Coming Talks: Friday July 18

Scheduled talks, times are Eastern Daylight Time

Mike Muno	Magnetars and Massive Stars	9:30a
Silvia Zane	A resonant cyclotron scattering model for the soft-ray spectra of magnetar candidates	10:30a
Valery Suleimanov	Models of magnetized neutron stars atmospheres	10:45a
Frank Haberl	X-ray observations of Isolated Neutron Stars	11:30a
David Kaplan	Optical/IR Observations of Isolated Neutron Stars	12:00p
Roberto Turolla	Surface emission from isolated neutron stars	12:30p
Elena Amato	Pulsar Wind nebulae: Theoretical Overview	2:00p
Patrick Slane	Observations of pulsar wind nebulae	2:30p
Rino Bandiera	On the peculiar shapes of some pulsar bow-shock nebulae	3:00p
Ingo Buesching	Cooling flows in Pulsar Wind Nebulae	3:15p
George Pavlov	Central Compact Objects in Supernova Remnants	5:00p
Jonathan Arons	Beam Filamentation Instability of Interacting Current Sheets in Striped Relativistic Winds: The Origin of Low Sigma?	5:30p
Ocker De Jager	Probing the birth periods and pair production multiplicities of neutron stars through multiwavelength observations of their wind nebulae.	5:45p
Nobuyuki Kawai	Jets and tori of a young pulsar PSR B1509-58	6:00p
Benjamin Owen	How LIGO can follow up high-energy observations of young neutron stars	6:15p

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?

Rino Bandiera: On the peculiar shapes of some pulsar bow-shock nebulae

Balmer lines, in particular Halpha are excellent tracers of bow-shock nebula. The "Guitar" nebula contains a fast moving pulsar moving toward the head of the Guitar. The field in X-rays contains a strange linear feature oriented about 120 degrees away from the pulsar's proper motion direction. The energetics of the pulsar spin-down does not seem to explain this elongated feature. He proposes the magnetic flux tubes of the ISM are directed along the direction of this feature. In this region, the gyration radius of electrons is comparable to the bow-shock size, and hence higher energy electrons would diffuse out the shock.

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.

In Montreal: My personal restaurants address book

As I am a local, I thought it would be interesting to share my personal restaurants address book. I hope this will inspire you...

Au Pied De Cochon: Quebec modern "terroir" food. This place is absolutely unique...
Located on Duluth street, east form St-Denis.

Steak et Frites (bring your own wine):
3 locations: L'hotel W (downtown), St-Paul street, Laurier street (Cote-des-Neiges and Parc)

M. Burger: Very innovative, "customizable" burgers
Located on Drummond, north from Maisoneuve.

For Grec food, the place to be is Duluth street, east from St-Denis. Several "bring your own wine" places.

Little Italy neighborhood is located around the Jean-Talon metro station (orange line). It's not downtown but it's the place to be for great Italian food (St-Laurent, corner Jean-Talon).

For some great Cuisine Francaise restaurants, I suggest to go on Rachel Street, east from St-Denis. Several "bring your own wine", very classy but affordable (CAN$ 20-30) restaurants (especially when you bring your wine). The "917" and "Poisson Rouge" are particularly recommended.

Last, not the least: Poutine! You have to try it, it's a local, greasy meal unique to Quebec. I suggest "La Banquise" corner Rachel and Papineau. See more at Montreal Poutine.

Jason Hessels: Uncovering the population of nearby neutron stars at low radio frequencies

Looking for nearby pulsars

• Look for pulsars at distances below 2 kpc.
  
• Get luminosity distributions.
• Since they're close, they can easily be follow up at other wavelengths.
• Look for pulsars at low frequencies. Low dispersion measurement (DM) sources easy to distinguish from noise. Low frequencies have not been exploited. Unfortunately, these sources will have many dispersion trials.

Galactice Plane Survey with the Green Bank Telescope (GBT)

• GBT survey at 350-MHz. Look for pulsars and radio transients. Sensitive to Low DM sources. The entire Northern Galactic Plane has been covered. 33 new pulsars so far! Examples of sources discovered because of low frequency observing:

1. PSR J0243+62 discovered in a single pulse search. P=592 ms. At a distance of 400 kpc. Very low luminosity L400~0.2 mJy kpc^2.
   
2. PSR J0054+66 also identified in a single pulse search. P=1.39 s. D~ 1kpc.

• GBT350 discoveries show there are still many nearby intermittent sources.
• These are the types of sources that the LOw Frequency ARray (LOFAR) will find.

Pulsars with LOFAR

• 30-80 MHz.
• 2x24 High band antennae (HBA) tiles.
• 96 Low Band Antennae (LBA) tiles.
• Ideal for surveying the local Galaxy for pulsars.
• Single pulses from B0329+54 found "blindly" using just 6 LOFAR HBA tiles.
• Simulations show that there is the potential to find +1000 new pulsars.
• LOFAR will be online soon and will provide unparalleled sensitivity.
  

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.

Rene Breton: "New test of gravity using eclipses in the double pulsar PSR J0737-3039A/B"

Rene Breton showed how he and collaborators have used the eclipses of pulsar A in the double pulsar system to perform a new test of gravity:

• Double pulsar system is observed almost edge on (i ~ 89 deg): eclipses of pulsar A by the magnetosphere of B.
• Eclipses last for ~30 seconds, no strong frequency dependence.
• Can see modulation during the eclipse at the spin rate of pulsar B.
• Synchrotron absorbtion in the magnetosphere of pulsar B.
• "rotating doughnut" model, very sensitive to the geometry of the system.
• Very good evidence for dipolar magnetic field.
• Kramer et al. 2006: measured 5 relativistic effects from pulsar timing (most precise test of GR in the strong-field regime)
• Breton et al. 2008 in Science: Eclipse profile changing with time: precession of pulsar B's spin axis measured: 4.77+0.66 deg/yr (consistent with GR prediction)

Some very nice movies showing this result are available here.

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?