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.

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.

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.

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.

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.

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.

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?

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.

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?