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23rd of March 2020
Radio and X-ray monitoring of an accreting millisecond X-ray pulsar in outburst
by Gusinskaia et al.
In August, the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) Xray and gamma-ray telescope discovered a new transient, IGR J17591−2342. The high absorption seen in the X-ray spectrum suggested the source is roughly at the distance of the Galactic centre (∼8 kilo-parsecs, or ~26,000 light years). Initial radio observations using ATCA measured a high flux density, typical of that seen in a BH-LMXB (Black Hole -- Low-Mass X-ray Binary). However, timing analysis of follow-up NuSTAR and NICER X-ray observations revealed 527-Hz coherent X-ray pulsations, showing that IGR J17591−2342 was an accreting millisecond X-ray pulsar (AMXP).

Early observations revealed that the source's radio emission was brighter than that of any other known neutron star low-mass X-ray binary (NS-LMXB) at comparable X-ray luminosity, assuming its likely distance. Gusinskaia et al. present the results of extensive radio and X-ray monitoring campaign of the 2018 outburst of IGR J17591-2342. The total of 10 quasi-simultaneous radio (VLA, ATCA) and X-ray (Swift-XRT) observations make IGR J17591-2342 one of the best-sampled NS-LMXBs. The figure above shows the X-ray and radio light-curves during the 2018 outburst of IGR J17591−2342. The top panel has blue squares representing the Swift-XRT (1 − 10 keV) X-ray light-curve (using the left-hand axis) and grey circles represent the NICER (0.2 − 12 keV) X-ray light-curve (using the right-hand axis). The centre panel shows the radio light-curve, with circles and diamonds representing ATCA and VLA observations, respectively. The bottom panel shows the inferred radio spectral index. More details are presented in the paper published in Monthly Notices of the Royal Astronomical Society.

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