A long time ago in a galaxy far, far away, the terrible beauty of a supernova is witnessed as it occurred almost 4 billion years ago. For a moment in time ASASSN-15lh, a super-luminous supernova and the most powerful supernova ever observed, was twice as luminous as the previous record holder at its peak light. Its power and brilliance are without precedent for a single object, blazing with the light of 570 billion suns, its radiant output exceeded that of our Milky Way galaxy by a factor of 20! Following the earth’s formation 4.6 billion years ago and 800 million years later, as single-cell life was just emerging here on earth, the supernova’s progenitor star ended it’s life in spectacular fashion, sending the light on its long journey, heralding what happened so long ago and so far away.
Occurring in June of last year (minus 3.8 billion years), ASASSN-15lh was detected using the All-Sky Automated Survey for Supernovae headquartered at the Ohio State University’s Department of Astronomy. ASASSN is an international collaboration of professional and amateur astronomers who have built a responsive and dynamic network of small to mid-sized telescopes that continuously monitor the sky for supernovae. The lead investigator in the study of ASASSN-15lh, published in the magazine Science, is Subo Dong of Peking University’s Kavli Institute for Astronomy and Astrophysics.
Near the Small Cloud of Magellan, ASASSN-15lh is located in the southern constellation of Indus, close to the constellation’s boundary with Tucana (the Toucan). It was first observed with the collaboration’s twin 14-cm (5 inch) telescopes on Cerro Telolo, Chile. Superluminous supernovae are a new class of supernova and are poorly understood. A number of different scenarios are postulated to describe their enormous power and we will describe them including the theory most astronomers believe explains the brilliance and power of ASASSN-15lh.
Based on its energy output at peak light, with no existing precedent, Dong and the majority of astronomers, postulate that the supernova remnant and source of its brilliance is 30 solar masses of radioactive Nickel (56) enhanced by the highly-magnetized, rapidly-rotating neutron star or Magentar – not your standard supernova remnant- the remnant that what was left of the star’s core after the bang. Nickel (56) is the heavy-metal, nuclear dead-end, end-product (got that?) of every high-mass star’s productive life; see this article for an in-depth discussion of heavy-element nucleosynthesis in stars, and this one that discusses nuclear fusion, the basis for all of it. The following is an excerpt from the recently published paper (some terms highlighted and explained below):
The power source for ASASSN-15lh is unknown. Traditional mechanisms invoked for normal SNe likely cannot explain SLSNe-I . The lack of hydrogen or helium suggests that shock interactions with hydrogen-rich circumstellar material, invoked to interpret some SLSNe, cannot explain SLSNe-I or ASASSN-15lh. SLSN-I post-peak decline rates appear too fast to be explained by the radioactive decay of Ni(56) – the energy source for Type Ia supernovae. Both the decline rate of the late-time light curve and the integral method will allow tests of whether ASASSN-15lh is powered by Ni(56), and we estimate that M⊙(30) of Ni(56) would be required to produce ASASSN-15lh’s peak luminosity. Another possibility is that the spindown of a rapidly-rotating, highly-magnetic neutron star (a magnetar) powers the extraordinary emission. To match the peak Bolometric luminosity and timescale of ASASSN-15lh, the light-curve models imply a magnetar spin period and magnetic field strength of P ≃ 1 ms and B ≃ 10^14 G, respectively, assuming that all of the spindown power is normalized in the stellar envelope.
SLSN-I: hydrogen-poor super luminous supernova
SLSNe-I: a hydrogen-poor Super-Luminous Supernova event
Ni(56): the nuclide Nickel-56
M⊙(30): 30 solar masses
spindown: the slowing down of (and in this case the conversion of rotational energy to radiant energy)
Bolometric luminosity: the object’s total luminosity over all spectral wavebands
P ≃ 1 ms: period of 1 milli-second; the object rotates once in one milli-second or 1/1000 of a second!
B ≃ 10^14 G: magnetic field strength of 100 trillion Guass!
stellar envelope: the envelope of gas surrounding the SN
With any discovery of this kind in astronomy, comparison of multiple images acquired over days, weeks, months or even years are required to find a change. In the case of Pluto, Clyde Tombaugh compared 2 plates from January 23 and January 29, 1930. In the case of this supernova and supernovae in general, “blinking” images in succession, comparing them and looking for changes in time is necessary. It used to be a manual process, such as was the case with Pluto. Modern blink comparators are integrated digital processors, nodes in a computer network with the ability to detect subtleties which most likely would have been missed by legacy equipment. The initial discovery images for ASASSN-15lh were obtained by twin 0.14 meter (5 inch) telescopes, part of the Las Cumbres Observatory Global Telescope Network (LCOGT) at Cerro Tololo Inter-American Observatory, atop 2.2 Km high Cerro Tololo Mountain, Chile. The possibility of a new supernova was announced by Subo Dong on 16 June 2015, only 11 days past peak light according to the latest telegram. The follow-up confirmation images were a Dark Energy Survey Observatory 4-meter archival image and an LCOGT 1-meter telescope image. Note the strong blue cast of the host galaxy in the 1-meter confirmation image. Following the confirmation, a new announcement was promulgated on 8 July 2015, heralding ASASSN-15lh as the most luminous supernova ever observed. From that telegram, the following excerpts provide some discovery context and insight into the nature of ASASSN-15lh:
ASASSN-15lh was reported in ATel #7642 as a probable supernova in an unknown redshift galaxy APMUKS(BJ) B215839.70-615403.9. Detailed analysis of ASAS-SN data reveal that it peaked around June 5, 2015.
All spectra reveal very blue, mostly featureless continuum, except for broad (OII) absorption features characteristic of hydrogen-poor super-luminous supernovae — SLSN-I (prototype SN 2005ap, Quimby et al., 2007, ApJ 668, L99).
Following the confirmation telegram, ASASSN-15lh was named according to the standard naming convention of all supernovae: SN-2015L. In their paper the authors neglect to specify the mass of the progenitor star, a very important detail. Lets take a look at that. There are two types of supernovae, Type I and Type II with subclasses for both.
White Dwarfs and Type Ia Supernovae
First it should be pointed out that a supernova is the actual occurrence, the event itself. What we see afterwards is the supernova remnant. Thus, what we see in the sky now is a supernova remnant; the supernova has already occurred. The most common Type I supernova is Ia with the progenitor a white-dwarf member of a parasitic white-dwarf/main sequence binary system. After accreting material from the larger, “normal” star, the white dwarf reaches a critical mass of about 1.38 solar masses, a mass approaching the Chandrasekhar Mass limit of 1.44 solar masses. As it does so, temperatures below the surface of the white-dwarf approach the 600,000,000 Kelvin carbon-fusion ignition temperature. If carbon fusion reactions begin, a runaway thermonuclear chain reaction ensues, consuming all the hydrogen, quickly producing all the elements up to and including Ni(56) which subsequently decays into Cobalt(56) and Iron(56), a processes that produces the characteristic light curve of a Type Ia supernova and the afterglow. Thus, the radioactive decay of Ni(56) is the source of the supernova’s luminescence. Sufficient energy is produced that will completely unbind the star in a Type Ia supernova. Because these supernovae always occur at the same point at the same mass threshold, their luminosity is consistent and, as such, they are used as accurate standard candles. This will only occur for carbon-oxygen white-dwarfs or the stellar remnants of sun-like stars. For stars further up on the mass scale, whose white-dwarfs stellar remnants are composed of elements slightly more massive than carbon, such as oxygen, magnesium or neon, this doesn’t occur because the accreting mass buildup is insufficient to trigger oxygen ignition and begin the thermonuclear chain reaction.
How Do We Know What Type it is?
SLSN-I is a subset of Type I supernovae. Since all the hydrogen that accreted onto the White Dwarf was consumed during the runaway thermonuclear chain reaction, producing everything in the element chain up to and including Nickel (56), there was no hydrogen or helium remaining and thus, the spectrum of SN-2015L exhibited only metals and heavy elements with the predominat emission being Ni(56). Mg, Si, C and Oxygen absorption spectra are exhibited with no Hydrogen or Helium at all. Any evidence of Hydrogen or Helium would be indicative of a circumstellar shell of these elements, telltale signs of a Type-II, core collapse event. Detailed description of this process can be found in this discussion of stellar nucleosynthesis.
Neutron Stars and Black Holes
Since the star now exceeds the Chandrasekhar limit, the limit above which electron degeneracy pressure is unable to prevent further collapse, the star collapses to form a neutron star, a uniquely bizarre object whose density is that of an atomic nucleus, an atomic nucleus 12 km in radius! This class of object is between the Chandrasekhar mass limit (1.44 solar masses) and 3 solar masses. A neutron star above this mass, where outwardly directed quantum degeneracy pressure is insufficient to halt further collapse, collapse continues to form a Black Hole.
Now that we’ve established a basis for the possible types of supernovae to choose from, we can return to our discussion of the progenitor’s mass. The scenario where Ni(56) decay provides the energy (gamma rays) and subsequent heating of material works just fine for typical Type Ia events but strains credibility when presented as a possibility for the observed output of SN-2015L unless the ejecta is greater than 30 M⊙ and even this is pushing the limits of what is theoretically possible. In order for the ejecta of just Ni(56) to be 30 M⊙, regardless of its radiant output, the total mass of the progenitor had to be considerably higher, probably approaching 100 M⊙. Stars in that mass range are exceeding rare but, since we’re looking back across almost 4 billion years of time to an earlier epoch in the universe’s history, a time when conditions were different and stars were thought to be hotter and more massive, such massive stars may have been more prevalent. A possibility suggested by Dong and his collaborators is the stellar remnant, the magnetar described above, is enhancing the Ni(56) emission of the 30 M⊙ Nickel ejecta. Again, even when including the magnetar, that possibility strains credibility when considering the enormous power output observed. Other possibilities include:
- a pair instability supernova, another in the class of Superluminous Supernovae. A pair instability supernova is a Type II, core-collapse event that occurs for behemoth stars in the 120 M⊙ range. In the current epoch, the theoretical mass limit for any star is in this range. These stars are so massive and require so much energy to maintain the balance between outward pressure and the crushing gravity of 120 solar masses, the enormous gamma ray flux generated breaks up the nuclei in the core and causes it (and the rest of the star) to literally implode in on itself; thus, we would expect to see some evidence of hydrogen or helium. Although the observed energy output of SN-2015L is consistent with a pair instability supernova, the odds are slim that it is since we don’t observe any evidence of hydrogen or helium.
the birth of a Quasar. SN-2015L occurred within 750 parsecs (2500 light years) of the host galaxy’s nucleus. The host galaxy is typical for an aging, large galaxy that is about 50% larger than our Milky Way with about 200 billion stars, a stellar population comparable to the Milky Way’s. In short, the host galaxy is not that extraordinary. The power output of SN-2015L is consistent with luminosities at the low end of a quasars output range. The typical luminosity of a quasar is roughly 100x that of a galaxy such as the Milky Way. It cannot be ruled out that the host galaxy contains an “active nucleus” a nucleus that has, at its heart, an active, super-massive black hole, the powerhouse that drives a quasar. Such a galaxy is said to be an “Active Galaxy” and is in class known as AGN’s (Active Galactic Nuclei). Quasars are a subclass of AGNs. That SN-2015L’s power output is consistent with that of a small quasar and that it occurred so close to the galaxy’s nucleus suggests this possibility. Additionally, there are similarities between the spectrum of a typical quasar and that of SN-2015L. Quasars typically exhibit a broad continuum across many wave bands with a significant blue “bump” (note the decidedly blue cast to the optical image). This excerpt from the paper underscores that similarity: “All spectra reveal very blue, mostly featureless continuum, except for broad (OII) absorption features characteristic of hydrogen-poor super-luminous supernovae“.
- that SN-2015L is a transient event associated with the coming to life of a supermassive black hole in the host galaxy’s nucleus.
In order to ascertain the true nature of SN-2015L, continued observations are required using large aperture, ground-based telescopes across multiple wavebands. As well, and specifically towards this end, follow-up observations with the Hubble Space Telescope are planned for next month (February 2016).
Not since SN-1987a has there been such interest in supernovae, no doubt because of the unprecedented power of SN-2015L.
Regarding supernovae and things that go bang in the night, here are a few to keep an eye on closer to home:
- Eta Carinae, a supermassive star in the 120-150 solar mass range that is, quite literally, in its death throes. Having already erupted within the last century, its final demise is imminent. No need to worry though, as it’s 7,500 light years away. It will put on quite a light show when it does “go bang” but that’s about all it will do (at least for us).
- Betelgeuse, the brilliant red supergiant star of Orion’s right shoulder, is a supernova candidate and is exhibiting contractions and changes (yes, it is so large that we can actually “see” it change) consistent with a final collapse. It could happen tomorrow or 1,000 years from now (1,000 years is a heartbeat in stellar evolutionary time scales).
WR-104, a bizarre and exotic binary star system composed of two hot, high-mass Wolf-Rayet stars with one so far gone that it’s visibly losing mass, presenting as a celestial pinwheel embedding the companion star (mass loss such as what’s observed with WR-104 occurs in the later stages of a high-mass star’s evolution, right before it goes bang!) A Wolf-Rayet star is a highly evolved, high-mass star that presents with either no hydrogen and, in some cases, no helium spectra. It’s so hot and so luminous that the radiation pressure has literally “pushed off” the star’s outer layers and, in the more extreme cases, exposes the star’s carbon fusing core! WR-104 is at a comparable distance as Eta Carinae at about 7,800 light years towards our Milky Way’s galactic center. There was an earlier concern that since WR-104 was basically a potential Gamma Ray gun (when the star(s) finally do go bang, a GRB- gamma ray burst will likely ensue) with earth in its cross hairs that, even at almost 8,000 light years distant, we would still be fried by the gamma rays. Follow-up studies using the twin Keck 10 meter telescopes on Mauna Kea have refined the alignment angle of the WR-104 system with the earth to be about 30 – 40º off center – so we’ll survive and be witness to a spectacular light show!
Questions often arise regarding the minimum safe distance an observer should be from a supernova. For a typical Type Ia or Type II supernova, a minimum distance of no less than 50 light years but preferably closer to 100 light years. For SN-2015L, there is no minimum safe distance; no distance is too far away! To illustrate this point, if SN-2015L were at the distance of the star Vega (the “Contact” star, about 30 light years distant), it would be as bright as the noon day sun with an apparent magnitude of -24! We and everything else out to about 200 light years would be fried from the gamma rays – from a distance of 30 light years! If it were at the distance of the galactic center (about 25,000 light years), it would present with the brightness of the first quarter moon at about magnitude -9 and, finally, if it were on the other side of the galaxy, at about 100,000 light years, it would present with the brightness of a crescent moon at magnitude -6 but concentrated into point!
“Imagination is more important than knowledge”
An index of all articles in this blog can be found here.