Einstein@Home discovery of the gamma-ray millisecond pulsar PSR J2039–5617 confirms its predicted redback nature

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 M⊙ < Mpsr < 1.6 M⊙, and a companion mass of 0.15–0.22 M⊙, 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.

Gamma-ray orbital modulation of the transitioning millisecond pulsar binary XSS J12270-4859

“…Our inspection of the timing solutions does not provide a definite test for them, and the current solutions may be slightly inaccurate. Small inaccuracy (e.g., ∼10 s phase offsets for PSR J2039−5617; Clark et al 2021) would not change our conclusions significantly since the gamma-ray modulation is detected regardless of the timing solutions we tested and so seems to be quite robust. However, phases of the gamma rays might have been mixed, which makes the light curve broader; the intrinsic gamma-ray light curve of J1227 may be somewhat narrower.…”

“…Our investigation of the radio timing solutions suggests that higher-order derivatives (e.g., higher than the second time derivatives) of the orbital frequency seem to be small as in PSR J2039−5617 (∆P B /P B ≤ 10 −6 ; Clark et al 2021). Otherwise, it is very difficult to maintain the phase alignment of the optical data over the 15 yr period (e.g., Fig.…”

“…Orbital periods of pulsar binaries can vary slightly over time, and radio timing solutions for J1227 obtained over short time intervals (c.f., >12.5 year of LAT op-eration) require one or two orbital-period derivatives (Roy et al 2015;de Martino et al 2020). They may reflect long-term changes of the orbital motion or a small but unpredictable variation as seen in other pulsar binaries (e.g., ∆P B /P B ≤ 10 −6 for PSR J2039−5617; Clark et al 2021). In the latter case, the short-term radio timing solutions may be valid only within the given intervals.…”

“…Pulsar wind particles are accelerated in the shock and emit synchrotron radiation, producing characteristic double peaks in the Xray light curve around a maximum (e.g., Roberts et al 2015). Gamma rays are produced primarily in the pulsar magnetosphere, but inverse-Compton scattering (ICS) of the stellar blackbody radiation by energetic particles in the shock or in the pre-shock pulsar wind can also produce gamma-ray radiation in the GeV band; this ICS emission may exhibit orbital modulation due to variation of the viewing geometry, having a maximum or minimum at the pulsar's conjunctions (e.g., An et al 2020;Clark et al 2021).…”

4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf

“…Nonetheless, additional radio searches that use our optical ephemerides for scheduling would be valuable, and we have recently begun a GBT program utilizing this strategy. An independent, promising route to a pulsar detection would be to search for γ-ray pulsations, with the computationally-expensive search (e.g., Nieder et al 2020;Clark et al 2021) constrained via our optical orbital parameters. The detection of the pulsar, combined with new high-resolution optical spectroscopy, could also allow a mass measurement of the neutron star, which is predicted to be high.…”