A re-establishing jet during an X-ray re-brightening of the Be/X-ray binary Swift J0243.6+6124

Abstract

Transient Be/X-ray binary systems, wherein a compact object accretes from a Becompanion star, can show giant and periastron outbursts. During the decay of their giant outbursts, some Be/X-ray binaries also show X-ray re-brightenings, the origin of which is not understood. Recently, we presented the discovery of a jet from a neutron star Be/X-ray binary, observed during the giant outburst of Swift J0243.6+6124. Here, we present continued radio monitoring of its 2017/2018 giant outburst decay and a re-brightening of this source. During the former, we observe a radio flare with a steep radio spectrum, possibly caused by interactions between discrete ejecta colliding with the pre-existing jet or the surrounding medium. During the X-ray re-brightening, we observe the radio jet turning on and off within days. Surprisingly, this re-establishing jet is as bright in radio as at the peak of the super-Eddington giant outburst, despite more than two orders of magnitude lower X-ray luminosity. In addition, the jet is only observed when the X-ray luminosity exceeds approximately 2 × 10 36 (D/5kpc) 2 erg/s. We discuss how such an X-ray threshold for jet launching might be related to the presence of a magnetic centrifugal barrier at lower mass accretion rates. We also discuss the implications of our results for the launch of jets from strongly magnetized neutron stars, and explore future avenues to exploit the new possibility of coordinated X-ray/radio studies of neutron star Be/X-ray binaries.

An Evolving Broad Iron Line from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

“…It is important to note that this pulsar launched a jet when the X-ray luminosity was ≥4×10 36 erg s −1 during giant and fainter outbursts (for a distance of 7 kpc; van den Eijnden et al 2018Eijnden et al , 2019. At the same time, the luminositydependent emission lines were observed in the spectral range of NICER.…”

“…We also explored the correlation between Fe-line emission and the radio flux density as observed with VLA ( Figure 6). The radio data used here are taken from van den Eijnden et al (2018) and van den Eijnden et al (2019). The VLA observations were quasi-simultaneous to NICER pointings (Ob-sID 1050390105, 1050390116, 1050390118, 1050390122, 1050390134, and 1050390143) between 2017 October and 2018 February.…”

“…x , consistent with both black hole and neutron star X-ray binaries. Further VLA monitoring found that the jet emission turned on and off within days (van den Eijnden et al 2019). The jet emission also appeared when the pulsar re-brightened in fainter X-ray outbursts in March and May 2018.…”

Chandra reveals a possible ultrafast outflow in the super-Eddington Be/X-ray binary Swift J0243.6+6124

“…We note that, as discussed extensively in van denEijnden et al (2018b) andvan den Eijnden et al (2019), the observed radio properties (flux densities, spectral shape, and evolution) through-MNRAS 000, 1-14(2018)…”

“…It reached super-Eddington X-ray luminosities during the peak of its outburst (van den Eijnden et al 2018b) and has been referred to as the first Galactic ULX pulsar by both Tsygankov et al (2018) and Wilson-Hodge et al (2018). Through Very Large Array (VLA) radio and Swift X-ray monitoring, Sw J0243 was found to launch a relativistic jet during its super-Eddington state (van den Eijnden et al 2018b), as well as at lower accreting rates (van den Eijnden et al 2019). This jet detection constituted the first from a strongly-magnetized neutron star, contrary to the predictions of neutron star jet formation theory (Blandford & Payne 1982;Massi & Kaufman Bernadó 2008).…”