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 \sim 254.3 Hz is the only transitional pulsar so far to have shown a full accretion episode , reaching an X-ray luminosity of \sim 10 ^ { 37 } 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 \sim 60 keV in both the ISGRI ( 10.9 \sigma ) and Fermi/GBM ( 5.9 \sigma ) bandpass .
The alignment of the ISGRI and Fermi GBM 20 – 60 keV pulse profiles are consistent at a \sim 25 \mu s level .
We compared the pulse profiles obtained at soft X-rays with XMM-Newton with the soft \gamma -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 \mu s , 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 } \approx 2.7 \times 10 ^ { 8 } g cm ^ { -2 } .
For such a helium burst the estimated recurrence time of \Delta t _ { rec } \approx 5.6 d is in agreement with the observations .