X-ray Polarization of the Black Hole X-ray Binary 4U 1630-47 Challenges Standard Thin Accretion Disk Scenario

Kavli Affiliate: Herman Marshall

| First 5 Authors: Ajay Ratheesh, Michal Dovčiak, Henric Krawczynski, Jakub Podgorný, Lorenzo Marra

| Summary:

Large energy-dependent X-ray polarization degree is detected by the Imaging
X-ray Polarimetry Explorer ({IXPE}) in the high-soft emission state of the
black hole X-ray binary 4U 1630–47. The highly significant detection (at
$approx50sigma$ confidence level) of an unexpectedly high polarization,
rising from $sim6%$ at $2$ keV to $sim10%$ at $8$ keV, cannot be easily
reconciled with standard models of thin accretion discs. In this work we
compare the predictions of different theoretical models with the {IXPE} data
and conclude that the observed polarization properties are compatible with a
scenario in which matter accretes onto the black hole through a thin disc,
covered by a partially-ionized atmosphere flowing away at mildly relativistic
velocities.

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