The transitional millisecond pulsar IGR J18245-2452 during its 2013 outburst at X-rays and soft gamma-rays

¹ Departement Physik, Universität Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
e-mail: vittorio-df@issibern.ch
² International Space Science Institute (ISSI), Hallerstrasse 6, 3012 Bern, Switzerland
³ SRON–Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁴ ISDC, Data centre for astrophysics, University of Geneva, Chemin d’Écogia 16, 1290 Versoix, Switzerland
⁵ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁶ Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
⁷ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone (Roma), Italy

Received: 10 February 2017
Accepted: 13 April 2017

Abstract

IGR J18245–2452/PSR J1824–2452I is one of the rare transitional accreting millisecond X-ray pulsars, showing direct evidence of switches between states of rotation-powered radio pulsations and accretion-powered X-ray pulsations, dubbed transitional pulsars. IGR J18245–2452 with a spin frequency of ~ 254.3 Hz is the only transitional pulsar so far to have shown a full accretion episode, reaching an X-ray luminosity of ~ 10³⁷ erg s^-1 permitting its discovery with INTEGRAL in 2013. In this paper, we report on a detailed analysis of the data collected with the IBIS/ISGRI and the two JEM-X monitors on-board INTEGRAL at the time of the 2013 outburst. We make use of some complementary data obtained with the instruments on-board XMM-Newton and Swift in order to perform the averaged broad-band spectral analysis of the source in the energy range 0.4–250 keV. We have found that this spectrum is the hardest among the accreting millisecond X-ray pulsars. We improved the ephemeris, now valid across its full outburst, and report the detection of pulsed emission up to ~ 60 keV in both the ISGRI (10.9σ) and Fermi/GBM (5.9σ) bandpass. The alignment of the ISGRI and Fermi GBM 20–60 keV pulse profiles are consistent at a ~ 25 μs level. We compared the pulse profiles obtained at soft X-rays with XMM-Newton with the soft γ-ray ones, and derived the pulsed fractions of the fundamental and first harmonic, as well as the time lag of the fundamental harmonic, up to 150 μs, as a function of energy. We report on a thermonuclear X-ray burst detected with INTEGRAL, and using the properties of the previously type-I X-ray burst, we show that all these events are powered primarily by helium ignited at a depth of y_ign ≈ 2.7 × 10⁸ g cm${-2}^{}$. For such a helium burst the estimated recurrence time of Δt_rec ≈ 5.6 d is in agreement with the observations.