Astronomy and Astrophysics – Astrophysics – High Energy Astrophysical Phenomena
Scientific paper
2011-12-22
Astronomy and Astrophysics
Astrophysics
High Energy Astrophysical Phenomena
Subimitted to ApJ
Scientific paper
The low mass X-ray binary IGR J17480-2446 in the globular cluster Terzan 5 harbors an 11 Hz accreting pulsar. This is the first object discovered in a globular cluster with a pulsar spinning at such low rate. The accreting pulsar is anomalous because its characteristics are very different from the other five known slow accreting pulsars in galactic low mass X-ray binaries. Many features of the 11 Hz pulsar are instead very similar to those of accreting millisecond pulsars, spinning at frequencies >100 Hz. Understanding this anomaly is very valuable because IGR J17480-2446 can be the only accreting pulsar discovered so far which is in the process of becoming an accreting millisecond pulsar. We first verify that the neutron star in IGR J17480-2446 is indeed spinning up by carefully analysing X-ray data with coherent timing techniques that account for the presence of timing noise. We then study the present Roche Lobe Overflow epoch and the two previous spin-down epochs dominated by magneto dipole radiation and stellar wind accretion. We find that IGR J17480-2446 is very likely a mildly recycled pulsar and suggest that it has started a spin-up phase in an exceptionally recent time, that has lasted less than a few 10^7 yr. We also find that the total age of the binary is surprisingly low (<10^8 yr) when considering typical parameters for the newborn neutron star and propose different scenarios to explain this anomaly.
Ali Alpar M.
J. van den Heuvel E. E. P.
Patruno Alessandro
van der Klis Michiel
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