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

TL;DR

This paper presents multi-instrument observations of the 2015 outburst of the accreting millisecond pulsar IGR J17511-3057, revealing spectral features, pulsations, and bursts, and comparing them to previous outbursts to understand the source's behavior.

Contribution

It provides detailed spectral and timing analysis of the 2015 outburst, including pulsation and burst detection, and compares these findings with prior outbursts to enhance understanding of the pulsar's accretion and spin evolution.

Findings

Spectral dominated by thermal Comptonization with electron temperature > 20 keV

Detection of broad Fe Kα emission line at 6.9 keV

Observation of five type-I X-ray bursts without photospheric radius expansion

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)E-9 erg/cm$^2$/s 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 ({\sigma} ~ 1 keV) emission line was detected at an energy (E = 6.9$^{+0.2}_{-0.3}$ keV) compatible with the K{\alpha} 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 3.5E-15 Hz/s. Interpreting this value in terms of electromagnetic spin down yields an upper limit of 3.6E26 G/cm$^3$ to the pulsar magnetic dipole (assuming a magnetic inclination angle of 30{\deg}). 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.