The 2015 outburst of the accreting millisecond pulsar IGR J17511–3057 as seen by INTEGRAL, Swift, and XMM-Newton

¹ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00044 Monte Porzio Catone (Roma), Italy
e-mail: alessandro.papitto@oa-roma.inaf.it
² Institut de Ciències de l’Espai (IEEC-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
³ ISDC Data Centre for Astrophysics, Chemin d’Ecogia 16, 1290 Versoix, Switzerland
⁴ European Space Astronomy Centre (ESA/ESAC), Science Operations Department, 28691 Villanueva de la Cañada, Madrid, Spain
⁵ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica – Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁶ Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain

Received: 2 February 2016
Accepted: 1 September 2016

Abstract

We report on INTEGRAL, Swift, and XMM-Newton observations of IGR J17511–3057 performed during the outburst that occurred between March 23 and April 25, 2015. The source reached a peak flux of 0.7(2) × 10^-9 erg cm^-2 s^-1 and decayed to quiescence in approximately a month. The X-ray spectrum was dominated by a power law with photon index between 1.6 and 1.8, which we interpreted as thermal Comptonization in an electron cloud with temperature >20 keV. A broad (σ ≃ 1 keV) emission line was detected at an energy (E = 6.9^+0.2_-0.3 keV) compatible with the K-α transition of ionized Fe, suggesting an origin in the inner regions of the accretion disk. The outburst flux and spectral properties shown during this outburst were remarkably similar to those observed during the previous accretion event detected from the source in 2009. Coherent pulsations at the pulsar spin period were detected in the XMM-Newton and INTEGRAL data at a frequency compatible with the value observed in 2009. Assuming that the source spun up during the 2015 outburst at the same rate observed during the previous outburst, we derive a conservative upper limit on the spin-down rate during quiescence of 3.5 × 10^-15 Hz s^-1. Interpreting this value in terms of electromagnetic spin-down yields an upper limit of 3.6 × 10²⁶ G cm³ to the pulsar magnetic dipole (assuming a magnetic inclination angle of 30°). We also report on the detection of five type-I X-ray bursts (three in the XMM-Newton data, two in the INTEGRAL data), none of which indicated photospheric radius expansion.