarkable uniformity in their evolution, strongly suggesting a common central engine powering the emission. One potential central engine capable of this is a highly magnetized, rapidly rotating neutron star, known as a magnetar. Magnetars can be formed by two compact objects coalescing, a scenario which is one of the leading progenitor models for short bursts in general. Assuming that a magnetar is formed, we gain a value for the magnetic field and late-time spin period for nine of the extended emission bursts by fitting the magnetic dipole spin-down model of Zhang and M´´ aros. Assuming that the magnetic field is constant, and the observed energy release during extended emission is entirely due to the spin-down of this magnetar, we then derive the spin period at birth for the sample. We find that all birth spin periods are in good agreement with those predicted for a newly born magnetar.

Magnetar powered GRBs: explaining the extended emission and X-ray plateau of short GRB light curves

Extended emission (EE) is a high-energy, early time rebrightening sometimes seen in the light curves of short gamma-ray bursts (GRBs). We present the first contiguous fits to the EE tail and the later X-ray plateau, unified within a single model. Our central engine is a magnetar surrounded by a fall-back accretion disc, formed by either the merger of two compact objects or the accretion-induced collapse of a white dwarf. During the EE phase, material is accelerated to super-Keplarian velocities and ejected from the system by the rapidly rotating (P � 1 10 ms) and very strong (10 15 G) magnetic field in a process known as magnetic propellering. The X-ray plateau is modelled as magnetic dipole spin-down emission. We first explore the range of GRB phenomena that the propeller could potentially reproduce, using a series of template light curves to devise a classification scheme based on phenomology. We then obtain fits to the light curves of 9 GRBs with EE, simultaneously fitting both the propeller and the magnetic dipole spin-down and finding typical disc masses of a few 10 −3 M⊙ to a few 10 −2 M⊙. This is done for ballistic, viscous disc and exponential accretion rates. We find that the conversion efficiency from kinetic energy toEM emission for propellered material needs to be & 10% and that the best fitting results come from an exponential accretion profile.

The accretion flows and evolution of magnetic cataclysmic variables

We have used a model of magnetic accretion to investigate the accretion flows of magnetic cataclysmic variables (mCVs). Numerical simulations demonstrate that four types of flow are possible: disks, streams, rings, and propellers. The fundamental observable determining the accretion flow, for a given mass ratio, is the spin-to-orbital-period ratio of the system. If intermediate polars (IPs) are accreting at their equilibrium spin rates, then for a mass ratio of 0.5, those with Pspin/Porb 0.1 will be disklike, those with 0.1 Pspin/Porb 0.6 will be streamlike, and those with Pspin/Porb ~ 0.6 will be ringlike. The spin-to-orbital-period ratio at which the systems transition between these flow types increases as the mass ratio of the stellar components decreases. For the first time we present evolutionary tracks of mCVs, which make it possible to investigate how their accretion flow changes with time. As systems evolve to shorter orbital periods and smaller mass ratios, in order to maintain spin equilibrium their spin-to-orbital-period ratio will generally increase. As a result, the relative occurrence of ringlike flows will increase, and the occurrence of disklike flows will decrease, at short orbital periods. The growing number of systems observed at high spin-to-orbital-period ratios with orbital periods below 2 hr and the observational evidence for ringlike accretion in EX Hya are fully consistent with this picture.

Swift observations of the X-ray and UV evolution of V2491 Cyg (Nova Cyg 2008 No. 2)

We present extensive, high-density Swift observations of V2491 Cyg (Nova Cyg 2008 No. 2). Observing the X-ray emission from only one day after the nova discovery, the source is followed through the initial brightening, the super-soft source phase and back to the pre-outburst flux level. The evolution of the spectrum throughout the outburst is demonstrated. The UV and X-ray light curves follow very different paths, although changes occur in them around the same times, indicating a link between the bands. Flickering in the late-time X-ray data indicates the resumption of accretion. We show that if the white dwarf (WD) is magnetic, it would be among the most magnetic known; the lack of a periodic signal in our later data argues against a magnetic WD, however. We also discuss the possibility that V2491 Cyg is a recurrent nova, providing recurrence time-scale estimates.

Short gamma-ray bursts in old populations: magnetars from white dwarf-white dwarf mergers

Recent progress on the nature of short duration γ-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 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 SGR 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.