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.

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.