Lara Sidoli: Transient outburst mechanisms

Supergiants Fast X-ray Transients (SFXTs) present short transient X-ray emissions (first observed: XTE J1739) associated with OB supergiant companions and have a high dynamical range of luminosity between outbursts and quiescent phases. Different mechanisms are evoked to explain the short outbursts. The most obvious one involved a spherically symmetric clumpy wind from the companion. Observations of IGR J11215 suggested a different mechanism involving an equatorial disk wind from the supergiant. Finally, the third mechanism involves an outburst being driven by a magnetic barrier in which the wind accretion is prevented by a high magnetic field (magnetar-like) and a long spin period. More data are still required and the present monitoring campaign with Swift should address what mechanism is best suited to explain the different behaviors observed among the SFXTs population.

Jonathan Arons: Beam Filamentation Instability of Interacting Current Sheets in Striped Relativistic Winds: The Origin of Low Sigma?

According to models, pulsar wind nebula behave as if the wind is weakly magnetized at termination shock. For the aligned rotator, the current sheet is flat and along the equator. In reality, pulsars are oblique rotators. The current sheet of such a pulsar travels away with the wind outflow and has a more complicated "wavy" or "striped" topology. One has to find a way of dissipating the striped sheets as they travel away in order to obtain a low magnetization at the termination shock. You may imagine these striped sheets as parallel slabs having anti-parallel magnetic fields, which generate, in the highly magnetized plasma in between, a current flow. Plausible magnetization dissipation mechanisms are investigated within this framework.

Roberto Turolla: "Surface emission from isolated neutron stars"

Roberto Turolla discussed the various exciting prospects, as well as the complications, in theoretically understanding the surface emission from isolated neutron stars. Whether these have an atmosphere or not is crucial for understanding their emission, but remains unknown.
One would expect there to be an atmosphere, but then why do we see a BB in the X-rays? The seven XDINSs, also known as the "magnificent seven" are still radio quiet (see results/poster by Joshi et al.). Finding more sources is crucial.

Valery Suleimanov: Models of magnetized neutron stars atmospheres

New model atmospheres for high magnetic fields (>10^12 G) have been computed for fully ionized hydrogen and helium atmospheres, and for partially ionized hydrogen amospheres. It was found that the inclination of the magnetic field was unimportant, but that vacuum polarization significantly affected the spectra in the case of large fields (polarization is important because magnetic fields introduce angle and polarization dependence in the opacities). Most of the resulting spectra have absobtion features due to proton cyclotron lines.

Andreas Reisenegger: Neutron star magnetic fields: a theoretical perspective

Neutron star matter is stably stratified by a composition gradient, i.e. is not a barotropic fluid, and the magnetic fields are weak (the fluid pressure is about 7 orders of magnitude greater than the magnetic pressure for magnetar strength fields). Stable hydromagnetic equilibria appear to exist, where the stable stratification plays an important role. Erosion of this stratification through beta decays or ambipolar diffusion (relative motion of charged particles and neutrons) allows magnetic field evolution.

David Eichler: A model for the large amplitude QPO luminosity variation in the tail of SGR giant flares

We heard some discussion earlier in the session about the seismic vibrations that have been detected in the aftermath of giant flares from magnetars. One of the big questions relating to the oscillations (which allow us to do seismology and study the interior of the stars) is how a vibration of the stellar surface can generate varying X-ray emission. Particularly challenging for theorists are the high amplitudes of the variations in the X-ray emission. These are far too large to be explained by physical motions of the star's crust (it would be ripped apart) - so you need some kind of amplification mechanism. David Eichler presented a new model that may resolve this problem. The model relies on the fact that torsional oscillations of the crust (twisting motions) will also force the magnetic field to twist and oscillate. The associated currents drive variations in density; resonant cyclotron upscattering then operates, with varying optical depth, to generate high amplitude variable X-ray emission. The nice thing about the model is that you don't need large amplitude crust movements to get much larger amplitude variations in the X-ray. David also pointed out that the energy deposited in the crust as the oscillations dissipate energy could be responsible for the observed afterglows.

Yuri Lyubarsky: Theoretical overview of magnetars

The strong magnetic fields (~10^15 G) in magnetars feed many types of activity, and the emission from magnetars is powered by the field rather than by rotation or accretion. By restricting electron motion, the magnetic fields also suppress the radiation cross-section, thus allowing large super-Eddington luminocities.

A 10^46 erg giant flare was seen from the magnetar SGR 1806-20 on Dec. 27th 2004, which provides a useful test of magnetar theory. Despite being ~100 times brighter than other flares from this source, the energy in the pulsating tail was similar to that of other flares. This is thought to be because the energy in the tail is set by the storage capacity in the magnetosphere, which mostly depends on the magnetic field. Mass ejection was also detected in connection with the giant flare, by resolving the ejected cloud using radio interferometry. Further, QPOs were detected in the tail of the giant flare.

QPOs open for the possibility of NS seismology. Intermittent QPOs can probe (perhaps) shear modes in the NS crust, while low frequency (~20Hz) QPOs can probe the Alfven speed in the core. Magnetic fields couple the crust to the core on timescales of 0.01-0.1s, and this must be considered when interpreting QPOs.

The trapped fireball model predicts frequency dependent radiation cross-section, and this produes a flat spectrum below BB peak. One should attempt to fit this to the spectrum of sources which have previously been fitted with a BB plus power law spectrum.

The magnetar paradigm for SGRs and AXPs is now unquestioned, and high quality spectral data require sophisticated models of the radiation transfer. X-ray polarimitry would be a great help, but is presently unavailable.

Maxim Lyutikov: Crab giant pulses at sub-nanosecond resolution: quantitative model of non-trivial spectrum

A model for the emission of giant pulses from the Crab pulsar is proposed by Lyutikov and is based on anomalous cyclotron resonance on the ordinary modes. His main conclusions are: 1. giant pulses come from closed field lines; 2. giant pulses are seen by chance, i.e. produced in narrow emission windows; 3. closed field lines are not dead, but populated by plasma with n >> n_{GJ}; 4. Earth analogues: magnetosperic hiss, roars.

Vyacheslav Zavlin: Pulsar high energy emission: Soft X-rays

This presentation was a broad overview of soft x-ray emission from pulsars/NSs. Observations of NSs in the soft X-ray band are important since the bulk of the flux from NSs falls in this range and since some NSs/pulsars have still not been detected outside this range. Both thermal and non-thermal emission can be observed. The thermal emission, due to internal heat (or from a small hot spot), is ideal to constrain the equation of state. It is important to note that a blackbody interpretation of the thermal radiation implies an emission area much smaller than with a H-atmosphere interpretation. The non-thermal emission can be due to the magnetosphere, pulsar wind nebula, or inter-binary shocks. Young objects (~1 kyr) are dominated by non-thermal emission, while middle-aged and old pulsar (>1 Gyr) are dominated by thermal radiation, from the surface and hot polar caps respectively. The author presented different objects that have been observed in different wavelength bands (Vela, PSR J0538+2817, Geminga, ...). In most cases,  polar caps (thermal emission) were (very likely) detected. However, non-thermal emission can be used in many cases to explain the X-ray emission. Finally, regarding the cooling models of pulsars/NSs, we notice large difference in the cooling curve whether or not proton superfluidity is included in the model.

Matthew Baring: Theoretical Perspectives on Non-Thermal Pulsar Emission

At present, there are over 1700 radio pulsars, around 30 optical pulsars, over 60 X-ray pulsars, and 7 gamma ray pulsars known. The main focus of this talk was how to distinguish between the polar cap and outer gap emission models, in particular how to do this using the 30MeV to 300GeV capabilities of the newly launched GLAST telescope.
The predicted populations of pulsars detectable with GLAST differ depending on whether one uses a polar cap model or an outer gap model. If pulsar emission originates from a polar cap, one expects 150-200 new pulsars detected with GLAST, while one expects 300-800 new pulsar detections if the emission originates from around the surface, as in the outer gap model.
The predicted spectrum of the Vela pulsar differs in the two models, and simulations indicate that GLAST should be able to resolve this difference.
In general, the polar cap model predicts that the highest detectable energies from pulsars decrease with increasing magnetic field, while the outer gap model predicts increasing maximum energy with increasing magnetic field. The present data cannot rule out either model on this basis, but GLAST data may be able to do so.
In the future, polarimetry in X-ray and soft gamma ray bands would be valuable in constraining pulsar emission mechanisms, by distinguishing between synchrotron and curveature polarization throgh the polarization swing profiles.

Ben Stappers: "Pulsar radio emission: overview of observational status"

• Better chance of finding drifting subpulses at longer observing wavelengths? (Weltevrede et al. 2006, 2007). Drifting a general property of radio emission from neutron stars?

• "swooshes": epsiodic emission in "new" region (pulse phase) turns on when the "normal" emission turns off. Hard to reconcile with carousel model.
• "flaring": similar to "swooshing" phenomenon, occasional bright pulses at different pulse longitudes.
  
• "RRAT-alogue" (catalogue of known RRATs): more than 24 RRATs now known (11 presented in original McLaughlin et al. paper). New, very nearby RRATs being found.
  
• Some RRATs show bursts of several pulses, closely spaced: "bursters".
  
• Four "intermittent pulsars" (e.g. PSR B1931+24) now known. Switch on/off on timescales of weeks to months and show different Pdot when on/off.
• Very high degree of modulation in single pulses of radio-emitting magnetar XTE J1810-197.
• Is there a continuum of processes across the different "off" states from the single/few pulse nulls, extreme nullers, RRATs, and intermittent pulsars?