ASASSN-15lh

ASASSN-15lh
Observation data (Epoch J2000)
Supernova type Hypernova SLSNe (Type Ic)
Host galaxy APMUKS(BJ) B215839.70−615403.9
Constellation Indus
Right ascension 22h 2m 15.45s[1]
Declination −61° 39′ 34.64″[1]
Discovery date June 15, 2015
Peak magnitude (V) 16.9
Redshift z=0.2326
Distance 1,171 megaparsecs
3.82 gigalight-years
Designations
SN 2015L

ASASSN-15lh (supernova designation SN 2015L[2]) is a superluminous supernova that was detected by the All Sky Automated Survey for SuperNovae (ASAS-SN) in 2015 in the southern constellation Indus.[1] The discovery, confirmed by ASAS-SN group with several other telescopes,[1] was formally described and published in a Science article[3] led by Subo Dong at the Kavli Institute of Astronomy and Astrophysics (Peking University, China) on January 15, 2016. ASASSN-15lh is the most luminous supernova ever detected; at its brightest it was approximately 50 times more luminous than the whole Milky Way galaxy,[4] with an energy flux 570 billion times greater than the Sun.[5][6] The peak absolute magnitude was −23.5, putting out 2.2×1038 watts. Energy radiated exceeded 1.1×1045 joules in the first fifty days.[3] The supernova was at redshift 0.2326, in a stagnant but luminous galaxy[3] some 3.8 billion light years from Earth.[6] 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."[7]

Discovery

A possible supernova was first noticed during an observation in June 2015 by ASAS-SN’s twin 14-cm telescopes in Chile; the team gave it the designation ASASSN-15lh.[8] The supernova appeared as a transient dot of light on an image and was confirmed with additional observations from other telescopes.[8] The spectrum of ASASSN-15lh was provided by the 2.5-meter du Pont Telescope in Chile.[8] 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 24 July, the event formally received the supernova designation SN 2015L from the Central Bureau of Astronomical Telegrams.[2]

At its peak, ASASSN-15lh was twice as luminous as the previous brightest superluminous supernova, iPTF13ajg.[9] Later on, other images were found to have been made of the supernova as early as May 8, 2015. At this stage the magnitude was 17.4.[3] From May 8 the 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]

Previously observed superluminous supernovae (SLSNs), also known as hypernovae, have been discovered in relatively small and "busy" star-producing galaxies, in contrast to ASASSN-15lh, which lies in a large and relatively calm galaxy of a type not previously known for producing SLSNs.[10] Such explosions are 100 to 1,000 times as powerful as typical supernovae.[4]

Light from the supernova showed some broad absorption lines due to singly ionized oxygen (O II) and magnesium (Mg II). No hydrogen or helium was detected.[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.[10] The constraints are such that the magnetar model has been described as "just barely" able to explain the data,[8] but is nevertheless still thought to be "the most plausible explanation for ASASSN-15lh’s unprecedented brightness".[11] If instead it was caused by the death of a huge star astronomers should eventually be able to detect the "fading afterglow caused by the gradual decay of 30 solar masses’ worth of radioactive nickel";[8] the data indicate that the supernova was hydrogen-poor,[1][8] and previously observed hydrogen-poor superluminous supernovae explosions cannot be fully explained by their radioactive nickel-56.[1] Still other scientists have suggested the explosion may mark the birth of a strange quark star.[12] Another mechanism for powering super-energetic supernovae is based on jets launched from a newly born black hole.[13]

Observations by the Hubble Space Telescope scheduled for February 2016 may give rise to another scenario, albeit one thought to be unlikely, that the supernova is in the nucleus of its galaxy and that its brightness is due to its interaction with a supermassive black hole there.[11]

Progenitor

ASASSN-15lh emitted a blue-colored light towards Earth.[1] The progenitor of ASASSN-15lh may have been a massive and rapidly rotating blue, hot star,[11] which appears to have shed its surface layers of hydrogen and helium not long before its explosion as those elements are absent in ASASSN-15lh.[11] It may have been a Wolf–Rayet star.[11]

Galaxy

The host galaxy for ASASSN-15lh is APMUKS(BJ) B215839.70−615403.9.[3] The host galaxy has visual magnitude 18.5 and is red in color. 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]

References

  1. 1 2 3 4 5 6 7 Young, Monica (July 12, 2015). "The Most Luminous Supernova". Sky & Telescope. Retrieved January 16, 2016.
  2. 1 2 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 (24 July 2015). Daniel W. E. Green, ed. "SN 2015L". Central Bureau Electronic Telegrams. Central Bureau for Astronomical Telegrams. 4120. Bibcode:2015CBET.4120....1D.
  3. 1 2 3 4 5 6 7 8 9 10 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.; Thompson, I. B.; et al. (January 15, 2016). "ASASSN-15lh: A highly super-luminous supernova". Science. 351 (6270): 257–260. arXiv:1507.03010Freely accessible. Bibcode:2016Sci...351..257D. doi:10.1126/science.aac9613.
  4. 1 2 Carnegie Institution for Science (January 14, 2016). "Most-luminous supernova ever discovered". phys.org. Retrieved January 15, 2016.
  5. ↑ Cowen, Ron (January 14, 2016). "Brightest-ever supernova still baffles astronomers". Nature. doi:10.1038/nature.2016.19176.
  6. 1 2 Bhanoo, Sindya N. (January 15, 2016). "Supernova Has Energy of Hundreds of Billions of Suns". The New York Times. Retrieved January 16, 2016.
  7. ↑ Boyle, Rebecca (January 14, 2016). "We've found the brightest ever supernova but can't explain it". New Scientist. Retrieved January 15, 2016.
  8. 1 2 3 4 5 6 Billings, Lee (January 14, 2016). "Found: The Most Powerful Supernova Ever Seen". Scientific American. Retrieved January 16, 2016.
  9. ↑ Young, Monica (January 15, 2016). "Brightest Supernova Baffles Astronomers". Sky & Telescope. Retrieved January 16, 2016.
  10. 1 2 Thomson, Jason (January 14, 2016). "Astronomers spot brightest supernova in history". The Christian Science Monitor. Retrieved January 15, 2016.
  11. 1 2 3 4 5 Smith-Strickland, Kiona (January 14, 2016). "Brightest supernova ever seen pushes theoretical models to the edge". Astronomy. Retrieved January 17, 2016.
  12. ↑ Dai, Zigao; Wang, S. Q.; Wang, J. S.; Wang, L. J.; Yu, Y. W. (31 August 2015). "Most Luminous Supernova ASASSN-15lh: Signature of a Newborn Rapidly-Rotating Strange Quark Star". arXiv:1508.07745Freely accessible.
  13. ↑ Gilkis, Avishai; Soker, Noam; Papish, Oded (4 November 2015). "Explaining the most luminous supernovae with an inefficient jet-feedback mechanism". arXiv:1511.01471Freely accessible.

Further reading

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