HEAPOW: Aspherical COWs? (2022 Jan 17)

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HEAPOW: Aspherical COWs? (2022 Jan 17)

In 2018, a strange sort of supernova was captured by telescopes around the world. This transient event, dubbed AT2018cow by odd astronomical naming conventions, was a rare event now known as a Fast Blue Optical Transient (or FBOT). Astronomers realized that the "cow" event was orders of magnitude more luminous than other more typical supernovae usually seen in the cosmic barnyard. Astronomers have not yet agreed on what AT2018cow is (or was). It's perhaps likely that the enormous outpouring of energy from the "cow" is being produced by directed beams of radiation from a compact object created by the death and collapse of a high mass star once the star ran out of thermonuclear fuel. The image above shows two possibilities. On the left, energy is beamed towards earth by a baby black hole at the center of supernova remnant. In this scenario, the directed energy beam is powered by the accretion of a disk of matter falling back onto the event horizon of a newborn baby black hole. The scenario shown on the right also has a directed energetic beam, but this time powered by the magnetic energy of a highly magnetized neutron star, or magnetar, left behind by the supernova. Astronomers are on the hunt for more such events to see if they can figure out if either scenario is correct. In 2020, a new X-ray transient source was found by the eROSITA X-ray telescope on the Russian-German Spektr-RG X-ray space observatory. This source was also detected as an FBOT by the telescopes of the Zwicky Transient Facility and dubbed AT2020mrf. Astronomers now know that the AT2020mrf explosion took place in a small star forming galaxy about 2 billion lightyears away, and saw that, astoundingly, this "cow-like" event is even brighter than the original "cow" - 10 to 100 times brighter in X-rays. Additional observations of "mrf" and the "cow", and new FBOTs, will help refine our understandings of stellar death and the birth pangs of neutron stars and black holes.

AAS: AT2020mrf: The Most X-ray Luminous Cow-like Supernova

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Author: Dr. Michael F. Corcoran

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Astronomers Find Most Luminous "Cow" to Shine in X-Rays
California Institute of Technology | Chandra X-ray Observatory | 2022 Jan 11

Credit: Bill Saxton, NRAO/AUI/NSF

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Another member of the new "Cow" class of supernova explosions has been discovered—the brightest one seen in X-rays to date. The new event, dubbed AT2020mrf, is only the fifth found so far belonging to the Cow class of supernovae. The group is named after the first supernova found in this class, AT2018cow, whose randomly generated name just happened to spell the word "cow."

What lies behind these unusual stellar explosions? New evidence points to either active black holes or neutron stars.

When a massive star explodes, it leaves behind either a black hole or a dead stellar remnant called a neutron star. Typically, these stellar remnants are relatively inactive and shrouded by material ejected in the explosion. But according to Yuhan Yao (MS '20), a graduate student at Caltech, Cow-like events have at their cores very active, and mostly exposed, compact objects that emit high-energy X-ray emission. ...

The first Cow event, AT2018cow, shocked astronomers when it was discovered in 2018: the stellar explosion was 10 times brighter in visible light than typical supernovae and faded more quickly. It also gave off a large amount of highly variable X-rays, leading astronomers to believe that they were directly witnessing the birth of a black hole or neutron star for the first time.

Another distinguishing factor of Cows is that they throw off heaps of mass before they explode, and this mass gets illuminated later, after the explosion. When the stars blow up, they generate shock waves that are thought to plow through the pre-existing material, causing them to glow in radio and millimeter-wavelength light.

AT2020mrf is the first to be found initially in X-rays rather than optical light. Yao and her colleagues spotted the event in July 2020 using X-ray data from the Russian--German Spektrum-Roentgen-Gamma (SRG) telescope. They checked observations taken in optical light by the Zwicky Transient Facility (ZTF), which operates from Caltech's Palomar Observatory, and found that ZTF had also spotted the event. ...

'Cow' supernova is brightest ever seen in X-ray observations
Space.com | Tereza Pultarova | 2022 Jan 13

The X-ray and Radio Loud Fast Blue Optical Transient AT2020mrf: Implications
for an Emerging Class of Engine-Driven Massive Star Explosions ~ Yuhan Yao et al

• arXiv.org > astro-ph > arXiv:2112.00751 > 01 Dec 2021

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<<The spherical cow is a humorous metaphor for highly simplified scientific models of complex phenomena. Originating in theoretical physics, the metaphor refers to physicists' tendency to reduce a problem to the simplest form imaginable in order to make calculations more feasible, even if the simplification hinders the model's application to reality.

American Astronomical Ass. spherical cow

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The phrase comes from a joke that spoofs the simplifying assumptions sometimes used in theoretical physics: Milk production at a dairy farm was low, so the farmer wrote to the local university, asking for help from academia. A multidisciplinary team of professors was assembled, headed by a theoretical physicist, and two weeks of intensive on-site investigation took place. The scholars then returned to the university, notebooks crammed with data, where the task of writing the report was left to the team leader. Shortly thereafter the physicist returned to the farm, saying to the farmer, "I have the solution, but it works only in the case of spherical cows in a vacuum."

It is told in many variants, including a joke about a physicist who said he could predict the winner of any race provided it involved spherical horses moving through a vacuum. A 1973 letter to the editor in the journal Science describes the "famous story" about a physicist whose solution to a poultry farm's egg-production problems began with "Postulate a spherical chicken". Alan Turing, in his 1952 paper "The Chemical Basis of Morphogenesis", asserted: "a system which has spherical symmetry, and whose state is changing because of chemical reactions and diffusion ... cannot result in an organism such as a horse, which is not spherically symmetrical.">>