Scientists have discovered an object in a cosmic collision that is denser than previously detected neutron stars but still far less dense than known black holes, challenging the accepted description of neutron stars and black holes, two of the many forms of massive, dying stars in the universe.
Physics professors Patrick Brady and听Jolien听Creighton and a group of 15 other researchers from 51猎奇鈥檚 Center for Gravitation, Cosmology and Astrophysics听contributed to the recent paper by the LIGO-Virgo Collaboration published in听The听Astrophysical Journal Letters听that investigated the collision.
The end game of massive stars often involves a smashup, called a 鈥渃ompact binary merger,鈥� that听is听so violent听it听produces听gravitational waves 鈥� ripples in space-time that can be detected by the LIGO听and听Virgo听detectors.
Now that these听mergers听can be detected,听scientists want to further听explore听them听using听quick follow-up听from telescopes听and instruments that听spot听forms of鈥痳adiation, such as visible light,鈥痳adio waves, gamma rays and鈥疿-rays.听This 鈥渕ulti-messenger鈥� investigation听is expected to reveal information that helps听physicists piece together听new knowledge about the universe.
In this recent paper, the researchers have detected a merger in which one object has a mass that defies the standard description. For decades astronomers have been puzzled by a gap that lies between the mass of neutron stars and black holes: The heaviest known neutron star is no more than 2.5 times the mass of our sun 鈥� or 2.5 鈥渟olar masses鈥� 鈥� and the lightest known black hole is about 5 solar masses.
This newly discovered object lies in this so-called 鈥渕ass gap.鈥�
In this conversation,听顿别别辫听颁丑补迟迟别谤箩别别, a听51猎奇听doctoral researcher and a听co-author on the paper,听discusses the mass gap and听explains听the importance of multi-messenger astronomy in听determining听whether听more听objects exist in the mass gap.
Scientists don鈥檛 know if the object in the听cosmic听merger that听LIGO/Virgo found is a neutron star or a black hole. But, since it falls into the 鈥渕ass gap,鈥� does听迟丑补迟鈥痬别补苍听there could be neutron stars that are much听denser听than we thought?听
That is certainly a possibility. This observation means that the lighter object 鈥� if it鈥檚 a neutron star 鈥� is the heaviest one detected so far. Theoretically, neutron stars could have masses听of听about 3. However, scientists have only observed them with masses up to 2.5.听So, this would change the present belief of how heavy neutron stars can be.
The other possibility is the existence of a lighter black hole. It is believed that black holes have solar masses of more than 5, the upper limit of the mass gap. This also is challenged听in case this object is a black hole.
What will scientists have to see in the future to鈥痵olve the mystery of whether this鈥痮bject was a neutron star or black hole?
Observations听with electromagnetic听radiation听are a confident way to solve this. If the neutron star was torn apart by the black hole, the dynamics of the remnant matter could produce light, similar to the neutron star merger GW170817 in 2017. Observation of light would mean this was a neutron star. But unfortunately, this particular event was pretty far away, so the light may be too dim.
There is also the possibility that the neutron star was not torn apart but swallowed by the black hole, in which case there is no light. In the future, if we have a better understanding of what a neutron star is made of听鈥撎齣ts 鈥渆quation of state鈥� 鈥撎齱e could听rule out black holes with听more听confidence.
Why is the mass of these objects important? What have we learned about super-massive objects by studying their movements in space?
The masses are a part of the whole story. Compact objects are interesting because they are probes of both extreme matter and extreme gravity.鈥疶hese conditions cannot be reproduced in an Earth-based lab. Therefore, dense compact objects are high-energy astrophysical laboratories. We need super-dense objects colliding in space to produce enough gravitational waves detectable by LIGO. Thus, binary compact object mergers are the most promising source in this regard.
Also, these compact object mergers produce not only gravitational waves听but also other electromagnetic messengers like gamma-ray bursts and fast-optical transients,听called听kilonovae. Currently, there is a global focus on听multi-messenger听observations听because each method provides different听information.
What aspect of this multi-messenger research effort do you work on?听
We stand to learn the most from听multi-messenger observations, but the duration of these events in the sky听is very short. We need rapid communication between LIGO and other partner observing facilities to coordinate.
I鈥檓 involved with the LIGO low-latency group, where I develop scientific tools that predict if a merger is worth following up. I also develop automated software infrastructure within LIGO to relay the discovery information to astronomers for follow-up.听Besides LIGO,听I听work with the Zwicky Transient Facility collaboration, an optical telescope听in California听that follows up gravitational-wave events to hunt the听elusive听kilonova.听With Zwicky, I work on the sensitivity of such telescopes and develop strategies for follow-up.
Besides Brady, Creighton and Chatterjee, 51猎奇 co-authors on the paper include:听 Caitlin Rose, Ignacio Magana Hernandez, Adam Mercer, Duncan Meacher, Mike听Manske,听Xiaoshu听Liu, Patrick听Brockill, Warren Anderson, Siddharth Mohite,听Shaon听Ghosh,听Shasvath听Kapadia, Tanner Prestegard, Sinead Walsh and Tom Downes.听
