[chris (OP)]

This is the leader from an article published today by Discover magazine:

Four billion years ago, an immense galaxy with a black hole at its heart spewed forth a jet of particles at nearly the speed of light. One of those particles, a neutrino that is just a fraction of the size of a regular atom, traversed across the universe on a collision course for Earth, finally striking the ice sheet of Antarctica last September. As it hit, a neutrino detector planted by scientists within the ice recorded the neutrino’s charged interaction, causing a blue flash of light that lasted just a moment. The results are published today in the journal Science.

I would appreciate the insights of the learned members of the forum to help me to understand more about this.

1) How do they know where it came from?

2) How do they know how old it is?

3) Why is this important?

[evan_au]

I can't find the article you mentioned, but this event might be like the "Big Bird"* event, at the Ice Cube detector at the South Pole.

1) How do they know where it came from?
Modern neutrino detectors are able to estimate the original path of the neutrino from the the Cherenkov radiation emitted by the debris (in the unlikely event that a neutrino interacts with an atomic nucleus in the detector).

They can use optical and radio telescopes to look where this neutrino came from, and sometimes manage to tie it to some observable event in the sky.
https://www.skyandtelescope.com/astronomy-news/where-big-bird-neutrino-may-come-from-0505201623/

2) How do they know how old it is?
If they can pin it to an outburst in a galactic black hole, they can measure the galactic redshift, and estimate the distance.

3) Why is this important?
Massively energetic neutrinos must originate in spectacularly energetic events. Some of these events are so energetic that astronomers are baffled about where the energy is coming from.

Neutrinos can escape from the center of imploding stars, and escape from the vicinity of a black hole shrouded by dust clouds in the center of a galaxy, telling us of events that would otherwise be invisible to astronomers.

They can travel farther across the galaxy than gamma rays, since gamma rays interact with the intergalactic medium, and slowly lose energy.
See: https://www.scientificamerican.com/video/big-bird-neutrino-is-linked-to-bright-blazar/
https://en.wikipedia.org/wiki/IceCube_Neutrino_Observatory#Results

*Apparently, they are naming spectacular events after Sesame Street characters....

[chris (OP)]

Thanks @evan_au

This is the reference to the paper: http://science.sciencemag.org/content/early/2018/07/11/science.aat2890.full

There's also a nice summary of the discovery by Science journalist Dan Clery: http://www.sciencemag.org/news/2018/07/ghostly-particle-caught-polar-ice-ushers-new-way-look-universe

Interestingly, they do say that the MAGIC telescope responded to IceCube's alert - when it saw the neutrino - and tracked a coincident burp of gamma rays to Blazar TXS0506+056; this suggests that a supermassive black hole there developed the cosmological equivalent of indigestion as it was consuming matter and belched out a jet of high energy matter, producing the "shock" that ultimately generated the neutrinos.

What fascinates me is that people are saying that the neutrinos can give us information about the structures in space that produced them because their poor interaction with other entities means that they travel vast distances unfettered.  Would you go along with that? Is there sufficient resolution in the limited number of daily detections to determine much of use?

[evan_au]

Thanks for the references.

their poor interaction with other entities means that they travel vast distances unfettered

I agree with that: neutrinos can peer into times and places that are inaccessible to other astronomical tools.

Astronomers are still excited by the cosmic microwave background radiation, which was emitted about 300,000 years after the Big Bang.

If one day we are able to detect the cosmic neutrino background radiation, we will be able to see events that happened  perhaps 1 second after the Big Bang.
See: https://en.wikipedia.org/wiki/Cosmic_neutrino_background

Is there sufficient resolution in the limited number of daily detections to determine much of use?

Nuclear weapons tests are also infrequent, high-energy events, which are (fortunately) rare enough that weapons control experts are very excited to learn everything possible about every individual event.

Similarly, astronomers get very excited about one big neutrino detection, or one gravitational wave event; it's like winning the lottery! Even with this sparse data, they are able to tell a lot about fundamental physics and the statistical distribution of energetic events in the universe.

The first successful solar neutrino detector was a large vat of dry-cleaning fluid. As I recall, they flushed the tank about every 3 months, and managed to find 2 or 3 atoms that had interacted with a neutrino. But even this was enough to hint at new physics: that neutrinos have mass.
See: https://en.wikipedia.org/wiki/Solar_neutrino_problem

The main benefit of these rare detections is that astronomers and research sponsors are willing to fund bigger detectors. These can change sparse events into a flood of events that starts to produce a more coherent picture.

Ice Cube is an example of this - with a cubic kilometer of ice as the detector, it is extremely sensitive, picking up almost 20,000 neutrinos per year (unfortunately, many of them are "local", produced in Earth's atmosphere by cosmic rays).

The number of supernovas expected to occur in our galaxy every century is far higher than the number that humans have actually observed. Perhaps they are hidden behind dust clouds in the plane of the galactic disk, or they collapse into a black hole before they emit the telltale flash? But I expect that we will have a clear detection of the next supernova that happens in our galaxy, even if we can't see it with the naked eye.

could mark the founding event of neutrino astronomy

I think that this title has already been snaffled, by the detection of a supernova in the Large Magellanic Cloud in 1987, which was observed simultaneously by 3 neutrino observatories, counting a total of 25 neutrinos.

With 25 neutrinos from a single event, that is enough to point a finger right at the source.
See: https://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions

[evan_au]

Is there sufficient resolution in the limited number of daily detections to determine much of use?

In some ways, the situation with neutrino telescopes is similar to the situation in the early days of cosmic ray detectors.

Researchers investigating high-energy cosmic rays (>10¹⁸ eV) have the problem that these events only occur about once per century per square kilometer.

The cosmic ray early detectors were less than a square kilometer - but they sparked enough interest to expand the detectors significantly. The Pierre Auger observatory now covers around 3000 square kilometers, with the expansion focussed on those rare, high-energy events. It can measure the energy and direction of the incoming cosmic rays.

There are plans that could expand the ice-cube detector to make it more sensitive to those rare high-energy neutrino  events.

See: https://en.wikipedia.org/wiki/Pierre_Auger_Observatory

neutrinos can give us information about the structures in space that produced them because their poor interaction with other entities means that they travel vast distances unfettered.

The incidence of cosmic rays tells us something about the distribution of high-energy events in the universe.
- However, they interact with the diffuse interstellar medium, which reduces their energy the farther they travel.
- They are also electrically charged, which means they will be diverted by interacting with magnetic fields of the Sun and the galaxy. So the direction of arrival does not tell us much about the direction of the source, which means we can't locate the source or estimate its distance.

So despite the rare detection of high-energy neutrinos, they can tell us a lot about the sources - their energy at the source, and the direction of the source. This is very valuable to astronomers.

And we can always expand the size of the neutrino detectors...