The Peculiar Evolutionary History of IGR J17480-2446 in Terzan 5

TL;DR

This paper investigates the unique evolutionary history of the low-mass X-ray binary IGR J17480-2446 in Terzan 5, revealing it as a mildly recycled pulsar in a very recent spin-up phase, challenging typical age expectations.

Contribution

The study provides the first detailed analysis of the spin evolution of IGR J17480-2446, suggesting it is a recently spun-up, mildly recycled pulsar with an unexpectedly young binary age.

Findings

IGR J17480-2446 is spinning up, confirmed by coherent timing analysis.

The pulsar is likely a mildly recycled neutron star.

The binary system's age is surprisingly low, less than 10^8 years.

Abstract

The low mass X-ray binary (LMXB) 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 LMXBs. 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 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 (NS) in IGR J17480-2446 is indeed spinning up by carefully analyzing 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 NS and propose different scenarios to explain this anomaly.