Discovery of slow X-ray pulsations in the high-mass X-ray binary 4U 2206+54

Abstract

Context. The source 4U 2206+54 is one of the most enigmatic high-mass X-ray binaries. In spite of intensive searches, X-ray pulsations have not been detected in the time range 10 −3 -10 3 s. A cyclotron line at ∼30 keV has been suggested by various authors but never detected with significance. The stellar wind of the optical companion is abnormally slow. The orbital period, initially reported to be 9.6 days, disappeared and a new periodicity of 19.25 days emerged. Aims. The main objective of our RXTE monitoring of 4U 2206+54 is to study the X-ray orbital variability of the spectral and timing parameters. The new long and uninterrupted RXTE observations allow us to search for long (∼1 h) pulsations for the first time. Methods. We divided the ∼7-day observation into five intervals and obtained time-averaged energy spectra and power spectral density for each observation interval. We also searched for pulsations using various algorithms. Results. We have discovered 5560-s pulsations in the light curve of 4U 2206+54. Initially detected in RXTE data, these pulsations are also present in INTEGRAL and EXOSAT observations. The average X-ray luminosity in the energy range 2-10 keV is 1.5 × 10 35 erg s −1 with a ratio F max /F min ≈ 5. This ratio implies an eccentricity of ∼0.4, somewhat higher than previously suggested. The power spectrum is dominated by red noise that can be fitted with a single power law whose index and strength decrease with X-ray flux. The source also shows a soft excess at low energies. If the soft excess is modelled with a blackbody component, then the size and temperature of the emitting region agrees with its interpretation in terms of a hot spot on the neutron star surface. Conclusions. The discovery of X-ray pulsations in 4U 2206+54 confirms the neutron star nature of the compact companion and definitively rules out the presence of a black hole. The source displays variability on time scales of days, presumably due to changes in the mass accretion rate as the neutron star moves around the optical companion in a moderately eccentric orbit. If current models for the spin evolution in X-ray pulsars are correct, then the magnetic field of 4U 2206+54 at birth must have been B 10 14 G.

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.…”

“…Importantly, both "short" (∼3.245 s) and "long" (∼2567 s) potential periodicities lie within the range of pulsational periods exhibited by neutron stars within Be X-ray binaries (e.g. Knigge et al 2011;Reig et al 2009) and we expand upon this further in Sect. 4.1.…”

“…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).…”

“…1.4 (Li & van den Heuvel 1999;Reig et al 2009) where B = 10 12 B 12 G is the dipolar field strength,Ṁ = 10 15Ṁ 15 g s −1 the mass accretion rate and R = 10 6 R 6 cm and M = 1.4 M 1.4 M the neutron star radius and mass. We adopt P spin ∼ 2567 s, (Reig et al 2009;Sect.…”

Exploration of spin-down rate of neutron stars in high-mass X-ray binaries

“…Diamonds and pluses mark the supergiant wind-fed HMXBs and supergiant fast X-ray transients, respectively, and crosses are for Roche lobe overflow systems. The two joined triangles represent 4U 2206+54 for the two suggested orbital periods (Reig et al 2009). The two joined asterisks represent IGR J01583+6713 for a possible orbital period in the range 3-12 days suggested by Wang (2010).…”