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In July 2015, the high-mass X-ray binary V 0332+53 underwent a giantoutburst, a decade after the previous one. This system hosts a stronglymagnetized neutron star. During the 2004--2005 outburst, an anti-correlationbetween the centroid energy of its fundamental cyclotron resonance scatteringfeatures (CRSFs) and the X-ray luminosity was observed. The long ($\approx 100$d) and bright ($L_{\rm x} \approx 10^{38}$ erg s$^{-1}$) 2015 outburst offeredthe opportunity to study during another outburst the unique properties of thefundamental CRSF and its dependence on the X-ray luminosity. The source wasobserved by the INTEGRAL satellite for $\sim 330$ ks. We exploit the spectralresolution at high energies of the SPectrometer on INTEGRAL (SPI) and the JointEuropean X-ray Monitors to characterize its spectral properties, focusing inparticular on the CRSF-luminosity dependence. We complement the data of the2015 outburst with those collected by SPI in 2004--2005 and left unpublished sofar. We find a highly significant anti-correlation of the centroid energy ofthe fundamental CRSF and the $3-100$ keV luminosity of $E_1 \propto-0.095(8)L_{37}$ keV. This trend is observed for both outbursts. We confirm thecorrelation between the width of the fundamental CRSF and the X-ray luminositypreviously found in the JEM-X and IBIS dataset of the 2004--2005 outburst. Byexploiting the RXTE/ASM and Swift/BAT monitoring data we also report on thedetection of a $\sim 34$ d modulation superimposed on the mean profiles androughly consistent with the orbital period of the pulsar. We discuss possibleinterpretations of such variability.

Two giant outbursts of V 0332+53 observed with INTEGRAL