Discovery of the redback millisecond pulsar PSR J1728-4608 with ASKAP
F. Petrou, Y. Wang, N. Hurley-Walker, S. McSweeney, L. Zhang, R. G. Key, J. Freeburn, B. W. Meyers, David L. Kaplan, A. Zic, Tara Murphy, D. Dobie, Y. Maan
TL;DR
This work reports the discovery of PSR J1728-4608, a redback millisecond pulsar identified in ASKAP image-domain data from the VAST/EMU surveys. A coordinated multi-telescope radio follow-up yields a precise phase-connected timing solution: a spin period of $P=2.86\ \mathrm{ms}$ in a $P_{ m orb}=0.210410559\ \mathrm{d}$ orbit with a minimum companion mass $M_c\approx0.13\ M_\odot$, and a dispersion measure $\mathrm{DM}=65.5\ \mathrm{pc\,cm^{-3}}$; the orbital period shows measurable variations likely driven by the companion’s gravitational quadrupole changes. The eclipse is frequency-dependent with an ingress duration scaling as $t_{\text{ingress}}\propto \nu^{n}$, where $n=-1.74$, and the eclipse is consistent with synchrotron absorption in the eclipsing material, supported by a measured DM_excess of $\mathrm{DM_{excess}}=2.0\pm1.2\ \mathrm{pc\,cm^{-3}}$ and electron column density $N_e\approx(5.9\pm3.6)\times10^{18}\ \mathrm{cm^{-2}}$. Optical Gaia data identify a counterpart within $0.5''$ (Gaia1) with a $G$-band light curve showing ellipsoidal modulation plus irradiation effects, situating the system among RB pulsars in a transitional regime between ellipsoidal and irradiation-dominated optical morphologies. A nearby Fermi source, $4FGL\,J1728.0-4606$, remains a plausible but unconfirmed gamma-ray association. Overall, PSR J1728-4608 advances our understanding of pulsar recycling, companion-driven eclipses, and plasma processes in tight RB binaries, with implications for population studies and the interpretation of radio and $\gamma$-ray emission from spider pulsars.
Abstract
We present the discovery of PSR J1728-4608, a new redback spider pulsar identified in images from the Australian SKA Pathfinder telescope. PSR J1728-4608 is a millisecond pulsar with a spin period of 2.86 ms, in a 5.05 hr orbit with a companion star. The pulsar exhibits a radio spectrum of the form $S_ν\propto ν^α$, with a measured spectral index of $α= -1.8(3)$. It is eclipsed for 42% of its orbit at 888 MHz, and multi--frequency image--domain observations show that the egress duration scales with frequency as a power law with index $n = -1.74$, where longer duration eclipses are seen at lower frequencies. An optical counterpart is detected in archival Gaia data within $0.5''$ of the radio position. It has a mean G-band magnitude of 18.8 mag and its light curve displays characteristics consistent with a combination of ellipsoidal modulation and irradiation effects. We also report the nearest Fermi $γ$-ray source, located 2$'$ away from our source, as a possible association. A radio timing study constrains the intrinsic and orbital properties of the system, revealing orbital period variations that we attribute to changes in the gravitational quadrupole moment of the companion star. At the eclipse boundary, we measure a maximum dispersion measure excess of $2.0 \pm 1.2 \ \mathrm{pc\ cm^{-3}}$, corresponding to an electron column density of $5.9 \pm 3.6 \times10^{18} \ \mathrm{cm^{-2}}$. Modelling of the eclipse mechanism suggests that synchrotron absorption is the dominant cause of the eclipses observed at radio wavelengths. The discovery and characterisation of systems like \psr\ provide valuable insights into pulsar recycling, binary evolution, the nature of companion-driven eclipses, and the interplay between compact objects and their plasma environments.