In my previous blog post, I introduced the General-relativistic Radiative Transfer Code, which is an important tool for computing the radiative properties from simulated results (see “a bridge between observation and numerical simulation”). Today, I would like to provide some examples of how the image of black hole shadow, photon ring and the spectrum would change depending on the different physical properties, some of which are very difficult to be constrained yet. 1. Black hole image in different frequency The well-known monumental image of the M87 black hole shadow, which was announced in 2019, was captured at 230 GHz by the telescopes in the Event Horizon Telescope array. The intrinsic…
Read More >>“The new Hubble”
On December 25th 2021, while many people were opening their Christmas presents, astronomers around the world were getting excited about another kind of present: the James Webb Space Telescope (JWST). Launched at 13:20 on Christmas day, this new telescope built by ESA, NASA, and the Canadian Space Agency, finally made it to space after years of delay due to its complexity and the number of tests it had to go through before being cleared to go up. It is the largest telescope ever sent in orbit and represents not only a real opportunity for astronomy but also a progress in technology and engineering. You are probably familiar with the Hubble…
Read More >>Stellar-mass black holes as particle accelerators
For over a century, scientists have been detecting elementary particles of extraterrestrial origin, known as cosmic rays. These particles carry energy that sometimes exceeds their rest mass by a million times. The mechanism behind this energization and the environmental properties capable of such a process remains unclear. Among these cosmic rays, the dominant population is that of protons. These protons carry energy up to a million times their rest mass (i.e., up to 106 GeV, where ~1GeV is the energy carried by protons at rest), and very likely they originate in our Milky Way. Despite decades of research, the source of cosmic rays remains unclear because, while cosmic rays propagate…
Read More >>Sound Echoes and Echoes in the Universe
When you stand on the top of a hill, and shout your name, after a short delay, you will recursively hear your bellow, although your voice is successively fainted. The returned sound is an echo. You might be curious, why cannot we hear an echo in a room? This is because the human ear cannot distinguish echo from the original sound if the delay is less than 0.1 seconds. The speed of sound through air is approximately 343 m/s, therefore, the reflector (the walls of a room) must be placed more than 17.2 m away from the sound source (a man who is also a recevier here) for the echo…
Read More >>How to Destroy a Black Hole
Black holes are highly destructive objects in the universe, e. g., a star can be destroyed if it gets too close to the extreme gravity of a black hole (figure 1). On the other hand, there is nothing we could throw at a black hole that would damage it, not even another black hole would do it, they will simply merge into a larger one (figure 2). However, there may be a way to destroy these objects, all we need to do is to wait. In 1974, Stephen Hawking realized that quantum fluctuations could lead a black hole to lose mass at a very low rate. These fluctuations continuously create…
Read More >>Scales, scales, scales?! (Part I)
When reading our blog posts, you might find yourself asking what the scale of the black holes, jets, or accretion disks we speak of are. If that is the case, then this post is for you! Given that Halloween just happened, let us choose our base unit as one Jack-o’-lantern pumpkin (JOLP) that is 20 kg heavy and 0.5 meters in diameter. For scale, roughly four JOLPs stacked on top of each other would span a human that is 1.8 meters tall and 80 kg heavy. Right, then how many JOLPs would we need to span the entire circumference of the Earth? Well, we would need about 80 million (80…
Read More >>The Science Behind the Solar Eclipse (Part II)
This is the second and final part on the series of blogposts titled “The Science Behind the Solar Eclipse”. In this post, I will build upon the ideas and concepts discussed in Part I of the post to describe how solar eclipses occur. So I strongly recommend reading through it here. Solar eclipses occur because of the Moon blocks the Sun, as viewed from some point on Earth. As elucidated in Part I, due to the varying distance between the Earth and the Moon, the Moon may block the Sun in its entirety or a portion of it. Another way to think about this is the geometry of the Earth,…
Read More >>Electron Microphysics in the global GRMHD simulation
It is widely believed that the luminosities of M87 and Sagittarius A*, which are the major targets of the Event Horizon Telescope collaboration, are significantly lower than their Eddington luminosities. It means that the mass accretion rates of these objects are so low that the material accreting onto the central supermassive black hole can be interpreted as the radiatively inefficient accretion flow (RIAF), which is geometrically thick and optically thin (See “disk thin / thick?!” for the disk models). In such a (geometrically) thick disk, it is generally acceptable to assume that the interactions between ions and electrons are very poor, so they are not in thermal equilibrium, resulting in…
Read More >>When you’re a JET…
No, not the West Side Story musical kind… (although they are awesome) The Jets gang from West Side Story. So our research group is called Jetset. We work on trying to figure out the physical processes that launch jets of matter and light from (very close to) black holes. So what are these jets actually? In the astrophysical sense, jets are linear beams of charged particles like protons and electrons (aka plasma) that are emitted along the rotation axis of an astrophysical object, like a black hole. Jets launched from black holes reach speeds close to the speed of light. These super fast moving outflows of plasma exhibit the effects…
Read More >>Black Holes, the most Powerful Engines in Nature
Energy source is an eternal topic of human development, as any activity in nature requires a supply of energy. In ancient times, nearly all production work relied on human or animal power, resulting in very low productivity for thousands of years. The Industrial Revolution in the 18th century changed this situation completely, replacing human labour by machines powered by burning fuels. Combustion of chemical fuels is the most widely adopted energy source in the current world, for both domestic and industrial purposes. In such a process, the chemical energy of the fuels are first transformed into thermal energy, which is then used to power various machines to do work. How…
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