INTEGRAL contributing to leapfrogging in hard X-ray polarimetry
The last 10−15 years has seen a leapfrog in the interest in X-ray
polarimetry; especially the field of hard X-ray polarimetry has been
quite active, which can be seen through the new developments of
dedicated polarimeters and a large number of new proposals to develop
hard X-ray polarimeters. There has also been a significant effort in
extracting polarization information from instruments which main purpose
is not focused on polarimetry measurements, such as those onboard
INTEGRAL. Measurements from these experiments in the last two decades
have provided some important scientific inputs in understanding emission
mechanism and geometry in X-ray pulsars, magnetic field structure in
Pulsar Wind Nebula, disk-jet interplay in black hole X-ray binaries,
hard X-ray emission mechanism in GRB prompt emission, and emission
process behind solar flares. One interesting and common feature in all
these measurements is a systematic increase in polarization within the
hard X-ray band which makes this energy regime extremely promising for
polarimetry experiments. A couple of examples are shown here.
The Crab pulsar and nebula is one of the brightest X-ray objects in our
sky. It comprises a fast rotating neutron star or pulsar surrounded by a
bright supernova remnant or nebula. The top two figures summarizes all
the polarization measurements (polarization fraction in top panel and
angle in bottom panel) of the total Crab - pulsar+nebula (left) and
only the nebula (right) by various instruments like CZTI on board
AstroSat, IBIS and SPI on board INTEGRAL, SGD on board Hitomi, PoGO+ and
PolarLight. When plotted against observed energy of measurement, one
can see a hint of increase in the polarization fraction in both the
total Crab and the nebula.
Cygnus X-1 is a bright black-hole binary system in our galaxy. There
have been some attempts to measure polarization in X-rays of this
object. The bottom figure summarizes all the polarization measurements
(polarization fraction in the top panel and angle in the bottom panel)
of Cygnus X-1 till date by various instruments like IBIS and SPI on
board INTEGRAL, OSO-8, and PoGO+. Except for the OSO-8 measurements
(shown by the black points), all other measurements correspond to the
hard spectral state of the source. When plotted against observed energy
of measurement, one can see an increase in the polarization fraction and
a swing in polarization angle at higher energies.
Credits:
"Hard X-ray Polarimetry − An overview of the method, science drivers and recent findings",
Tanmoy Chattopadhyay,
2021, accepted for publication in Journal of Astrophysics and Astronomy
https://arxiv.org/abs/2104.05244
