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All in the family: Kin of gravitational-wave source discovered

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Posted October 18, 2018

In October 2017, astronomers excitedly announced the first detection of electromagnetic waves, or light, from a gravitational wave source. Now a year later astrophysicists, including from the University of Bath, have discovered a cosmic relative to that historic event.

Both events are cosmic explosions, or ‘gamma-ray bursts’, triggered by the merging of two neutron stars and providing brief glimpses of a moment where the laws of physics are pushed to extreme limits.

Cosmic look-alikes: This image shows data from NASA’s Chandra X-ray Observatory (purple in the inset boxes) alongside an optical image of GRB150101B from the Hubble Space Telescope. The field of view from Chandra is outlined as a box on the Chandra image.

Follow-up analysis of a gamma ray burst detected in 2015 shows that the event, called GRB150101, shares remarkable similarities to the source discovered in 2017. The latest study concludes that these two events may be directly related.

Dr Hendrik van Eerten, a member of the University of Bath’s Astrophysics Group, was a part of an international team studying the GRB150101 event and the 2017 event, known as GW170817.

He said: “We have been remarkably lucky to find archival data of such quality that we now are able to interpret it using what we learned from last year’s neutron star merger observations: if it had not been for the initial confusion caused by an unrelated nearby source of X-rays during the 2015 discovery, instruments would not have been pointed at this part of the sky for as long as they have.”

The discovery was made using data from telescopes including NASA’s Chandra X-ray Observatory and Neil Gehrels Swift Observatory, the NASA/ESA Hubble Space Telescope (HST), and the Discovery Channel Telescope (DCT).

“It’s a big step to go from one detected object to two,” said Dr Eleonora Troja, lead author of the study from NASA’s Goddard Space Flight Center in Greenbelt, MD. “Our discovery tells us that events like GW170817 and GRB150101 could represent a whole new class of erupting objects that turn on and off and might actually be relatively common.”

Dr Troja and her colleagues think both GRB150101B and GW170817 were most likely produced the same way: by the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet, or beam, of high-energy particles. The jet produced a short, intense burst of gamma rays, a high-energy flash that can last only seconds. GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.

The apparent match between GRB150101 and GW170817 is striking: both produced an unusually faint and short-lived gamma ray burst, and both were a source of bright, blue optical light and long-lasting X-ray emission. The host galaxies are also remarkably similar; both are bright elliptical galaxies with a population of stars a few billion years old and display no evidence for new stars forming.

“We have a case of cosmic look-alikes,” said co-author Dr Geoffrey Ryan of the University of Maryland, College Park. “They look the same, act the same and come from similar neighbourhoods, so the simplest explanation is that they are related.”

In the cases of both GRB150101 and GW170818, the explosion was likely viewed “off-axis,” that is, with the jet not pointing directly towards the Earth. The discovery of GRB150101 represents only the second time astronomers have ever detected an off-axis short gamma-ray burst.

While there are many commonalities between GRB150101 and GW170817, there are two very important differences. One is their location. GW170817 is about 130 million light years from Earth, while GRB150101 lies about 1.7 billion light years away. Even if Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) had been operating in early 2015, it probably would not have detected gravitational waves from GRB150101 because of its greater distance.

“The beauty of GW170817 is that it gave us a set of characteristics, kind of like genetic markers, to identify new family members of explosive objects at even greater distances than LIGO can currently reach,” said co-author Luigi Piro of National Institute for Astrophysics in Italy (INAF).

The optical emission from GB150101 is largely in the blue portion of the spectrum, providing an important clue that this event is a so-called kilonova, as seen in GW170817. A kilonova is an extremely powerful explosion that not only releases a large amount energy, but also produces important elements like gold, platinum, and uranium that other stellar explosions do not.

It is possible that a few mergers like the ones seen in GW170817 and GRB150101 had been detected before but had not been identified with other telescopes. Without detections at longer wavelengths like X-rays or optical light, GRB positions are not accurate enough to determine what galaxy they are located in.

The other important difference between GW170817 and GRB150101 is that without gravitational wave detection, the team does not know the masses of the two objects that merged. It is possible that the merger was between a black hole and a neutron star, rather than two neutron stars.

“We need more cases like GW170817 that combine gravitational wave and electromagnetic data to find an example between a neutron star and black hole. Such a detection would be the first of its kind. Our results are encouraging for finding more mergers and making such a detection.”

Source: University of Bath

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