Abstract
ABSTRACT The Fermi Large Area Telescope gamma-ray source 3FGL J2039.6–5618 contains a periodic optical and X-ray source that was predicted to be a ‘redback’ millisecond pulsar (MSP) binary system. However, the conclusive identification required the detection of pulsations from the putative MSP. To better constrain the orbital parameters for a directed search for gamma-ray pulsations, we obtained new optical light curves in 2017 and 2018, which revealed long-term variability from the companion star. The resulting orbital parameter constraints were used to perform a targeted gamma-ray pulsation search using the Einstein@Home-distributed volunteer computing system. This search discovered pulsations with a period of 2.65 ms, confirming the source as a binary MSP now known as PSR J2039–5617. Optical light-curve modelling is complicated, and likely biased, by asymmetric heating on the companion star and long-term variability, but we find an inclination i ≳ 60°, for a low pulsar mass between $1.1\, \mathrm{M}_{\odot } \lt M_{\rm psr} \lt $ 1.6 M⊙, and a companion mass of 0.15–$0.22\, \mathrm{M}_{\odot }$, confirming the redback classification. Timing the gamma-ray pulsations also revealed significant variability in the orbital period, which we find to be consistent with quadrupole moment variations in the companion star, suggestive of convective activity. We also find that the pulsed flux is modulated at the orbital period, potentially due to inverse Compton scattering between high-energy leptons in the pulsar wind and the companion star’s optical photon field.
Highlights
Millisecond pulsars (MSPs) are old neutron stars that have been spun-up to millisecond rotation periods by the accretion of matter from an orbiting companion star (Alpar et al 1982)
The most compelling evidence for this “recycling” scenario comes from the discovery of three transitional MSPs, which have been seen to switch between rotationally powered MSP and accretion-powered low-mass X-ray binary (LMXB) states (Archibald et al 2009; Papitto et al 2013; Bassa et al 2014; Stappers et al 2014)
Using a directed search for gamma-ray pulsations running on the distributed volunteer computing system Einstein@Home, we have confirmed the redback nature of the candidate binary system within 4FGL J2039.5−5617
Summary
Millisecond pulsars (MSPs) are old neutron stars that have been spun-up to millisecond rotation periods by the accretion of matter from an orbiting companion star (Alpar et al 1982). The most compelling evidence for this “recycling” scenario comes from the discovery of three transitional MSPs, which have been seen to switch between rotationally powered MSP and accretion-powered low-mass X-ray binary (LMXB) states (Archibald et al 2009; Papitto et al 2013; Bassa et al 2014; Stappers et al 2014). In their rotationally powered states, these transitional systems all belong to a class of interacting binary MSPs known as “redbacks”, which are systems containing an MSP in orbit with a low-mass (0.1 M M 0.4 M ) non-degenerate companion star (Roberts 2013). Traditional “acceleration” search methods for binary pulsars (Ransom et al 2002) are only optimal when the integration time is 10% of the orbital period, leading to an additional sensitivity loss to spiders, which often have orbital periods of just a few hours
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