Gamma-ray burst and its visible afterglow seen simultaneously with gravitational waves from the merger of two neutron stars
Simultaneous detections of a short Gamma-Ray Burst by Fermi/GBM, INTEGRAL, and as gravitational wave by LIGO/Virgo, followed by MPG/GROND multi-wavelength observations heralds the start of a new era in astronomy
On 17 August 2017, for the first time, light – or electromagnetic radiation to be precise – has been associated with a gravitational wave signal. The GBM instrument aboard the Fermi satellite was the first to trigger on a new source at highly energetic gamma rays, which was soon confirmed by the INTEGRAL satellite. Subsequent observations by GROND later captured a fading transient at optical and near-infrared wavelengths. Scientists at the Max Planck Institute for Extraterrestrial physics contributed to this exciting discovery from the start and confirmed its origin as being the merger of two neutron stars. This discovery confirms that such a merger is indeed the precursor to short gamma-ray bursts and that the explosion following this merger – a so-called kilonova – is the source of heavy elements in the Universe.
On 17 August 2017 at 12:41:06 UTC the Fermi Gamma-Ray Burst Monitor (GBM) on board the Fermi satellite detected and triggered on a short Gamma-Ray Burst, which received the catalogue number GRB 170817A.
“At first, the GBM detection of that particular Gamma-Ray Burst did not really seem extraordinary,” recalls Andreas von Kienlin, scientist at the Max Planck Institute for Extraterrestrial Physics (MPE) the astronomer on duty at the time the signal arrived, and one of the members of the team at MPE who helped build the GBM instrument. “We routinely detect GRBs – about four or five per week.” What he didn’t know was that, at about the same time - on 17 August 2017 at 12:41:04 UTC to be precise - the Advanced LIGO and Virgo gravitational wave observatories had seen a distinctive gravitational-wave signal from the exact same event, which was revealed after the rapid-response team manually inspected the data from all three detectors following the GRB alert.
“When we learned that there was a trigger for gravitational waves at the same time, we immediately knew that this was a historical event” says von Kienlin.
For the first time ever, both gravitational waves and electromagnetic radiation has been detected from the same, cataclysmic astronomical event. Also for the first time, the source of the gravitational waves was not two colliding black holes but two neutron stars, remnants of massive stars after the gravitational collapse at the end of their stellar evolution.
A further surprise is that the gamma-rays and the gravitational waves were not detected at exactly the same time:
“The time delay of two seconds between the gravitational wave blip and the gamma-ray burst is a great new constraint for theories of what happens after two neutron stars collide,” adds von Kienlin. “These and the other follow-up observations provide us with unique insights into the physics in and around this event.”
High-energy radiation from the merger was also seen by INTEGRAL, another gamma-ray mission featuring an MPE instrument. The Anti-Coincidence Shield of the SPI Spectrometer on-board INTEGRAL saw the immediate signal and INTEGRAL then carried out pointed follow-up observations with all of its instruments. The probability that the temporal and spatial association of the GRB and the gravitational wave signal happened by chance is 1 in 200 million – you’d have a better chance to win the lottery.
“The signal observed with INTEGRAL was not very bright, but we can confirm the Fermi/GBM event with a completely independent gamma-ray detection,” states Roland Diehl, scientist at MPE and Co-Principal Investigator for INTEGRAL/SPI. “This puts the association of the gamma-ray burst with the gravitational signal on firm ground and means that we can associate a short gamma-ray burst for the first time clearly with a neutron star collision.”
Both Fermi/GBM und INTEGRAL/SPI with its ACS are instruments designed and built by MPE in collaboration with partners in the USA and France respectively. In addition, the institute was able to contribute to the optical follow-up observations with its GROND instrument attached to the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile. The simultaneous gravitational wave and gamma-ray signal led to an alert to observers around the globe about 40 minutes after its detection, and less than 11 hours later a new, bright optical source was seen by several teams in the lenticular galaxy NGC4993, about 130 million light-years from Earth. No object had been seen at this location previously. This means that a new optical source, a Gamma-Ray Burst and a gravitational wave detection all originate from the same source: the merger of a neutron star binary system.
Neutron stars are the extremely dense remnants of massive stars, which collapsed under gravity as they ran out of fuel for the nuclear fusion that fires in their interiors. The merger of two such neutron stars (or one neutron star and a black hole) has been proposed to explain short Gamma-Ray Bursts – in theory. The joint observations of characteristic signals from the event on 17th August now confirm this definitively.
Subsequent observations at optical, infrared as well as X-ray and radio wavelengths completed the picture and provide evidence that the transient was powered by the radioactive decay of heavy elements formed during the cataclysmic event. Continuous GROND follow-up of the optical and near-infrared transient for two weeks after the event proved pivotal in understanding the synthesis of heavy atomic nuclei in the neutron star-neutron star merger. Compact binary mergers of this type have long been hypothesised to be the prime source of heavy elements in the Universe, and the unique GROND dataset provided the observational data needed to test these models.
“We were able to identify clear signatures of the transient emission that was predicted by theoretical models: we saw the so-called kilonova,” says Janet Ting-Wan Chen from MPE, who performed the observations with GROND and is second author on the paper presenting the kilonova discovery.
“This first concordant observation of an astronomical source with both electromagnetic and gravitational waves gives us a detailed picture of this event from three minutes prior to the merger to several weeks later,” says Jochen Greiner, the MPE scientist responsible for the building of GROND and organizer of the Fermi symposium this week. “Over and above this single event, it marks the start of a new era in astronomy.”