Speaker
Description
X-ray binaries (XRBs) hosting a stellar-mass black hole (BH) or neutron star produce strong emission through accretion of matter from a companion star onto the compact object. This process can also lead to the formation of fast, collimated outflows, called jets. Most BH-XRBs are transient systems, producing steady compact jets during quiescence and hard X-ray states, and discrete ejecta during transitions to softer, brighter states. Radio polarimetric observations of these jets provide a valuable probe of their magnetic field structure and geometry. Although synchrotron emission is theoretically expected to reach linear polarisation fractions up to ~70% at radio frequencies, polarised emission has only been detected in a limited number of XRBs, typically at levels below 20%. This discrepancy is likely due to a combination of instrumental and physical depolarisation effects.
4U1630–47 is a recurrent BH-XRB transient that undergoes outbursts every ~600 days. During its most recent outburst in April 2025, the source reached an exceptionally bright radio state, with a peak flux density of 26 mJy at 1.3 GHz: the highest radio flux measured from this system to date. In this talk I will present the results of our multi-wavelength campaign of 4U1630–47 in both X-rays and radio using the Swift satellite and the MeerKAT interferometer.
We found that the unusual radio behaviour is likely linked to the peculiar X-ray evolution of the outburst, during which the source remained in an atypical hard state and never transitioned to the soft state. Radio polarimetric results show an intrinsic polarisation degree of ~20% and Rotation Measure RM ~1600 rad/m2, among the highest ever observed from an XRB. Moreover, the detection of temporal variability in the RM suggests that the observed depolarisation is likely driven by internal effects within the jet plasma itself, highlighting the potential of radio polarimetry as a powerful tool to probe the composition and physical properties of XRB jets.