
What Is a Blazar? It’s Like Staring Down the Barrel of a Black Hole - eaguyhn
http://blogs.discovermagazine.com/crux/2018/07/12/what-is-a-blazar/
======
AndrewKemendo
I'm always astounded by the pictures that are associated with black holes.
Absolutely none of them are representative of what we've actually
seen...because we've never directly imaged a black hole.

I'm not even sure if we can image it, because theoretically, we're imaging the
_stuff around them_ aka the event horizon or other matter which is being acted
on by the black hole.

I mention this because it's one of the best examples Along with the electron)
where we have such a strong desire to create visual representation of
something we can't actually perceive because our minds _require_ visual
representation to understand the concept.

~~~
myWindoonn
Well, slight semantic nuances of English aside, we absolutely have taken
pictures of black-hole-emitted radiation; see [0] for an example. Search for
photos of X-ray emissions. It's true that most of the energy seen from the
direction of a black hole will be leftover scraps from its messy eating
habits, but Hawking radiation should be a thing (although we'll likely never
observe it above the CMBR), and rotating black holes should also be a thing.

If you decide to go the fuzzball route, then perception without direct
visualization isn't too hard, aside from the holographic principle, which
helpfully justifies the idea that fuzzballs are a place where 3D movement
becomes 2D movement. On a fuzzball, you might imagine strings moving around on
the surface, but there's no meaningful way to move further into the fuzzball
nor to detach from the surface and leave, aside from getting lots of energy
and popping off as Hawking radiation/firewall. Incidentally, 2D movement is
something that we have tons of intuition for already!

[0] [https://www.space.com/35231-deepest-x-ray-image-black-
holes....](https://www.space.com/35231-deepest-x-ray-image-black-holes.html)

Edit: Many folks would consider our gravitational-wave telescopes, like LIGO,
to give us data which could be interpreted visually. [1]

[1]
[https://www.youtube.com/watch?v=I_88S8DWbcU](https://www.youtube.com/watch?v=I_88S8DWbcU)

~~~
AndrewKemendo
This is exactly my point and the semantics do matter here because they relate
to the epistemic nature of the content. Namely that humans can't directly
interpret X-Rays (or other non-visible light emissions).

We have to convert X-Ray emissions into images or mathematical representations
which we then interpret visually/logically. So there is some level of
information compression which we have to do just to be able to do some
interpretation.

This is not a hit on them at all, in fact it's a genius workaround. All I
really want to point out here really is the limits to our perception and the
lengths that we go to, in order to transform non-perceivable phenomena into
into modalities that we can perceive. It's an exercise in investigating the
boundaries of our epistemic capabilities.

~~~
SiempreViernes
Strictly speaking we don't see colours at all, we just hallucinate them from
the monochrome response in three (or fewer) filters. So, for example, we
commonly confuse mixes of different colours with a colour between them.

In this strict frame of mind, it is not a trivial epistemic statement to say
that "the highlight colour at yellow.com is yellow", since your screen can not
physically emit yellow light.

~~~
AndrewKemendo
Yes, the rabbit hole goes deep in the perception question. Which is why I
think it's important that we recognize the nested abstractions when we are
trying to describe concepts - especially when they are interpreted from things
that we don't have even fundamental perceptual abstractions for.

------
SiempreViernes
Worth keeping in mind is that it is only slightly better than even odds this
was actually an _astrophysical_ neutrino.

~~~
Jabbles
Very much better than even odds.

 _We report the detection of a high-energy neutrino by IceCube and the
multiwavelength /multi-instrument observations of a flaring γ-ray blazar, TXS
0506+056, which was found to be positionally coincident with the neutrino
direction (16). Chance coincidence of the IceCube-170922A event with the flare
of TXS 0506+056 is statistically disfavored at the level of 3σ in models
evaluated below, associating neutrino and γ-ray production._

~~~
SiempreViernes
Supposing it was astrophysical, the chance coincidence is low yes, but:

> _The muon-neutrino astrophysical spectrum, together with simulated data, was
> used to calculate the probability that a neutrino at the observed track
> energy and zenith angle in IceCube is of astrophysical origin. This
> probability, the so-called signalness of the event (14), was reported to be
> 56.5% (17)._

Meaning there is about even odds this was not from outer space, but from a
cosmic ray hitting the atmosphere, and we know cosmic rays point back at
nothing in particular.

~~~
perl4ever
But does that probability take into account the coincidence of the putative
source being lined up, or not?

~~~
SiempreViernes
No, that is purely the chance that they correctly identified the event as
being signal, it doesn't take into account known sources.

Given that the background is basically isotropic, I doubt adding in a source
catalogue will move the probability much.

~~~
perl4ever
You're saying there are blazars everywhere, such that no matter the
trajectory, it would match to something? That doesn't sound right.

~~~
SiempreViernes
No, I'm saying cosmic rays come from anywhere (as the get deflected by
magnetic fields) and when hitting the atmosphere they can create high energy
neutrinos that can look like they are from space despite actually being from
the _atmosphere_.

------
mirimir
Could someone please ELI5 the distinction between blazars and quasars? I get
that quasars are radio sources. But so are blazars, right? Is it that blazars
are pointed right at us, whereas quasars aren't necessarily?

~~~
teh_klev
According to:

[https://en.wikipedia.org/wiki/Quasar](https://en.wikipedia.org/wiki/Quasar)

 _In the 1980s, unified models were developed in which quasars were classified
as a particular kind of active galaxy, and a consensus emerged that in many
cases it is simply the viewing angle that distinguishes them from other active
galaxies, such as blazars and radio galaxies._

..also...

 _A minority of quasars show strong radio emission, which is generated by jets
of matter moving close to the speed of light. When viewed downward, these
appear as blazars ..._

To the best of my knowledge and interpretation they can be a type of "Quasar"
but are really a type of galaxy with an active galactic nucleus (AGN) where
their jets point towards Earth:

[https://en.wikipedia.org/wiki/Blazar](https://en.wikipedia.org/wiki/Blazar)

 _A blazar is an active galactic nucleus with a relativistic jet (a jet
composed of ionized matter traveling at nearly the speed of light) directed
very nearly toward the Earth._

[https://en.wikipedia.org/wiki/Active_galactic_nucleus#Types_...](https://en.wikipedia.org/wiki/Active_galactic_nucleus#Types_of_active_galaxy)

~~~
mirimir
Thanks. I did skim the Wikipedia articles, but wasn't left with a clear
understanding.

But OK, I get that they're all galaxies with AGNs aka massive black holes. And
that blazars are the ones with a jet pointed right at us.

But now I'm curious. When a jet is pointed right at us, we see visible, x-ray
and gamma radiation, and relativistic particles. But is longer wavelength
radiation also emitted mainly on the jet axis?

~~~
teh_klev
I'd be a complete fraud if I said I could answer that :)

~~~
mirimir
Yeah, me too :)

But this is interesting:
[https://www.bu.edu/blazars/VLBAproject.html](https://www.bu.edu/blazars/VLBAproject.html)

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dghughes
I'm just as amazed that a place like the IceCube Neutrino Observatory could be
built.

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mchahn
So it emits particles at near-light-speeds aimed towards us. Some questions
(possibly stupid).

Are they still at near-light-speed when the get to us?

How can you detect something going so fast? Aren't there weird relativistic
behaviors?

------
SteveParker60
dickbar on top

dickbar on bottom

dickbars to the sides

~~~
dredmorbius
[https://outline.com/http://blogs.discovermagazine.com/crux/2...](https://outline.com/http://blogs.discovermagazine.com/crux/2018/07/12/what-
is-a-blazar/)

------
walrus01
Blazer?

[http://en.wikipedia.org/wiki/Chevrolet_Blazer](http://en.wikipedia.org/wiki/Chevrolet_Blazer)

While its gravitational density may approach a black hole, it's not an
astronomical phenomenon.

