Letters to the Editor

The spin and orbit of the newly discovered pulsar IGR J17480-2446

¹ Dipartimento di FisicaUniversità degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
e-mail: apapitto@oa-cagliari.inaf.ig
² INAF – Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra ( CA), Italy
³ Dipartimento di Scienze Fisiche ed Astronomiche, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
⁴ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy

Received: 21 October 2010
Accepted: 28 November 2010

Abstract

We present an analysis of the spin and orbital properties of the newly discovered accreting pulsar IGR J17480-2446, located in the globular cluster Terzan 5. Considering the pulses detected by the Rossi X-ray Timing Explorer at a period of 90.539645(2) ms, we derive a solution for the 21.27454(8) hr binary system. The binary mass function is estimated to be 0.021275(5) M_⊙, indicating a companion star with a mass larger than 0.4 M_⊙. The X-ray pulsar spins up while accreting at a rate of between 1.2 and 1.7 × 10^-12 Hz s^-1, in agreement with the accretion of disc matter angular momentum given the observed luminosity.

We also report the detection of pulsations at the spin period of the source during a Swift observation performed  ~2 d before the beginning of the RXTE coverage. Assuming that the inner disc radius lies in between the neutron star radius and the corotation radius while the source shows pulsations, we estimate the magnetic field of the neutron star to be within  ~2 × 10⁸ G and  ~2.4 × 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.