"The Next Frontier" --LIGO May Be On the Verge of Announcing Major Gravitational Wave Discovery from Exotic New Source
At 6:07 today, August 25, LIGO, the Laser Interferometer Gravitational-Wave Observatory in Louisiana and Washington state, tweeted: "A very exciting @LIGO - VIRGO observing run is drawing to a close."
“It would be an incredible advance in our understanding,” says Stuart Shapiro, an astrophysicist at the University of Illinois at Urbana–Champaign.
Public records show that telescopes around the world including the Hubble Space Telescope have been looking at the same galaxy, NGC 4993, since last week, but astronomers caution that they could have been picking up signals from an unrelated source.
LIGO has detected gravitational waves — ripples in the fabric of space-time — three times so far emerging from colliding black holes. Since the initial discovery, scientists have been hoping to detect ripples from another cosmic cataclysm, such as the merger of neutron stars, remnants of large stars that exploded but were not massive enough to collapse into a black hole.
Such an event should also emit radiation across the electromagnetic spectrum — from radio waves to γ-rays — which telescopes might be able to pick up.
On August 16, reports Nature, astronomer J. Craig Wheeler of the University of Texas at Austin began the public rumour mill when he tweeted, "Rumor of exciting new LIGO source." Wheeler followed up two days later, tweeting:
“New LIGO. Source with optical counterpart. Blow your sox off!”
An hour later, reports the journal Nature, astronomer Peter Yoachim of the University of Washington in Seattle tweeted that LIGO had seen a signal with an optical counterpart (that is, something that telescopes could see) from a galaxy called NGC 4993, which is around 130 million light years away in the southern constellation Hydra. “Merging neutron-neutron star is the initial call”, he followed up.
If gravitational-wave researchers saw a signal, adds Nature, it is plausible that they could know very quickly whether it emerged from colliding black holes or neutron stars, because each type of event has its own signature, even though data must be studied carefully to be more precise about an event’s origin.
It’s also possible that LIGO’s sister observatory Virgo in Pisa, Italy, which has been helping LIGO to hunt for gravitational waves since August, might have spotted the event, which would give researchers more confidence about its source.
Virgo has an average sensitivity for neutron-star mergers of only 25 million to 27 million parsecs, but in some regions of the sky, it can see farther, up to 60 million parsecs away, physicist Giovanni Losurdo, who led the detector's upgrade work, told Nature.
Both Wheeler and Yoachim declined to comment, and Wheeler later apologized on Twitter. “Right or wrong, I should not have sent that tweet. LIGO deserves to announce when they deem appropriate. Mea culpa,” he wrote.
Astronomers, who do not want to be identified, say that NASA’s Fermi Gamma-ray Space Telescope is rumoured to have spotted γ-rays emerging from the same region of sky as the potential gravitational-wave source, which would be consistent with expectations that neutron-star collisions may be behind the enigmatic phenomena known as short γ-ray bursts (GRBs), which typically last a couple of seconds and are usually followed by an afterglow of visible light and sometimes, radio waves and x-rays, lasting up to a few days. But even if the Fermi telescope has seen a GRB, it would not be able to pinpoint its origin with high precision, astronomers caution. A senior Fermi member declined to comment on this possibility.
Evidence soon emerged of telescopes turning to look at NGC 4993 after an alert, adds Nature: On 22 August, a Twitter feed called Space Telescope Live, which provides live updates of what the Hubble Space Telescope observes, suggested that a team of astronomers was looking at a binary neutron-star merger using the probe’s on-board spectrograph, which is what astronomers would normally use to look at the afterglow of a short GRB.
The Hubble tweet has been deleted, but public records confirm that multiple teams have used the Hubble Space Telescope over the last week to examine NGC 4993, and state as their reason that they are trying to follow up on a candidate observation of gravitational waves.
Additional publicly available records from other major astronomy facilities — including the European Southern Observatory’s Very Large Telescope and the world’s premiere radio observatory, the Atacama Large Millimeter/submillimeter Array (ALMA), both in Chile — show that those also targeted NGC 4993 on 18 and 19 August.
Today, August 25, LIGO and Virgo will end their current data-collecting run and posted its top-level update, saying: “Some promising gravitational-wave candidates have been identified in data from both LIGO and Virgo during our preliminary analysis, and we have shared what we currently know with astronomical observing partners. We are working hard to assure that the candidates are valid gravitational-wave events, and it will require time to establish the level of confidence needed to bring any results to the scientific community and the greater public. We will let you know as soon we have information ready to share.”.
David Shoemaker, a physicist at the Massachusetts Institute of Technology who is LIGO’s spokesperson adds:"It will take time to do justice to the data, and ensure that we publish things in which we have very high confidence."
Image credit: LIGO/Aurore Simonnet/Sonoma State University