Following the February discovery, a second signal was found by the same LIGO team in the ‘noise’, by looking for a pulse of waves of increasing frequency in the two detectors. This pulse is longer (~55 waves compared with ~10 previously) and the frequency change (‘chirp’) indicates binary components of smaller mass (total ~20M⊙ compared with ~60M⊙).
The new LIGO paper again asserts coalescing Black-Holes is the only explanation, saying the masses are higher than the maximum for canonical neutron stars. The last stage ‘ringdown’ signal, which could distinguish the form of the merging object, is minor and well below detection threshold.
It retains this position as originally agreed by its thousand authors, despite some critical voices eg. in Physics World of 29th April reporting Vitor Cardoso on the gravastar interpretation “Are ‘gravastars’ mimicking…“. That it’s vital to detect the ringdown part of the signal is stressed by a leading expert Remo Ruffini (“What we can really infer from GW150914?“). B. Sathyaprakash of Cardiff’s LIGO team admitted (to Physics World) “Our signal is consistent with both the formation of a black hole and a horizonless object – we just can’t tell.” ‘Horizonless’ models of super-compact objects include gravastars and do not suffer from the infinite time dilution at black-hole horizons:
The new LIGO paper shows weakness in stating the only alternative binary components are neutron stars and not admitting to excluding the ‘exotic’ alternatives – lest people point out that Black Holes are ‘exotic’. It’s weak also in not admitting that the ‘ringdown’ signal of black-hole coalescence is below their detection threshold.
The statement in mid-May by Ruffini and colleagues thus remains unchallenged:
“… the signal around 150 Hz occurs just at the limit of the sensitivity of LIGO… not sufficient to determine the astrophysical nature of GW 150914, nor to assess that it was produced by a binary black-hole merger leading to a newly formed black-hole.”
NOTE The topic ‘mentors’ of physicsforums didn’t like me questioning the Black-Hole interpretation. They first stopped an informative exchange https://www.physicsforums.com/threads/in-ligos-pulse-how-much-comes-from-bh-merging-inspiraling.865876/ on grounds of unpublished ‘personal speculation’. When I showed Cardoso (and others) had already published, they allowed the new thread https://www.physicsforums.com/threads/was-the-ligo-team-over-hasty-to-claim-black-holes-confirmed.872264/, citing Physics World and Ruffini’s new paper on arXiv, but then blocked me from continuing the discussion. It smells of a conspiracy to give LIGO prizes for discovering black-holes, when there’s no such ‘discovery’ – LIGO’s gravitational pulse indicated the spin-down and merging of two compact bodies in a binary system. Unexpectedly massive bodies indeed, at 10-30 times the Sun’s mass M⊙, something for theorists to explain.