We present a simultaneous periodic and aperiodic timing study of the accreting millisecond X-ray pulsar SAX J1808.4-3658. We analyze five outbursts of the source and for the first time provide a full and systematic investigation of the enigmatic phenomenon of the 1 Hz flares observed during the final stages of some of the outbursts. We show that links between pulsations and 1 Hz flares might exist, and suggest they are related with hydrodynamic disk instabilities that are triggered close to the disk-magnetosphere boundary layer when the system is entering the propeller regime.

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The past two decades have witnessed a renewed interest in low frequency radio astronomy, with a particular focus on frequencies above 30 MHz e.g., LOFAR (LOw Frequency ARray) in the Netherlands and its European extension ILT, the International LOFAR Telescope. However, at frequencies below 30 MHz, Earth-based observations are limited due to a combination of severe ionospheric distortions, almost full reflection of radio waves below 10 MHz, solar eruptions and the radio frequency interference (RFI) of human-made Email addresses: m.j.bentum@tue.nl (M.J. Bentum), m.k.verma@student.tudelft.nl (M.K. Verma), r.t.rajan@tudelft.nl (R.T. Rajan), boonstra@astron.nl (A.J. Boonstra), C.J.M.Verhoeven@tudelft.nl (C.J.M. Verhoeven), e.k.a.gill@tudelft.nl (E.K.A. Gill), a.j.vanderveen@tudelft.nl (A.J. van der Veen), h.falcke@astro.ru.nl (H. Falcke), m.kleinwolt@astro.ru.nl (M. Klein Wolt), b.monna@hyperiontechnologies.nl (B. Monna), s.engelen@hyperiontechnologies.nl (S. Engelen), j.rotteveel@isispace.nl (J. Rotteveel), lgurvits@jive.eu (L.I. Gurvits)

The moon is a unique location in our solar system and provides important information regarding the exposure to free space that is essential for future human space exploration to mars and beyond. The active broadband (1 kHz−100 MHz) tripole antenna now envisaged to be placed on the European Lunar Lander located at the Lunar South Pole allows for sensitive measurements of the exosphere and ionosphere, and their interaction with the Earths magnetosphere, solar particles, wind and CMEs and studies of radio communication on the moon, that are essential for future lunar human and science exploration. In addition, the lunar South pole provides an excellent opportunity for radio astronomy. Placing a single radio antenna in an eternally dark crater or behind a mountain at the south (or north) pole would potentially provide perfect shielding from man-made radio interference (RFI), absence of ionospheric distortions, and high temperature and antenna gain stability that allows detection of the 21 cm wave emission from pristine hydrogen formed after the big bang and into the period where the first stars formed. A detection of the 21 cm line from the moon at these frequencies would allow for the first time a clue on the distribution and evolution on mass in the early universe between the Epoch of Recombination and Epoch of Reionization (EoR). Next to providing a cosmological breakthrough, a single lunar radio antenna would allow for studies of the effect of solar flares and Coronal Mass Ejections (CMEs) on the solar wind at distances close to earth (space weather) and would open up the study of low frequency radio events (flares and pulses) from planets such as Jupiter and Saturn, which are known to emit bright (kJy−MJy) radio emission below 30 MHz (Jester and Falcke, 2009). Finally, a single radio antenna on the lunar lander would pave the way for a future large lunar radio interferometer; not only will it demonstrate the possibilities for lunar radio science and open up the last unexplored radio regime, but it will also allow a determination of the limitations of lunar radio science by measuring the local radio background noise.