The pulse profile and spin evolution of the accreting pulsar in Terzan 5, IGR J17480-2446, during its 2010 outburst

Abstract

(abridged) 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 RXTE, Swift and INTEGRAL observations performed between October and November, 2010. By measuring the pulse phase evolution we conclude that the NS spun up at an average rate of <nu_dot>=1.48(2)E-12 Hz/s, compatible with the accretion of the Keplerian angular momentum of matter at the inner disc boundary. 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 neutron star 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 kT_e~3 keV. At the same time, the pulse amplitude decreases from 27% to few per cent 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 onto the NS surface.

To appear in MNRAS

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• NuSTAR and XMM-Newton broad-band spectrum of SAX J1808.4-3658 during its latest outburst in 2015
• Accreting pulsars: mixing-up accretion phases in transitional systems
• Discovery of a millisecond pulsar in the 5.4 day binary 3FGL J1417.5-4402: observing the late phase of pulsar recycling
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• Study of the accretion torque during the 2014 outburst of the X-ray pulsar GRO J1744-28
• Rapid X-ray variability properties during the unusual very hard state in neutron-star low-mass X-ray binaries
• An XMM-Newton and INTEGRAL view on the hard state of EXO 1745-248 during its 2015 outburst
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• X-ray burst ignition location on the surface of accreting X-ray pulsars: Can bursts preferentially ignite at the hotspot?
• The near-IR counterpart of IGR J17480-2446 in Terzan 5
• Discovery of 3 s pulsations from the Brightest Hard X-ray Source in M31
• On the formation of the peculiar low-mass X-ray binary IGR J17480$-$2446 in Terzan 5