Changes in the cyclotron line energy on short and long timescales in V 0332+53

Abstract

We present the results of the pulse-amplitude-resolved spectroscopy of the accreting pulsar V 0332+53 using the NuSTAR observations of the source in 2015 and 2016. We investigate the dependence of the energy of the cyclotron resonant scattering feature (CRSF) as a function of X-ray luminosity on timescales comparable with the spin period of the pulsar within individual observations, and the behavior on longer timescales within and between the two observed outbursts. We confirm that in both cases the CRSF energy is negatively correlated with flux at luminosities higher than the critical luminosity and is positively correlated at lower luminosities. We also confirm the recently reported gradual decrease in the line energy during the giant outburst in 2015. Using the NuSTAR data, we find that this decrease was consistent with a linear decay throughout most of the outburst, and flattened or even reversed at the end of the 2015 outburst, approximately simultaneously with the transition to the subcritical regime. We also confirm that by the following outburst in 2016 the line energy rebounded to previous values. The observed behavior of the CRSF energy with time is discussed in terms of changes in the geometry of the CRSF forming region caused by changes in the effective magnetospheric radius.

Cyclotron lines in highly magnetized neutron stars

“…V 0332+53 is singled out by the fact that it is the only source to date in which we find both E cyc / L x correlations: the negative at high luminosities and the positive at low luminosity Vybornov et al 2018). It is also the only one with a very strong negative correlation, accompanied by a second source -SMC X-2 -which shows a much weaker dependence (see Fig.…”

“…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 Ferrigno et al 2016a;Vybornov et al 2018) (see also the discussion in Sect. 4.4).…”

First characterization of Swift J1845.7–0037 with NuSTAR

“…Source spectra were extracted from a region of 54 radius around the position of Swift J1845.7−0037, whereas background spectra were extracted from a circular region of 147 radius away from the source. The extraction regions were optimized to increase the signal to noise ratio in the hard energy band ≥ 40 keV as described in (Vybornov et al 2018). The spectra for the two NuSTAR modules (FPMA and FPMB) were extracted independently and modeled simultaneously in the 3−79 keV range.…”

The First NuSTAR Observation of 4U 1538–522: Updated Orbital Ephemeris and a Strengthened Case for an Evolving Cyclotron Line Energy

“…La have argued that a similar decay in CRSF energy is present in Vela X-1, based on Swift/BAT observations. Finally, the BeXRB V 0332+53 displayed hysteresis in its CRSF evolution over the course of its 2015 giant outburst (despite there being no sign of this behavior during its 2003-2004 outburst), with a markedly lower CRSF energy at the end of the outburst compared to the beginning ; furthermore, the CRSF energy seems to have rebounded back to its pre-outburst state by the time of its 2016 outburst (Doroshenko et al 2017;Vybornov et al 2018).…”

“…The transient BeXRB V 0332+53 did experience an increase in CRSF energy between its 2015 and 2016 outbursts(Vybornov et al 2018), but this was following the decrease in CRSF energy over the 2015 outburst -V 0332+53's very high flux, transient nature, and much shorter timescale for CRSF variations makes it unlikely that its changes have similar origins to 4U 1538−522's. 4 see Appendix F ofMihara (1995); since this measurement uses a Gaussian-profile CRSF and an npex continuum, it should only be directly compared to the third column in Table 3 -care should be taken when comparing it to the rest of the work cited here due to the different continuum model.…”