Exploring the potential for ultra-relativistic jets in Scorpius X-1 with low angular resolution radio observations

TL;DR

This study tests the existence of ultra-relativistic flows (URFs) in Sco X-1 by combining archival VLBI-based slow-jet kinematics with new high-time-resolution VLA radio monitoring. It models flare sequences as URF launches interacting with the slow jet, using MCMC to infer plausible Lorentz factors and assessing whether relativistic beaming could hide URFs from low-resolution data. The analysis yields URF candidates with $Γ$ values exceeding 2 in several flare sequences (up to $Γ>8.1$ for some cases), while Beaming can suppress the observed flux, explaining non-detections. The work highlights the potential ubiquity of fast jets in neutron-star binaries and the importance of simultaneous high-resolution and low-resolution campaigns to disentangle jet physics and beaming effects.

Abstract

Scorpius X-1 (Sco X-1) is a neutron star X-ray binary in which the neutron star is accreting rapidly from a low mass stellar companion. At radio frequencies, Sco X-1 is highly luminous and has been observed to have jet ejecta moving at mildly relativistic velocities away from a radio core, which corresponds to the binary position. In this Letter, we present new radio observations of Sco X-1 taken with the Karl G. Jansky Very Large Array. Using a fast imaging method, we find that the 10 and 15GHz data show a number of flares. We interpret these flares as the possible launching of fast jets ($βΓ$>2), previously observed in Sco X-1 and called ultra-relativistic flows, and their interaction with slower moving jet ejecta. Using the period between successive flares, we find that it is possible for the fast jets to remain undetected, as a result of the fast jet velocity being sufficiently high to cause the jet emission to be beamed in the direction of the motion and out of our line of sight. Our findings demonstrate that the ultra-relativistic flows could be explained by the presence of fast jets in the Sco X-1 system.

Figures (6)

• Figure 1: A schematic of Sco X-1 showing the positions relative positions of the core, approaching and receding slow jets and the effects of the URF on the system. The URF (indicated by the diagonal arrows) is launched from the core at the time of the core's flare and then propagates out to the slow jets where they then cause flares, first in the approaching and receding jet.
• Figure 2: Two plots showing the observed separation, in mas, of three pairs of radio lobes from the core of Sco X-1, as a function of time. Overlaid is a linear fit to data at both frequencies which corresponds to proper motion in mas/day. We assumed the proper motion was constant. Upper panel: Pairs 1 and 2, observed in June 1999. Lower panel Pair 3, observed in February 1998.
• Figure 3: The light curves produced from the VLA data presented in Section \ref{['sec:obs']} at 10 and 15 GHz. Each point corresponds to 60 seconds of data. The light curves show that the emission from Sco X-1 is brighter at lower frequencies but both light curves demonstrate strong flaring behaviour.
• Figure 4: Decomposition of the components in the 10 GHz band. Five flares (labelled 1 to 5) are identified and fitted with broken power laws, and the remaining baseline flux is fitted linearly. Note that the flux after the fifth flare was not included in the linear fit, as the Ku band data suggests there may be a flare, but we were not able to clearly identify it in the X band.
• Figure 5: Corner plots of the derived posterior distribution for the three fastest URF candidates. The plots show how inferred velocity depends on the angle against the plane of the sky of the jet. For URFs 2 and 3, the inferred velocity depends strongly on angle.