MeerKAT caught a Mini Mouse: serendipitous detection of a young radio pulsar escaping its birth site
Abstract:In MeerKAT observations pointed at a Galactic X-ray binary located on the Galactic plane, we serendipitously discovered a radio nebula with cometary-like morphology. The feature, which we named 'the Mini Mouse' based on its similarity with the previously discovered 'Mouse' nebula, points back towards the previously unidentified candidate supernova remnant G45.24+0.18. We observed the location of the Mini Mouse with MeerKAT in two different observations, and we localized with arcsecond precision the 138-ms radio pulsar PSR J1914+1054g, recently discovered by the FAST telescope, to a position consistent with the head of the nebula. We confirm a dispersion measure of about 418 pc cm-3 corresponding to a distance between 7.8 and 8.8 kpc based on models of the electron distribution. Using our accurate localization and two period measurements spaced 90 d apart, we calculate a period derivative of (2.7 ± 0.3) × 10 -14 s s-1. We derive a characteristic age of approximately 82 kyr and a spin-down luminosity of 4 × 1035 erg s-1. For a pulsar age comparable with the characteristic age, we find that the projected velocity of the neutron star is between 320 and 360 km s-1 if it was born at the location of the supernova remnant. The size of the proposed remnant appears small if compared with the pulsar characteristic age; however, the relatively high density of the environment near the Galactic plane could explain a suppressed expansion rate and thus a smaller remnant.
Bursts from Space: MeerKAT – the first citizen science project dedicated to commensal radio transients
Abstract:The newest generation of radio telescopes is able to survey large areas with high sensitivity and cadence, producing data volumes that require new methods to better understand the transient sky. Here, we describe the results from the first citizen science project dedicated to commensal radio transients, using data from the MeerKAT telescope with weekly cadence. Bursts from Space: MeerKAT was launched late in 2021 and received ∼89 000 classifications from over 1000 volunteers in 3 months. Our volunteers discovered 142 new variable sources which, along with the known transients in our fields, allowed us to estimate that at least 2.1 per cent of radio sources are varying at 1.28 GHz at the sampled cadence and sensitivity, in line with previous work. We provide the full catalogue of these sources, the largest of candidate radio variables to date. Transient sources found with archival counterparts include a pulsar (B1845-01) and an OH maser star (OH 30.1–0.7), in addition to the recovery of known stellar flares and X-ray binary jets in our observations. Data from the MeerLICHT optical telescope, along with estimates of long time-scale variability induced by scintillation, imply that the majority of the new variables are active galactic nuclei. This tells us that citizen scientists can discover phenomena varying on time-scales from weeks to several years. The success both in terms of volunteer engagement and scientific merit warrants the continued development of the project, while we use the classifications from volunteers to develop machine learning techniques for finding transients.
MAXI J1848-015: The first detection of relativistically moving outflows from a globular cluster X-ray binary
Abstract:Over the past decade, observations of relativistic outflows from outbursting X-ray binaries in the Galactic field have grown significantly. In this work, we present the first detection of moving and decelerating radio-emitting outflows from an X-ray binary in a globular cluster. MAXI J1848−015 is a recently discovered transient X-ray binary in the direction of the globular cluster GLIMPSE-C01. Using observations from the Karl G. Jansky Very Large Array, and a monitoring campaign with the MeerKAT observatory for 500 days, we model the motion of the outflows. This represents some of the most intensive, long-term coverage of relativistically moving X-ray binary outflows to date. We use the proper motions of the outflows from MAXI J1848−015 to constrain the component of the intrinsic jet speed along the line of sight, β int cos θ ejection , to be =0.19 ± 0.02. Assuming it is located in GLIMPSE-C01, at 3.4 kpc, we determine the intrinsic jet speed, β int = 0.79 ± 0.07, and the inclination angle to the line of sight, θ ejection = 76° ± 2°. This makes the outflows from MAXI J1848−015 somewhat slower than those seen from many other known X-ray binaries. We also constrain the maximum distance to MAXI J1848−015 to be 4.3 kpc. Finally, we discuss the implications of our findings for the nature of the compact object in this system, finding that a black hole primary is a viable (but as-of-yet unconfirmed) explanation for the observed properties of MAXI J1848−015. If future data and/or analysis provide more conclusive evidence that MAXI J1848−015 indeed hosts a black hole, it would be the first black hole X-ray binary in outburst identified in a Galactic globular cluster.
Time-dependent visibility modelling of a relativistic jet in the X-ray binary MAXI J1803-298
Tracking the motions of transient jets launched by low-mass X-ray binaries (LMXBs) is critical for determining the moment of jet ejection, and identifying any corresponding signatures in the accretion flow. However, these jets are often highly variable and can travel across the resolution element of an image within a single observation, violating a fundamental assumption of aperture synthesis. We present a novel approach in which we directly fit a single time-dependent model to the full set of interferometer visibilities, where we explicitly parametrize the motion and flux density variability of the emission components, to minimize the number of free parameters in the fit, while leveraging information from the full observation. This technique allows us to detect and characterize faint, fast-moving sources, for which the standard time binning technique is inadequate. We validate our technique with synthetic observations, before applying it to three Very Long Baseline Array (VLBA) observations of the black hole candidate LMXB MAXI J1803−298 during its 2021 outburst. We measured the proper motion of a discrete jet component to be 1.37 ± 0.14 mas h−1, and thus we infer an ejection date of MJD 59348.0+0.05-0.06,which occurs just after the peak of a radio flare observed by the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/Sub-Millimeter Array (ALMA), while MAXI J1803−298 was in the intermediate state. Further development of these new VLBI analysis techniques will lead to more precise measurements of jet ejection dates, which, combined with dense, simultaneous multiwavelength monitoring, will allow for clearer identification of jet ejection signatures in the accretion flow.