PSR J0024$-$7204ai: a massive, eccentric binary system in the globular cluster 47 Tucanae

Abstract

In this paper we present PSR J0024$-$7204ai, a 13.026-ms binary pulsar recently discovered in the globular cluster 47 Tucanae by the MeerKAT radio telescope. This is the slowest spinning pulsar known in this globular cluster, and has a $\sim1.67$-day orbit with an eccentricity of $e\approx0.18$. Although it was not yet possible to derive an unambiguous phase-connected timing solution, by combining detections obtained from MeerKAT and archival Parkes data we were able to measure the rate of advance of periastron to high significance, $\dotω$ = 0.1601 $\pm 0.0046$ deg yr$^{-1}$. This value implies a total system mass of $2.41 \pm 0.11\, \mathrm{M}_\odot$ (68.3\% C. L.), which, when combined with the binary mass function, gives a maximum pulsar mass of $\sim 1.7 \, \mathrm{M}_\odot$ and a minimum companion mass of $\sim 0.7\, \mathrm{M}_\odot$. Apart from being the slowest pulsar in 47~Tucanae, its orbit is by far the most eccentric and its companion is the most massive among all known binary pulsars in this globular cluster. One possibility is that system is an old MSP - Carbon-Oxygen White Dwarf binary, whose orbit was perturbed by stellar dynamical interactions in the cluster core. Further follow-up observations of this system will be essential for a more detailed characterisation of this system and its evolution.

Figures (7)

• Figure 1: Period-Acceleration diagram for 47 Tuc ai. The data points are the barycentric spin periods and line-of-sight accelerations as measured on seven different epochs where the pulsar was blindly detected. The best-fit parametric curve (solid black line), from equations 3 and 4 from 2001MNRAS.326..901Freire_b, largely deviates from a simple ellipse, implying a mild orbital eccentricity ($e \approx 0.18$).
• Figure 2: SEEKAT Localization of 47 Tuc ai, with the 1-$\sigma$ uncertainty on the localized position are shown as magenta coloured ellipse. The gray ellipses are beams from the observation 26U1 in which the pulsar was detected at the highest S/N, and were used for localizing the pulsar as discussed in sec. \ref{['sec:localization']}. The center of the GC 47 Tuc (central cross), the core radius (blue circle) and the positions of some of the known pulsars in 47 Tuc are shown for the reference. The image covers an area of 1 arcmin $\times$ 1 arcmin around the center of the GC $00^\mathrm{h}\,24^\mathrm{m}\,05.67^\mathrm{s}$, $$-$72^\circ\,04'\,52.6"$2012AJ....143...50Woodley.
• Figure 3: Evolution of pulse profile of PSR 47 Tuc ai with observing frequency. The profile components are marked with Comp1 and Comp2. Each subplot shows the pulse profile at different observing frequencies in black and 2-component Gaussian fits to the profile in dotted red lines. The profiles have been normalized in the amplitude. The results obtained from fit - relative amplitudes, widths and separation between the components are reported in Table \ref{['table:prof_evol']}.
• Figure 4: Variation of relative amplitudes, widths and separation between two components of the pulse profile with respect to observing frequency. The fitted profiles at different frequency are shown in Fig. \ref{['fig:combined_profiles']} ans fitting statistics are listed in table \ref{['table:prof_evol']}. It can be seen from the upper subplot that the relative amplitude of first component increases as a function of observing frequency.
• Figure 5: Post-fit timing residuals of 47 Tuc ai as per the timing solution in table \ref{['table:timing_sol']}. Groups of ToAs are marked in different colours as follows: Red - MeerKAT (jumped); Blue - MeerKAT phase-connected ; Black - Parkes Multibeam.