We analyse the spectral and pulse properties of the 11 Hz transient accreting pulsar, IGR J17480−2446, in the globular cluster Terzan 5, considering all the available Rossi X-Ray Timing Explorer, Swift and INTEGRAL observations performed during the outburst shown between 2010 October and November.
By measuring the pulse phase evolution we conclude that the neutron star (NS) spun up at an average rate of Hz s−1, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. This confirms the trend previously observed by Papitto et al., who considered only the first few weeks of the outburst. Similar to other accreting pulsars, the stability of the pulse phases determined by using the second harmonic component is higher than that of the phases based on the fundamental frequency. Under the assumption that the second harmonic is a good tracer of the NS spin frequency, we successfully model its evolution in terms of a luminosity-dependent accretion torque. If the NS accretes the specific Keplerian angular momentum of the in-flowing matter, we estimate the inner disc radius to lie between 47 and 93 km when the luminosity attains its peak value. Smaller values are obtained if the interaction between the magnetic field lines and the plasma in the disc is considered.
The phase-averaged spectrum is described by thermal Comptonization of photons with energy of ≈1 keV. A hard to soft state transition is observed during the outburst rise. The Comptonized spectrum evolves from a Comptonizing cloud at an electron temperature of ≈20 keV towards an optically denser cloud at kTe≈ 3 keV. At the same time, the pulse amplitude decreases from 27 per cent to few per cent, as already noted by Papitto et al., and becomes strongly energy dependent. We discuss various possibilities to explain such a behaviour, proposing that at large accretion luminosities a significant fraction of the in-falling matter is not channelled towards the magnetic poles, but rather accretes more evenly on to the NS surface.