INTEGRAL/IBIS observations of V404 Cygni polarimetric properties during its 2015 giant flares
2017 July 18
2017 August 23
V404 Cygni is a low mass X-ray binary believed to host a black hole of 12 solar masses. After 25 years of quiescence, V404 Cygni entered into a period of outburst during which it produced flares largely exceeding its Eddington luminosity. The brightness of the most luminous flares detected with the IBIS telescope has hampered the standard data treatment and produced large telemetry gaps. Polarimetric parameters were thus not measurable with the same data analysis package used with success in the studies of the Crab and Cygnus X-1. We were then forced to develop and implement a special INTEGRAL/IBIS/Compton analysis routine. Our findings show that this new process improved significantly our polarimetry sensitivity not only for very bright flaring events as for V404 Cygni but equally on more standard sources as the Crab nebula. In this paper, we compare the results of our new method with the already published Cygnus X-1 polarization. We show that V404 Cygni were highly polarized (95 ± 35 %) during INTEGRAL revolution 1555 (i.e. 2015, June 20th – 22nd) with a polarization angle of 160°, consistent with infrared measurement made at the WHT on June 23rd. This achieved sensitivity opens a new window for polarimetric studies with INTEGRAL/IBIS at energies above 200 keV.