IGR J17591-2342 is a 527-Hz accreting millisecond pulsar in outburst

Following the INTEGRAL discovery of the new transient IGR J17591-2342 and its precise localization by the Neil Gehrels Swift observatory (ATel #11941, #11942), we asked for target-of-opportunity observations with NuSTAR and NICER. NuSTAR observed the source from 2018-08-13 at 22:36 to 08-14 at 14:26 for an exposure of 27 ks. NICER observed from 2018-08-15 at 00:01 to 08-15 at 14:08:00 for a total exposure of 7.2 ks. Timing analysis of both data sets revealed that the new transient is an accreting millisecond pulsar (AMSP) in outburst with a spin frequency of ~527 Hz, an orbital period of 0.37 days, projected semi-major axis of ~1.23 lt-s, and ascending node epoch MJD 58343.7. The 3-30 keV flux as measured by NuSTAR was 4.2 ×10-10 erg/s/cm-2, corresponding to a luminosity of 3×1036erg/s at 8 kpc distance. The 0.1-10 keV flux measured by NICER was 1.3×10-10 ergs/cm2/s. The spectrum could be described with absorbed thermal Comptonization plus a soft black-body, as usually observed for this class of sources. The significance of the periodicity detection is more than five sigmas in <~ 1 ksec exposures in both datasets, but the spin and orbital parameter values should be considered as preliminary at the current status of analysis. We note that the ATCA radio flux reported in ATel #11954 is about three times larger than for other two AMSP (see e.g., Fig. 6 in Tudor at al., 2017). Multi-wavelength observations are encouraged to follow up the source outburst evolution together with the approved Swift monitoring. We thank the Swift, NuSTAR and NICER teams for scheduling and performing these observations on a short notice. NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station; the NICER mission and portions of the NICER science team activities are funded by NASA.