A newly discovered accreting pulsar in Terzan 5

The 11 Hz accreting pulsar, IGR J17480-2446, was recently discovered in the globular cluster Terzan 5. The analisys of the Doppler shifts induced by the orbital motion reveals how the neutron star belongs to a ∼ 21.3 hr binary system with a companion star of 0.4-1.5 M. The X-ray pulsar spins up while accreting at an average rate of 1.48(2)×10−12 Hz s−1, in agreement with the accretion of disc matter angular momentum given the observed luminosity. From the presence of pulsations at different accretion rates we constrain the magnetic field to lie within ∼2×108 and ∼2×1010 G. From this estimate, the value of the spin period and of the observed spin-up rate, we associate this source with the still poorly sampled population of slow, mildly recycled, accreting pulsars. The pulse profile is generally modelled by using two harmonic components, and a timing analysis performed on the pulse phases shows how the second harmonic is much less affected by timing noise than the first one. The temporal evolution of the pulse phases computed over the second harmonic is also much more in agreement with the expected dependence of the spin-up rate on the accretion luminosity. The assumption that this harmonic component is a good tracer of the neutron star spin frequency opens the possibility of a dynamical measure of the accretion torque exerted on this neutron star during accretion.