Letter to the Editor

Swift monitoring of the new accreting millisecond X-ray pulsar IGR J17511-3057 in outburst

E. Bozzo¹, C. Ferrigno^1,2, M. Falanga³, S. Campana⁴, J. A. Kennea⁵ and A. Papitto^6,7

¹ ISDC data centre for astrophysics, University of Geneva, Chemin d'Écogia 16, 1290 Versoix, Switzerland e-mail: Enrico.Bozzo@unige.ch
² IAAT, Abt. Astronomie, Universität Tübingen Sand 1, 72076 Tübingen, Germany
³ International Space Science Institute (ISSI) Hallerstrasse 6, 3012 Bern, Switzerland
⁴ INAF – Osservatorio Astronomico di Brera, via Emilio Bianchi 46, 23807 Merate (LC), Italy
⁵ Department of Astronomy and Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁶ Universitá degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
⁷ INAF – Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy

Received: 21 October 2009
Accepted: 28 November 2009

Abstract

Context. A new accreting millisecond X-ray pulsar, IGR J17511-3057, was discovered in outburst on 2009 September 12 during the INTEGRAL Galactic bulge monitoring programme.

Aims. To study the evolution of the source X-ray flux and spectral properties during the outburst, we requested a Swift monitoring of IGR J17511-3057.

Methods. In this paper we report on the results of the first two weeks of monitoring the source.

Results. The persistent emission of IGR J17511-3057 during the outburst is modelled well with an absorbed blackbody ( keV) and a power-law component (Γ–2), similar to what has been observed from other previously known millisecond pulsars. Swift also detected three type-I Xray bursts from this source. By assuming that the peak luminosity of these bursts is equal to the Eddington value for a pure helium type-I X-ray burst, we derived an upper limit to the source distance of ~10 kpc. The theoretically expected recurrence time of the bursts according to the helium burst hypothesis is 0.2–0.9 days, in agreement with the observations.