Radio and X-ray monitoring of the accreting millisecond X-ray pulsar IGR J17591−2342 in outburst

Abstract

IGR J17591−2342 is a new accreting millisecond X-ray pulsar (AMXP) that was recently discovered in outburst in 2018. Early observations revealed that the source's radio emission is brighter than that of any other known neutron star low-mass X-ray binary (NS-LMXB) at comparable X-ray luminosity, and assuming its likely ∼ > 6 kpc distance. It is comparably radio bright to black hole LMXBs at similar X-ray luminosities. In this work, we present the results of our extensive radio and X-ray monitoring campaign of the 2018 outburst of IGR J17591−2342. In total we collected 10 quasisimultaneous radio (VLA, ATCA) and X-ray (Swift-XRT) observations, which make IGR J17591−2342 one of the best-sampled NS-LMXBs. We use these to fit a power-law correlation index β = 0.37 +0.42 −0.40 between observed radio and X-ray luminosities (L R ∝ L X β ). However, our monitoring revealed a large scatter in IGR J17591−2342's radio luminosity (at a similar X-ray luminosity, L X ∼ 10 36 erg s −1 , and spectral state), with L R ∼ 4×10 29 erg s −1 during the first three reported observations, and up to a factor of 4 lower L R during later radio observations. Nonetheless, the average radio luminosity of IGR J17591−2342 is still one of the highest among NS-LMXBs, and we discuss possible reasons for the wide range of radio luminosities observed in such systems during outburst. We found no evidence for radio pulsations from IGR J17591−2342 in our Green Bank Telescope observations performed shortly after the source returned to quiescence. Nonetheless, we cannot rule out that IGR J17591−2342 becomes a radio millisecond pulsar during quiescence.

High-energy characteristics of the accretion-powered millisecond pulsar IGR J17591−2342 during its 2018 outburst

“…The distance to IGR J17591−2342 derived in this work of 7.6 ± 0.7 kpc puts the source in the Galactic bulge at a Galactic center distance that is consistent with the I-K color value given in Nowak et al (2019). It also means that the radio-jet of IGR J17591−2342 shines very brightly compared to other neutron star LMXBs, placing it in the L X -L R diagram at parameter space locations that is typically occupied by black-hole LMXBs (see Russell et al 2018;Gusinskaia et al 2020). The extreme brightness of radio-jet is unlikely to be due to beaming because of the detection of winds from the system (Nowak et al 2019).…”

“…The total hydrogen column density N H of (2.09 ± 0.05) × 10 22 cm −2 derived in this work, however, is considerably lower, but still compatible at a 2σ confidence, compared to the values derived by other groups analysing spectral data of IGR J17591−2342 using a different combination of high-energy instruments (see e.g. Russell et al 2018;Sanna et al 2018;Nowak et al 2019;Gusinskaia et al 2020). The difference can be explained by the degeneracy between the N H and kT bb,seed fit parameters which become clear through the inclusion of highquality spectral data from XMM-Newton RGS 1 & 2 and EPIC-MOS2 for energies below ∼1 keV in this work.…”

The Flare-dominated Accretion Mode of a Radio-bright Candidate Transitional Millisecond Pulsar

“…Figure 10. L X -L R plot containing 3FGLJ0427.9−6704, IGR J17591-2342 (Gusinskaia et al 2020), three tMSPs, and other low-mass X-ray binaries obtained from the database of Bahramian et al (2018). 3FGLJ0427.9−6704 sits close to the black hole relation at L X ∼10 33 erg s −1 and is the only low-L X neutron star yet observed to do so.…”

“…However, our understanding of radio emission from accreting neutron stars has been slower to develop, with evidence emerging in the last decade of a much more complex situation for neutron stars than for black holes. It is clear that neutron stars do not follow a single relation between radio and X-ray luminosity, but show a wide range of radio-loudness at all L X >10 34 erg s −1 (e.g., Migliari & Fender 2006;Migliari et al 2011;Tudor et al 2017;Gallo et al 2018;Gusinskaia et al 2020).…”

“…The only other accreting neutron star shown to sit close to the black hole radio/X-ray correlation is the AMXP IGR J17591-2342, but this is at much higher L X 10 35 erg s −1 , and its distance is also not yet well-constrained (Gusinskaia et al 2020). This paper points out that there is no obvious reason why this AMXP should be much more radio-loud than other similar systems, and some candidate explanations such as the spin rate of the neutron star or possible beaming are not consistent with the data.…”