1720024

We propose to continue the study of the long-term evolution of the cyclotron line energy Ecyc in Her X-1. The long-term decay of Ecyc discovered in data taken between 1993 and 2012 had amounted to ~ 5 keV over 20 yrs. The observing campaign in 2016 coordinated between NuSTAR and INTEGRAL had provided evidence that the decay has ended. This has been confirmed by further NuSTAR observations showing that the flux corrected cyclotron line energy is very stable at ~37.5 keV - actually since about 2012. It is now the question whether we will (at some time) see a new increase similar to the one that was observed between 1990 and 1993 due toa possible evolution of the magnetic field configuration at the accretion mounds formed by matter accumulated in the polar regions of the neutron star. The remarkable spectral stability shown so far by Her X-1 and its relatively high X-ray flux at high energies during the Main-On states make this source a good calibration target for IBIS/ISGRI whose performances have recently become more problematic due to the aging of the instrument. In this regard we have started a collaboration with the INTEGRAL team at ISDC using the Her X-1 data obtained in the previous cycle of observations carried out with INTEGRAL and NuSTAR and we believe it will be useful to continue this effort for a successful future use of IBIS/ISGRI. For both reasons science and inter-calibration we therefore propose two further INTEGRAL observations of 200 ks each (about half a year apart) of Her X-1 during AO-17 (with both SPI and IBIS operational) in coordination with observations by NuSTAR. [truncated! Please see actual data for full text]