Coming Talks: Saturday July 19

Last day of talks for session E11. All times are Eastern Daylight Time.

Anna Watts	Type I bursts and burst oscillations	9:30a
Michiel van der Klis	Accreting millisecond X-ray pulsars	10:00a
Mariano Mendez	Aperiodic variability in low-mass X-ray binaries	10:30a
Robert Hynes	Multiwavelength Observations of Neutron Star Transients	11:30a
Diego Altamirano	Millihertz quasi-periodic oscillations and their relation with Type-I X-ray Bursts: an intimate relation?	12:00p
Piergiorgio Casella	Discovery of two intermittent accreting millisecond X-ray pulsars	12:15p
Teo Muñoz-Darias	Bowen Blend Echo-tomography of Neutron Star LMXBs	12:30p
A. Nepomuk Otte	Detection of the Crab pulsar above 25 GeV with the MAGIC telescope	12:45p
Tatehiro Mihara	Cyclotron line studies of the X-ray binary pulsars	2:00p
Lara Sidoli	Transient outburst mechanisms	2:30p
Ignacio Negueruela	Multiwavelength observations of HMXBs: Constraints on the accretion environment of neutron stars	5:00p
Shin Watanabe	X-ray spectroscopy of NS environments in HMXBs	5:30p
Ramanpreet Kaur	Chandra and XMM-Newton observations of 7 enigmatic faint persistently slow pulsators	6:00p
Dae-Sik Moon	Space Mid-Infrared Spectroscopy for Understanding the Evolution of Highly-Obscured Neutron Star X-ray Binaries	6:15p

Benjamin Owen: How LIGO can follow up high-energy observations of young neutron stars

Ben discussed LIGO's attempts to do searches for gravitational wave emission from isolated spinning neutron stars, where the position is known but where you have to search over an unknown spin frequency. The poster child source for this type of search is Cas A. LIGO are interested in targeting (a) isolated non-pulsing neutron stars (like some of the CCOs), (b) as yet unseen neutron stars in pulsar wind nebulae/small supernova remnants, (c) massive star-forming regions in which a lot of neutron stars might be born, and (d) globular cluster cores. He also advertised the upcoming LIGO/Neutron Star astronomer meeting to be held in January 2009.

In Montreal: Fireworks at La Ronde

La Ronde amusement park is host of the Loto-Quebec's International Fireworks Competition. Events are on Wednesdays and Saturdays at 10 pm (rain or shine), with competitors representing a different country each time. This Saturday, July 19, will feature Howard & Sons Pyrotechnics Australia. You may buy a ticket and go watch the fireworks on site, but I would rather suggest you just walk East on Ste-Catherine Street to Papineau Street near the Jacques-Cartier Bridge (it's a nice ~15 minutes walk from the Palais des Congres area) and the whole street is blocked to cars from St-Denis Street. Follow this link to see some gorgeous pictures from this year's previous events.

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.

George Pavlov: Central Compact Objects in Supernova Remnants

George reviewed the status of observations of Compact Central Objects (CCOs) - radio and gamma-ray quiet X-ray sources that are found close to the center of supernova remnants. Evidence is emerging that at least two of them are 'antimagnetars' (gamnetars?) - neutron stars that are born spinning slowly but with unusually low magnetic fields compared to most radio pulsars. The youngest of the CCOs (in the Cas A supernova remnant) seems to be different - and could well be a quiescent magnetar. Single temperature blackbody spectral fits for this object suggest a very small radius (which might imply that it is a quark star). More complex spectral fits involving two components with different temperatures, however, can give an acceptable fit to a neutron star equation of state. Ultimately phase-coherent timing analysis (with XMM/Chandra), and the detection and modeling of spectral lines, are the best way to figure out the nature of these objects.

Patrick Slane: Observations of pulsar wind nebulae

Pat discussed the geometry of pulsar wind nebulae (PWN), such as their jet and torus structure. The jets, for example, are due to the the fact that the wind termination shock is farther from the pulsar at equator than along the axis. The pulsar dumps energetic particles into the PWN. Where the synchrotron lifetime of the particles is comparable to the age of the supernova, a break appears in the spectrum. Energetic electrons produce both the synchrotron and inverse Compton (IC) emission observed in PWN. By measuring both emission mechanisms, using gamma-ray and X-ray observations, one can constrain the magnetic field of the nebula. The interaction between the PWN and its surrounding supernova remnant may explain why PWN seem distorted. Because PWN evolve within a supernova remnant a reverse shock "slams" into the PWN. Turbulent and asymmetric structures are due to this reverse shock. This may explain why TeV sources are offset from pulsars.

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.

David Kaplan: "Optical/IR/UV Observations of Isolated Neutron Stars"

David nicely showed how optical/IR/UV observations are providing unique information about neutron star spectral energy distributions and energetics. These objects are hard to observe (B magnitudes of 25-28), but this is still very worthwhile. So far, 6 isolated neutron stars have optical counterparts. Optical/IR/UV have in fact given very strong evidence that INSs are indeed neutron stars. A major outstanding issue is the "optical excess" (X-ray determined blackbody does not fit well at longer wavelengths). Also, it is still unknown whether the optical or UV is pulsed. Can we constrain atmospheric models using these multi-wavelength data?

Frank Haberl: X-ray observations of Isolated Neutron Stars

The author presented a global view of X-ray observations of neutrons stars and the possible information that one can deduce from them. Evidence of multicomponent Xray spectra and pulsation of the Xray flux are evidence for a non-uniform temperature on the surface of the neutron star. Three middle aged pulsars (The three musketeers) present two thermal components (from the surface and a hot spot) and a power-law component (most probably coming from the magnetosphere). Pulse phase spectroscopy from Chandra and XMM showed that the hot and the cold blackbody components are not in phase.

XMM observations of the 7 known isolated neutron stars showed that they do not have pure blackbody spectra, indicating absorption features (even multiple lines). Several origins for those lines were mentioned, e.g. Cyclotron resonance, atomic lines transitions (Hydrogen). Those isolated neutron stars also provide a unique ways of measuring their magnetic fields using 2 independent method (magnetic dipole breaking and proton cyclotron absorption). Observations of RX J0720.4-3125 put forward the evidence for precession of the neutron star and the evidence of 2 polar caps.

Finally, the author showed with cooling curves analysis the evidence of magnetic field decay. The main conclusion from this presentation is that a NS model with a uniform temperature and a dipolar magnetic field is far too simple.

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.

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.

A. Nepomuk Otte: Detection of the Crab pulsar above 25 GeV with the MAGIC telescope

The Crab pulsar was detected with ~6 sigma significance at >25GeV using MAGIC. Detection of the Crab has been called the holy grail of ground based gamma ray astronomy, since sensitivity below 100GeV is very difficult to achieve using the air Cherenkov detectors. The interpulse amplitude was larger than the main pulse, indicating that the interpulse has a much harder spectrum than the main pulse.

At COSPAR: Mission updates

We've just heard that RXTE is likely to be extended to August 2009 - unfortunately not long enough for full overlap with LIGO's next science run, but long enough to give some overlap with GLAST. In Session E15 we heard about exciting new mission proposals including AXTAR (an advanced X-ray timing mission), NICE (which aims to explore neutron star properties), and GEMS (investigating gravity and extreme magnetism) - but the big topic was the possible merger of Constellation-X and XEUS.

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.

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?