Dr. Alexander Cooper

Abstract:

ABSTRACT Precessing relativistic jets launched by compact objects are rarely directly measured, and present an invaluable opportunity to better understand many features of astrophysical jets. In this Letter we present MeerKAT radio observations of the neutron star X-ray binary system (NSXB) Circinus X-1 (Cir X-1). We observe a curved S-shaped morphology on $\sim 20\, \rm arcsec\, (\sim 1\:\text{pc})$ scales in the radio emission around Cir X-1. We identify flux density and position changes in the S-shaped emission on year time-scales, robustly showing its association with relativistic jets. The jets of Cir X-1 are still propagating with mildly relativistic velocities $\sim 1\:\text{pc}$ from the core, the first time such large scale jets have been seen from a NSXB. The position angle of the jet axis is observed to vary on year time-scales, over an extreme range of at least $110^\circ$. The morphology and position angle changes of the jet are best explained by a smoothly changing launch direction, verifying suggestions from previous literature, and indicating that precession of the jets is occurring. Steady precession of the jet is one interpretation of the data, and if occurring, we constrain the precession period and half-opening angle to $>10$ yr and $>33^\circ$, respectively, indicating precession in a different parameter space to similar known objects such as SS 433.

Abstract:

Abstract Black hole X-ray binaries in outburst launch discrete, large-scale jet ejections which can propagate to parsec scales. The kinematics of these ejecta appear to be well described by relativistic blast wave models original devised for gamma-ray burst afterglows. In previous kinematic-only modelling, a crucial degeneracy prevented the initial ejecta energy and the interstellar medium density from being accurately determined. In this work, we present the first joint Bayesian modelling of the radiation and kinematics of a large-scale jet ejection from the X-ray binary MAXI J1535-571. We demonstrate that a reverse shock powers the bright, early ejecta emission. The joint model breaks the energetic degeneracy, and we find the ejecta has an initial energy of E0 ∼ 3 × 1043 erg, and propagates into a low density interstellar medium of nism ∼ 4 × 10−5 cm−3. The ejecta is consistent with being launched perpendicular to the disc and could be powered by an efficient conversion of available accretion power alone. This work lays the foundation for future parameter estimation studies using all available data of X-ray binary jet ejecta.

Abstract:

Recently, a class of long-period radio transients (LPTs) has been discovered, exhibiting emission thousands of times longer than radio pulsars. These findings, enabled by advances in wide-field radio surveys, challenge existing models of rotationally powered pulsars. Proposed models include highly magnetized neutron stars, white-dwarf pulsars and white-dwarf binary systems with low-mass companions. Although some models predict X-ray emission, no LPTs have been detected in X-rays despite extensive searches Here we report the discovery of an extremely bright LPT (10–20 Jy in radio), ASKAP J1832−0911, which has coincident radio and X-ray emission, both with a 44.2-minute period. Its correlated and highly variable X-ray and radio luminosities, combined with other observational properties, are unlike any known Galactic object. The source could be an old magnetar or an ultra-magnetized white dwarf; however, both interpretations present theoretical challenges. This X-ray detection from an LPT reveals that these objects are more energetic than previously thought and establishes a class of hour-scale periodic X-ray transients with a luminosity of about 1033 erg s−1 linked to exceptionally bright coherent radio emission.

Abstract:

Blast wave models are commonly used to model relativistic outflows from ultra-relativistic gamma-ray bursts (GRBs), but are also applied to lower Lorentz factor ejections from X-ray binaries (XRBs). Here, we revisit the physics of blast waves and reverse shocks in these systems and explore the similarities and differences between the ultra-relativistic () and moderately relativistic () regimes. We first demonstrate that the evolution of the blast wave radius as a function of the observer frame time is recovered in the on-axis ultra-relativistic limit from a general energy and radius blast wave evolution, emphasizing that XRB ejections are off-axis, moderately relativistic cousins of GRB afterglows. We show that, for fixed blast wave or ejecta energy, reverse shocks cross the ejecta much later (earlier) on in the evolution for less (more) relativistic systems, and find that reverse shocks are much longer lived in XRBs and off-axis GRBs compared to on-axis GRBs. Reverse shock crossing should thus typically finish after 10–100 of days (in the observer frame) in XRB ejections. This characteristic, together with their moderate Lorentz factors and resolvable core separations, makes XRB ejections unique laboratories for shock and particle acceleration physics. We discuss the impact of geometry and lateral spreading on our results, explore how to distinguish between different shock components, and comment on the implications for GRB and XRB environments. Additionally, we argue that identification of reverse shock signatures in XRBs could provide an independent constraint on the ejecta Lorentz factor.