Abstract
ABSTRACT We present an optical, X-ray, and γ-ray study of 1SXPS J042749.2-670434, an eclipsing X-ray binary that has an associated γ-ray counterpart, 4FGL J0427.8-6704. This association has led to the source being classified as a transitional millisecond pulsar (tMSP) in an accreting state. We analyse 10.5 yr of Fermi LAT data and detect a γ-ray eclipse at the same phase as optical and X-ray eclipses at the >5 σ level, a significant improvement on the 2.8 σ level of the previous detection. The confirmation of this eclipse solidifies the association between the X-ray source and the γ-ray source, strengthening the tMSP classification. However, analysis of several optical data sets and an X-ray observation do not reveal a change in the source’s median brightness over long time-scales or a bi-modality on short time-scales. Instead, the light curve is dominated by flickering, which has a correlation time of 2.6 min alongside a potential quasi-periodic oscillation at ∼21 min. The mass of the primary and secondary stars is constrained to be $M_1=1.43^{+0.33}_{-0.19}$ M⊙ and $M_2=0.3^{+0.17}_{-0.12}$ M⊙ through modelling of the optical light curve. While this is still consistent with a white dwarf primary, we favour the tMSP in a low accretion state classification due to the significance of the γ-ray eclipse detection.
Highlights
‘Redbacks’ are binary star systems that have a neutron star (NS) primary and a low-mass, near-main sequence companion
These three systems have been observed to transition between a radio loud state, where the pulsar is detectable at radio wavelengths and there is no evidence for active accretion from the secondary, and a low-mass X-ray binary (LMXB) state, where emission from the radio pulsar is quenched and material flows from the secondary through the inner Lagrange point towards the NS primary, where it builds an accretion disc
The underlying orbital modulation coupled with the recent distance estimate to the system measured by Gaia suggests that the primary is a neutron star, but the classification of J0427 as a transitional millisecond pulsar (tMSP) in an accreting state still hinges on the observed γ -ray emission from the source, with no other evidence to support the classification as a tMSP over a regular LMXB
Summary
‘Redbacks’ are binary star systems that have a neutron star (NS) primary and a low-mass, near-main sequence companion. 3 ‘redback’ systems have become increasingly important in understanding the evolution of MSPs in binary systems: PSR J1023+0038 (Archibald et al 2009; Stappers et al 2013), IGR J18245-2452 (Papitto et al 2013), and PSR J1227-4853 (Bassa et al 2014) These three systems have been observed to transition between a radio loud state, where the pulsar is detectable at radio wavelengths and there is no evidence for active accretion from the secondary, and a low-mass X-ray binary (LMXB) state, where emission from the radio pulsar is quenched and material flows from the secondary through the inner Lagrange point towards the NS primary, where it builds an accretion disc. The underlying orbital modulation coupled with the recent distance estimate to the system measured by Gaia suggests that the primary is a neutron star, but the classification of J0427 as a tMSP in an accreting state still hinges on the observed γ -ray emission from the source, with no other evidence to support the classification as a tMSP over a regular LMXB
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