In May 2023, the LIGO-Virgo-KAGRA collaboration’s fourth observing run brought about an exciting discovery: the mass-gap event GW230529. A mass-gap event happens when one, or both, of the objects in the gravitational wave-emitting binary falls between the upper limit of mass for a neutron star (roughly three solar masses), and the lower limit of mass for a black hole (roughly five solar masses). These events are intriguing because we do not know the nature of the mass-gap object: are they a heavy, unstable neutron star? Or perhaps an unusually light black hole?
Usually, as was the case for GW190814, it is the secondary, lighter object in the binary which falls inside this mass gap and merges with a black hole. However, GW230529 intrigued the community because here it was the heavier object in the mass gap, and the secondary was a neutron star. It seems to be almost certainly a neutron star-black hole (NSBH) merger, which are always intriguing events for follow-up due to their rarity. Among several possible explanations for how this event came about––from exotic stellar physics to a chain of mergers preceding this one––an alternative explanation has also been proposed: gravitational lensing.
Gravitational lensing magnifies events, and therefore makes them appear closer than they actually are. Because of this, if we don’t know that an event is lensed (and thus estimate the distance to the event wrong), it will also look heavier than it is in reality. For this reason, regular binary neutron star (BNS) mergers can look like heavier events, and fall into the mass-gap. Mass-gap events are therefore of particular interest to the lensing community to look into: is GW230529 a lensed BNS event?
Finding out whether a single event is lensed is difficult, but we look for certain signatures in the signal, as well as check against the expected statistics of lensing. So what’s did we find?
Statistics
There are a great many population models about the masses and distribution of these types of events, as well as the rate of lensing of such events. To begin with, lensing is already very rare: typical estimates would say that only 1/1000 events are lensed, and an event such as this would need a fairly high magnification to be observed at the masses we see. Also, higher magnifications would cause the secondary object to become smaller than the mass of the Sun, which is also very unlikely! So, to remain consistent with both observations and statistics, the probability that this event is lensed just based on our knowledge of GW event populations is low.
Signatures
BNS events differ from binary black hole (BBH) events in a key manner: BNSs can experience deformation due to the gravity of their companion. This effect is called tidal deformation. Since this tidal effect does not depend on the redshift (and therefore, the distance) to the event, estimating the distance to the event incorrectly does not change the observed tidal deformability. We can measure the deformability from the GW itself, but also calculate it from the masses––and if we have estimated the masses wrong in the case of lensing, we would see that the values do not match as they should. This could be an indicator of lensing.
We can also search for signatures from lensing itself. Sometimes, lensing can cause a phase shift in the image known as a Morse phase shift. We can re-analyze the GW and search for this shift. There is no evidence of such a shift in the analyses. Since strong lensing can cause multiple images of the same event, we can search for these other counterparts in the data; because of the need for high magnifications to match this event to a BNS, we would expect other images to be very close to the event in time; unluckily, no such corresponding images were found in our data. We can also test for microlensing: signatures of lensing from masses too small to cause strong lensing, that leave signs on the waveform. Similarly, there’s no evidence for any microlensing signatures, either.
The verdict?
While we may not yet have a simple explanation for GW230529, it is unlikely that lensing is the answer. Based on our understanding of astrophysical populations, of lensing statistics, and our searches for specific lensing signatures, we do not find any convincing evidence that would indicate lensing. You can read the complete analyses here. It is therefore unlikely that GW230529 is a BNS simply dressed-up as a mass-gap event, and instead remains an intriguing event in many other ways––the search continues!