Starship Asterisk*

An aerial image of the Cerro Tololo Inter-American Observatory in Chile, which hosts the All Sky Automated Survey for SuperNovae, among other telescopes.

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Astronomers have spotted what seems to be the brightest supernova ever discovered: an exploding star that shines brighter than 500 billion Suns. Don’t go looking for it with binoculars though: its light has taken 2.8 billion years to travel to Earth and, at such a distance, the supernova is only visible through a telescope.

The ASASSN-15lh supernova was first picked up on 14 June by two telescopes operated by the All Sky Automated Survey for SuperNovae at the Cerro Tololo Inter-American Observatory in Chile. In reports posted on the Astronomer’s Telegram — an online bulletin service — on 8 July, astronomers led by Subo Dong at the Kavli Institute for Astronomy and Astrophysics at Peking University in Beijing report that they caught it about nine days after its brightness peaked. ...

Using larger telescopes to follow up the sighting, Dong and his colleagues from the United States and Chile estimate that the stellar explosion is the most extreme instance yet of a superluminous supernova. A few dozen of these enormous blasts, one hundred times brighter than ordinary supernovae, have been spotted in the past decade — and ASASSN-15lh is about twice as bright as any of them. ...

• arXiv.org > astro-ph > arXiv:1507.03010 > 10 Jul 2015

[img3="An artist's impression of the record-breakingly powerful, superluminous supernova ASASSN-15lh as it would appear from an exoplanet located about 10,000 light years away in the host galaxy of the supernova. (Credit: Beijing Planetarium / Jin Ma)"]http://www.sciencemag.org/sites/default ... nova_3.jpg[/img3][hr][/hr]

Kaboom! Astronomers have found the most violently explosive supernova so far detected in the history of the universe. Supernovae are already some of the brightest events out there but in recent decades astronomers have seen a rare new class of blasts, super-luminous supernovae (SLSNe)—sometimes dubbed hypernovae. The new discovery was spotted last June by the All Sky Automated Survey for SuperNovae (ASAS-SN), a system of eight small 14-centimeter telescopes at two sites in Chile and Hawaii that can scan the entire sky every 2 to 3 days. At its peak, ASAS-SN-15lh, as the new supernova is known, was twice as luminous as any previously seen, thousands of times brighter than a normal supernova, and outshone our entire Milky Way galaxy by 50 times. (The artist’s impression above shows what it would look like from an exoplanet 10,000 light years away in its home galaxy.) But, as the ASAS-SN team describe online today in Science, more detailed study of the object and its surroundings with larger telescopes is confounding theorists. ASAS-SN-15lh appears to fall into a class called a hydrogen-poor SLSN which theorists believe occurs when an old star, run out of fuel, creates a supernova blast while collapsing into a highly-magnetized neutron star, known as a magnetar. The magnetic energy from the magnetar—so the theory goes—then powers up the still-expanding supernova making it unusually bright. However, this sort of SLSN is expected to form in small, dim dwarf galaxies full of young stars but ASAS-SN-15lh is in a large, bright galaxy with little star formation. So, back to the drawing board.

• Science 351(6270):257 (15 Jan 2016) DOI: 10.1126/science.aac9613
  arXiv.org > astro-ph > arXiv:1507.03010 > 10 Jul 2015 (v1), 07 Oct 2015 (v2)

Ann wrote:
We are often told that a supernova can shine as brightly as an entire galaxy, and I have often thought that this must refer to a small galaxy, considerably fainter than the Milky Way. According to the Wikipedia article about supernova type Ia, the typical absolute visual magnitude of a such a supernova is −19.3. But according to a Wikipedia article about absolute magnitude, the absolute magnitude of the Milky Way is −20.5, and the absolute magnitude of giant Virgo cluster elliptical galaxy M87 is −22. So the Milky Way would be about three times brighter than a typical supernova type Ia, and M87 would be 12-13 times brighter than a normal SN type Ia. (You will have to excuse my shabby math here.)

Yes, but the peak absolute magnitude of ASASSN-15lh was −23.5. That makes it fully three magnitudes brighter than the entire Milky Way, almost 16 times brighter than our entire galaxy! And it was one and a half magnitudes brighter than giant elliptical M87, which I believe means that it was almost 4 times brighter than M87. That is just incredible!

http://arxiv.org/abs/1507.03010 wrote:
ASASSN-15lh appears to be hosted by a luminous galaxy (M_K ~ -25.5) with little star formation.

alcor wrote:
As Dong writes in his preprint (1507.03010), is the supernova also known as 2015L.

http://www.rochesterastronomy.org/sn2015/index.html wrote:

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2015L (= ASASSN-15lh), ATEL 7642 discovered 2015/06/14.250
by All Sky Automated Survey for SuperNovae (ASAS-SN)

Found in an anonymous galaxy at R.A. = 22h02m15s.45, Decl. = -61°39'34".6
Located 0".41 east and 0".04 south of the center of the host galaxy