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[Music]
I have the great pleasure to introduce
our next two speakers Ana Rudolf who's a
scientist working at the ne B Institute
in Copenhagen she's an astrop partical
physicist um and uh Sylvia Sue who's uh
working at DIY and is an astrop prodical
physicist as well the two of them are
going to be talking about gamma ray
bursts and boats which to my great
surprise has nothing to do with floating
vessels um so um no so as an avid us
Sports follower I know about goats which
is also an acronym that is very similar
to this one no I'm curious to see what
we'll learn about um Gamay burst today
please welcome Anika and
[Applause]
Sylvia
okay let's go um we're super happy to be
here I'm Ana I'm a theoretical astrop
partical physicist this is Sylvia she's
an astronomer so we brought together
Theory and observations to bring to you
gy bursts which are our passion or
actually more our job um we're going to
be talking today about the most extreme
explosions that you can imagine in the
universe they outshine the complete sky
during their short duration they're
extremely energetic and specifically we
want to talk about an event that
happened last year that was
record-breaking in all senses it even
made it to the common news headlines as
you can
see so very bright very energetic super
fancy astropath physics but also we want
to talk about a
boat before we can do that um we will
need a bit of a his a bit of an
introduction and that is going to be
given by
Sylvia yeah so let's set the stage let's
turn back time a little bit and we're
going to go back to the middle of the
last century to a very uh not very
well-known event called the Cold War so
during this time of course everyone is
trying to build nuclear weapons once you
build a nuclear weapon you also have to
test that weapon and you also want to
make sure you know if your the other
team is testing nuclear weapons right so
in order to do that you could launch
satellites that are uh with the goal of
looking for ex for uh evidence of
nuclear testing on Earth and this might
be evidenced in the form of uh bright
flashes of very energetic photons known
as gamma rays and before I go further
anytime you see um an IM with a number
next to an image that's just a reference
for that image if you want to go find
out more information about it anytime
you see an r with a number that's a
reference to further reading um and this
is all in the description I believe of
the event as well or of the talk as well
as the slides so lots more you can read
and look at if you want to so going back
to the Cold War satellites called The
vea Satellites were launched to look for
signs of nuclear testing so they're
looking for flashes of Gamay from the
atmosphere and instead they find flashes
of Gamay from the universe so somewhere
in the universe something is producing
these bursts of Gamay or very creatively
we call them Gamay bursts
this what you're seeing here is a plot
from the discovery paper from the very
first paper and this might be the I
think this is the very first Gamay birst
that was detected by us um and again
very creatively you see this fancy name
6772 what kind of very cryptic serial
number is that yeah it happened in 1967
in July uh thei uh so this is so now you
know the naming nomenclature for grbs
it's very
straightforward um yeah yeah so we were
all we I wasn't alive then everyone in
the community was very excited and
surprised by these Gamay bursts from
space and now come the question well
what are these and it took a while to
answer this question of what are these
where are they coming from um because
once you discover something that you
weren't expecting you then have to go
back to your funding agencies and say
can you please give us more money you
take that money to your engineers and
say can you please build us this new
thing and then you have to send it off
into space it's a whole set of processes
and so for a couple of decades Gamay
bursts or grbs we're still you know we
didn't know too much about them and this
changed in the early '90s when we
finally started launching Satellites
with telescopes that were aimed at
detecting and finding and characterizing
and observing these Gamay bursts uh
satellites such as the Compton Gamay
Observatory and so the number of grbs
that we knew about and that we could
really uh observe that we really
observed just exploded in the early '90s
and once we had more information once we
had more observations of of grbs of gam
bursts we started to tease out uh some
facts about them we started to
understand them a bit better and these
flashes of gamas it turns out they can
look very different they could be as
short as a second long or even less than
a second long they could be as long as
minutes so there's a wide variety of
behaviors you could find in a gatar
burst and this flash of Gamay burst we
call the prompt emission just because it
comes very promptly it's very short
quick um and what you're seeing in the
middle here is just a bunch of examples
of what we call light curves and this
just tells you how bright something is
over time so you see there's a variety
of behavior in these grb light
curves but there's more information you
can get from a light curve than just how
bright was it over
time you can see that some of these
Peaks are really short and in fact if
you dig down into the data the Peaks can
be as short as milliseconds so that's
very short it's a little surprising
because it's coming from space some
somewhere so somehow a very short burst
of energy is coming to us from
space if you then add up all of the
energy if you take all of the photons
that your detector measures and you kind
of extrapolate and you do some
calculations you find that the energy
measured by your detector can be as high
as 10^1 Jew per cenim squ wa what does
that mean this number is 100 times
hundreds of times as brighter than the
average steady Source or the brightest
steady Source in the sky so this means
that when a Gamay burst occurs for your
Gamay detector it's hundreds of times
brighter than everything else anything
else around it so it really is a
spectacular
event from these gamma bursts we were
also able to get a bit more information
about where are they coming from because
main question is are they coming from
inside our galaxy or are they coming
from other galaxies if they're coming
from inside of our galaxy then we should
see more Gamay bursts where the Milky
Way has a lot of stars and so when we
look at the Milky Way we should be
seeing a lot of gam bursts as well if we
plot those on top of each other if
they're coming from outside of our
universe then they should be distributed
throughout the sky we should see as many
above as below we should see them coming
from everywhere and so in fact when we
looked at where the gamar bursts were
coming from they are coming from all
over the
sky and so this is a a plot of basically
many years of observations of Gamay
bursts we're pretty clear we're pretty
sure they're coming from everywhere so
they're not just coming from the Milky
Way but what does that mean that means
they could be coming from very they
could be very close to us or they could
be very far away from us and we still
don't have the Smoking Gun of well how
what is the distance to these gamy
bursts so that had to wait a little bit
longer until we had observatories that
could really locate where the grb was
coming from so these are observatories
such as beox and other observatories
later these are ones that could really
locate where the grb was this is
important because once you know where a
grb is where where it went off you could
go back to it hours later or days later
you could go back to that part of the
sky you can take an image of that sky
with your X-ray or your Gamay instrument
and you could see is there's still
something there because we already know
with a Gamay burst you get a bright
flash of Gamay fine but then we learned
that after this flash of gam of of gamma
raay disappears you still have some
residual fading
emission and this emission this what we
call the Afterglow emission we sometimes
detect it for hours but we sometimes
detected for months after the grb first
went
off and with the afterglows this is
again light curve so just brightness
over time you see they all start out
pretty high and they all Decay but the
way the Decay is a little bit different
although there are some patterns um and
so with grbs not only do you have prompt
emission that might look different you
also have Afterglow emission that might
look a little different a bonus when you
have observatories that can really tell
where the the grb is pointed or where
the G grb is located those observatories
can now communicate with other
observatories including ones that are
good at taking Spectra now what is a
spectrum so a spectrum just basically if
you just take a look at an optical
Spectrum um basically all of these
objects in the universe they shine they
shine uh across a wide range of
wavelengths of different colors in the
optical Spectrum um and depending on
what material is there in that object or
around it you also see these dark
features um
and these features again depend on the
material so they're linked to certain um
atoms or certain molecules and so we
know we understand very well where those
features should occur so uh what
wavelength they should be
at but when you take a spectrum of an
object and you put it in another galaxy
we know that all of the galaxies are are
in the universe the universe is
expanding so those galaxies are moving
farther and farther away from us what
this means is that those features move
to longer wavelengths or to the redder
part of the spectrum and so these
features are red shifted and by finding
the distance between when where you
expect them to be and what they actually
are that tells you the distance to your
gamy burst that tells you the red shift
and so when we did that we found that
Gamay bursts were located at Red shifts
of all the way up to the early universe
so red Chi's greater than one this
translates to when the universe was you
know uh only 8 billions of years old
instead of 13 something billions of
years old all the way up to when the
universe was less than a billion years
old so these are really Gamay bursts
these are really objects going back in
Cosmic
time so these were the clues that we had
we knew that gamma reverse would uh
showed rapid variation so smaller than a
millisecond quite small we also knew how
much energy we could measure by the
detectors um but we also knew that these
really rapid really energetic objects
were coming from billions of light years
away so anuka what does that tell us
then about what a gamma rur actually is
okay um let's first take these Clues
step by step the first one the rapid
variability it kind of means that
something very compact has to be
involved something that is capable of
generating a variable signal on a such
short time scale typical objects here
are black holes or neutron stars the
small black holes the Stellar Mass ones
not the big ones the center of
galaxies in addition if we know the
energy
detected on Earth and we know how far
away they are we can calculate how much
energy that is
intrinsically and that's it as it turns
out it's quite a lot of energy and this
hints towards the fact that the um
emission is not isotropic but instead
it's focused in a jet that goes out of
that Central object on the bottom and on
the
top by the energy those Garay photons
have we also can kind of deduce that
this jet must be moving at relativistic
speed so close to the speed of um light
okay in the next few slides I'm going to
be walking you a through how do we think
a GB is made like from what generates
the compact object how do we get the Jet
and how does the jet produce the gamma
ray what do we think generates the this
new compact object well with the with a
population of GBS that we finally had
end of the '90s people soon realize that
we don't have one population of GBS but
instead there is two this can be easiest
seen by the distribution of the duration
um on the one hand you have short
duration Garay burst which last less
than 2 seconds that's the blue
population here and by some theoretical
arguments but also by observational
constraints we are not quite sure that
these shorter GBS they come from the
merger of two compact objects so you
have two neutron stars that merge or
neutron star in a black hole in both
cases you create a new compact object
which then makes your
GB the most spectacular confirmation for
this was when in 2017 we caught the
gravitational wave of a Newton star
merger and we could actually associate
it with a short GB that's I mean in my
view that that means okay we're sure
this is the theory right yeah okay um on
the other hand the long
GBS there the compact object is
generated in the collapse of a very
massive
star um again here we have some
theoretical arguments but also we have
Supernova associations that support
this okay so these are kind of the
progenitor systems this is what was
there before the GB but we need a
compact objects how do we get that in
the two cases let's start with the short
ones right so you have a neutron
standard Neutron star or neutron star in
the black hole they INSP spiral they get
closer and at some point either the
neutron stars merge or the neutron star
is tirely disrupted and torn apart by
the black hole and then maybe through a
brief period of a hyper massive not so
stable neutron star in the end you end
up with a black hole that is the new
Central black object that we were
looking for and in both cases because
there was these neutron stars to begin
with and there was a lot of stuff you
typically have some accretion going on
around this black hole now what com
comes in handy is that we know or we
have theories since the' 70s actually
that if you have a black hole which is
magnetized and which spins very fast
that can you can launch a jet great
there's also some people who say okay
this jet could be launched by the hyper
massive neutron star that is in between
there but it's not so clear and the
theory is not so um so well confirmed
also by simulations
what would be the picture in case of the
long jaes in in case of the collapse of
the very massive star so in this case
obviously we start with a very massive
star here we we have a wola wola is a
very fancy name for a star which has
lost all his hydrogen and this hydrogen
floats out away from the star in
something that we call a Stellar Wind it
just move means that stuff is moving
away and you only have the inner core of
the ver very massive star in the
center now um for a very very long time
that star with this St wind is
stable however at some point um
something very common for stars happen
which is the Stella collapse what does
that mean so typically a stable
star is in pressure balance you have the
gravitational pressure from outside that
pressure is in and from the inside you
have um the radiation pressure and they
are they balance each other and nothing
happens but then at some point the
radiation pressure is not large enough
anymore and the inward pressure
dominates and the whole thing collapses
and does something that probably you've
heard of it goes boom and it does a
supernova but not only does it do that
actually what it leaves behind is yay a
black hole that is somehow surrounded by
The Leftovers of the star that we had in
the beginning and again the black hole
it can launch the jet that we were
searching
for now this is very like pretty
schematic pictures how would we actually
confirm this the problem with gabes is
that they are very tiny in size so if
you have a black hole in the center of a
of a galaxy it's a huge system and
actually you can take pictures in
different wavelengths of the jet now
recently they also started taking
pictures of the of the bottom of the jet
gb's Stellar Mass black H holds a lot
smaller and also more far away so no way
you're going to see that jet instead we
have to rely on simulations of these
systems the good news is that over the
last decades there were amazing advances
in these fields and we are now able to
generate self-consistently jets in grmhd
simulations and now I'm not sure if this
going to work now I need to move here
right yeah um so what you do in these
simulations typically
um is that you have
no um you start with some kind of system
that you know it's going to be there so
for example you have some envelope and
you put in a black hole and then the
system and and some kind of magnetic
field configuration and then the system
like here uh the neutron star merger
itself consistently generates the Jet
and it you are able to follow the jet
propagation through the surrounding
material and it gets free and everything
very pretty the same actually also works
for collapsars again here you see that
there is a black hole that we put in the
simulation by hand in the beginning then
around it is the rest what was left over
from the star and the simulation
self-consistently generates the jet
which pierces through this uh Stellar
envelope and nowadays we can even uh
take into account the interaction
between the Jet and this envelope which
has substantial effects on the jet at
some point again it breaks free and you
see this wobbly thing um this is
actually the Gib jet that is supposed to
shine later
on okay great so we have the central
black the central black hole and we have
the
jet but now we need to understand how
the jet shines what produces the gamma
rays that we can observe on
Earth um in order to to understand this
We Take a Hint from observations and we
look at the
Spectra in astrop particle physics we
categorize
Spectra in in a very simple way on the
one hand we have thermal Spectra and on
the other hand we have non-thermal
Spectra thermos Spectra come from the
motion of hot particles for example in
our sun and for thermal Spectrum if you
know the temperature you know exactly
how many number how many photons you
have in at which energy they very well
characterized and very deterministic in
that sense on the other hand non-thermal
Spectra look quite different and I'm
showing here on the left typical uh
Spectra for GBS and the first thing that
you can already see is that it's quite
different so obviously it's not a
thermal one you can also see that they
are not this spiky structure but instead
you have these straight lines now these
straight lines are here in a log log
representation that's actually a power
LW Decay something that is also
important for observations later is that
you have a lot more photons at the low
energies than at the high energies so
whatever kind of detector you build you
need to compensate for
that but overall we know from the
Spectra that there must be some
efficient particle acceleration going on
in
GBS now how do we accelerate particles
in space
the prime Contender is always shocks a
shock occurs if you have some kind of
plasma and it hits a slower
plasma and that interaction occurs um
very fast then it cannot pressure
equilibrate and we have an astrophysical
shock like what you can see here it's um
a supernova shock actually but it's also
in there in other objects now if you
have around that chock electrons and
protons and you also have some magnetic
fields then these electrons and protons
they can bounce back and forth the shock
repeated times and every time they cross
the shock they get more energetic and in
the end I can get a non-thermal particle
distribution and that non-thermal
particle distribution can produce
non-thermal radiation like the one that
we observe in
Gs so what does it mean do we actually
know how GVS shine
well the first thing that we know is
that very close to the central black
hole they don't shine at all because
it's super dense no way your photons are
never going to get out there close to
the central black hole nothing is
shining but then if you get a bit
further out with your jet
then hello ah okay then um particles may
be accelerated within the Jet and it
could indeed be those shocks like you
could imagine a jet where you have um
parts that are moving slower and parts
that are moving faster and you create
these shocks that I was just talking
about and you can have the acceleration
around
it however I have to say that for JS we
don't really know it could also be that
there is magnetic reconnection going on
and that is accelerating the particles
honestly The Prompt phase it's still a
bit of a
mystery something that we understand a
lot better is the Afterglow because here
you really have that nice um shock that
uh we were seeing before because the jet
it rushes into the medium that is around
it and as it rushes into that Medium you
at the front of the uh of the jet you
actually have a shock front and indeed
the theory that we have for the particle
acceleration it works out nicely and we
can describe well the Afterglow spectrum
and the temporal
Behavior
so um let's bring bring it back because
now um you hopefully have an idea of how
a g looks like let's maybe bring it back
uh to observations nowadays and for this
just a brick quick recap in which energy
ranges are we now expecting to see a
gab so the prompt emission by all the
mechanisms that I described to you it
will mostly shine in gamma rays well
it's still a gumy burst right it
probably it's also going to shine at a
bit lower energies but the main amount
of energy by all the mechanisms it's
dumped in the higher energy range um the
Afterglow instead uh it it also starts
with a Broadband Spectrum with at very
high energies when the Jud is still very
energetic but then as the jet is slowed
down over time over the days over the
months the photons also become less
energetic because the whole system
carries less
energy and maybe I can give back back to
you now and you tell us because we we
ended up end of the '90s maybe you can
understand now how we detect photons
nowadays right yeah okay right we got to
move on from the 90s I guess yeah right
so we just heard about what we expect to
see from a grb okay how do we actually
see those photons from those
grbs um and so one thing you can see
here here is a sort of schematic view of
the electromagnetic spectrum where you
can see the optical photons in the
middle the ones we can see with our eyes
at around one electron volt in energy
and then on the on the far ends we see
radio waves from grbs at 10 -12 electron
volts so much smaller much less
energetic on the higher end we see very
high energy gamma rays again astronomers
are very creative with our names so the
really high energy ones we call very
high energy these have 10 12 electron
volts of energy so that's 10 -2 10^ the
positive 12 that's 10^ the
24 times steps no 24 steps of 10 in
between right so that's a huge energy
range over which we want to observe gam
bursts so we need a lot of different
kinds of observatories and telescopes to
do this so what kind of Observatory you
build will depend on what wavelength or
what energy range you care about and
also what the atmosphere is doing at
those energies because the atmosphere is
transparent to some parts of the
electromagnetic spectrum we know it's
mostly transparent to Optical photons
because we can see you know stars and
the Sun and stuff um and it's mostly
transparent to a large part of the radio
spectrum a radio part of the spectrum
because that's I mean that's how we
communicate with satellites and GPS
satellites and everything like that um
but there's a lot in between on the high
energy side where the atmosphere is
opaque so those waves those that
radiation cannot penetrate through uh
which is a good thing because you know
we have to live on earth or whatever and
we don't want all this radiation it's
fine
so depending on where you're looking
that will tell you what kind of
telescope you want to build and where so
you might build your large radio dishes
or your radio uh dish arrays on the
ground um you might also build these
very large um optical telescopes on the
ground or you might go into space like
hobble um depending on what you want to
look for pros and cons to both but if
you want to look for gamma ray bursts if
you want to detect those gamma ray
bursts in gamma rays or high energy
x-rays you really need to go into space
so how do we detect gamma rays then well
let's start by talking about how we
detect Optical photons so photons we see
with our eyes these are photons again
with energies at around one electron
volt so in general for an optical Photon
you have some sort of focuser usually
these days it's a lens it used to be a
mirror but so you have a lens of some
sort this focuses all of those photons
that come in into one single point or
plate or sort of General location
um onto something on a photosensitive
surface so this is a surface that can
exactly detect these photons and it says
ahuh I detected a red one a green one a
blue one um and then you get an electric
signal out of that and then you can save
all these signals together and you build
up your picture so this is roughly how
our eyes work this is also very simple
view of how cameras and backyard
telescopes
work um if you want to go to higher
energies if you want to go to gamma rays
at 1 kilo electron volt or a mega
electron volts or 10 three or 10 the six
electron volts you have to use different
strategies if you're at a Kila electron
volt or a mega electron volt you might
use scintillator detectors so what a
scintillator is it's just something that
glows when it's hit by a photon of a
certain energy um so in this case we're
interested in centiles that glow when
they're hit by that things that glow
when they're hit by photons of one k or
or one me in energy now this glow is
just Optical photons these photons are
then collected and go into some sort of
signal multiplier or photo or photo
multiplier um and this sort of increases
the strength of the signal so you can
really measure it and then at the end
you have an electric signal that comes
out that you can work backwards and
figure out the properties of your
original Photon and this is uh what
happens on some of the detectors on the
firmy uh Gamay Space Telescope which is
one of the biggest or one of the main
gamma burst detectors right now um and
there are all these uh silic uh
scintillating cintilla detectors spread
out across F me all over it so that you
can see the whole sky in combination um
and so here the photon enters from the
left signals right out from the right so
this works when you're at one kilo
electron volt one Mega electron volt if
you want to go up higher to 1 Giga
electron volt so 10 the 9 electron volts
um you have to use a different
strategy you might then
instead have a chunk of very uh dense
heavy um high atomic number material so
this causes this means that your Photon
your Gamay comes in it will convert into
electron positron pair and then that
electron and that positron that will
travel through your detector and when
you have an electron and positron
traveling through your detector that
changes you know that that creates holes
and that changes um the electrical
configuration in your device then you
can follow those paths and you can
figure out well where those uh charged
particles were coming from and therefore
where that where that original Photon
was coming from and therefore you get
out again an electric signal to
understand the properties of the
original gamma ray and here we're seeing
um a cutaway view of the other telescope
on fmy that uh detects Gamay at around
one gig electron volt using exactly this
method um and how big are these how big
are the uh these sorts of gam detectors
well here are two pictures of fmy in the
clean room before launch um with a
couple of humans to scale and you can
see that the these detectors are you
know 1 m 2 m in length width height so
these are fairly big um and we need
volumes that are this big in order to
have enough stopping power because it's
hard to stop Gamay they have a lot of
energy so you really need to build build
a big detector to stop gamma rays which
means at some point you can't build a
detector in space really big enough to
stop gamma rays of even higher and
higher energies because then you go back
to your funding agency and they say no
you cannot launch that um
and so if you want to detect photons of
let's say one Terra electron volt so
we're at 10 to the 12 electron volts now
you actually have to come back down to
earth to do so um and I said okay well
the Earth's atmosphere is opaque to
photons of the energy well then how do
we actually use it well what this means
the Earth's atmosphere being opaque it
means that when a gamma ray with an
energy of a Terra electron volt when it
comes in it hits the particles in the
Earth's atmosphere and it doesn't
survive and in particular it turns into
a Cascade of subatomic particles and
these particles extremely energetic
going very close to the speed of light
and as this whole shower propagates the
shower grows bigger and bigger you have
more and more of the subatomic particles
that are extremely energetic and if you
have a couple of very well-placed water
tanks let's say those particles are
going to go through the water tank um
and but they're not going to lose a lot
of energy or they're not going to slow
down when they hit the water cuz they're
particles um but light that goes through
that water will slow down a lot um due
to the density of water what this means
is that these particles when they hit
the water they're actually traveling
faster than the speed of light in water
and they create a flash of blue light
called cherenov radiation which is
really just the light equivalent of a
sonic boom so Sonic Boom to sound
chairing COV radiation to light so this
is a flash of blue light and if you have
a lot of very strategically placed water
tankes with a lot of very strategically
carefully tuned Electronics inside um
then you can see the pattern of which
water tanks are hit by chanov light and
how much and when um and this lets you
work backwards to figure out the
properties of the original Photon that
hit the Earth's atmosphere and turn into
a particle uh
Cascade and the most recent most
up-to-date version of this type of
protector is called the large high
altitude air shower Observatory or lasso
located in the highlands of China um and
on the left here you see an overhead
view of what one of these might look
like of what one of these observatories
look like looks like so there are tons
of water tanks all over the place as
well as other kinds of detectors in this
Observatory um and so yeah so this
brings us I guess relatively up to date
yeah now we can actually talk about why
we wanted to give that talk and the
boats and all that maybe okay okay you
go great so we talked about gam burst in
general what are we doing to observe
them blah blah blah um now let's go back
to this one very spectacular Gamay burst
that happened in October of last year
that created all of these uh very
exciting um headlines so this is what
the public was seeing very a lot of
excitement what was going what was going
on on astronomer computers what were
astronomers seeing during this time
let's pull the curtain back a little bit
so astronomers with our supercomputers
and our Computing clusters and our high
Computing throughput BL algorithm blah
Blas uh we were refreshing a web page um
to see text documents so to be clear
there are a lot of ways that telescopes
communicate with each other and uh
autonomously rapidly uh very clever ways
um but also the slow humans have to
communicate with each other as well and
we do that through text documents um to
a one Central server uh and we just call
these circulars
so just walking through a couple of the
things that happened so as I was
refreshing the page and seeing what was
going on that day um there was first
this circular by this one satellite
Swift which detects a lot of Gamay
bursts and the Swift people said okay we
found something we thought it came from
the Galaxy but now we're not so sure so
we think now it might be a Gamay burst
if it's a Gamay burst it's extremely
energetic um and so everyone else should
go look at it to see if we're right and
to see if it is extremely special
um this was followed by another
satellite fir me uh again one of these
big um this uh prolific gam burst
detectors the the firmy people said yeah
we also saw this grb um it's among the
brightest we've ever detected so it's
definitely something special everyone
please go look at it followed by another
message saying not only do we detect
this grb we definitely detect high
energy emission so we detect G Giga
electron volt photons from it um and so
everyone started to get excited about
this one
grb um and other Gamay observatories
also jumped in and said we see these two
um and by the way if you want to see
sort of the full set of messages if you
go to r21 you can see all the text
documents in all their Glory of
astronomers talking to each other across
the
world um so a little while later an
optical Observatory
BLT they were able to take a spectrum of
this grb and this was key because this
finally told us how far away the grb is
and they found it's at a shift of 0.151
which is a light travel time of 2 * 10 9
years which sounds really far but
remember we detect grbs out to Red ships
of 1 2 3 4 five so 0.151 is actually
right in our backyard so it's extremely
close we already knew was extremely
bright so here is something that's
pretty
special and confirmed by other
observatories lots of other people
talking about it the circular that
really had the whole community in a
tizzy was this one circular uh this one
message by lasso so the uh Observatory
we talked about that can detect teev
photons and they said that they detected
this grb at energies above 500 gev even
above 10 teev this was exciting because
this is the first time we've detected
photons at these high energies from a
grb which means this is the first time
we've observed a grb at that extreme of
an energy this is also the very first
time we've ever detected a grb using
this type of Observatory so this is a
really exciting
time and there was a also lots of other
I mean again I don't want to Short
Change anyone but there's tons of other
uh discussions and observatories um
monitoring this grb um as well as
continued observations um and because it
was so bright someone called it the
brightest of all time so that's where we
end up with the boat so that's where we
are told you astronomers are really
great at naming things so this is the
boat okay but all time is a long
we're finally at the boat but all time
is a really long time what does this
mean brightest of all time what is that
really so in order to get an idea for
how bright something is and how long all
time is we can compare it to previous
record Breakers so if we look at the
prompt admission which is that initial
flash of Gamay We compare it to the
previous recordbreaker from
2013 so this was a grb previous record
holder that we said oh my God nearby
ordinary monster it saturated all our
detectors oh we we'll never get one such
as nice as this one um so we were all
very impressed by this grb in 2013 when
we compare this one to the new one easy
easily dwarfed the last one right the
peak is twice as high you know we it
it's it's much brighter than before so
we have a much nicer grb um but this is
not the end of the story because in fact
it was so bright it was too bright it
caused problems for our detect for the
most sensitive detectors so going back
to these scintilating detectors or
scintillator detectors these are ones
where the photon comes in converts to an
electric signal um through a glowing
crystal and you get and you get some
information out the electric signal the
shape of that pulse depends on the
properties of the initial Photon and it
takes a little while for your system to
then um go back and relax again but this
amount of time is like microsc which is
usually fine that's not a problem right
it's a problem if you have too many
photons coming in at once because then
all of these pulses are piling onto each
other and so multiple pulses are hitting
before you can read one out so you're
really under counting how many photons
you truly have and when you correct for
that under
counting we're in a different ballpark
all together we can barely you know the
old one we barely even care about so
this new grb clearly The Prompt mission
was really bright the Afterglow mission
was also so bright that we thought maybe
something is wrong with our detectors
maybe it's too bright um what what
you're seeing here are the Afterglow
light curves so brightness over time for
a large population of grbs and this
latest one in pink and at the very
beginning it was 10 times brighter than
anything else we ever saw hundreds of
times brighter thousands of times
brighter than the average grb so this is
something that's clearly very special on
Earth and we can quantify how special on
Earth because if you take a look at how
much energy your detector H your
detector measured from this grb you can
you can get a sense for okay the the
ones that are uh in general we expect to
detect a lot of mediocre grbs not very
energetic ones the really energetic ones
should be pretty rare and we can
quantify how rare that is based on the
uh what our observatories actually saw
over the years and when we do that we
see that this grb on the left hand side
this is the plot for the prompt emission
this grb shows you know we have this
expectation of how many bright grbs we
would get and this grb which is
indicated by the stars is much Above
This expectation and when we quantify
how far above this expectation we
actually are we have a once in a 10,000
year event so this is what all time
means it's 10,000 years um this is
pretty long compared to an astronomer's
uh uh career um so fine pretty special
average astronomer um but okay you might
say wait a second but that's just
because it was particularly close if I
were farther away maybe this grb
wouldn't be so special so can we in some
way figure out how special the grb is
intrinsically
yeah sure because honestly the
observational part not so much mine so
let's move it back to the source and
what you can see here is the intrinsic
energy of the grb on the left hand side
of the prompt emission on on the right
hand side it's the flux transferred back
close to the go what you can see on the
left on the y axis is um a distribution
of of gos a lot of them that we have
observed in their um isotropic energy
and you can see yes the boat it's kind
of an outlier like I think it's the I
think it has the highest intrinsic
isotropic energy that we've ever
detected but there are some others that
were pretty pretty close so yes it's
super energetic but not extreme in the
sense of we would never expect it what
makes it so so unique is the combination
of the low red shift and or the the
closed distance and this intrinsic
energy and this is also confirmed by the
Afterglow which if you convert it to the
intrinsic Afterglow to the intrinsic
energy flux well it's
just among the others right it's not so
special anymore something to point out
here also about this prompt phase um
energy is that what I'm showing here is
the isotropic energy now yes GVS have
Jets which are not isotropic but cated
but very often we just don't know how
big that jet that actually is what's the
opening angle so what we do instead is
we calculate what would be the energy if
it were emitting on a sphere just not to
make the error of assuming something of
course the true energy of the event is
just the small percentage that is
concentrated in that jet and as it turns
out there are some hints from
observations that for the boat the jet
was spectacularly narrow which also
means that the percentage that was in
that jet it wasn't that much so in that
sense the G was not that extremely
energetic just it was very
narrow okay so that means there was
nothing so interesting about the
GB well indeed actually there was and
the most surprising and exciting
announcement for theorist was indeed
this very very high energy observation
because that was unseen and typically
new data means we can get some new
insights the first thing that you might
ask is was that um TV observation was it
the prompt Mission or the Afterglow and
for the prompt Mission Again to to
phrase it again for the promptu mission
the the event was very common in a sense
like all the other characteristics the
temporal and also the Spectrum it was
kind of what we would expect for a GB
that comes from a massive star collapse
only we had really bad data because what
Sylvia explained with these pileups it
means that there's bad time intervals
during which you can't really use the
data great on the other hand okay so
this is the prompt uh the TV emission it
also started during the prompt phase but
it showed a very distinct uh time
profile if you would if you expect the
um TV emission to come from The Prompt
face then it should be somehow
correlated in the temporal profile since
we don't see that and since
that uh that decay of the mission is
very indicative of an Afterglow we are
quite sure that this new TV observation
is associated with the Afterglow of the
event with the onset of the Afterglow so
with the onset of the deceleration of
that
jet so was there something new that we
could make out of it
yes so be they released the data in two
chunks the first one was going up to
roughly 5 TV that's the symbols that are
not filled and for these symbols that
are not filled they've they built a very
nice physically motivated model that
could explain not only the their data
but also the the data at lower energy so
x-rays everything that was there at the
same time then at a later point they
released the DAT these the filled data
points that are the really really
highest Energies and what you can see
here already is the model these lines
that they produced before they don't
explain that new data that the very high
energy ones now unfortunately as it
turns out this kind of model you cannot
just tweak it and make it extend to
higher energies because remember it's a
physically motivated model you have some
boundaries of what is feasible and what
can happen like the processes they have
their
limits okay so you may think [ __ ] my
model it doesn't it doesn't explain the
data anymore
I can tell you there's a very very easy
fix as a theorist you just introduce a
new component something new shining at a
different wavelength and you add it on
top it's very
common um what what what could be that
new
component well the most for me the most
logical thing that it could be and also
papers that that did that used that kind
of approach is maybe um this new
component
arises if we increase the model
complexity if we account for some some
aspect that we didn't account before an
example could be right now in the
modeling a lot of times we just take the
jet as a uniform thing what if the jet
was not uniform what if for example it
had a narrow cone core that is very
energetic and then wings that are less
energetic and the additional signal
could come from these wings or what if
protons were accelerated on top of
electrons and they could that that um
additional signal okay but this is my
kind of favorite way to approach this
there's Al also other people who do
particle physics um more specifically um
beyond the standard model particle
physics and they in this additional
component they see something exotic like
um Dark Matter Axion all of that and yes
of course I'm I'm I'm very biased here
right I come from a certain point of
view and maybe they are also right just
in my point of view I think we should
first explore a more complex setup when
we model the grb and if all of that
doesn't work out then then maybe resort
to the non-standard
things there is actually one last
explanation that is not modeling
connected but rather how we correct the
data because what happens at the highest
energies is that photons are absorbed on
their way to Earth and as it turns out
how much they are observed observed
absorbed uh we don't really know because
we are we don't have very precise
measurements of all the photon fields in
the universe and if for example we use a
different model for that observ for that
absorption we could also make it work
like in my view on the right hand side
the model always almost explains the
data
know so a lot of different
possibilities and coming to summarize
what does it mean where do we stand the
astronomer
astronomers they gave us boat but in my
personal point of view they gave us this
and now it's up to us to piece it
together and to find out what collimated
that jet and what made that additional
component and many more papers are to
follow
[Applause]
okay thank you both uh up until this
point we have a little bit of time left
for questions so if you have questions
in the room please come to one of the
four microphones and we'll just uh get
started with a question from our signal
Angel relaying a question from the
internet hello thank you for the amazing
talk um so the internet um would love to
um understand a little bit better how
the um propagation velocities of the
X-ray fronts work how does all brings
together with the gbrs uh grb sorry um
and and what is this thank
you sorry could you repeat that question
yeah please um so the you have these
x-ray fronts around the grv as far as I
understood so my physics is not so
strong but um and the internet is
interested in um how these propagation
fronts actually work how we absorb them
and so on are you referring to the the
last point that an was talking about yes
okay the absorption ah the absorption
that happens um that is actually the
very high very high energy gamar raay
what happens is that they interact so
all in the universe you have background
Fields Photon background fields that we
call extragalactic Background Fields
they are of very low energy the very
high energy photons that come from very
far far away objects they can interact
with these and they surpass the energy
threshold to do electron positron pair
creation and since for the photon that
creates the electron positron pair this
is a very catastrophic thing to happen
because afterwards is not there anymore
instead you have the two leptons they
are absorbed um and the level of this
absorption just depends on well the
background field that it it encounters
on the way that it could interact with
and we don't have very good measurements
of these that's the
issue right please um hik thank you for
the talk um um I wanted to ask how wide
are these gamma ray burs because you
said they are very narrow and not
isotropic and how in other words how
easy is it to miss them here on Earth oh
quite quite easy um so up to 10° I would
say we don't have very good measurements
of a jet opening angles it's how thing
to to actually measure typically I would
say something like 5° opening angle and
if if you're worried about uh gam rers
destroying Earth um so so right so part
of the issue is I mean short answer no
um but part of it is because okay the
opening angles the Jets are narrow so
that already lowers your chances um also
gam births as we heard come from
especially the collapsing star ones they
come from the most uh really massive
stars U most extreme Stars which are
already rare um and those types of stars
are aren't really formed in galaxies
similar to the Earth or to similar to
the Milky Way and galaxies around us so
those are more common much farther away
um so all of that combined very unlikely
I don't know the exact number very
unlikely um but if a gam burst were
really close to Earth yeah it would just
you know vaporize everything everything
would die if we were in this
intermediate distance um that's actually
worse because it would cause the
nitrogen and the oxygen in the
atmosphere to turn into I think nitrous
dioxide which is a brown gas it's the
one that's not laughing gas it's the
other one the less fun one it's a brown
gas everything dies uh so you would hope
it'd be really close or really far away
luckily they're all really far away it's
fine okay thank
you all right let's move to mic three hi
thanks for the talk uh so my question is
related to multimessenger
astronomy and in particular has has this
phenomenon being observed through
perhaps gravitational waves or different
things that might shine a light to
explain why it was so different um this
one no gravitational wave so it's a
massive star collab um this is not
easily observable by gravitational waves
at
least most of the time most of the time
it doesn't produce the gravitational
waves um that we would observe different
frequency range and also different
process than the neutron D merger there
were um multimessenger also mean means
neutrinos uh Ice Cube was looking for an
I mean they're always looking for an Eng
it's always operational they didn't see
any anything
um it did not it does this gives some
constraints in my view however they are
not spectacular it's something very
common the constraints that you get out
of it are
normal thanks all right another question
from our signal
Angel um and that's a very practical one
so is there any good sources on data for
realtime Garay bursts that that one can
follow like a map or something like this
that you could recommend yeah actually
uh if you're interested in gamar burst
there's this one uh recent website um uh
project called Astro Colibri um you can
just Google it they're also on social
media um and so they're really good for
uh uh getting all of this information
together into a single place so you
don't have to scour through um lots of
text files um but then you can see you
know a map of the sky and you can say
I'm interested in gam burst from this
day to this day oh I like this one let
me find out more about it um so that's
that's Astro kri c o l i b r i I think
um so if you Google that or search for
that you will find uh a good resource
there awesome let's go to the mic in the
back yes hello thanks you one very nice
talk um there are a couple of cubat
missions uh with uh small Garay
detectors claiming that they also want
to detect Garay burst and map them and
we need more of that to cover the whole
sky so my question is um have these
small detectors the capability of
actually producing meaningful data yeah
absolutely um the the so the the cube
set right the part of the problem right
now is that uh the governments don't
really want to put up large uh gam burst
detectors anymore I don't know it's fine
um they want to look at other things
like exoplanets um but so instead yeah
you would need maybe a lot of smaller
detectors so these Cube SATs um and we
there so this brly bright grb was
actually detected by a cubat um grb
Alpha and this data is also um or this
uh information is also there in the the
sort of list of circulars um if you want
to take a look at it um and I think the
cube stats will play more and more
important role as our current satellites
are aging um and Cube sets are very
cheap to put up and so if you have a lot
of them then you can really cover like
as you say a large part of the sky and
you really get good coverage good
information about it then so um yeah
Cube sets absolutely crucial and uh will
become even more so in the next
decade okay microphone number
one um I hope this is not a very silly
question but is it thinkable that we
could perhaps uh also observe the
intermediate space between the grbs and
us so bluntly could we see Dark
Matter uh the that we could observe this
the space information about like I guess
like an AG if it gives some information
about um what's in between us and the
grb sort of like with graval
gravitational lenses yes I mean
something that definitely comes into
mind is absorption um because what we
can for example that very right JY was
actually behind the Milky Way somehow
and that meant that it underwent a lot
of absorption due to dust and this
produced very very clear features that
helped Rec construct the dust in the
Milky Way on on the way so this is some
way that it could work uh the
gravitational lensing I don't think it's
something very commonly done in g i
people look for it but we haven't yeah I
don't know what the likelihood is or
anything like that it is something that
people look for yeah no I just meant I
didn't mean it would be gravitational
lenses just like some kind of disturbing
no it's a it's a fair question though it
is something that people look for so
it's a fair question yeah yeah and I
mean my answer would probably be dust
absorption is something something that
happens and that is also used in order
to constrain what happens in
between okay fa
thanks okay let's go to mic number three
uh yeah hi thanks for the talk um one
question I had is you said that these
Gamay bursts are actually focused in
some way but I understand the Jets that
are exiting your black hole those are
matter based
um uh that Jets of some sort and then in
there the electrons flow that will
actually produce the photons that will
then yeah reach us but What mechanisms
focuses the photons ah um okay so first
of all in the beginning the the jet is
not met dominated because it's produced
by the magnetic fields that are dragged
around the black hole and in in the
beginning you TP if you go for the black
hole explanation of all of it um you
have a jet that is dominated well most
of the the energy is carried by the
magnetic fields it then picks up U
material as it propagates outwards and
also this propagation through the the
form ofar that was there and also other
stuff that was around this focuses the
jet now what happens is with the
focusing of the photons is that if you
have electrons that move in that jet and
that move relativistically their
emission is by special rela relativity
arguments focused okay um along their
allance Factor but this is yeah focusing
due to the DU to the motion of the whole
thing basically okay
thanks do we have another question from
the
internet yes um so you already went into
the um yeah into the things being hit by
a Garay burst um so are there any other
correlations um with gay burst and other
observable
phenomena um with gam burst um so I
think we do this more often with
Supernova um the you know related
phenomena um they're more common um and
they're Closer by and they also are
isotropic and so it's much easier to be
affected by a supernova than a gamma
rsted um but so with that you would um
for Supernova you do see signs such as
um I think in either really old trees or
layers of the ice you see some uh
certain types of Isotopes um in some
elements that in a certain pattern um
because these elements were hit by the
energy from the Supernova um in a in a
way that you can really calculate in a
well- defined way um uh and so if you
know if we did have a a grb in our re in
our history um you would basically I
think just try to look for certain
combinations of isotopes that are hard
to form unless you have a big dep
positive energy in that
way all right we have time for one quick
last question so uh Mike for please uh
hi thank you so much for the talk uh my
question is related to spectroc uh
spectr
spectr thank you um so what kind of
lines do you follow and do you see signs
of exotic uh particle reactions in the
in the beams I think in the Spectra you
mostly I think exotic um interactions
would give you a big headache um when
you do spectroscopy spect spectrometry
when you take
yeah I agree when you take Spectra um
you're really depending on um known lies
of known lines of known processes so um
I think these are usually um uh
electrons changing energy levels in uh
hydrogen right hydrogen is a b one um so
ionized hydrogen yeah yeah yeah so you
have the lman alpha y blah yeah I
see I think we're out of time anyone
else who has questions just come over
there and we can explain whatever you
need to be explained of course awesome
yeah as you said we are at time thank
you again
[Applause]
[Music]
Ania