News and Views

Author: Jian-Ping Yuan

Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, China; yuanjp@xao.ac.cn

Globular clusters (GCs) have been found to exhibit remarkably different physical conditions compared to those in the Galactic disk (e.g. Ransom 2008). The extraordinarily high stellar density near the central core of GCs exceeds the typical values in the Galactic plane by several order of magnitude. This provides exceptionally ideal conditions for gravitational interactions between stellar systems. Thus, GCs are highly efficient factories for the formation of exotic systems, among which are radio pulsars.

Pulsars in GCs are highly interesting because they can help to shed light on exotic stellar evolution scenarios as well as the physics of dense matter, accretion and gravity. The population of radio pulsars in GCs shows sharp contrasts compared to that in our Milky Way, and some GC pulsars are even found to exhibit negative period derivative. The latter are caused by the line-of-sight acceleration due to the gravitational field of the cluster. The long-term timing of these pulsars opens up opportunities to study the dynamics and interstellar medium of the clusters, and to search for hypothetical intermediate-mass black holes (IMBHs).

The effect on pulsar timing due to acceleration is relatively small (<1 ns) for pulsars in the Galactic field. However, this effect is likely non-negligible for pulsars in GCs. Freire et al. (2017) measured the changes in orbital period for pulsars in 47 Tuc, and the real line-of-sight acceleration in the gravitational field of the cluster. Dai et al. (2020) determined the acceleration of pulsars in the GC Omega Centauri.

Xie & Wang (2020) investigate the effect of pulsar transverse acceleration, and their simulation indicates that the timing residual can reach the 100 ns level for GC pulsars due to this effect. The study suggests that pulsar transverse acceleration may be detectable for pulsars in close GCs. Although detailed investigations are currently limited, this effect could be tested with giant radio telescopes, such as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and Square Kilometre Array (SKA). FAST, which possesses unrivaled sensitivity, has discovered new pulsars in GCs (e.g. Pan et al. 2020, Wang et al. 2020). The detection of pulsar transverse acceleration may contribute to the improvement of pulsar timing precision, which is helpful for detection and characterization of low-frequency gravitational waves using a pulsar timing array (PTA).

References

• Ransom S. M., 2008, in Vesperini E., Giersz M., Sills A., eds, Proc. IAU Symp. 246, Dynamical Evolution of Dense Stellar Systems. Cambridge University Press, Cambridge, UK, p. 291

• Freire, P.~C.~C. and Ridolfi, A. and Kramer, M. and Jordan, C. and Manchester, R.~N. and Torne, P. and Sarkissian, J. and Heinke, C.~O. and {D'Amico}, N. and Camilo, F. and Lorimer, D.~R. and Lyne, A.~G., 2017, MNRAS, 471, 857-876

• Dai, Shi and Johnston, Simon and Kerr, Matthew and Camilo, Fernando and Cameron, Andrew and Toomey, Lawrence and Kumamoto, Hiroki, 2020, ApJL, 888, L18

• Xie, Yi, and Wang, Li-Chun, 2020, RAA, 20, 191

• Pan, Zhichen and Ransom, Scott M. and Lorimer, Duncan R. and Fiore, William C. and Qian, Lei and Wang, Lin and Stappers, Benjamin W. and Hobbs, George and Zhu, Weiwei and Yue, Youling and Wang, Pei and Lu, Jiguang and Liu, Kuo and Peng, Bo and Zhang, Lei and Li, Di, 2020, ApJL, 892, L6

• Wang, Lin and Peng, Bo and Stappers, B.~W. and Liu, Kuo and Keith, M.~J. and Lyne, A.~G. and Lu, Jiguang and Yu, Ye-Zhao and Kou, Feifei and Yan, Jun and Jiang, Peng and Jin, Chengjin and Li, Di and Li, Qi and Qian, Lei and Wang, Qiming and Yue, Youling and Zhang, Haiyan and Zhang, Shuxin and Zhu, Yan and {FAST Collaboration}, 2020, ApJ, 892, 43