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China's FAST telescope finds the lowest-mass double neutron star yet measured
Astronomers timing PSR J1856-0039 with China's FAST telescope found the lowest combined mass of any known double neutron star system. The orbit shrinks as general relativity predicts, and formation models now have a lighter pair to account for.
The Scientist · Science desk
What happened
- Researchers at the Chinese Academy of Sciences and Beijing's State Key Laboratory of Radio Astronomy and Technology reported the result in Physical Review Letters.
- The two stars complete an orbit every 2.36 hours, the second-shortest period among confirmed double neutron star systems.
- FAST observed the system in 17 sessions between 2020 and 2025, yielding 253 measurements of pulse arrival times.
- According to the researchers, the system ranks second among confirmed double neutron stars in the strength of its relativistic effects.
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Why it matters
- constraint Any model of how double neutron stars form and evolve now has to be able to produce a pair lighter than any previously weighed.
- capability Han says the pair has the highest potential of any for detecting frame-dragging, so further FAST timing could add a fourth relativistic effect to the three already measured.
- precedent Han's FAST survey has found about 900 pulsars; this one became a precision mass measurement after five years of timing, so other faint finds in that sample are candidates for the same treatment.
The masses come from pulse timing, and the design checks itself. By logging when each radio pulse reached FAST, the team followed both the pulsar's spin and its motion around its companion [7]. Shifts in those arrival times exposed three effects that general relativity predicts: the orbital period getting shorter, the orbit's closest point slowly turning, and the Einstein delay, a small timing offset [8]. The team used the theory to turn those effects into a mass for each star. It then compared the measured orbital shrinkage with the rate predicted for those masses [10]. Two unknown masses against three measured effects leave one measurement over, and that spare one is the independent check on the theory [1].
The stars go around each other about ten times a day [2]. "From these, we have determined the individual masses of both neutron stars and find that their combined mass is the lowest yet measured for any DNS system," JinLin Han, a co-author of the paper, told Phys.org [11].
It is also a faint target. Han put its mean flux density at about 0.1 mJy, varying from one session to the next [13]. "FAST's exceptional sensitivity enables high signal-to-noise ratio (S/N) detection of this relatively faint source," he said [18]. The timing record averages about 15 arrival times per observing session [3], collected from the first detection on May 4, 2020 onward [5]. Han said five years of monitoring produced precise measurements of all three relativistic effects [19].
Agreement with general relativity is only as tight as the error bar on the measured orbital decay. The Phys.org account does not report that uncertainty, the two masses, or the merger time the team estimated [15].
"In this paper, we present these results and discuss their implications for neutron star formation and binary evolution," Han said [16]. I'd be slower to carry the system into gravitational-wave population estimates. In my view a rate estimate needs more than one binary, and it needs this binary's merger time [15].
What to watch
- Independent timing of PSR J1856-0039 with another radio telescope, which would check the FAST mass solution on a separate instrument.
- Further low-mass double neutron stars from the FAST survey, which would turn one extreme system into a sample that population estimates can use.