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XPoSat - Unveiling the Mysteries of Astronomical X-rays

Cosmos is full of awe and wonders, and we are fortunate to have a decent brain that allows us to study the nature of the universe and quench our thirst for curiosity. I'm sure you wonder how we study this vast universe while being so small. To answer this, let's take a deep dive into how XPoSat will unravel the mysteries of the cosmos in X-rays.

Light, which is essentially electromagnetic radiation, has different wavelengths and frequencies. Based on these characteristics, we categorize them and refer to the combination as the Electromagnetic Spectrum. X-rays are part of this spectrum, falling under the category of 0.01-10 nm wavelength. You may have heard of X-rays when someone breaks a bone. Yes, X-rays are high-energy radiation with very low wavelengths that help us discover the wonders of the universe. One space observatory that has been doing this for over two decades is the Chandra X-Ray Observatory.

On January 1st 2024, at 9:10 am, ISRO is set to launch India's first space observatory dedicated to studying the universe in X-rays. It's called the X-ray Polarimeter Satellite or XPoSat. This technological marvel will study various dynamics of bright astronomical X-ray sources such as black holes, neutron stars, active nuclei (AGNs), and pulsars. But before diving into XPoSat, let's first understand what X-ray astronomy is.

Intro to X-Ray Astronomy

Observing different planets, galaxies, and constellations with the help of a telescope at night has always been fun, right? So let me ask you, how do you observe these objects during the night sky? No, I don't mean with your eyes. Please answer in terms of frequency and wavelength. Yes, you are correct. When we observe the night sky with our naked eyes or with the help of an optical telescope, we are using visible light. Now, what if I told you that we can observe in all different available wavelengths? That's true, and it's quite interesting, isn't it? Now, I know a question might arise: "Parth, we cannot see things with wavelengths other than visible light, so how do we do that?" Right? Before answering this question, I want you to perform a small task. Find a television remote nearby and get your phone. Open the camera on your phone and focus it on one of the LEDs on the television remote. Click a button, and you may observe some violet light in your camera, right? But wait, you cannot see it with your naked eyes, correct? Yes, that LED emits infrared (IR) light. You can't see it with your naked eyes, but your mobile phone's camera can. Similarly, we observe the universe with different wavelengths using instruments and satellites like XPoSat, Astrosat, or other observatories.

So, if we observe the Universe in X-rays, it is known as X-ray Astronomy. Now, let me introduce you to the concept of dimensions in X-ray Astronomy. Dimensions in X-rays refer to the different parameters we use to solve mysteries of the Cosmos. These dimensions include techniques like Polarimetry, Photometry, Imaging, and Spectroscopy. The explanation of these techniques is beyond the scope of this article, so if you want to know more, you can check it out on the internet.

Since the birth of X-ray Astronomy, we have achieved tremendous growth in the sensitivity of X-ray observations. As a result, three dimensions, namely photometry, imaging, and spectroscopy, are well-developed subjects. However, there has been only one measurement of polarization in X-ray Astronomy, which was carried out on the Crab Nebula over three decades ago (as of 2021). Now, you might be wondering how polarization plays an important role in X-ray Astronomy. Let's dive deeper into it.

Why Polarization is important for X Ray Astronomy ?

Before we delve into why it is important, let's first understand what the 4th Dimension of Cosmic Observation is, which is polarization. Polarization refers to the orientation of the oscillations of a wave and is a property of certain types of waves, such as electromagnetic waves like light. Simply put, polarization describes the direction in which the wave vibrates or oscillates as it travels through space.

Understanding how something vibrates or oscillates can unlock a wealth of knowledge for us. It helps us comprehend the behavior of objects under extreme conditions, such as super-strong magnetic fields and intense gravity. By providing two independent parameters—the degree and angle of polarization—it helps us constrain the physical model for the X-ray source.

While we have made significant progress in studying X-rays in various aspects (such as their speed of change, origin, and composition), determining their polarization has lagged behind. It's like having a high-speed computer but using an outdated keyboard. We have not been able to keep up with the advancements in X-ray astronomy. Therefore, understanding X-ray polarization will aid us in unraveling the mysteries of the Universe.

Polarimetric observations of accreting Galactic Black Hole systems are also very interesting because they provide a unique opportunity to test some of the predictions of general relativity, which are inaccessible by any other means. Another set of interesting targets for polarization observations are the cosmic acceleration sites such as supernova remnants and jets in Active Galactic Nuclei (AGN) or micro-quasars. Polarization observations of these sites will provide direct information about the geometry of the shocked sites as well as the structure and intensity of magnetic fields therein.

So if we want to summarize why polarization is important for X-ray Astronomy, here’s how we will do that: It can give us valuable insights into:

  • The strength and distribution of magnetic fields in the sources, by which we can create physical models

  • Geometric anisotropies in the sources

  • Their alignment with respect to the line of sight

  • The nature of the accelerator responsible for energizing the electrons taking part in radiation and scattering.

Payloads on XPoSat

There are mainly two Payloads onboard XPoSat : 1) Polix and 2) XSPECT

XPoSat Schematic diagram
XPoSat Schematic diagram


The X-ray polarimeter (POLIX) is a scientific payload on XPoSat that operates based on the Thomson X-ray polarimeter technique—a technique known to us for about 100 years. It is a polarimeter designed to work in the energy band of 8-30 keV, and in the case of Compton Scattering, it can extend beyond 30 keV. The payload was developed by the Raman Research Institute (RRI), Bangalore, in collaboration with the U R Rao Satellite Centre (URSC). It is capable of detecting a minimum polarization of 2-3% for approximately the 40 brightest X-ray sources over the planned mission life of 5 years.

POLIX Specification

Before delving into the workings of POLIX, let's first understand how the Thomson X-ray polarimeter operates. We have a low atomic mass scatterer that scatters light coming from the source. Its atomic mass is low to minimize the absorption of photons by the scatterer itself. If a fraction of photons is polarized, they will tend to move in a direction where their electric field vector does not change. All of this is then detected by detectors. The intensity distribution of the scattered photons is measured as a function of the azimuthal angle. Polarized X-rays will produce an azimuthal modulation in the count rate, as opposed to the uniform azimuthal distribution of count rate for unpolarized X-rays. This is how the Thomson X-Ray Polarimeter works. Now let's understand the working of POLIX.

Working of Polix

So there is one Collimator on Top. It is used to Compensate for inaccuracy in satellite pointing and to attain constant effective area. The construction uses aluminium honeycomb, sandwitched between copper plates with circular holes. The holes on the front plate has slightly smaller diameter than the holes on the back plate which leads to a flat topped response. Since aluminium does not provide enough X-ray absorption above 15 keV, a silver coating of ∼ 10 micron thickness is provided on the aluminium surface to increase absorption. This construction has the advantage of small weight per unit area coverage. It restrict field of view to 3 degree x 3 degree.

POLIX is like having four detectors, each with its own special eyes (to detect) and brain (to understand). These detectors can figure out where X-ray photons land by using a cool trick called 'charge division' with a set of resistive anode wires connected in series. The brain of POLIX, or it’s analog electronics, does a lot of important jobs like making sure the detectors have enough power, amplifying the signals when an X-ray hits, and turning all the information into a language scientists can understand. It's like these detectors not only see where X-rays are landing but also have their own mini-computers to handle all the important tasks, like making sure they're powered up and turning the X-ray information into something scientists can use.

Electronics Engineering Group of RRI has carried out the design and development of both the analog and digital electronics systems for this instrument, meeting stringent requirements of a typical space mission. EEG has developed in-house a complete electronics system for (i) detector operation, (ii) pulse processing and digitization, (iii) on-board data handling, (iv) housekeeping, and (v) control

POLIX, VIewing Axis and different parts

The digital brain (Digital Electronics system) of POLIX is like a super-smart computer that does a bunch of cool stuff. It has a special logic to avoid mistakes (anti-coincidence logic), turns the X-ray signals into digital language (digitization of pulse amplitudes), and creates data in different ways from the four detectors. RRI-EEG, which company behind the scenes, has created a digital system for POLIX that's even more complicated than what other space experiments use.

The POLIX detectors are designed and fabricated in collaboration with the MES and are wired and assembled in EEG. Each detector has about 400 wires of 25 and 50 micron diameters precisely wired and soldered onto a frame. EEG has developed expertise over several years, of making it reliable so that the large number of wires sustains satellite launch vibration and thermal cycling in space for several years. It will be observing one source for about 1-4 weeks. So this was about Polix now lets dive deeper into Second Payload which is XSPECT.


"The XSPECT (X-ray Spectroscopy and Timing) mission is like having a cool tool that tells us a lot about soft X-rays. It's a part of the X-ray Polarimeter Satellite (Polix) mission, and both of them work together with their special angles to give us even more information. After India's first mission to study the universe in many ways, a payload called XSPECT is suggested to focus on timing and spectroscopy for low-energy X-rays.

XSPECT Detector package

XSPECT is a cool tool that lets us watch space things for a long time and study how they change in the X-ray range from 0.8 to 15 keV. It's like having a cool detector that can tell us a lot about the energy and timing of X-rays. This cool detector has a special device called Swept Charge Devices (SCDs; CCD-236), which is like a variant of X-ray CCDs. These devices can quickly read information (10-100 kHz) and give us decent details about the X-rays without using fancy lenses. They're also unique because they don't need much cooling, unlike regular X-ray CCDs. XSPECT, with its special lens and field-of-view, is perfect for studying soft X-ray timing, which is like what LAXPC does for high-energy X-rays on ASTROSAT. It's like having two superheroes detectors—one for high-energy X-rays and one for soft X-rays—working together to understand the whole picture.

Some electronics of XSPECT

The main goals of XSPECT are to figure out how X-ray sources behave over a long time by looking at both the timing and the changes in their spectra. When XSPECT works with the X-ray polarimeter, it's like having a super combo that can study everything about the X-rays—how much energy they have, when they arrive, and even which way they vibrate. It's like having a superhero team that can tackle all the challenges and mysteries of the Universe in X-rays! If you enjoyed it, please follow zetagravit on X and Insta for daily space updates. You can also subscribe to our newsletter with your email to receive new articles directly into your inbox.

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