ASASSN-15lh

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ASASSN-15lh
ASASSN-15lhLightCurve.png
A visual band light curve for ASASSN-15lh, plotted from data published by Godoy-Rivera et al. (2017). [1] The purple line shows the brightness of the host galaxy.
SLSNe (Type Ic), SNSLSN-I  OOjs UI icon edit-ltr-progressive.svg
Right ascension 22h 2m 15.45s [2]
Declination −61° 39′ 34.64″ [2]
Distance1,171 megaparsecs
3.82 gigalight-years
Redshift 0.2326  OOjs UI icon edit-ltr-progressive.svg
HostAPMUKS(BJ) B215839.70−615403.9 [3]
Peak apparent magnitude 16.9
Other designationsSN 2015L
[ edit on Wikidata]

ASASSN-15lh (supernova designation SN 2015L) is an extremely luminous astronomical transient event discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN), with the appearance of a superluminous supernova event. It was first detected on June 14, 2015, located within a faint galaxy in the southern constellation Indus, and was the most luminous supernova-like object ever observed. [4] At its peak, ASASSN-15lh was 570 billion times brighter than the Sun, and 20 times brighter than the combined light emitted by the Milky Way Galaxy. [4] The emitted energy was exceeded by PS1-10adi.

Contents

The nature of ASASSN-15lh is disputed. The most popular explanations are that it is the most luminous type I supernova (hypernova) ever observed, or a tidal disruption event around a 108 M☉ supermassive black hole. [5] Other hypotheses include: gravitational lensing; a quark nova inside a Wolf–Rayet star; or a rapid magnetar spindown.

Discovery

A possible supernova was first noticed during an observation in June 2015 by ASAS-SN's twin 14-cm telescopes located at Cerro Tololo Inter-American Observatory, Chile; the team gave it the designation ASASSN-15lh. [6] It appeared as a transient dot of light on an image and was confirmed with additional observations from other telescopes. [6] The spectrum of ASASSN-15lh was provided by the 2.5-meter du Pont Telescope in Chile. [6] The Southern African Large Telescope was used to determine the redshift, and hence the distance and luminosity. [3] The Swift space telescope also contributed observations. [3] On July 24, the event formally received the supernova designation SN 2015L from the Central Bureau of Astronomical Telegrams. [7]

Later, other images were found to have been made of ASASSN-15lh as early as May 8, 2015. At this stage the visual magnitude was 17.4. From May 8 the possible supernova brightened until it reached a peak brightness of magnitude 16.9 on June 5. By September the brightness had dropped to magnitude 18.2. [3] There was an unusual "rebrightening" of up to 1.75 magnitudes at blue and ultraviolet wavelengths, starting about 90 days after the maximum. This coincided with a plateau in the bolometric luminosity that lasted for 120 days. [1]

Properties

Based on its redshift and location projected on the nucleus of a large galaxy, the distance of ASASSN-15lh is calculated at 1,171 Mpc, in a large luminous galaxy. [3]

At its peak, the absolute magnitude of ASASSN-15lh in the AB magnitude system u band was −23.5. Its bolometric luminosity is twice that of the previous brightest type-I superluminous supernova, iPTF13ajg. [8] At its brightest, it was approximately 50 times more luminous than the whole Milky Way galaxy, [9] with an energy flux 570 billion times greater than the Sun. [10] [11] The total energy radiated in the first 50 days exceeded 1.1×1045 joules. [3] According to Krzysztof Stanek of Ohio State University, one of the principal investigators at ASAS-SN, "If it was in our own galaxy, it would shine brighter than the full moon; there would be no night, and it would be easily seen during the day." [12]

The spectrum of ASASSN-15lh was relatively featureless, with no hydrogen or helium lines, but two very broad absorption bands. Ionised magnesium absorption doublets were detected and used to confirm the redshift at 0.2326. [3]

The temperature of ASASSN-15lh at the time of maximum luminosity was 20,000 K, although it was hotter earlier in the outburst. By 50 days after the peak, the temperature had declined to 11,000 K and then remained relatively constant. The radius of ASASSN-15lh at peak brightness was over 70,000 R☉. [3] The mass of ASSASN-15lh is 40 M☉

Host galaxy

The host galaxy for ASASSN-15lh is APMUKS(BJ) B215839.70−615403.9, much larger and more luminous than the Milky Way. The host galaxy has visual magnitude 18.5 and is red in color with a low rate of star formation. It maintained a steady brightness until the supernova lit up. The strongest parts of the galaxy's spectrum have wavelengths around 1 μm in the near infrared. [3]

Suggested mechanisms

The precise mechanism underlying the very large ASASSN-15lh explosion is still unknown, with speculation ranging from the presence of very large quantities of decaying nickel-56 to the amplifying effects of a magnetar. Its unusual location in a relatively quiescent galaxy may offer clues for scientists to discover and observe similar events. [13]

Superluminous supernova

The initial hypothesis was that ASASSN-15lh was the most extreme superluminous supernova (SLSN) so far seen, but it was recognized as being unusual in several respects. The spectrum did not closely match other type I SLSNe and previous SLSNe have been discovered in relatively small active star-producing galaxies, not in the central regions of large galaxies. [13] The double-peaked light curve is not expected from a SLSN and the total energy output approaches theoretical limits. [3]

The lack of hydrogen and helium features in the spectrum suggest an explosion originating in an object lacking both hydrogen and helium, which would imply a highly stripped star such as a massive Wolf–Rayet star. The energetics of the explosion would require a massive star.

Magnetar

One model for unusually luminous supernovae involves the conversion of rotational energy from a rapidly-spinning neutron star into polar jets that heat surrounding material. Again, the energy produced by ASASSN-15lh strains the theoretical limits of this type of explosion and the detailed properties are difficult to reproduce with a magnetar model. [3]

Quark nova

One unusual explanation for ASASSN-15lh is a quark-nova within the supernova explosion from a WO-type Wolf-Rayet star. The quark nova is produced by the neutron star remnant of the supernova and occurs a few days after the core collapse of the Wolf-Rayet star. This can reproduce many of the unusual features of the observed event but is somewhat speculative and not widely accepted. [14]

Tidal disruption event (TDE)

One known method for producing extremely large amounts of energy is the tidal disruption of objects such as stars by a supermassive black hole. ASASSN-15lh occurred in the nucleus of a large passive galaxy where a supermassive black hole is likely. A black hole of the mass expected in the host galaxy of ASASSN-15lh would normally swallow stars without a visible flare. The conditions for the production of a highly luminous flare from a TDE around a black hole of the expected mass are unusual, but a rapidly-spinning Kerr black hole might be able to disrupt a star with a mass similar to the sun outside the event horizon and produce a hot accretion disc and luminous transient. It could also account for the temperature changes, rebrightening, and unusual spectral evolution. [15] However, lack of hydrogen and/or helium lines in the spectra of ASASSN-15lh poses a major problem for the TDE scenario.

Gravitational lensing

Unexpectedly bright visible objects can be produced by gravitational lensing of very distant objects by extremely massive objects closer to Earth. However, this usually occurs with objects much more distant than ASASSN-15lh, and there are no observations indicating the presence of a galaxy cluster suitable to produce a lensing effect. [3]

Related Research Articles

<span class="mw-page-title-main">Supernova</span> Astrophysical phenomenon

A supernova is a powerful and luminous explosion of a star. A supernova occurs during the last evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed to form a diffuse nebula. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

<span class="mw-page-title-main">Indus (constellation)</span> Constellation in the southern celestial hemisphere

Indus is a constellation in the southern sky first professionally surveyed by Europeans in the 1590s and mapped on a globe by Petrus Plancius by early 1598. It was included on a plate illustrating southern constellations in Bayer's sky atlas Uranometria in 1603. It lies well south of the Tropic of Capricorn but its triangular shape can be seen for most of the year from the Equator. It is elongated from north to south and has a complex boundary. The English translation of its name is generally given as the Indian, though it is unclear which indigenous people the constellation was originally supposed to represent.

<span class="mw-page-title-main">Superluminous supernova</span> Supernova at least ten times more luminous than a standard supernova

A super-luminous supernova is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae. Like supernovae, SLSNe seem to be produced by several mechanisms, which is readily revealed by their light-curves and spectra. There are multiple models for what conditions may produce an SLSN, including core collapse in particularly massive stars, millisecond magnetars, interaction with circumstellar material, or pair-instability supernovae.

<span class="mw-page-title-main">Messier 82</span> Starburst galaxy in the constellation Ursa Major

Messier 82 (also known as NGC 3034, Cigar Galaxy or M82) is a starburst galaxy approximately 12 million light-years away in the constellation Ursa Major. It is the second-largest member of the M81 Group, with the D25 isophotal diameter of 12.52 kiloparsecs (40,800 light-years). It is about five times more luminous than the Milky Way and its central region is about one hundred times more luminous. The starburst activity is thought to have been triggered by interaction with neighboring galaxy M81. As one of the closest starburst galaxies to Earth, M82 is the prototypical example of this galaxy type. SN 2014J, a type Ia supernova, was discovered in the galaxy on 21 January 2014. In 2014, in studying M82, scientists discovered the brightest pulsar yet known, designated M82 X-2.

<span class="mw-page-title-main">Pinwheel Galaxy</span> Galaxy in the constellation Ursa Major

The Pinwheel Galaxy is a face-on, unbarred, and counterclockwise spiral galaxy located 21 million light-years from Earth in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 and was communicated that year to Charles Messier, who verified its position for inclusion in the Messier Catalogue as one of its final entries.

<span class="mw-page-title-main">NGC 300</span> Galaxy in the constellation Sculptor

NGC 300 (also known as Baldwell 70 or the Sculptor Pinwheel Galaxy) is a spiral galaxy in the constellation Sculptor. It was discovered on 5 August 1826 by Scottish astronomer James Dunlop. It is one of the closest galaxies to the Local Group, and it most likely lies between the latter and the Sculptor Group. It is the brightest of the five main spirals in the direction of the Sculptor Group. It is inclined at an angle of 42° when viewed from Earth and shares many characteristics of the Triangulum Galaxy. It is 94,000 light-years in diameter, somewhat smaller than the Milky Way, and has an estimated mass of (2.9 ± 0.2) × 1010M☉.

<span class="mw-page-title-main">SN 2006gy</span> 2006 hypernova in constellation Perseus

SN 2006gy was an extremely energetic supernova, also referred to as a hypernova, that was discovered on September 18, 2006. It was first observed by Robert Quimby and P. Mondol, and then studied by several teams of astronomers using facilities that included the Chandra, Lick, and Keck Observatories. In May 2007, NASA and several of the astronomers announced the first detailed analyses of the supernova, describing it as the "brightest stellar explosion ever recorded". In October 2007, Quimby announced that SN 2005ap had broken SN 2006gy's record as the brightest-ever recorded supernova, and several subsequent discoveries are brighter still. Time magazine listed the discovery of SN 2006gy as third in its Top 10 Scientific Discoveries for 2007.

<span class="mw-page-title-main">SN 2005ap</span> Energetic supernova ievent of 2005 in constellation Coma Berenices

SN 2005ap was an extremely energetic type Ic supernova in the galaxy SDSS J130115.12+274327.5. With a peak absolute magnitude of around −22.7, it is the second-brightest superluminous supernova yet recorded, twice as bright as the previous record holder, SN 2006gy, though SN 2005ap was eventually surpassed by ASASSN-15lh. It was initially classified as type II-L, but later revised to type Ic. It was discovered on 3 March 2005, on unfiltered optical images taken with the 0.45 m ROTSE-IIIb telescope, which is located at the McDonald Observatory in West Texas, by Robert Quimby, as part of the Texas Supernova Search that also discovered SN 2006gy. Although it was discovered before SN 2006gy, it was not recognized as being brighter until October 2007. As it occurred 4.7 billion light years from Earth, it was not visible to the naked eye.

<span class="mw-page-title-main">SCP 06F6</span> Astronomical object discovered in 2006

SCP 06F6 is an astronomical object of unknown type, discovered on 21 February 2006 in the constellation Boötes during a survey of galaxy cluster CL 1432.5+3332.8 with the Hubble Space Telescope's Advanced Camera for Surveys Wide Field Channel.

<span class="mw-page-title-main">SN 2011fe</span> Supernova in the Pinwheel Galaxy

SN 2011fe, initially designated PTF 11kly, was a Type Ia supernova discovered by the Palomar Transient Factory (PTF) survey on 24 August 2011 during an automated review of images of the Messier 101 from the nights of 22 and 23 August 2011. It was located in Messier 101, the Pinwheel Galaxy, 21 million light years from Earth. It was observed by the PTF survey very near the beginning of its supernova event, when it was approximately 1 million times too dim to be visible to the naked eye. It is the youngest type Ia ever discovered. About 13 September 2011, it reached its maximum brightness of apparent magnitude +9.9 which equals an absolute magnitude of about -19, equal to 2.5 billion Suns. At +10 apparent magnitude around 5 September, SN 2011fe was visible in small telescopes. As of 30 September the supernova was at +11 apparent magnitude in the early evening sky after sunset above the northwest horizon. It had dropped to +13.7 as of 26 November 2011.

<span class="mw-page-title-main">Tidal disruption event</span> Pulling apart of a star by tidal forces when it gets too close to a supermassive black hole

A tidal disruption event (TDE) is a transient astronomical source produced when a star passes so close to a supermassive black hole (SMBH) that it is pulled apart by the black hole's tidal force. The star undergoes spaghettification, producing a tidal stream of material that loops around the black hole. Some portion of the stellar material is captured into orbit, forming an accretion disk around the black hole, which emits electromagnetic radiation. In a small fraction of TDEs, a relativistic jet is also produced. As the material in the disk is gradually consumed by the black hole, the TDE fades over several months or years.

The All Sky Automated Survey for SuperNovae (ASAS-SN) is an automated program to search for new supernovae and other astronomical transients, headed by astronomers from the Ohio State University, including Christopher Kochanek and Krzysztof Stanek. It has 20 robotic telescopes in both the northern and southern hemispheres. It can survey the entire sky approximately once every day.

<span class="mw-page-title-main">V5856 Sagittarii</span> Nova that occurred in 2016

V5856 Sagittarii, also known as Nova Sagittarii 2016 Number 4, was the 4th and brightest nova that occurred in the constellation Sagittarius during 2016. It was discovered by the All Sky Automated Survey for SuperNovae on 25.02 October 2016, at which time it had an apparent visual magnitude of 13.7. It was independently discovered by Yukio Sakurai of Mito, Ibaraki, Japan on 26.38 October 2016, by which time it had reached magnitude 10.4. It reached its peak brightness of magnitude 5.4, making it visible to the naked eye, on 8 November 2016. The nova occurred within a region of the sky monitored by the OGLE microlensing experiment, and that group reported that no star brighter than magnitude 22 was seen at the nova's position prior to its eruption.

<span class="mw-page-title-main">Hypernova</span> Supernova that ejects a large mass at unusually high velocity

A hypernova is a very energetic supernova which is believed to result from an extreme core collapse scenario. In this case, a massive star collapses to form a rotating black hole emitting twin astrophysical jets and surrounded by an accretion disk. It is a type of stellar explosion that ejects material with an unusually high kinetic energy, an order of magnitude higher than most supernovae, with a luminosity at least 10 times greater. Hypernovae release such intense gamma rays that they often appear similar to a type Ic supernova, but with unusually broad spectral lines indicating an extremely high expansion velocity. Hypernovae are one of the mechanisms for producing long gamma ray bursts (GRBs), which range from 2 seconds to over a minute in duration. They have also been referred to as superluminous supernovae, though that classification also includes other types of extremely luminous stellar explosions that have different origins.

<span class="mw-page-title-main">PS1-10adi</span> 2010 transient event in the constellation Delphinus

PS1-10adi is an unusual kind of highly energetic optical transient discovered by the Pan-STARRS survey on 15 August 2010. The explosion or transient event emitted 2.3×1052 ergs (2.3×1045 Joules), exceeding ASASSN-15lh. It may be a superluminous supernova or a stellar disruption event. The magnitude of the explosion challenges the limits of the current models for theoretical physics.

<span class="mw-page-title-main">SN 2018cow</span> Supernova event of June 2018 in the constellation Hercules

SN 2018cow was a very powerful astronomical explosion, 10–100 times brighter than a normal supernova, spatially coincident with galaxy CGCG 137-068, approximately 200 million ly (60 million pc) distant in the Hercules constellation. It was discovered on 16 June 2018 by the ATLAS-HKO telescope, and had generated significant interest among astronomers throughout the world. Later, on 10 July 2018, and after AT 2018cow had significantly faded, astronomers, based on follow-up studies with the Nordic Optical Telescope (NOT), formally described AT 2018cow as SN 2018cow, a type Ib supernova, showing an "unprecedented spectrum for a supernova of this class"; although others, mostly at first but also more recently, have referred to it as a type Ic-BL supernova. An explanation to help better understand the unique features of AT 2018cow has been presented. AT2018cow is one of the few reported Fast Blue Optical Transients (FBOTs) observed in the Universe. In May 2020, however, a much more powerful FBOT than AT 2018cow was reportedly observed.

ASASSN-19bt was a tidal disruption event (TDE) discovered by the All Sky Automated Survey for SuperNovae (ASAS-SN) project, with early-time, detailed observations by the TESS satellite. It was first detected on January 21, 2019, and reached peak brightness on March 4. The black hole which caused the TDE is in the 16th magnitude galaxy 2MASX J07001137-6602251 in the constellation Volans at a redshift of 0.0262, around 375 million light years away.

SN 2016aps is the brightest and most energetic supernova explosion ever recorded. It released more energy than ASASSN-15lh. In addition to the sheer amount of energy released, an unusually large amount of the energy was released in the form of radiation, probably due to the interaction of the supernova ejecta and a previously lost gas shell.

An Intermediate Luminosity Optical Transient (ILOT) is an astronomical object which undergoes an optically detectable explosive event with an absolute magnitude (M) brighter than a classical nova (M ~ −8) but fainter than that of a supernova (M ~ −17). That nine magnitude range corresponds to a factor of nearly 4000 in luminosity, so the ILOT class may include a wide variety of objects. The term ILOT first appeared in a 2009 paper discussing the nova-like event NGC 300 OT2008-1. As the term has gained more widespread use, it has begun to be applied to some objects like KjPn 8 and CK Vulpeculae for which no transient event has been observed, but which may have been dramatically affected by an ILOT event in the past. The number of ILOTs known is expected to increase substantially when the Vera C. Rubin Observatory becomes operational.

<span class="mw-page-title-main">NGC 5996</span> Spiral galaxy in the constellation Serpens

NGC 5996 is a barred spiral galaxy in the constellation Serpens. The galaxy lies about 125 million light years away from Earth, which means, given its apparent dimensions, that NGC 5996 is approximately 65,000 light years across. It was discovered by William Herschel on March 21, 1784.

References

  1. 1 2 Godoy-Rivera, D.; Stanek, K. Z.; Kochanek, C. S.; Chen, Ping; Dong, Subo; Prieto, J. L.; Shappee, B. J.; Jha, S. W.; Foley, R. J.; Pan, Y.-C.; Holoien, T. W.-S.; Thompson, Todd. A.; Grupe, D.; Beacom, J. F. (2017). "The unexpected, long-lasting, UV rebrightening of the superluminous supernova ASASSN-15lh". Monthly Notices of the Royal Astronomical Society. 466 (2): 1428. arXiv: 1605.00645 . Bibcode:2017MNRAS.466.1428G. doi: 10.1093/mnras/stw3237 . S2CID   119253925.
  2. 1 2 Young, Monica (July 12, 2015). "The Most Luminous Supernova". Sky & Telescope. Retrieved January 16, 2016.
  3. 1 2 3 4 5 6 7 8 9 10 11 12 Dong, S.; Shappee, B. J.; Prieto, J. L.; Jha, S. W.; Stanek, K. Z.; Holoien, T. W.- S.; Kochanek, C. S.; Thompson, T. A.; Morrell, N.; et al. (January 15, 2016). "ASASSN-15lh: A highly super-luminous supernova". Science . 351 (6270): 257–260. arXiv: 1507.03010 . Bibcode:2016Sci...351..257D. doi:10.1126/science.aac9613. PMID   26816375. S2CID   31444274.
  4. 1 2 Jessica Orwig (January 14, 2016). "Astronomers are baffled by a newly discovered cosmic explosion that shines 570 billion times brighter than the sun". Business Insider . Retrieved March 22, 2016.
  5. ↑ Leloudas, G.; Fraser, M.; Stone, N. C.; van Velzen, S.; Jonker, P. G.; Arcavi, I.; Fremling, C.; Maund, J. R.; Smartt, S. J.; Krìhler, T.; Miller-Jones, J. C. A.; Vreeswijk, P. M.; Gal-Yam, A.; Mazzali, P. A.; De Cia, A.; Howell, D. A.; Inserra, C.; Patat, F.; de Ugarte Postigo, A.; Yaron, O.; Ashall, C.; Bar, I.; Campbell, H.; Chen, T.-W.; Childress, M.; Elias-Rosa, N.; Harmanen, J.; Hosseinzadeh, G.; Johansson, J.; Kangas, T.; Kankare, E.; Kim, S.; Kuncarayakti, H.; Lyman, J.; Magee, M. R.; Maguire, K.; Malesani, D.; Mattila, S.; McCully, C. V.; Nicholl, M.; Prentice, S.; Romero-Cañizales, C.; Schulze, S.; Smith, K. W.; Sollerman, J.; Sullivan, M.; Tucker, B. E.; Valenti, S.; Wheeler, J. C.; Young, D. R. (December 12, 2016). "The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole". Nature Astronomy. 1 (1): 0002. arXiv: 1609.02927 . Bibcode:2016NatAs...1E...2L. doi:10.1038/s41550-016-0002.
  6. 1 2 3 Billings, Lee (January 14, 2016). "Found: The Most Powerful Supernova Ever Seen". Scientific American . Retrieved January 16, 2016.
  7. ↑ S. Dong; B.J. Shappee; J.L. Prieto; S.W. Jha; K.Z. Stanek; T.W.-S. Holoien; C.S. Kochanek; T.A. Thompson; N. Morrell; I.B. Thompson; U. Basu; J.F. Beacom; D. Bersier; J. Brimacombe; J.S. Brown; P. Chen; E. Conseil; A.B. Danilet; E. Falco; D. Grupe; S. Kiyota; G. Masi; B. Nicholls; F. Olivares; G. Pignata; G. Pojmanski; G.V. Simonian; D.M. Szczygiel; P.R. Wozniak (July 24, 2015). Daniel W. E. Green (ed.). "SN 2015L". Central Bureau Electronic Telegrams. 4120. Central Bureau for Astronomical Telegrams: 1. Bibcode:2015CBET.4120....1D.
  8. ↑ Young, Monica (January 15, 2016). "Brightest Supernova Baffles Astronomers". Sky & Telescope. Retrieved January 16, 2016.
  9. ↑ Carnegie Institution for Science (January 14, 2016). "Most-luminous supernova ever discovered". phys.org. Retrieved January 15, 2016.
  10. ↑ Cowen, Ron (January 14, 2016). "Brightest-ever supernova still baffles astronomers". Nature. doi:10.1038/nature.2016.19176. S2CID   182077393.
  11. ↑ Bhanoo, Sindya N. (January 15, 2016). "Supernova Has Energy of Hundreds of Billions of Suns". The New York Times. Retrieved January 16, 2016.
  12. ↑ Boyle, Rebecca (January 14, 2016). "We've found the brightest ever supernova but can't explain it". New Scientist. Retrieved January 15, 2016.
  13. 1 2 Thomson, Jason (January 14, 2016). "Astronomers spot brightest supernova in history". The Christian Science Monitor. Retrieved January 15, 2016.
  14. ↑ Ouyed, Rachid; Leahy, Denis; Welbanks, Luis; Koning, Nico (2016). "The Superluminous (Type I) Supernova ASASSN-15lh : A case for a Quark-Nova inside an Oxygen-type Wolf-Rayet supernova remnant". arXiv: 1611.03657 [astro-ph.HE].
  15. ↑ Leloudas, G.; Fraser, M.; Stone, N. C.; Van Velzen, S.; Jonker, P. G.; Arcavi, I.; Fremling, C.; Maund, J. R.; Smartt, S. J.; et al. (2016). "The superluminous transient ASASSN-15lh as a tidal disruption event from a Kerr black hole". Nature Astronomy. 1: 0002. arXiv: 1609.02927 . Bibcode:2016NatAs...1E...2L. doi:10.1038/s41550-016-0002. S2CID   73645264.

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