Silvia Zane: A resonant cyclotron scattering model for the soft X-ray spectra of magnetar candidates (Talk given by Roberto Turolla)

The goal of providing the model presented here is to explain why the 0.5-10 keV emission is well represented by a Blackbody + power law component. The main idea suggests that the magnetic field is twisted inside the object. This required a supporting current that could be the cause of the X-ray luminosity increase that we observe, the cause of spectral hardening and of spin down torque increase. The resonant cyclotron scattering model developed can be applied to all magnetars (2 tabulated models available for XSPEC). Further investigation of the effects of QED cross section are required to take into account ultra-relativistic electrons. In addition, the author evoked the need to understand the cause of long term variability of AXPs and the high energy tail observed. Finally, the issue of a possible external field also have to be investigated.

Mike Muno: Massive Stars and Magnetars

Mike Muno cites Heger et al (2003), which claimed neutron stars can form from stars with initial masses > 25 solar masses, if they have high (i.e. solar) metalicities. But there's scant data supporting this. Muno's serendipitous discovery of a magnetar in Westerlund 1 (which has ~100 stars with M>35 Msun; age 3.6 Myr) supports this. Two other magnetars (SGR 1806-20 and SGR 1900+14) may also be associated with young clusters (and so, have massive star progenitors).

Muno recently searched 506 Chandra and 441 XMM observations near the galactic plane for new magnetars with 5<P<30 sec, finding none. With the known objects, he places a "standard AXP" birth rate of 0.003-0.016/yr; and estimates there are 59(+92,-32) total "standard AXPs" in the galaxy. For transient AXPs, the birth rate is 0.008-0.06/yr, and a total number of ~600 in the galaxy. At least 10% of neuron stars are born as magnetars. New transient magnetar searches are needed to firm these uncertain numbers.

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.

Joseph Gelfand: Radio Emission from the Magnetar SGR 1806-20 Giant Flare

The 2004 Decemeber 27 giant flare from SGR 1900+14 was the most energetic giant flare ever observed. The radio light curve had a t^-1.5 to ^ 2.2 dependence 9 to 25 days after the event. After that it had a t^-3 dependence. The source rebrightened 25 to 35 days later. After 35 days the flux decreased as t^-1. The radio spectrum had an average spectral index of -0.7+/-0.3. This is what is expected from shock heated electrons. In the first few days after the event, not much motion was observed in the position of the radio nebula; however, 9 to 31 days later, constant proper motion was observed at 1/2 of the expansion rate. This motion was observed along the major axis. No coherent motion 31 days later. All these results were found via modeling the UV data. The emission seems to be a one-sided outflow. He interprets this behaviour as being due to the giant flare ejecting material into the surroundings. The collision compressed ejecta into a thin shell. What is the ejecta? Either the ejection of a magnetic flux loop or baryons ablated off the surface of the neutron star. Both give ejecta mass of 10^24.5 g.

Peter den Hartog: Different spectral components revealed in the high-energy pulse profiles of Anomalous X-ray Pulsars

4 AXPs have been detected above 10 keV, 3 with INTEGRAL and 1 with the High Energy X-ray Timing Experiment (HEXTE) aboard the Rossi X-ray Timing Explorer (RXTE). For AXPs 4U 0142+61 and 1 RXS J1708-40 he finds peak energies of 279 keV and 287 keV respectively. Pulsed emission from 1 RXS J1708-40 is observed upto 270 keV. Broadband phase resolved spectroscopy of this source revealed that different components of its pulse profile vary with energy. In only 0.1 in pulse phase the spectrum of the source goes from very hard to very soft. He concludes that the pulsed spectra of AXPs are much more complex than previously thought. Many spectral components are required to explain the pulse shapes of these sources. These observations can be used to test the geometries of the magnetospheres of these sources. If you would like a copy of Peter's thesis, email him at Hartog@sron.nl.

Fotis Gavriil: Review of Magnetar X-ray Observations

Fotis Gavriil gave a summary of X-ray (and some other wavelength!) observations of magnetars, which are neutron stars with extremely strong magnetic fields. There are now 4 confirmed Soft Gamma Repeaters (SGRs), with 1 candidate, and 10 confirmed Anomalous X-ray Pulsars (AXPs), with 1 candidate. Three of the confirmed sources, and one of the candidates, are associated with supernova remnants, implying that they are young stars. For an up to date summary of all known magnetars see this website.

Fotis gave a nice overview of all of the great things that we have learned about magnetars from X-ray observations with RXTE, including the persistent pulsations, the short repeating X-ray flares, the rare giant gamma-ray flares, and their long-term X-ray outbursts. Of particular interest is the high level of flux variability, and changes in pulse profile. Both types of source also show major timing noise (particularly the SGRs), and glitches have now been detected in all of the AXPs for which coherent timing is available. With regard to the short X-ray flares, he noted that the AXPs burst less often but can have much longer bursts (minutes as opposed to less than a second for the SGRs). He made a special point about the common statement that SGR bursts are more energetic, noting that this could just be because the SGRs are more prolific bursters, since high energy bursts are rarer.

Fotis also discussed in detail the emerging connections between the high field radio pulsars (one of which has now been found to show magnetar like X-ray flares) and the magnetars (some of which show transient radio pulsations). He argued that we now seem to be seeing a continuum of behavior, and pointed out that the high magnetic field radio pulsars have not been observed in X-ray very often, so we may have missed other transient magnetar like episodes. Something for future missions!

Wrapping up, he posed a number of questions for the future. How are magnetars born? What is the reason for their inherent variability? How common are they in the Galaxy? What is the source for their high energy emission? What is the connection between the magnetars and the rotation powered radio pulsars? And do other young, highly magnetized rotation powered pulsars exhibit magnetar like behavior?

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.