INTEGRAL's Strong Hard X-ray Polarization from Cyg X-1 in a New Light
Cygnus X-1 is the brightest and persistent black hole X-ray binary in our galaxy
composed of a ~21 solar mass black hole orbiting around a ~40 solar mass O9.7 supergiant
star. It is also the source of bipolar powerful ejections, therefore called a micro-quasar.
The bipolar ejections or the jets are important to understand the accretion-ejection
coupling in the black hole. Is the jet also a contributing factor to the observed hard
X-ray emission? Resolving this question may lead to a significant improvement in our
understanding of the jet formation, energetics, magnetic field strength and geometry,
as well as the intricate interplay between the corona and the jet of the black hole. To
arrive at the answer, however, one needs to measure the polarization of the X-ray emission.
In the six decades since the birth of X-ray astronomy, there have only been 3 or 4 reports
on X-ray polarization measurement of Cygnus X-1. After a few initial attempts back in the
1970s, OSO-8 estimated polarization of 2.4% ± 1.1% at 2.6 keV and 5.3% ± 2.5% at 5.2 keV.
IBIS and SPI, onboard INTEGRAL independently estimated high polarization for this source
at ∼67% with sky polarization angle of 224 degrees (north-east) at energies above 400 keV,
while the emission in 250 to 400 keV being weakly polarized or unpolarized (Laurent et al.,
2011; Jourdain et al., 2012a). The results were interpreted as jet origin of the photons
corroborated by simultaneously observed two distinct spectral components of the source, a
thermal Comptonization component at energies below 400 keV and a power law component with
a power law of photon index 1.6 beyond 400 keV, primarily due to synchrotron radiation from
the jet (Laurent et al., 2011). PoGO+, a dedicated balloon-borne hard X-ray polarimeter
(19 - 181 keV), observed an upper limit of the polarization in the hard state of 4.8% at a
position angle of 154 ± 31 degrees (Chauvin et al. 2019). These results favor the hard X-ray
flux to be originating from an extended corona which has a small fraction of the reflection
component in the hard X-ray band (Chauvin et al., 2018, 2019). More recently, the soft X-ray
polarimeter IXPE reported the 4 ± 0.2% polarization in the energy range of 2-8 keV and the
polarization angle is aligned with the outflowing radio jet. They explained their results
by considering the outflowing corona which is extended perpendicular to the jet axis.
A new study conducted by Indian's AstroSat's Cadmium Zinc Telluride Imager (CZTI), reported
around 24% polarized high energy X-rays strongly suggesting that the mechanism of X-ray
emission more energetic than 200 keV is from the jet, possibly synchrotron radiation in an
ordered magnetic field. Notably, the polarization of X-rays originating from the corona, due
to the scattering of local thermal radiation, is expected to be less than 10%. Moreover, the
CZT imager detected high X-ray polarization only in the state that exhibits strong radio
emission from the jet. For the first time, therefore, one can confirm the direct connection
of the hard X-ray emission to the relativistic jet.
The image shows the polarization degree (top panel) and angle (bottom panel) of Cygnus X-1
across various spectral states (Pure hard, Intermediate hard, Intermediate soft, Averaged hard,
and Soft states) from all the available measurements - IXPE (Krawczynski et al. 2022), IBIS
(Laurent et al. 2011) and SPI (Jourdain et al. 2012) on board INTEGRAL, OSO-8 (Long et al. 1980),
PoGO+ (Chauvin et al. 2018) and AstroSat-CZTI. The polarization angle is measured counterclockwise
from the local north to northeast direction for all the instruments. When plotted against the
observed energy of measurement, plot reveals an apparent increase in the polarization degree and
a swing in polarization angle at higher energies. The presence of two distinct polarization angle
distributions indicates different origins for the radiation below and above ∼200 keV.
Reference:
"High hard X-ray polarization in Cygnus X-1 confined to the intermediate hard state: evidence for a variable jet component.",
Chattopadhyay, Tanmoy, Abhay Kumar, A. R. Rao, Yash Bhargava, Santosh V. Vadawale, Ajay Ratheesh, Gulab Dewangan, Dipankar Bhattacharya, N. P. S. Mithun, and Varun Bhalerao,
The Astrophysical Journal Letters 960, no. 1 (2023): L2, DOI 10.3847/2041-8213/ad118d
