XMM-Newtonspectroscopy of the accreting magnetar candidate 4U0114+65

Abstract

Aims. 4U0114+65 is one of the slowest known X-ray pulsars. We present an analysis of a pointed observation by the XMM-Newton X-ray telescope in order to study the nature of the X-ray pulsations and the accretion process, and to diagnose the physical properties of the donor's stellar wind. Methods. We analysed the energy-resolved light curve and the time-resolved X-ray spectra provided by the EPIC cameras on board XMM-Newton. We also analysed the first high-resolution spectrum of this source provided by the Reflection Grating Spectrometer. Results. An X-ray pulse of 9350 ± 160 s was measured. Comparison with previous measurements confirms the secular spin up of this source. We successfully fit the pulse-phase-resolved spectra with Comptonisation models. These models imply a very small (r ∼ 3 km) and hot (kT ∼ 2−3 keV) emitting region and therefore point to a hot spot over the neutron star (NS) surface as the most reliable explanation for the X-ray pulse. The long NS spin period, the spin-up rate, and persistent X-ray emission can be explained within the theory of quasi-spherical settling accretion, which may indicate that the magnetic field is in the magnetar range. Thus, 4U 0114+65 could be a wind-accreting magnetar. We also observed two episodes of low luminosity. The first was only observed in the low-energy light curve and can be explained as an absorption by a large over-dense structure in the wind of the B1 supergiant donor. The second episode, which was deeper and affected all energies, may be due to temporal cessation of accretion onto one magnetic pole caused by non-spherical matter capture from the structured stellar wind. The light curve displays two types of dips that are clearly seen during the high-flux intervals. The short dips, with durations of tens of seconds, are produced through absorption by wind clumps. The long dips, in turn, seem to be associated with the rarefied interclump medium. From the analysis of the X-ray spectra, we found evidence of emission lines in the X-ray photoionised wind of the B1Ia donor. The Fe Kα line was found to be highly variable and much weaker than in other X-ray binaries with supergiant donors. The degree of wind clumping, measured through the covering fraction, was found to be much lower than in supergiant donor stars with earlier spectral types. Conclusions. The XMM-Newton spectroscopy provided further support for the magnetar nature of the neutron star in 4U0114+65. The light curve presents dips that can be associated with clumps and the interclump medium in the stellar wind of the mass donor.

Accreting magnetars: linking ultraluminous X-ray pulsars and the slow pulsation X-ray pulsars

“…• The long period neutron star 4U 0114+65 (with rotational period about 9500 s) may be an accreting magnetar candidate (Sanjurjo-Ferrrin et al 2017). Observationally, possible signatures of a transient disk were also found in this system (Hu et al 2017).…”

“…(a) ULX pulsars may be accreting magnetars with high mass accretion rates. (b) The slow pulsation X-ray pulsars may be accreting magnetars with low mass accretion rates, including AX J1910.7+0917, 4U 0114+65 (Sanjurjo-Ferrrin et al 2017), 4U 2206+54 (Reig et al 2012) etc. Some supergiant fast X-ray transients (SFXTs, e.g.…”

No magnetars in ULXs

“…Such superluminous supernovae disrupt the binary and produce isolated magnetars. In principle, a small fraction of binaries could survive the catastrophe: for example the very slow (pulse period ∼ 2.6h) X-ray pulsar in the high-mass X-ray binary 4U 0114+65 could have a magnetar-strength magnetic field (Sanjurjo-Ferrrín et al 2017, and references therein). The problem with this channel as a model for PULX formation is that strong neutron-star fields ∼ 10 15 G decay quickly ( a few Myr) by several orders of magnitude (Mereghetti, Pons, & Melatos 2015).…”

Radiography in high mass X-ray binaries

“…The classic SgXB 2S0114+650 (also known as LSI+65 010) is an excellent candidate to apply the model presented in this paper, thanks to its grazing orbit. Sanjurjo-Ferrrín et al (2017) provide a ratio a/R * of 1.34-1.65. Given the high mass ratio (Reig et al 1996), the absence of accretion disk (Koenigsberger et al 2003) and the moderate X-ray luminosity (1.4 • 10 36 erg • s −1 , Sanjurjo-Ferrrín et al 2017), we can safely assume that the star does not fill its Roche lobe.…”