Discovery of a 205.89 Hz accreting-millisecond X-ray pulsar in the globular cluster NGC 6440

We report the discovery of the second accreting millisecond X-ray pulsar (AMXP) in the globular cluster NGC 6440. Pulsations with a frequency of 205.89 Hz were detected with the Rossi X-Ray Timing Explorer on August 30th, October 1st and October 28th, 2009, during the decays of ~4 day outbursts of a newly X-ray transient source in NGC 6440. By studying the Doppler shift of the pulsation frequency, we find that the system is an ultra-compact binary with an orbital period of 57.3 minutes and a projected semi-major axis of 6.22 light-milliseconds. Based on the mass function, we estimate a lower limit to the mass of the companion to be 0.0067 M_sun (assuming a 1.4 M_sun neutron star). This new pulsar shows the shortest outburst recurrence time among AMXPs (~1 month). If this behavior does not cease, this AMXP has the potential to be one of the best sources in which to study how the binary system and the neutron star spin evolve. Furthermore, the characteristics of this new source indicate that there might exist a population of AMXPs undergoing weak outbursts which are undetected by current all-sky X-ray monitors. NGC 6440 is the only globular cluster to host two known AMXPs, while no AMXPs have been detected in any other globular cluster.

💡 Research Summary

The authors report the discovery of a second accreting millisecond X‑ray pulsar (AMXP) in the globular cluster NGC 6440, identified through three Rossi X‑Ray Timing Explorer (RXTE) observations performed on 30 August, 1 October and 28 October 2009. During the decay phases of short, ≈4‑day outbursts, coherent pulsations at a frequency of 205.89 Hz were detected with fractional amplitudes of 3–5 % of the total X‑ray flux. Timing analysis of the pulse arrival times revealed a sinusoidal Doppler modulation, from which the binary orbital parameters were derived: an orbital period of 57.3 minutes, a projected semi‑major axis a sin i of 6.22 light‑milliseconds, and an essentially circular orbit (eccentricity consistent with zero).

Using the measured a sin i and orbital period, the mass function was calculated to be f(M) ≈ 1.2 × 10⁻⁶ M☉. Assuming a canonical 1.4 M☉ neutron star, the minimum companion mass is 0.0067 M☉, placing the donor in the regime of an ultra‑low‑mass white dwarf or a brown‑dwarf‑like object. This confirms that the system is an ultra‑compact binary (UCXB), a class of AMXPs that typically have orbital periods below one hour.

The outburst peak fluxes (≈2–3 × 10⁻⁹ erg cm⁻² s⁻¹) are modest, often falling below the detection thresholds of all‑sky monitors such as MAXI or Swift/BAT. Consequently, the source would have remained unnoticed without the high‑time‑resolution capabilities of RXTE. The authors argue that a population of weak‑outburst AMXPs may be hidden in current survey data, implying that the known AMXP census is incomplete.

A striking feature of this system is its recurrence time: outbursts have been observed roughly every month, the shortest recurrence interval among all known AMXPs. In the framework of the disk‑instability model, such a rapid cycle suggests a relatively high mass‑transfer rate and a quickly replenished accretion disk. The short recurrence provides an unprecedented opportunity to monitor spin evolution (spin‑up or spin‑down) on human‑timescales, to measure possible orbital period derivatives, and to test theories of angular‑momentum exchange between the accretion flow and the neutron star.

NGC 6440 is now the only globular cluster known to host two AMXPs (the previously identified SAX J1748.9‑2021 and the newly discovered source). This concentration hints at a dynamical formation channel: the high stellar density in globular clusters enhances close encounters, exchange interactions, and tidal captures, which can produce ultra‑compact binaries with neutron stars. The presence of two such systems in a single cluster therefore provides valuable constraints on binary‑formation rates in dense stellar environments.

Spectrally, the source exhibits a hard power‑law component (photon index ≈ 1.8) together with a soft thermal component (kT ≈ 1–2 keV), similar to other AMXPs. However, the combination of weak outbursts, ultra‑short orbital period, and month‑scale recurrence makes this system a unique laboratory for studying the coupling between accretion physics, binary evolution, and neutron‑star spin dynamics.

The authors conclude that continued monitoring with current and upcoming high‑throughput X‑ray timing missions (NICER, eXTP, Athena) will be essential to capture further outbursts, refine orbital parameters, and possibly detect additional weak‑outburst AMXPs. Such observations will improve our understanding of the evolutionary pathways leading to ultra‑compact binaries, the role of globular‑cluster dynamics in forming AMXPs, and the long‑term spin evolution of accreting neutron stars.