I don't know, but . . .

The headline read "Rare glimpse of exploding star." The AP story (Adithi Ramakrishnan penned it) was about supernova 2021yfj, whose spectra is dominated by silicon, sulphur, and argon (not hydrogen, helium, carbon, or oxygen). This seems to imply that the outer layers of the supernova had been stripped away before it blew--which is odd, but lets us know what was underneath them.

The AP story says it is "located in our Milky Way galaxy." That sounded very interesting--IceCube looks out for neutrinos from supernovas too. Most of a supernova's neutrinos aren't very energetic ones, but they make so many that the detector should see a significant diffuse glow in the ice--provided the supernova is close enough. At least within our galaxy close enough. Did they see it?

Wait, 2021yfj? 4 years ago? I'd have heard something about this years ago, whether yes or no.

Yes. AP/Ramakrishnan screwed up. Estimated distance over 600 mega parsecs. Not anywhere nearby--200 billion light years. Our galaxy is only 100,000 across.

?

Too bad. Still, and interesting observation.

This is cool. A research team discovered that a blazar's jet cone is pointed almost directly at us--we can see inside. The image above is their calculation of the magnetic field direction based on the polarization of light. Different regions along the jet are thought responsible for producing all manner of radiation, from radio waves to neutrinos. When such an energetic (Doppler shift of 30?) beam strikes gas clouds, the collisions produce new particles (e.g. pions), which when they decay produce neutrinos--which IceCube has detected coming from this blazar.

SciTech Daily has an article on ALMA's work trying to locate dark matter. The Figure 1 doesn't seem to appear in the paper itself. The figure looks very odd. The caption says it depicts calculated fluctions in dark matter density--it looks grid-like. I assume there's more elsewhere. The figures in the paper don't look quite so regular. Regularity like that of Figure 1 makes me suspicious--is it an artifact of their procedure?

Unfortunately I'm not familiar with the tools described in the paper, so I can't guess.

If their work pans out, they're getting much better resolution than before thanks to a more sophisticated analysis. I'll bet a number of the MOND people are already all over it, so we should find out soon.

I've always liked meteors. Tonight as we lay out on a baseball field in the shadow of the light pole, one came so straight at us that it just looked like a star that brightened and then vanished. I've written about the one that I "heard"--it was almost straight at me too. From little squibs that you aren't sure you saw to bright green slashes across the sky--it's fun to watch. Standing leads to a crick in the neck, though--lie back.

I think my favorite was when we were camping at Governor Dodge park, talking to a couple and their daughter about meteors. The daughter had never seen one and didn't know what they were, and the parents weren't quite sure either--and just as we spoke one lit up the sky in front of the daughter and me. She was so excited: "Was that a meteor!?" Perfect timing. It was behind the parents' heads--they missed it.

It's about 47M light years away, has an active black hole at the center, and gamma rays from the activity seem to be absorbed on the way out. But the absorbtion may enhance the emission of neutrinos. IceCube spotted them. This isn't the first time IceCube spotted a source--that would be TXS 0506+056--but it is the nearest.

Information on more sources is supposed to be released shortly--I'll update with the link when I get it.

UPDATE: Personal. I'm retired and not in the loop anymore, and I got the information pretty much when everyone else did. I have several questions--what sort of activity do other candidate galaxies show, are the other candidates different in gamma emission, are the rates consistent with the diffuse background (Olber's paradox--there should be lots more galaxies out there, and even ones we can't see should contribute some chance of neutrinos) (I didn't remember Edgar Allen Poe's contribution to astronomy.) And how much gas and dust does one need to block or transmute the gamma rays while not also blocking the high energy neutrinos--which interact much more strongly than low energy ones from the Sun or reactors?

Milky Way’s Graveyard of Dead Stars Found – First Map of the “Galactic Underworld” "A new study creates the first map of our galaxy’s ancient dead stars." OK, it's a Halloween themed clickbait headline, so maybe I shouldn't be surprised, but when I see the word "found" I expect some data. Nope, it's a model--"Here's where we expect to see old black holes and neutron stars" My first clue was the images of the Milky Way: edge-on and face-on. Neither of those exist; we just have models. Bummer. I expected better from scitechdaily.

Taken for what it really is, it's an interesting prediction of what the distribution should be--but unfortunately they predict that the relics would be spread out over an even larger area than the visible Milky Way, so they'd be harder to find that we might have hoped. It seems that a supernova that produces a black hole is apt to give it quite a kick--one was observed going fast enough to escape the host galaxy entirely.

and figure which way they're going.

Some years ago we were working with Sloan Digital Sky Survey on a software management design. I thought the project was quite cool, and if I hadn't been working on CDF I might have been interested in joining. "The Sloan Digital Sky Survey has created the most detailed three-dimensional maps of the Universe ever made, with deep multi-color images of one third of the sky, and spectra for more than three million astronomical objects." (read "galaxies and quasars", not stars) (When our sons were little, I tried to put glow in the dark stars on their ceiling to display the Southern Sky, but it never looked quite right and I gave up 1/4 of the way through.)

It looks like it might get a little complementary competition from WEAVE, a multi-object survey spectrograph that is supposed to take spectographs of as many as 1000 different stars in our galaxy in a single exposure. Of course that only gives the star's relative speed towards or away from us, and doesn't tell us anything about side-to-side motion, but that's important already (SDSS does that too). The transverse (side to side) motion we can measure with enough patience--a few hundred thousand years should be good enough to measure most of them. Stars in our galaxy are close enough that you don't worry about the expansion contribution to red shift (they can estimate gravitational contributions).

WEAVE is going after stars to try to get a handle on how things are moving in our Milky Way

Unfortunately the SDSS only gets part of the sky, thanks to the Milky Way getting in the way. They've over 4 million galaxies. WEAVE will have some blind spots thanks to dust clouds, and uncertain regions ditto, but they should be able to improve the current catalog a lot.

I wonder how a VR view of the local stars would look. You'd only see all the familiar constellations from one vantage point, of course--what would you see in the sky if you moved your POV elsewhere?

Would you put a bubblegram of the local stars on your desk? I wish I'd finished the ceiling, even though it didn't look right and faded decades ago...

You've seen the stories suggesting that an meteor air-burst destroyed Tel el-Hammam, which might have been Sodom, as well as trashing Jericho--though less emphatically. The combination of shattering, intense heat, and traces of odd minerals does seem to suggest something not within the military technology of the age. Pottery doesn't melt very easily--but have a look at what confined heat can do. People do worry about fire damage in tunnels and how to avoid/repair damage.

Tunguska would have been unpleasant to be underneath too. "Recent estimates place Tunguska-sized events at about one every thousand years." Um. The statistics are low, but spotting 2 in 4000 years, subject to a) not happening over the ocean and b) happening where people would notice, suggests to me a rate at least 10 times higher.

If you know nothing of astronomy: The first thing you notice (after day/night, of course) is the phases of the moon. That's pretty easy to figure out. Eclipses happen now and then--without a little math and a lot of measurements over time it's hard to predict those, and maybe they don't matter a lot, since everything seems fine afterwards. (I'm not talking about seasons and weather--you learn subtle signs about those independently of understanding the sky.)

The next thing you notice--if you are far enough north--is that the days aren't the same length all the time. At 5 degrees north or south, the difference is about an hour. That may or may not be very noticeable, but by 10 degrees it is 2 hours and I think that's enough of a difference that you can tell. The Sun isn't always in the same position when it sets, though if you live in a forest you might not be able to see that. If you live in the mountains it can be very obvious that sunset is at a different place on the distant slope than it was last month.

The ancient "observatory" closest to the equator seems to be the Chankillo towers at about 9.56 degrees away--and it is in an area with high hills and mountains.

If you live at the equator, unless someone has given you a reason to look, I don't think you'd spontaneously notice the changes with the sun. The planets are another matter, of course.

The Fermi detector has spotted a number of TeV blazars (very high energy photons) in the sky. Jamie Holder suggested that these bright-but-unseen-by-human-eyes objects deserved their own constellation. (BTW, TeV means Tera-electron-Volt)

How do you see magnetic fields when they're too far away to stick a probe in, and too weak to polarize light? Like, for example, the magnetic fields in the voids between clusters of galaxies...

One thing you can look for is gamma rays from blazars.

It turns out that light can scatter from charged particles, but it can also scatter off other light particles. The effect isn't nearly so strong, but it's there. And if a gamma ray of sufficient energy collides with one of the background microwaves, it can "pair-produce". It might just simply scatter, of course, in which case it loses energy and the microwave photon gains it. But pair-production contributes quite a bit. Now, a positron from that pair-production will often interact with an electron to produce a pair of photons, and the electron scatter off other matter and result in a photon too.

The first thing to notice here is that these new photons have lower energy than the original gamma ray (conservation of energy). The second thing to notice is that they'll be going in more or less the same direction as the electron or positron.

Now in the absence of a magnetic field, those electrons and positrons would keep going in pretty much the same direction as the original high-energy gamma ray. But if there is such a field, it will bend the particles into new directions, and the gamma rays that result from their interactions won't point back to that blazar.

Quanta has a description and picture.

So, when you look at the spectrum of gamma rays from a blazar, some of the low energy gamma rays shouldn't be there. The model say there should be more than we see.

Arguing from an absence isn't very robust--there might be some other reason for the missing gamma rays. The models can be wrong.

But if you look near a blazar, maybe you can see an excess of low energy gamma rays there, bent off into a halo around the star. It looks like you can.

The intergalactic magnetic fields they infer are tiny: of the order 10^-17 to 10^-15 Gauss. That turns out to be big enough to solve another: the universe seems to have expanded faster than you'd expect (this has nothing to do with inflation, which I don't want to try to defend).

So, a little halo of light (well, gamma-ray light) may be telling us something about a region we'll never be able to visit.

You may have heard of the DAMA dark matter search experiment. In the Grand Sasso lab they put 100kg of extremely pure scintillator, and looked for annual variations in the rate of signals.

The basic idea is: assume the dark matter in the galaxy is more or less like a fog that the orbiting stars sweep through. We know the direction our Sun is going, and we know the Earth's orbit. At one point in the orbit we're heading in the same direction as the Sun's motion, and at the opposite point we're heading away. Dark matter particles scattering off nuclei when we're going faster should involve a bigger kick than those when we're going slower--and that bigger kick should appear as more light. (Of course, if the dark matter is orbiting the galactic center at about the same speed as the Sun, you won't see much energy from the collisions--but you might still see a little variation.)

So, if on the average they see more light (energy deposited) at the times of the year when the Earth is moving faster wrt the galactic center, that might be evidence for dark matter.

They claim to see that. They've been more than a little reluctant to show their raw data, though, and the backgrounds must be huge (cosmic rays, radioactivity in the rock, residual impurities, etc).

The plots look great though. The problem is that they suggest interaction rates so high that other experiments should be able to see it too--and they don't.

There's an interesting paper from INFN (Buttazzo et al) that suggests that the annual variation (this story has some nice explanatory plots) is an artifact of the way they analyze their data. The background rate is too high to see anything clearly, so they average the background year by year, subtract that from the data on a every-few-months binning, and accumulate those residuals for a bunch of different years.

That seems OK, but what happens if the background rate grows with time? (e.g. impurities on the surface of the crystals migrate into the bulk volume, or helium migrating into the phototubes causing more after-pulsing)

Answer: you get residuals that look like a sawtooth: negative at the start of the reference period and positive at the end. If you use the DAMA start and end points and fit with a sinusoid, you get a peak at about the same place DAMA does.

Since we've never seen their raw data, or their analysis chain in any detail, we've no idea whether their background rate does grow over time.

There should be a simple check that lets them keep their data secret--use different start and end reference months, or accumulate over 2 years instead of 1 (you'll get worse statistics that way, but the sawtooth would then have a period of 2 years instead of 1. If the sinusoid remained with a period of 1 year and the same peak, even if it looked muddier than it does now, they might have something. If it had a 2-year variation (in the second case) or the peak appeared in a different month (for the first cross-check), they have zip.

I look forward to hearing what DAMA has to say.

A fireball over Kyoto in 2017 has been analyzed. Several sites got views of it, and they reconstructed its trajectory and think it came from "near-Earth asteroid 2003 YT1." As we saw with the Ryugu touchdown, asteroids can have lots of loose chunks, and I suppose that, something like comets though not as dramatic, bits can come off when they warm up in the Sun. Bits of me peel off when I spend too much time in Sun too.

It wasn't a huge chunk--they think only a few cm--but it's cool that they were able to track down where it probably came from. "The parent body 2003 YT1 could break up, and those resulting asteroids could hit the Earth in the next 10 million years or so, especially because 2003 YT1 has a dust production mechanism." (Dust production mechanism means stuff comes off it.)

When I was in high school, our senior class went to the highlands in Nimba county for--I suppose it was a field trip. We were there overnight, touristing about a bit. The evening dinner was noisier than I cared for, and the cat who walked by himself went outside for a little peace and quiet.

The stars were quite clear, and the air was a bit thinner. As I watched the sky, a meteor came almost straight down at me and burst.

I heard it. Simultaneously with the burst.

No, I did not have long frizzy hair, but there was a metal wire fence nearby.

Yes, you can hear them sometimes. With a track along the sky, some people hear crackling and popping. The topic is getting more serious attention, but the effect has been observed for years: China 817AD, England 1719AD.

According to this paper the meteor track can be a meter wide. That's potentially a lot of ionized gas. If an oxygen atom boils off the meteor and is ionized, it is initally moving at about 1.1 to 7.2 E7 meters/second. The Earth's magnetic field isn't very strong--2.5-6.5 microTesla--but with such a large speed the V cross B isn't negligable. The initial force is of order 2 to 75 E-19 Newtons, which given that the oxygen atom is only about 2.7 E-26kg, gives quite the acceleration.

The full MHD solution is way harder than I can solve on the back of an envelope, but that initial acceleration sounds pretty promising.

Suppose you have a cylinder of ionized gas moving at meteor speeds. It will slow down very rapidly, but in the meantime MHD forces will push the positive ions one way and the electrons the other. When they slow down enough, they will pull back together to recombine. If this second timescale is long enough, you should get a column of postive and a column of negative charges moving toward each other, which should create a low frequency radio pulse.

The only problem with this model is that it doesn't work: radio wave generation is rare!

You can detect meteor tracks from the way they reflect radio waves, but they usually don't seem to make any themselves. For starters, this naive model assumes no turbulance, which isn't even close to reality where gas gets mixed quickly. And it doesn't deal well with a bang at the end. But it gives an idea of what some of the forces are.

Now that I think of it, I wonder how much lightning you get along meteor tracks. They're far higher than thunderclouds, but the sprites are quite high too, and there have to be interesting electric fields with the sprites. Whether the same sorts of fields exist away from above thunderclouds I don't know.

You have heard that LIGO is a gravity wave detector, and has spotted signals consistent with black hole mergers. They've actually spotted a number of things, and with the Virgo detector running now, they're able to narrow down the window to tell where things come from.

A few days ago they spotted what looks like a neutron star merger. A neutron star has lots of ordinary matter to play with, so you expect a more brilliant explosion than with black holes--and there was a gamma ray signal within 2 seconds of that neutron star merger event. The merger proper is the last act of the event, and a lot of the fireworks come along with the initial splashing.

LIGO and Virgo and IceCube and FERMI and others send messages to each other when something interesting happens. Initially IceCube didn't get a good estimate, but on followup we found a neutrino track about 360 seconds before the merger. We issued our own bulletin.

6 minutes seems like a pretty long time, but even before the neutron stars touch they could be distorting each others' surfaces though tidal effects and blasting stuff around, or having dramatic magnetic field interactions (which could accelerate protons to wild speeds), or some other general relativistic interactions that I'm not thinking of yet. I don't have a good intuition for what those should look like.

That should generate lots of gamma rays too, but there could be so much stuff clouded around that a lot of the gamma rays might not get through. (Think of it like not seeing a fire because of the smoke.)

I suspect that more refined analysis will not make the time agreement closer, and that what will really help us decide what's going on is finding more events like it, and learning what the timing patterns are. So far the event rate seems high enough that we can be optimistic. But it will take a few years.

I forgot the charger for the laptop, so I had to try to think things out instead of looking them up.

What would meteors made of chunks of stuff like Bennu seems to be do when they hit the atmosphere? The soft conglomeration stuff should shatter easily and vaporize quickly, while the denser rock should punch through deeper. If the soft and hard stuff are made of different minerals, you should see different spectra at different points along the trail.

I tried to come up with a design to collect lots of data quickly, by having the telescope direction follow the radiant point, using a "cone" of mirrors inside to reflect concentric circles of light onto collector mirrors (of truly weird shape) and then onto diffraction gratings for on-line analysis. You'd get fairly coarse track resolution, of course. The whole business could be triggered by sensing whether there was a flash of light or not. (Too long a flash is an airplane or firefly, too short is an artifact or a coincidence. No flash means no meteor--just background.)

It could work. I think. There might be too much smearing of the image, though.

It turns out that this sort of analysis has already been done. If you shine the light on a diffraction grating, instead of getting a line in the sky you get a wide spectral smear, sort of like a feather--if you are lucky and the meteor's track is parallel to your grating. All it needs is lots of pictures and lots of patience and careful study of photographs of the steaks.

And different meteor showers do have different compositions. And there can be a difference in what appears at higher altitudes, as shown in the last image on that page.

Of course the atmosphere glows too when it gets hot, and there's black body radiation as well as the spectral lines, so it isn't trivial to parse out the details.

The Astronomy Picture of the Day of Bennu is very curious. It looks like a scree field, and it probably is scree all the way down. But the rocks are interesting. Much of the stuff lying around is clearly pitted and "space-weathered" and looks like concretions of smaller bits of stuff--the sort of thing you might expect from gentle gravity pulling dust and stuff together. But some show clean cleavage, as though a hard rock was cracked. If asteroids were built of accumulated cruft, like comets are supposed to be, all of it should be rough-looking.

Meteorites tell us that there are some hard bits in the asteroids--indeed some theories hold that a planetoid, big enough to have the heat and pressure to make rocks, broke up to make the asteroid belt--and all that frail-looking stuff suggests that a lot of the meteor gets splattered away high up in the atmosphere.

The meteor trail in the air is mostly glow from heated oxygen and nitrogen, but I'd expect some amount of light from the frying meteor as well. If (That's a big if. As it says at the link, usually you don't have a spectrometer pointed at the meteor trail.) you could compare the spectrum at the start and at the end, and subtract off the atmospheric contribution, you might be able to tell the difference between the composition of the soft cruft and the harder bits.

It turns out there's actually an IceCube connection to the black hole picture story.

The data had to be shipped back to the analysis center by freight, not by internet. If you look closely at the side of the box he's resting the server on, that's got a penguin in a tux and A-379-S on the side; it's one of our crates.(*)

The data transfer rate of a jet loaded with disks is very good. The transfer rate from the Pole by satellite is pretty slow, though between some US sites it can be much better. For example from NERSC to Madison it would take a little less than 6 months do move 5 PB. And 5TB is supposedly the total data set from all sites, not the amount of data each one sent.

If you remember your galaxy positions better than I do, you might wonder how it is that the South Pole Telescope managed to help image M87. (The EHT telescopes are in blue on that image.) After all, there's a planet in the way.

I haven't read half of their papers yet, but I know how I'd use the station. When you want to understand your detector, you need to know what it sees when part of it isn't working, and you want to know what it looks like if there is extra information. You can't bring Palomar along with you on a trip, but you can compare what your telescope sees with what Palomar sees when you are looking at the same thing, and that tells you something about what your scope can see by itself.

I'll bet they tried to use the telescopes to look at several nearby sources, and analyzed the data with and without the ones that couldn't see M87. That would have helped them calibrate their system--and it would have been even more fun if the source was near enough that they could have an optical comparison as well. That way they could get an estimate for what the resolution was--and you don't have a measurement if all you have is a number. You also need to know the uncertainty on that number. I gather that they deliberately blurred the image they reconstructed to match the actual resolution, so that people wouldn't think they saw structure in what were accidental artifacts of reconstruction. (People translate noisy pictures into "aliens on Mars" all the time.)

(*) Actually, I suspect that the photographer wanted a picture of him beside an Antarctic box, any box. The story just talks about shipping packs disk drives around, not servers--not even disk servers. (We also used Cabbage Cases). We ship our data on disks also, in two copies in case one disk fails (and they do). The summarized stuff we beam back by satellite, but sometimes you need the gory details.

Kudos to the team! I'm still reading the papers linked to here, and don't have anything to comment so far. This was a tough analysis.

They had 4 independent teams, blinded to each other's work, using different approaches, so that they could be sure they were doing it right. And, FYI, that ring you see? The light doesn't come straight. Photon paths are strongly bent around the black hole. And light coming from the side of the plasma disk that is headed our way has a huge Doppler boost compared to light from the receding side.

UPDATE: At the journal club today, Halzen said that one of the journalists at the press conference asked why the picture was so blurry. We got a good laugh out of that. If I understand correctly, this is the highest resolution image ever taken.

Today's Astronomy Picture of the Day is of globular cluster M15.

The variable stars are easy to spot in the animated gif, and there are a lot of them. Go watch it for a minute.