2017A&A...603A..16D

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 ∼254.3Hz is the only transitional pulsar so far to have shown a full accretion episode, reaching an X-ray luminosity of ∼10³⁷erg/s 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 0.4-250keV. 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 ∼60keV in both the ISGRI (10.9σ) and Fermi/GBM (5.9σ) bandpass. The alignment of the ISGRI and Fermi GBM 20-60keV pulse profiles are consistent at a ∼25µs 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 150µs, 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~=2.7x10⁸g.cm^–2. For such a helium burst the estimated recurrence time of Δt_rec~=5.6d is in agreement with the observations.