Accreting magnetars: a new type of high-mass X-ray binaries?

Abstract

The discovery of very slow pulsations (Pspin=5560s) has solved the long-standing question of the nature of the compact object in the high-mass X-ray binary 4U 2206+54 but has posed new ones. According to spin evolutionary models in close binary systems, such slow pulsations require a neutron star magnetic field strength larger that the quantum critical value, suggesting the presence of a magnetar. We present the first XMM-Newton observations of 4U 2206+54 and investigate its spin evolution. We find that the observed spin-down rate agrees with the magnetar scenario. We analyse ISGRI/INTEGRAL observations of 4U 2206+54 to search for the previously suggested cyclotron resonance scattering feature at ~30 keV. We do not find a clear indication of the presence of the line, although certain spectra display shallow dips, not always at 30 keV. The association of these dips with a cyclotron line is very dubious because of its apparent transient nature. We also investigate the energy spectrum of 4U 2206+54 in the energy range 0.3-10 keV with unprecedented detail and report for the first time the detection of very weak 6.5 keV fluorescence iron lines. The photoelectric absorption is consistent with the interstellar value, indicating very small amount of local matter, which would explain the weakness of the florescence lines. The lack of matter locally to the source may be the consequence of the relatively large orbital separation of the two components of the binary. The wind would be too tenuous in the vicinity of the neutron star.Comment: accepted for publication in MNRA

The paradigm of atypical HMXRBs: 4U 2206+54

“…Thanks to the large amount of data present in the INTEGRAL/ISGRI archive, we can study the long term evolution of the pulse period of 4U 22067+54, see Figure 3. We see that for the If we compare that to the work of [10], we can check that if we superimposse the same period depicted in our Figure 3, in their Figure 2, we clearly see that the measurements of the pulse period of 4U 2206+54 that they show are also compatible with a plateau of values fluctuating around an average pulse period. Very likely, the pulse period has not been monotonically increasing, but evolving irregularly with, at least, spin-up and plateau periods.…”

“…It has been proposed that the neutron star in 4U 2206+54 could have a high magnetic field (see [7] and [10]). We will review the two observational facts leading to this proposal, namely, the possible presence of a Cyclotron Resonant Scattering Feature (CRSF) in the spectra of 4U 2206+54, and the pulse period evolution of the source.…”

The VLT-FLAMES Tarantula survey

“…In each of the systems named above the exceptionally long pulsational periods of the neutron stars are explicable under the assumption that they had been significantly spun down, first via magnetic dipole radiation and subsequently the propellor mechanism (Li & van den Heuvel 1999;Reig et al 2009Reig et al , 2012Popov & Turolla 2012). Operating on comparatively short timescales (∼10 4 −10 5 yr; Popov & Turolla 2012), this would also be consistent with the relatively unevolved nature of VFTS 399 (cf.…”

“…However, with P orb 19.25 days, and P spin ∼ 5560 s (Reig et al 2009(Reig et al , 2012, the O9.5 Ve+neutron star binary 4U 2206+54 demonstrates that such a combination of long pulsation but short orbital period is viable, although this would require VFTS 399 to host the third most slowly rotating neutron star of any high mass X-ray binary, behind 4U 2206+54 and 2S 0114+650 (P orb ∼ 11.59 day and P spin ∼ 2.7 h; Crampton et al 1985;Corbet et al 1999;Farrell et al 2008). In this regard we note that both the X-ray luminosity and aperiodic variability of VFTS 399 are replicated in 4U 2206+54 − which is assumed to be powered by direct wind-fed accretion (Negueruela & Reig 2001) − while the primaries in both systems resemble one another Walborn et al 2014).…”

“…Reig et al (2012) demonstrate that the accretor in 4U2206+54 still supports an extreme (∼10 14 G) B-field under this formulation, although this requires an anomalously slow primary wind (which is observed in this system; Ribó et al 2006). Clearly, an orbital solution for VFTS 399 and a determination of the wind parameters of the primary (mass loss rate and velocity at the orbital radius) in order to determineṀ would provide an empirical test of the extreme B-field hypothesis, as would a measure of the long term spin-period history of the putative neutron star, from which an estimate of B may also be obtained.…”