A distant relative to GW170817, the first source identified to emit gravitational waves and light, has been discovered. Reported in the journal Nature Communications (arXiv.org preprint), the discovery of the new source, called GRB 150101B, was made using data from several ground- and space-based telescopes, including NASA’s Chandra, Hubble, Fermi telescopes, Neil Gehrels Swift Observatory and the Discovery Channel Telescope.
This image shows data from NASA’s Chandra X-ray Observatory (purple in the inset boxes) in context with an optical image of GRB 150101B from the NASA/ESA Hubble Space Telescope. Image credit: X-rays – NASA / CXC / GSFC / UMC / E. Troja et al; optical and infrared – NASA / STScI.
GRB 150101B was first reported as a gamma-ray burst detected by NASA’s Fermi Gamma-ray Space Telescope in January 2015.
This detection and follow-up observations at other wavelengths show the event shares remarkable similarities to GW170817, the neutron-star merger and gravitational wave source discovered by LIGO and Virgo observatories in 2017.
“It’s a big step to go from one detected object to two. Our discovery tells us that events like GW170817 and GRB 150101B could represent a whole new class of erupting objects that turn on and off in X-rays and might actually be relatively common,” said lead author Dr. Eleonora Troja, a researcher at NASA’s Goddard Space Flight Center and the University of Maryland at College Park.
GRB 150101B and GW170817 were most likely produced by the same type of event: the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet of high-energy particles. The jet produced a short, intense burst of gamma rays.
GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.
The apparent match between GRB 150101B 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 lasting a few days, and X-ray emission lasted much longer.
Their host galaxies are also remarkably similar: both are bright elliptical galaxies with a population of stars a few billion years old and displaying no evidence for new stars forming.
“We have a case of cosmic look-alikes. They look the same, act the same and come from similar neighborhoods, so the simplest explanation is that they are from the same family of objects,” said study co-author Dr. Geoffrey Ryan, a scientist at the University of Maryland at College Park.
In the cases of both GRB 150101B and GW170817, the slow rise in the X-ray emission compared to most gamma-ray bursts implies that the explosion was likely viewed ‘off-axis,’ that is, with the jet not pointing directly towards the Earth.
The discovery of GRB 150101 represents only the second time astronomers have ever detected an off-axis short gamma-ray burst.
While there are many commonalities between these events, there are two very important differences.
One is their location: GW170817 is about 130 million light years from Earth, while GRB 150101B lies about 1.7 billion light years away.
The optical emission from GB 150101B is largely in the blue portion of the spectrum, providing an important clue that this event involved a so-called kilonova (an extremely powerful explosion that not only releases a large amount energy, but may also produce important elements like gold, platinum, and uranium), as seen in GW170817.
The other important difference between GW170817 and GRB 150101B is that without gravitational wave detection, the study authors don’t 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,” said Dr. Hendrik Van Eerten, a researcher at the University of Bath.
“Our results are encouraging for finding more mergers and making such a detection.”
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E. Troja et al. 2018. A luminous blue kilonova and an off-axis jet from a compact binary merger at z = 0.1341. Nature Communications 9, article number: 4089; doi: 10.1038/s41467-018-06558-7
