Disc–jet coupling in low-luminosity accreting neutron stars

Abstract

In outburst, neutron star X-ray binaries produce less powerful jets than black holes at a given X-ray luminosity. This has made them more difficult to study as they fade towards quiescence. To explore whether neutron stars power jets at low accretion rates (L X 10 36 erg s −1 ), we investigate the radio and X-ray properties of three accreting millisecond X-ray pulsars (IGR J17511-3057, SAX J1808.4-3658 and IGR J00291+5934) during their outbursts in 2015, and of the non-pulsing neutron star Cen X-4 in quiescence (2015) and in outburst (1979). We did not detect the radio counterpart of IGR J17511-3057 in outburst or of Cen X-4 in quiescence, but did detect IGR J00291+5934 and SAX J1808.4-3658, showing that at least some neutron stars launch jets at low accretion rates. While the radio and X-ray emission in IGR J00291+5934 seem to be tightly correlated, the relationship in SAX J1808.4-3658 is more complicated. We find that SAX J1808.4-3658 produces jets during the reflaring tail, and we explore a toy model to ascertain whether the radio emission could be attributed to the onset of a strong propeller. The lack of a universal radio/X-ray correlation, with different behaviours in different neutron star systems (with various radio/X-ray correlations; some being radio faint and others not), points at distinct disk-jet interactions in individual sources, while always being fainter in the radio band than black holes at the same X-ray luminosity.

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

“…Alternatively, the X-ray wind itself could be responsible for boosting radio luminosity via wind-jet interaction. This was previously suggested as a possible explanation for the observed radio flares in SAX J1808.4−3658 (Tudor et al 2017). Such a scenario could also be a potential explanation for the large variability of the radio emission of IGR J17591−2342, which appears to be the brightest during the first three radio epochs.…”

“…In the last few years, new and upgraded radio telescopes -like the Karl G. Jansky Very Large Array (VLA), the Australia Telescope Compact Array (ATCA), and MeerKAT -coupled with flexible X-ray observatories such as the Neil Gehrels Swift X-ray Telescope (Swift-XRT), have provided the sensitivity and rapid response required to probe a steadily increasing number of known Galactic NS-LMXBs. This includes accreting millisecond X-ray pulsars (AMXPs; Tudor et al 2017), where the accretion flow is channeled by the NS's magnetic field (producing coherent X-ray pulsations: e.g., Wijnands & van der Klis 1998; see Patruno & Watts 2012 for a review), as well as non-pulsing NS-LMXBs -both of which have been observed in the hard and soft X-ray spectral states (e.g., Gusinskaia et al 2017). NS-LMXBs have been observed during their bright (Xray luminosity LX ∼ 10 36−38 erg s −1 ) accretion outbursts (e.g., Migliari & Fender 2006), and as they fade back to quiescence.…”

“…There is currently no clear physical picture explaining this observed diversity, but perhaps it is unsurprising given that NS-LMXBs have a range of NS spin rate, magnetic field, and magnetospheric inclination with respect to the disk. In the case of AMXPs, Tudor et al (2017) question whether it is even appropriate to fit a single LR-LX correlation over a broad luminosity range (> 1 dex, i.e. an order of magnitude) because these systems show very large scatter.…”

The MAVERIC survey: a hidden pulsar and a black hole candidate in ATCA radio imaging of the globular cluster NGC 6397

“…The light-green triangles mark known transitional millisecond pulsars (Hill et al 2011;Papitto et al 2013;Deller et al 2015;Bogdanov et al 2018). The deep-blue squares and pink stars show quiescent/hard-state NSs and accreting millisecond X-ray pulsars (AMXPs), respectively (Migliari & Fender 2006;Tudor et al 2017). The yellow triangles show upper limits of L R of two X-ray transients (Tetarenko et al 2016;Ludlam et al 2017).…”

Quasi-simultaneous radio and X-ray observations of Aql X-1 : probing low luminosities

“…Due to these time constraints and their relative radio faintness, only a few NS-LMXBs have been observed in the luminosity range LX < 10 36 erg s −1 . In the case of non-pulsating NS-LMXBs, these observations led exclusively to non-detections (Gusinskaia et al 2017;Tetarenko et al 2016;Tudor et al 2017). However, there are two classes that potentially stand out in this low X-ray luminosity regime (Gallo et al 2018): accreting millisecond Xray pulsars (AMXPs; systems that exhibit coherent millisecond X-ray pulsations as a result of accreting material channelling onto the magnetic polar caps of the NS; Wijnands & van der Klis 1998;Patruno & Watts 2012) and transitional millisecond pulsars (tMSPs; NSs that switch between rotation-powered radio millisecond pulsars and an accretiondisc state; Papitto et al 2013;Jaodand et al 2018).…”