Speaker
Description
X-ray binaries (XRBs) hosting a stellar mass black hole (BH) or a neutron star exhibit a powerful emission triggered by accretion of matter onto the compact object from the companion star; this process may lead to the formation of fast, collimated outflows, called jets. Most BH-XRBs are transients, producing steady, mildly relativistic compact jets during quiescence and the low-luminosity 'hard' state, while they launch discrete, apparently superluminal ejecta with a steep synchrotron spectrum during transitions to brighter, softer states. Polarimetric observations of these jets can provide information about their magnetic field structure and geometry. However, despite the theoretical expectation of up to ~70% linear polarisation from synchrotron emission at radio frequencies, such emission has been detected in only a handful of XRBs, and typically at levels below ~20%. This discrepancy is likely due to a combination of instrumental and physical depolarisation effects, including beam depolarisation and Faraday rotation.
4U 1630–47 is a recurrent BH-XRB transient that undergoes outbursts every ~600 days. During its most recent outburst in April 2025, it reached an unusually bright radio state, with a peak flux of 26 mJy at 1.3 GHz, the highest measured to date from this source. We monitored 4U 1630–47 in both X-rays and radio using the Swift satellite and the MeerKAT interferometer. I will present the results of this multi-wavelength campaign, with particular emphasis on the radio polarimetry results. We observed strong depolarisation at lower frequencies, indicative of an extremely high Rotation Measure (RM) – potentially the largest ever detected in an X-ray binary. RM-synthesis analysis suggests that the depolarisation is most likely due to internal Faraday rotation. Modelling the RM, we constrained the de-rotated polarisation angle and measured the fractional polarisation of the source. This study demonstrates that MeerKAT is an excellent instrument for investigating the polarisation properties of X-ray binaries in the radio band, paving the way for even more detailed studies with SKA.
| Topics | Transients & GW follow-up |
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