- 3494 - KILONOVA - learning how stars explode? Scientists have learned a lot about neutron star mergers and kilonovae since the 1998 first discovery. We know they can create either a single massive neutron star or collapse into a blackhole.
--------------------- 3494 - KILONOVA - learning how stars explode?
- When two neutron stars collide, it creates a “kilo nova“. The event creates both gravitational waves and emissions of electromagnetic energy. In 2017 the “LIGO-Virgo gravitational-wave observatories” detected a merger of two neutron stars about 130 million light-years away in the galaxy NGC 4993. The merger is called “GW170817“, and it remains the only cosmic event observed in both gravitational waves and electromagnetic radiation.
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- When stars are very large they have so much gravity that their nuclear fusion burns all of their fuel and they collapse and explode. It is called a “supernova” when this occurs. Atomic elements are spread all over the universe and what is left behind is a core “Neutron Star“. Neutron because the electrons are smashed into the protons in all the atoms and they become neutrons.
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- Kilonova is a supernova creatred when two neutron stars collide and merge into an even larger explosion, an even bigger supernova. Astronomers have watched the expanding debris cloud from the kilonova for years. A clearer picture of what happens in the aftermath is emerging.
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- This is about the emergence of a new source of X-rays from the binary neutron star merger “GW170817”.
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- Over the years, astronomers have trained a whole suite of scientific eyes on the expanding cloud, uncovering more and more detail about these cosmic calamities, kilonovas.
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- GW170817 has allowed astronomers to observe the gravitational waves and electromagnetic radiation resulting from this merger. The gravitational waves tell researchers about pre-merger activity, and the electromagnetic observations tell them about the post-merger physical properties.
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- When two neutron stars merge, it produces a cloud of debris and a burst of light. The model for neutron star mergers involves synthesized radioactive nuclei that provide a long-term heat source for the expanding debris envelope.
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- The optical and infrared light in the kilonova comes from the decay of elements like platinum and gold created during the merger. When LIGO and Virgo detected gravity waves from GW170817, other telescopes detected the optical and infrared light hours later.
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- Only 10.9 hours after triggering the largest astronomical search in history, the “Swope” 1-meter telescope at the Las Campanas Observatory in Chile discovered GW170817’s afterglow. Four days later, the afterglows were dimming in brightness.
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- The Chandra X-ray Observatory was also watching. Chandra saw nothing at first, which was unusual. Scientists expect kilonovae to produce x-rays in jets of high-energy particles. Now scientists think there was a jet, but it wasn’t pointed at Earth.
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- Chandra eventually detected x-rays when the jets impacted the surrounding gas and dust, causing them to widen and slow down. Then later in 2018, the x-ray emissions declined again.
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- The x-rays have remained stable since the end of 2020. Scientists think there could be two explanations for the steadying of x-ray emissions:
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- On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer both detected gravitational waves from the collision between these two neutron stars.
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- Within 12 hours observatories had identified the source of the event within the lenticular galaxy NGC 4993. The associated stellar flare, a kilonova, is clearly visible in the Hubble observations. This is the first time the optical counterpart of a gravitational wave event was observed. Hubble observed the kilonova gradually fading over the course of six days, between 22 and 28 August 2017.
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- On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer detected gravitational waves from the collision between these two neutron stars. Within 12 hours, observatories identified the event’s source within the lenticular galaxy NGC 4993.
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- The associated stellar flare, a kilonova, is visible in the Hubble observations. This is the first time astronomers have observed the optical counterpart of a gravitational wave event. Hubble observed the kilonova gradually fading over the six days between 22 and 28 August, 2017.
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- The first explanation is that a shock is involved, akin to a sonic boom. When the cloud of debris from the kilonova slams into gas around GW170817, the material is heated. The temperature is enough to produce x-rays and that can account for the steady kilonova afterglow Chandra detected.
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- The second explanation is entirely different. It says that the neutron star merger collapsed into a remnant blackhole. Material falling into the blackhole is heated enough to emit x-rays, a known phenomenon around other blackholes.
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- These astronomers point out that they have never observed a kilonova afterglow nor accretion-powered emissions like this before. Further observations should determine the cause of the x-ray afterglow. Astronomers will continue to observe GW170817 in both x-rays and radio waves.
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- If the glow comes from the kilonova, the radio emissions should brighten in the coming months and years. But if the glow comes from material falling into a blackhole, then the x-rays should stay steady or decline rapidly, but there’ll be no radio emissions over time.
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- Measuring the time of the peak of the kilonova afterglow, which probed the ejecta dynamics independent of shock microphysics, would offer a unique opportunity to do calorimetry of the kilonova’s fastest ejecta.
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- This is important because it relates to whether or not the merger left a remnant blackhole. If there’s a high-velocity tail in the ejecta, it can create excessive x-ray emissions that argues against the prompt collapse of the merger remnant into a blackhole.
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- Astrophysicists know that blackholes emit electromagnetic radiation in x-ray wavelengths. The Chandra X-ray Observatory has imaged many of them.
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- On the other hand, that same ejecta might emit a constant, or declining, source of X-ray emission in the next thousands of days that is not accompanied by bright radio emission. This would show that the merger collapsed into a blackhole.
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- When LIGO detected the first gravitational waves in 2016, they opened a new window into the Universe. One hundred years before their detection, Einstein predicted them in his general theory of relativity.
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- Since the first detection, LIGO and Virgo have detected many more blackholes and neutron star mergers. The combination of gravitational wave detections and quick and enduring follow-up electromagnetic observations have confirmed some theoretical work, including discovering that kilonovae produce heavy elements.
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- The spectrum and flux evolution of the kilonova emission from GW170817 was in agreement with theoretical predictions, demonstrating that mergers of neutron stars are one of the major sources of heavy elements in our Universe.
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- In 2019, using data from the X-shooter instrument on ESO’s Very Large Telescope, researchers found signatures of strontium formed in the GW170817 neutron-star merger.
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- Scientists have learned a lot about neutron star mergers and kilonovae since the 1998 first discovery. We know they can create either a single massive neutron star or collapse into a blackhole.
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- We know the merger can create an extraordinarily powerful magnetic field that’s trillions of times more potent than Earth’s puny magnetic field and that they can make that field in milliseconds.
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- We know they can produce gamma-ray bursts and that kilonovae can synthesize heavy elements like strontium.
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- Observations of GW170817 are mapping an uncharted territory of the binary neutron star merger that have far-reaching theoretical implications.
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March 7, 2022 KILONOVA - learning how stars explode? 3494
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