4U 1556–60 as a very faint neutron star X-ray binary at 700 pc with an undetected radio jet

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam Postbus 94249 1090 GE Amsterdam, The Netherlands
² Department of Physics and Astronomy, Texas Tech University Lubbock TX 79409-1051, USA
³ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Via U. La Malfa 153 I-90146 Palermo, Italy
⁴ INAF Osservatorio Astronomicodi Cagliari Viadella Scienza 5 09047 Selargius (CA), Italy
⁵ Hamburger Sternwarte, University of Hamburg Gojenbergsweg 112 21029 Hamburg, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 October 2025
Accepted: 10 February 2026

Abstract

Context. 4U 1556–60 is a low-mass X-ray binary that was discovered more than 50 years ago as a persistent X-ray source; however, very little was known about it. Recently, Gaia obtained a parallax for the optical counterpart that places 4U 1556–60 at a distance of only about 700 pc, making it one of the closest X-ray binaries known to date. This close distance drastically alters what was previously assumed about the source.

Aims. We revisit 4U 1556–60 in light of the newly determined distance of 700 pc, reinterpreting its literature and presenting new X-ray and radio observations to better understand various characteristics of the system.

Methods. We investigated the optical spectra and luminosity and the X-ray spectral and timing properties, and we performed the first targeted radio observation for the source in 45 years. These can be used to infer binary and accretion properties from independent methods.

Results. We conclude that a scenario in which 4U 1556–60 is a candidate ultracompact neutron star X-ray binary at a distance of ∼700 pc is able to explain the observed properties of the source. It resides at a persistent X-ray luminosity of ∼2 × 10³⁴ erg s⁻¹, an unusual value for a typical X-ray binary, but similar to several ultracompact systems. The ratio of the X-ray to optical luminosity is very high, also suggesting a physically small accretion disk. The radio jet is undetected with a very deep upper limit of 3 × 10²⁵ erg s⁻¹, which is about 10³ times fainter than the expected black hole jet correlation, strongly indicating a neutron star accretor. The X-ray spectrum is dominated by a power law, and the X-ray timing properties are also consistent with observations of other very low accretion rate X-ray binaries. No spin or orbital periodicity are found in the X-ray data. Future observations, especially to determine its orbital period, will further aid in understanding 4U 1556–60.