Multi wavelength and multi messenger astronomy

10 min readMar 18, 2019

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Let me start this post with a part of a verse from Rig Veda, one of the most ancient Indian texts:

एकं सद विप्रा बहुधा वदन्ति | Rig Veda 1.164.46

The verse essentially translates to: Reality is one, the wise speak of it in many ways. It gives one of the most important lessons that can be given to any person — what one ‘perceives’ as truth may only be a part of the ‘complete truth’. The information one possesses must be corroborated in as many ways as possible, to make sure what one has is as close to the complete truth as possible. Thus, one collects as much evidence — for any event under investigation — as possible, looks at the particular event from many possible viewpoints, and tries to gauge what the complete truth is.

Something similar occurs in science — more so in astronomy.

Ever since humans have ‘looked upwards’, and tried to gauge the motion and properties of the celestial bodies, we have seen certain objects are brighter than the others. Is it because the objects are closer? Or are they intrinsically brighter? Humans have pondered over such questions for long, and based ‘astronomy’ on what is seen.

We humans ‘see’ only a small part of the electromagnetic spectrum — the part called ‘Visible light’. This is shown better in the figure below:

Fig.1: The electromagnetic spectrum. The panels (from top) indicate (i). Whether a given wavelength penetrates the Earth’s atmosphere, (ii). Wavelength of the radiation, (iii). Approximate scaling along the spectrum, (iv). Corresponding frequency of the a given wavelength, (v). Temperature (actually, the peak temperature for a black body) corresponding to the given wavelength. Taken from EarthSky (NASA, Wikipedia)[2].

As seen above, we humans can see only in a minuscule portion of the enormous electromagnetic spectrum — this draws us to a conundrum: if an object ‘A’ appears brighter than an object ‘B’ visually, which would be brighter if the same objects were seen in X-rays (let’s say, by an alien species which see only X-rays)? Will A be still brighter than B?

We cannot say. We could probably guess something given observation in at least one more wavelength, but the actual behaviour of A vis-a-vis B cannot be extrapolated based on visual information alone! And here entered the relevance of what is know as ‘Multiwavelength observation’. But to appreciate this, we must take a detour to history!

Some time in the year 1800, William Herschel — musician, amateur astronomer, renowned for his discovery of the planet Uranus — performed a simple experiment with light. He took sunlight, let it pass through a prism, and put a thermometer at the various colours coming out of the prism. To his surprise, he found the thermometer shows a maximum temperature at a region just outside the red . Thus he dubbed these rays as ‘calorific rays’, and are today known as ‘Infrared rays’ (IR)[3].

Hot objects emit IR light. Thus, work was underway to detect IR radiation from different objects, and gradually entered mainstream astronomy due to the thermocouple developed by Nicholson and Pettit[4]. This was the start of understanding the truth from a different perspective — IR.

Meanwhile, in the 1930s, Karl Jansky performed a serendipitous extraterrestrial radio observation. This result was taken as a stepping stone by Grote Reber, who single-handedly built a 9.5 meter radio telescope to perform Galactic radio observations, spending out of his own pocket[5]. And thus started the field of Radio astronomy!

Meanwhile, Herbert Friedman at America started pioneering the use of Sounding rockets for observation of extraterrestrial objects. He was trying to understand the effect of solar radiation on the ionosphere of Earth (which is a layer of the atmosphere filled with ions), and wanted to go at higher altitudes to obtain X-rays emitted by the Sun[6]. This started the observations of an object in X-rays — now known as X-ray astronomy!

Highly energetic processes (and thus, very high frequency radiation) are not so common in the universe — thus, people were skeptical on the observation of the universe in Gamma rays. However, after theoretical considerations, there arose an interest in highly-energetic processes in the 1960s. And hence, the first Gamma-ray observation was done by Phillip Morrison — where the event was a highly energetic solar flare. The field of Gamma ray astronomy was born!

By 1980s, astronomy meant not just observation in visible light — but all over the spectrum ranging from Radio to Gamma. We took a couple of steps closer to the complete truth, it would seem.

How closer did we get to the complete truth, given these Multiwavelength observations? We were able to discern a whole lot more regarding the structure and properties of objects we observe. For example, we all know the Crab Nebula (Messier 1):

Fig.2: Multiwavelength observation of the Crab Nebula, taken from [7]. Radio image: Red is higher intensity, blue is lower; IR,Visible: These are composite images from various bands, with redder being larger wavelength; UV,X-ray and Gamma: higher intensity is brighter.

The beauty above is the Crab nebula in multiple wavelengths. The nebula offers a wealth of information regarding its structure at multiple wavelengths. What we understand from here is:

The red regions in the Radio observations indicate something called Synchrotron radiation. This occurs when charged particles, at relativistic speeds (reaching near speed of light) are deflected by strong Magnetic fields. One may look here for a brief introduction to various sources of radiation due to charged particles. Thus, it is a measure of (i). Amount of charged particles, (ii). their velocities and (iii). Magnetic field strength. Synchrotron emission is one type of non-thermal emission.
IR emission occurs when dust particles absorb UV light, get heated, and re-emit them. Thus, IR emission is a marker for the presence of dust in an astrophysical object. In the IR image, the redder parts indicate dust, but the bluer parts indicate Synchrotron emission from charged particles.
The Visible image shows the composition of the nebula — one can see the gaseous filaments falling inwards from the expanding shell of the Supernova which formed the Nebula. The entire image shows the interaction of the expanding shell of Supernova with the Interstellar medium (ISM).
The UV image might not be rich, but they indicate hot gas — gas hot enough to emit in UV. The UV image also shows the background stars — these are hot, young stars, and are generally used as markers for star formation in Galaxies. Hence, while we have warm dust particles recycling UV and X-ray radiation, we also see the native UV radiation from hot gas, which seems to follow the IR emission!
The X-ray observation is interesting. One can see an elongated structure at 7'o clock from the center — this is a jet emission by the central Pulsar of the Crab Nebula. There were literally no markers for the presence of a jet in the other images, while in here we clearly see material being pumped out in the direction of 7'o clock and 1'o clock (look hard!). The emission seems to come from Bremsstrahlung (God I tripped over my tongue!), which is essentially radiation from particles when stopped/deflected suddenly. Some basic information on Bremsstrahlung may be found here. On the whole, X-ray emission tells us about hot gas, jets, and possibly the amount of charged particles in the gas with their temperature.
The Gamma emission peaks at the center — this basically tells us a lot of nuclear fusion happening at the center. This emission actually can give us an estimate on the dominant process of nucleosynthesis, via fusion in a Supernova. Such an observation was used to later understand the formation of Gold and Platinum in the Neutron star-Neutron star merger event of 2017 (as seen in one of my previous posts).

The references for the above explanation can be found below[7][8][9].

Thus, we see a wealth of information being available by simply looking at the object in different wavelengths. However, one is tempted to think if photons are the only source of information we can obtain from the universe — are there other messengers which can be used to uncover more information regarding our universe?

Turns out there are. There are particles we cannot see, which go through us all the time — called Neutrinos. These are uncharged, light particles which travel very close to the speed of light, and hardly interact with any matter. Such particles are produced in Weak decays[10], and are markers for nuclear processes, particle interactions. The hypothesis of the existence of neutrino by Wolfgang Pauli is an exciting story, but I shall pass[11].

There is yet another messenger of the cosmos — the Gravitational waves (GW). These are, speaking crudely, ripples (or waves, rather) in space-time which squish and stretch space itself. With the detection of the first wave on 2015, there came a chance to get one more marker to understand physical processes. Mind you, GW strains are hyper-small, and they might be expected only from the mergers of massive objects, supernovae, and possibly from the Big Bang.

When we use light in conjunction with these messengers, we arrive at Multimessenger Astronomy. Let us look at two famous instances of Multimessenger astronomy in our recent past.

Blazar observation with Fermi and IceCube

IceCube is a neutrino observatory based at Amundsen-Scott South Pole station in Antarctica. The observatory contains an array of light sensors beneath the South Pole station, occupying almost a cubic kilometer of volume!

Neutrinos are chargeless, light particles which do not interact much with matter. The more mass in the path of the neutrino, the more is its probability of interacting with the mass. Thus, by keeping the detectors inside ice, the neutrinos which arise from the southern sky travel more distance, thereby increasing the probability of interaction.

Fig.3: IceCube Neutrino observatory station. The detectors are beneath the surface! Image from Space.com[12]

It so happens that nuclear processes and decays generate neutrinos, and most of the neutrinos that go through us (yes! there are millions of neutrinos travelling through us as we speak!) are from the Sun, or due to the interaction of cosmic rays with the Earth’s atmosphere. Some neutrinos, however may be either directly or indirectly due to highly energetic processes in the cosmos. One such process is called a Blazar[13].

Blazars are highly energetic processes which are presumably high-energy jets from a Supermassive Black Hole (SMBH). The exact physical process is not known, though they produce extremely energetic events, and are bright enough in the Gamma rays.

Sometime in September 2017, IceCube detected an extremely energetic neutrino zooming through its detectors, and traced it location back onto the Sky. The location was somewhere near the constellation of Orion, and near simultaneously, a bright flare was seen by Fermi, the Gamma ray telescope. The neutrinos had come from an elliptical galaxy named TXS 0506+056, from where the Blazar was also seen. It also seemed the Blazar was a recurring event, and IceCube archival data had detections of energetic neutrinos every time a Blazar occured[14]! This was a pretty neat correlation!

Scientists are still skeptical if the neutrinos had come directly from the Blazar, or due to another event activated by the Blazar. Nevertheless, modelling is already underway to constrain (i). How strong these Blazar jets might be, and (ii). What might the structure of the region surrounding the Blazar be[15].

Let’s have a look at one more event which caused ripples in the fabric of space-time!

Neutron star- Neutron star merger

LIGO had, in September 2015, detected the merger of two Black holes via the GW they generate. It was a validation of the Theory of General Relativity, put forward by Einstein in 1916. However, in August 2017, when LIGO was observing for mergers, they detected a signature which lasted for more than 100 seconds. Coordinating with the Virgo setup in Italy, they were able to triangulate onto the possible location of the source of wave, and notified EM telescopes for observation. Thus, one of the first observations was done by Fermi — the Gamma ray telescope. What it saw was something called a Kilonova. Till date, researchers hypothesized the Kilonova to be due to the r-process, which generates heavy elements like Gold and Platinum. And this was the first time the physical process behind Kilonova production was narrowed down, and followed up in other wavelengths. The presence, and quantification of amount of Gold, and other heavy elements were done using observations in near-IR. Radio observations of this event went over for more than a year after the merger occurred, thus providing clues towards the exact mechanism of NS-NS mergers[16][17][18]!

This event is so important to the progress of astronomy, that it actually merits a meme:

Fig.4: I had to put this. It is informative enough to show the time span of observation of the source, and meme enough to give a few laughs. Meme is indeed worthy =D! Source: Gravitational waves memes.

Well, now is the time to accumulate as many messengers of observation as possible, to get a more clearer picture of the various physical processes that happen in the Universe. The examples presented here are certainly not the only ones, but these are probably the more easier ones to present.

One ‘dream’ of astrophysicists would be to perform an observation of a Neutron star- Neutron star merger in GW, Neutrino, and EM. This will enable researchers to obtain a handle in answering a fundamental question about Neutron stars — what are they made of? What is the state of matter inside of these dense objects?

One may very well try to look for other messengers in astronomy, and who might know, one day we may be able to perceive the complete truth as it is, after all!

Multi wavelength and multi messenger astronomy

Blazar observation with Fermi and IceCube

Neutron star- Neutron star merger

References

Written by Vishal Upendran