Two giant outbursts of V0332+53 observed with INTEGRAL

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 ≈ 10 38 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 We complement the data of the 2015 outburst with those collected by SPI in [2004][2005], which have so far been left unpublished.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 E 1 ∝ −0.095(8)L 37 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.

A broad-band self-consistent modelling of the X-ray spectrum of 4U 1626−67

“…Therefore, we applied a gain correction to the BAT spectrum and we left the depth of the cyclotron line in the BAT spectrum as a free parameter. We obtained for the BAT matrix an offset of ∼ 1.2 keV (as similarly found by Ferrigno et al 2016;Doroshenko et al 2017) with respect to FPMAB, while the slope was consistent with unity.…”

Cyclotron lines in highly magnetized neutron stars

“…V 0332+53 displays normal as well as giant outbursts. Occurrences of the latter have been observed in 1973in (Ferrigno et al 2016a. During giant outbursts the source can become one of the most luminous X-ray sources in the Galaxy, reaching a few times 10 38 erg s −1 .…”

“…A qualitative discussion of results from pulse phase-resolved spectroscopy of this outburst in terms of the reflection model for cyclotron line formation has been presented (Poutanen et al 2013;Lutovinov et al 2015). Swift, INTEGRAL, and NuSTAR observations of the most recent giant outburst in 2015 also showed the negative correlation and provided evidence that the overall line energy, and thus the associated B-field, was lower just after the outburst, indicating some decay over the time of the outburst (Cusumano et al 2016;Ferrigno et al 2016a;Vybornov et al 2018) (see also the discussion in Sect. 4.4).…”

Inversion of the decay of the cyclotron line energy in Hercules X-1

“…The two data points shown in Fig. 1 are therefore from SPI (see also Ferrigno et al 2016), screened and reduced according to procedures described by Churazov et al (2011).…”