Unveiling the redback nature of the low-mass X-ray binary XSS J1227.0−4859 through optical observations★

Abstract

The peculiar low mass X-ray binary XSS J12270-4859, associated with the Fermi/LAT source 2FGL J1227.7-4853, was in a X-ray, gamma-ray and optical low-luminosity persistent state for about a decade until the end of 2012, when it has entered into the dimmest state ever observed. The nature of the compact object has been controversial until the detection of a 1.69ms radio pulsar early 2014. We present optical spectroscopy and optical/near-IR photometry during the previous brighter and in the recent faint states. We determine the first spectroscopic orbital ephemeris and an accurate orbital period of 6.91246(5) h. We infer a mid G-type donor star and a distance d= 1.8-2.0 kpc. The donor spectral type changes from G5 V to F5 V between inferior and superior conjunction, a signature of strong irradiation effects. We infer a binary inclination 45 o i 65 o and a highly undermassive donor, M 2 ∼ 0.06 − 0.12 M ⊙ for a neutron star mass in the range 1.4-3 M ⊙ . Thus this binary joins as the seventh member the group of "redbacks". In the high state, the emission lines reveal the presence of an accretion disc. They tend to vanish at the donor star superior conjunction, where also flares are preferentially observed together with the occurrence of random dips. This behaviour could be related to the propeller mechanism of the neutron star recently proposed to be acting in this system during the high state. In the low state, the emission lines are absent at all orbital phases indicating that accretion has completely switched-off and that XSS J12270-4859 has transited from an accretion-powered to a rotation-powered phase.

NuSTAR and Parkes observations of the transitional millisecond pulsar binary XSS J12270–4859 in the rotation-powered state

“…. Similar transitions were also observed in IGR J1825-2452, in the M28 globular cluster (Papitto et al 2013) and in the galactic field X-ray source XSS J12270-4859 (henceforth J1227) Bogdanov et al 2014;de Martino et al 2014). The transitions occured on timescales much shorter than secular evolution, likely controlled by variations in the mass transfer rate from the late-type companion.…”

Orbital Dynamics of Candidate Transitional Millisecond Pulsar 3FGL J1544.6-1125: An Unusually Face-on System

“…This is because the average density of the star is set entirely by the orbital period (Frank et al 2002). In the case of XSS J12270-4859, for example, the orbital period of ≈ 0.7 hours implies a density similar to a K3V/K4V star, but the spectral type is much earlier, at F5V-G5V, depending upon phase (de Martino et al 2014). XSS J12270-4859 therefore does not fall on the Main Sequence relation and so must be undermassive.…”

X-ray coherent pulsations during a sub-luminous accretion disc state of the transitional millisecond pulsar XSS J12270−4859

“…According to analytical calculations made by Poutanen & Beloborodov (2006), the ratio of the amplitude of the second harmonic to the fundamental observed from a fast pulsar whose antipodal spots are always visibile, is c 2 /c 1 = 1/2(tan i tan θ), where i is the binary inclination and θ is the spot colatitude. de Martino et al (2014) constrained the inclination of XSS J12270-4859 between 45 and 65…”

“…These include, (i) IGR J18245-2452, a binary of the globular cluster M28 that turned on as a bright (L X ≈ 10 36 erg s −1 ) AMSP in 2013, and was observed as a rotationally-powered MSP a few years before, and a few weeks after the accretion event (Papitto et al 2013); (ii) PSR J1023+0038, an MSP that had a sub-luminous ( 10 34 erg s −1 ) accretion disk in 2000/2001 (Archibald et al 2009), and that have entered back again in such a state in 2013 Stappers et al 2014); (iii) XSS J12270-4859, an LMXB that remained for a decade in a subluminous disk accretion phase (Saitou et al 2009;de Martino et al 2010de Martino et al , 2013, characterized by correlated X-ray and UV flux variability (de Martino et al 2013) and by bright radio and GeV emission (de Martino et al 2010;Hill et al 2011); during December 2012 it then transited to an MSP state characterized by the detection of 1.69 ms radio pulsations (Roy et al 2014), a fainter Xray and gamma-ray emission (Tam et al 2013;Bassa et al 2014;Bogdanov et al 2014;Xing & Wang 2014), and the absence of an accretion disk (de Martino et al 2014). …”