Observations of V0332+53 during the 2015 outburst using Fermi/GBM, MAXI, Swift and INTEGRAL

Abstract We present the light curves, spectra and hardness–intensity diagram (HID) of the high-mass X-ray binary V0332+53 using Fermi/GBM, MAXI, Swift/BAT and INTEGRAL through its 2015 Type II outburst. We observe characteristic features in the X-ray emission (2–50 keV) due to periastron passages, the dynamical time-scale of the accretion disc, and changes within the accretion column between a radiation-dominated flow and a flow dominated by Coulomb interactions. Based on the HID and the light curves, the critical luminosity is observed to decrease by ∼5 per cent to 7 per cent during the outburst, signalling a decrease in the magnetic field.