Pulsating iron spectral features in the emission of X-ray pulsar V 0332+53

Abstract

We present results of phase- and time-resolved study of iron spectral features in the emission of the Be/X-ray transient pulsar V 0332+53 during its type II outburst in 2004 using archival RXTE/PCA data. Coherent pulsations of both fluorescent iron line at ≈6.4 keV and neutral iron K-edge at ≈7.1 keV have been detected throughout the entire outburst. The pulsating iron K-edge is reported for the first time for this object. Near the peak of the outburst, the 3–12 keV pulse profile shows two deep, Fmax/Fmin ∼ 2, and narrow dips of nearly identical shape, separated by exactly Δφ = 0.5 in phase. The dip spectra are nearly identical to each other and very similar in shape to the spectra outside the dips. The iron K-edge peaks at the phase intervals corresponding to the dips, although its optical depth τK ∼ 0.05 is by far insufficient to explain the dips. The iron line shows pulsations with a complex pulse profile without any obvious correlation with the total flux or optical depth of the K-edge. Accounting for the component associated with reprocessing of the pulsar emission by the surface of the donor star and circumstellar material, we find a very high pulsation amplitude of the iron line flux, Fmax/Fmin ∼ 10. We demonstrate that these properties of V 0332+53 can not be explained by contemporary emission models for accreting X-ray pulsars and speculate about the origin of the observed iron spectral features.

Timing Properties of the X-Ray Accreting Pulsar 1A 0535+262 Studied with Insight-HXMT

“…V0332+53 is the only source where the transition from sub-to supercritical accretion has been unambiguously observed previously (Doroshenko et al 2017;Vybornov et al 2018;Staubert et al 2019). No detailed analysis of the pulse profile shape was possible in this case, although Tsygankov et al (2006) and Bykov et al (2021) did report hints of pulse profile changes.…”

Pulsed Iron Line Emission from the First Galactic Ultraluminous X-Ray Pulsar Swift J0243.6+6124

“…This may also lead to the pulsed iron K-edge, which is significant, as shown by the reduction of χ 2 (138) when adding an edge model in the spectral fitting. Although similar pulse variations were detected in V 0332+53 (Bykov et al 2021), also a Be/X-ray transient pulsar, the accretion curtain origin in their work seems out of place here. The reprocessing of the optically thick envelope is difficult for forming the edge structure, and in such a high state, it is difficult to observe the neutral iron atoms in this interior region.…”

“…Therefore, it is possible for us to analyze the ionization degree, kinematics, and geometric distribution of materials around the radiation area through the spectral structure shown by the iron lines, especially in neutron star systems, usually with complex accretion structures. In fact, pulsed iron line emissions associated with rotation have been detected in some high-mass X-ray binaries, such as Cen X-3 (Day et al 1993), Her X-1 (Choi et al 1994), LMC X-4 (Shtykovsky et al 2017), 4U 1538-522 (Hemphill et al 2014), GX 301-2 (Liu et al 2018), and V 0332+53 (Bykov et al 2021), but none of them are PULXs. The known PULXs are distant (>80 kpc, except our target), and the limited number of pulsed photons makes it difficult to support the analysis of the emission line.…”

Accreting strongly magnetised neutron stars: X-ray Pulsars

“…For instance, the pulsating iron line was detected in LMC X-4 [194], Cen X-3 [41], GX 301−2 [121,237], Her X-1 [33], and 4U 1538−522 [76]. Most recently variability of iron line equivalent width and iron K-edge at ∼7.1 keV with NS spin and orbital period were discovered in the transient XRP V 0332+53 [211,24]. X-ray energy spectra of pulsating ULXs are softer than the spectra of normal XRPs.…”