INTEGRAL discovers Polarization Flip in Microquasar Swift J1727.8-1613
Microquasars are known to produce strong hard X-ray emission during their
outbursts, which is thought to be a mix of inverse Compton and synchrotron
emission. Although challenging to measure properly, polarization is a
promising tool to distinguish between them.
Previous studies using the INTEGRAL/IBIS Compton mode have shown some
sources to be highly polarized (>50%) during their Hard State while some
show weaker or no polarization (INTEGRAL POM
April 2011,
March 2023).
In August 2023 the new source Swift J1727.8-1613 went into outburst and
was identified as one of the brightest Microquasar ever detected by
INTEGRAL. The source displays a high polarization fraction compatible with
synchrotron emission both in the Hard Intermediate state (HIMS, left
spectrum and polarigram) and the beginning of the Soft Intermediate state
(SIMS, right). More importantly it was discovered that the polarization
angle changed orientation during the transition to the SIMS, lining up
itself with the direction of the compact radio jet within less than a few
degrees (see polar plot in the middle panel).
This can be interpreted as a change in the orientation of the magnetic
field producing the synchrotron emission. In the SIMS the magnetic field
would be perpendicular to the compact jet, and the coincidence with a radio
flare tells us this change most likely happened within the jet during an
ejection.
This new and unexpected behavior has yet to be fully explained by current
accretion-ejection models, and a generalization among microquasars remains
to be confirmed.
Credits: Background image: ESA
Reference:
“INTEGRAL/IBIS polarization detection in the hard and soft intermediate
states of Swift J1727.8−1613”,
T. Bouchet, J. Rodriguez, F. Cangemi, P. Thalhammer, P. Laurent, V. Grindberg, J. Wilms, K. Pottschmidt,
2024, A&A
doi: 10.1051/0004-6361/202450826
