Saturday, July 19, 2008


With that, the meeting is over. Thanks are due to our intrepid bloggers: René Breton, Fotis Gavriil, Sebastien Guillot, Christian Haakonsen, Jason Hessels and Anna Watts. They did an outstanding job of giving you the quick punchline on the talks.

Ramanpreet Kaur : Chandra and XMM-Newton observations of 7 enigmatic faint persistently slow pulsators

The author presented the analysis of two (out of seven) faint slow pulsators with period greater than 150 sec, precluding that they belong to the HMXB, LMXB or IP (intermediate polar) class. Using XMM and Chandra, spectral, timing and positional data were obtained in order to classify the source. The first source, SAX J2324.4-6200, is most probably a Be/X-ray source (HMXB) based on the X-ray spectrum or an very uncommon LMXB. The second slow pulsator, SAX J1452.8-4959 shows no pulsation and have a very faint counterpart, suggesting a possible LMXB or IP classification. However, the timing analysis is uncertain, indicating that the source could be of a different class (e.g CV, background AGN, ...). The X-ray to optical/IR flux ratio for these sources is not known yet.

Ignacio Negueruela: Multiwavelength observations of HMXBs: Constraints on the accretion environment of neutron stars

Most HMXBs are X-ray pulsars, and strong magnetic fields somehow inhibit jet formation, so no radio emission is detected. The nature of the donor determines the main properties of the HMXB, and low resolution spectroscopy in the optical, or IR (if optically obscured), is used to characterise the donor. High quality spectra are needed to determine luminocity class and mass loss rate, but this is difficult (especially in IR) as large telescopes are needed.

In some HMXBs, the orbital parameters can be determined. For SMC X-1, the neutron star mass is quite robustly shown to be less than 1.2 solar masses, in line with a general trend of neutron stars in HMXBs being less massive than those in LMXBs (though mass errors are large, and the sample small). The mass of the donor star in 4U1700-37 is measured to be 58 solar masses, so it appears that not all massive stars form black holes. The neutron star in this HMXB is found to be 2.4 solar masses, though the errors are large.

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.

Teo Muñoz-Darias: Bowen Blend Echo-tomography of Neutron Star LMXBs

Teo gave an overview of a technique which picks out reprocessed emission lines (the Bowen blend) from the irradiated companion star in a neutron star binary. The continuum emission, by contrast, is swamped by the contribution from the disk. Using simultaneous X-ray/optical observations you can look for delayed echoes of X-ray variability. The time delays then tell you about the properties of the binary system as a function of orbital phase. The group are currently writing up an analysis of 4U 1636-536 (for which, fortuitously, they observed 3 reprocessed X-ray bursts), to complement an analysis of Sco X-1 published last year. In both cases the observed delays are consistent with reprocessing from the companion star for the Bowen blend lines, and the accretion disk for the continuum.

Piergiorgio Casella: Discovery of two intermittent accreting millisecond X-ray pulsars

There are a total of 8 known millisecond X-ray pulsars, which is fewer than expected from the hundreds of candidate LMXBs. A detailed search of the entire RXTE archive revealed intemittent millisecond X-ray pulsations from Aql X-1 (550Hz) and SAX J1748.9-2121 (442Hz). In the case of Aql X-1 the pulsations were only seen in 0.01% of total observation time, which indicates that longer observations may reveal millisecond pulsations in several sources where none are presently detected.

Diego Altamirano: "Millihertz quasi-periodic oscillations and their relation with Type-I X-ray Bursts: an intimate relation?"

Diego discussed whether there is a strong link between the mHz QPOs seen in some LMXBs and the occurence of Type I X-ray bursts. Until now we haven't been able to predict X-ray bursts, the recurrence time of which depend on a lot of unknown parameters. However, mHz QPOs, sometimes drifting in frequency, disapear just before an X-ray burst. This is the first time that something in the persistent emission seems to predict an X-ray burst (though mHz QPOs are not always seen before a burst). It appears that one won't see a burst unless the frequency of the mHz QPO drops below some threshold (9mHz appears to be the "magic number" in e.g. 4U1636-53). How do we interpret this? It could possibly be due to marginally stable burning.

Are mHz QPOs a new way to study X-ray bursts and changes in spectral states? Why is 9mHz the "magic number" and why there is frequency drift remains unknown.

Robert Hynes: Multiwavelength Observations of Neutron Star Transients

Rob took us away from the X-ray, and discussed the ways in which observations in the radio, IR, optical and UV are helping us to figure out the physics of neutron star binaries. Such multi-wavelength studies are building up evidence for the presence of jets, and innovative techniques like Doppler tomography and echo-mapping are generating new constraints on binary properties including neutron star mass and the size of the orbit. He also presented exciting new evidence for optical mHz Quasi-Periodic Oscillations in 4U 0614+09 - a phenomenon that has previously only been observed in 4U 1626-67. Wrapping up, he pointed out that most multi-wavelength work so far has focused on the persistent systems. There is a lot of work to be done to extend these studies to the transient systems.

Coming Talks: New Afternoon Schedule for Saturday July 19

All times are Eastern Daylight Time.

Tatehiro Mihara Cyclotron line studies of the X-ray binary pulsars 2:00p
Lara SidoliTransient outburst mechanisms 2:30p
Shin Watanabe X-ray spectroscopy of NS environments in HMXBs3:00p
Ignacio Negueruela Multiwavelength observations of HMXBs: Constraints on the accretion environment of neutron stars 4:00p
Ramanpreet Kaur Chandra and XMM-Newton observations of 7 enigmatic faint persistently slow pulsators 4:30p
Dae-Sik Moon Space Mid-Infrared Spectroscopy for Understanding the Evolution of Highly-Obscured Neutron Star X-ray Binaries4:45p
Meeting Closed

At COSPAR: International X-ray Observatory

In discussions with Mariano Mendez and Luigi Piro, they mentioned the following:

ESA has decided that XEUS as proposed is too expensive, and it has been dropped from the ESA long-range plan. NASA and ESA have met over the past few days, and have agreed to jointly explore an international X-ray observatory, which would replace Con-X and XEUS; the agreement for doing so has been written up, and it is expected to be signed today.

Michiel van der Klis: Accreting millisecond X-ray pulsars

Accreting millisecond X-ray pulsars (AMPs) are predicted by the recycling scenario but were unsuccessfully detected in the early searches (with EXOSAT) due to an insufficient timing resolution. RXTE allowed the detection of the first AMP: SAX J1808.4. Now, 10 AMPs are known (including some intermittent and some sporadic) with frequencies between 182 and 600 Hz. Their spectral energy distribution is quite hard and indicates Compton scattering shock interactions. In addition, we observed smooth sinusoidal pulse profiles. However, the pulse shape sometimes changes (e.g. for SAX J1808).

Intermittent and sporadic pulsars can provides clues to why most NS-LMXBs do not show AMPs behaviors. This can be explained by several reasons: a relatively low mass accretion rate, a weak B-field, scattering, a wandering hot spot, ... AMPs also allow for high precision timing and calculation of orbital ephemerids (although complicated by a pulse changes and a short detectability). Moreover, we observe an important phase residual (not fully understood) suggesting possible torque or propeller effects.

The author presented results from SAX J1808 (long term spin down, change in the orbital period) and finished with a quick overview of burst oscillations and kHz QPOs.

Anna Watts: "Type I bursts and burst oscillations"

Anna reviewed the X-ray bursts which are observed from roughly half the known NS LMXBs. There is a rich variety of phenomena observed in these bursts, and new types of bursts are still emerging (e.g. burst triplets). These new types of bursts are revealing new burning regimes. However, there is still no satisfactory explanation for the burst oscillations, which are confidently observed in 12 sources. These burst oscillations show great variety in behaviour from source to source, which makes it difficult to provide a singular explanation.

Friday, July 18, 2008

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 MendezAperiodic 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 HMXBs5:30p
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 Binaries6:15p

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.