High-energy characteristics of the accretion-powered millisecond pulsar IGR J17591−2342 during its 2018 outburst

XMM-Newton, NICER, NuSTAR, and INTEGRAL view of the 0.3–300 keV X-ray band

¹ SRON-Netherlands Institute for Space Research, Sorbonnelaan 2, Utrecht 3584, CA, The Netherlands
e-mail: L.M.Kuiper@sron.nl
² Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, 117997 Moscow, Russia
⁴ International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
⁵ International Space Science Institute Beijing, No. 1 Nanertiao, Zhongguancun, Haidian District, 100190 Beijing, PR China
⁶ School of Physics and Astronomy, Monash University, Australia, Clayton, VIC 3800, Australia
⁷ Monash Centre for Astrophysics, Monash University, Australia, Clayton, VIC 3800, Australia
⁸ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
⁹ Department of Physics, Xiangtan University, Xiangtan 411105, PR China

Received: 25 February 2020
Accepted: 16 June 2020

Abstract

IGR J17591−2342 is an accreting millisecond X-ray pulsar, discovered with INTEGRAL, which went into outburst around July 21, 2018. To better understand the physics acting in these systems during the outburst episode, we performed detailed temporal-, timing-, and spectral analyses across the 0.3–300 keV band using data from NICER, XMM-Newton, NuSTAR, and INTEGRAL. The hard X-ray 20–60 keV outburst profile covering ∼85 days is composed of four flares. Over the course of the maximum of the last flare, we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data, posing constraints on the source distance. We derived a distance of 7.6 ± 0.7 kpc, adopting Eddington-limited photospheric radius expansion and assuming anisotropic emission. In the timing analysis, using all NICER 1–10 keV monitoring data, we observed a rather complex set of behaviours starting with a spin-up period (MJD 58345–58364), followed by a frequency drop (MJD 58364–58370), an episode of constant frequency (MJD 58370–58383), concluded by irregular behaviour till the end of the outburst. The 1–50 keV phase distributions of the pulsed emission, detected up to ∼120 keV using INTEGRAL ISGRI data, was decomposed in three Fourier harmonics showing that the pulsed fraction of the fundamental increases from ∼10% to ∼17% going from ∼1.5 to ∼4 keV, while the harder photons arrive earlier than the soft photons for energies ≲10 keV. The total emission spectrum of IGR J17591−2342 across the 0.3–150 keV band could adequately be fitted in terms of an absorbed COMPPS model yielding as best fit parameters a column density of N_H = (2.09 ± 0.05) × 10²² cm⁻², a blackbody seed photon temperature kT_bb, seed of 0.64 ± 0.02 keV, electron temperature kT_e = 38.8 ± 1.2 keV and Thomson optical depth τ_T = 1.59 ± 0.04. The fit normalisation results in an emission area radius of 11.3 ± 0.5 km adopting a distance of 7.6 kpc. Finally, the results are discussed within the framework of accretion physics- and X-ray thermonuclear burst theory.