Alessandro Papitto (University of Cagliari, Italy)

The spin evolution of the Terzan 5 accreting pulsar, IGR J17480-2446, during its 2010 outburst.

We present an analysis of the pulse properties of the 11 Hz pulsar recently discovered in the globular cluster
Terzan 5, IGR J17480-2446, as observed by RXTE, Swift and INTEGRAL. The observed evolution of the
pulse phases clearly indicate how the source spins up at a rate compatible with the accretion of the
Keplerian angular momentum of the disc matter. Similarly to other accreting pulsars, pulse phases computed
over higher order harmonics appears as more stable than those of the fundamental frequency. By
considering the second harmonic phases, we apply a physically motivated torque model to the observed spin
frequency evolution of the source and estimate the size of the inner disc radius around the pulsar. This
estimate is then compared to the values derived from the modelling of the kHz QPO and of the broad iron
emission line observed from the source.
We also report about the evolution observed in the pulse profile properties during the outburst rise,
contemporary to a softening of the source energy spectrum. We discuss various possibilities to explain such
a behaviour, among which we propose that at a large X-ray luminosity the geometry of accretion changes,
and a significant fraction of the accreting matter falls more evenly onto the neutron star surface, rather than
being channelled to the magnetic poles.