Two giant outbursts of V0332+53 observed with INTEGRAL

¹ ISDC, University of Geneva, chemin d’Écogia, 16 1290 Versoix, Switzerland
e-mail: carlo.ferrigno@unige.ch
² Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls Universität, Sand 1, 72076 Tübingen, Germany
³ European Space Astronomy Center (ESA/ESAC), Science Operations Department, Villanueva de la Cañada, 28691 Madrid, Spain
⁴ D Karl Remeis-Sternwarte and Erlangen Center for Astroparticle Physics, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ CRESST, Department of Physics, and Center for Space Science and Technology, UMBC, Baltimore, MD 21250, USA
⁶ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ François Arago Center, APC – Astroparticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire De Paris, Sorbonne Paris Cité, Paris, France

Received: 6 May 2016
Accepted: 12 July 2016

Abstract

Context. In July 2015, the high-mass X-ray binary V0332+53 underwent a giant outburst, a decade after the previous one. V0332+53 hosts a strongly magnetized neutron star. During the 2004–2005 outburst, an anti-correlation between the centroid energy of its fundamental cyclotron resonance scattering features (CRSFs) and the X-ray luminosity was observed.

Aims. The long (≈100 d) and bright (L_x ≈ 10³⁸ erg s^-1) 2015 outburst provided the opportunity to study the unique properties of the fundamental CRSF during another outburst and to study its dependence on the X-ray luminosity.

Methods. The source was observed by the INTEGRAL satellite for ~330 ks. We exploit the spectral resolution at high energies of the SPectrometer on INTEGRAL (SPI) and the Joint European X-ray Monitors to characterize its spectral properties, focusing in particular on the CRSF-luminosity dependence. We complement the data of the 2015 outburst with those collected by SPI in 2004–2005, which have so far been left unpublished.

Results. We find a highly significant anti-correlation of the centroid energy of the fundamental CRSF and the 3–100 keV luminosity of E₁ ∝ −0.095(8)L₃₇ keV. This trend is observed for both outbursts. We confirm the correlation between the width of the fundamental CRSF and the X-ray luminosity previously found in the JEM-X and IBIS dataset of the 2004–2005 outburst. By exploiting the RXTE/ASM and Swift/BAT monitoring data, we also report on the detection of a ~34 d modulation superimposed on the mean profiles and roughly consistent with the orbital period of the pulsar. We discuss possible interpretations of such variability.