With upcoming missions such as IXPE and eXTP, we are standing on the very verge of the age of X-ray polarimetry. However, none of these future mission will cover the energies above 400 keV. Especially for X-ray binaries, measurements at such high energies are crucial: this is where we see an excess emission, the so-called "hard tail". Detection of polarization and its variability can constrain the origin of this excess emission. It can, in particular, answer the question whether this is the high-energy contribution of the jets that we see in the radio in the low/hard emission states. Luckily, for exceptionally bright sources and/or long exposures, polarization measurements in the very hard X-ray/soft gamma-ray band are possible already today with INTEGRAL.Previously, we have utilized the Compton mode of the IBIS telescope onboard INTEGRAL to show that the hard tail in the black hole high mass X-ray binary Cygnus X-1 is highly polarized, a result that has been later independently confirmed by others using the INTEGRAL/SPI telescope. First measurements of state-resolved polarization confirmed a hard tail component and a high polarization fraction above 400 keV in the hard state, i.e., when we observe radio jets. In the soft state, when the radio emission from jets is either absent or strongly suppressed, the analysis was hampered by a lack of INTEGRAL observations. Here, we present an updated re-analysis of all INTEGRAL observation to date, including 6 years of new observations. We detect a hard tail component in both, hard and soft, states using IBIS and SPI and discuss implications of this result.
AAS/High Energy Astrophysics Division
- Pub Date:
- March 2019