Superluminous supernova

Last updated
NASA artist's impression of the explosion of SN 2006gy, a superluminous supernova SN 2006gy, NASA illustration.jpg
NASA artist's impression of the explosion of SN 2006gy, a superluminous supernova

A super-luminous supernova (SLSN, plural super luminous supernovae or SLSNe) is a type of stellar explosion with a luminosity 10 or more times higher than that of standard supernovae. [1] 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 (CSM model), or pair-instability supernovae.

Contents

The first confirmed superluminous supernova connected to a gamma ray burst was not found until 2003, when GRB 030329 illuminated the Leo constellation. [2] SN 2003dh represented the death of a star 25 times more massive than the Sun, with material being blasted out at over a tenth the speed of light. [3]

Stars with M ≥ 40 M☉ are likely to produce superluminous supernovae. [4]

Classification

Discoveries of many SLSNe in the 21st century showed that not only were they more luminous by an order of magnitude than most supernovae, their remnants were also unlikely to be powered by the typical radioactive decay that is responsible for the observed energies of conventional supernovae.[ verification needed ]

SLSNe events use a separate classification scheme to distinguish them from the conventional type Ia, type Ib/Ic, and type II supernovae, [5] roughly distinguishing between the spectral signature of hydrogen-rich and hydrogen-poor events.[ verification needed ]

Hydrogen-rich SLSNe are classified as Type SLSN-II, with observed radiation passing through the changing opacity of a thick expanding hydrogen envelope. Most hydrogen-poor events are classified as Type SLSN-I, with its visible radiation produced from a large expanding envelope of material powered by an unknown mechanism. A third less common group of SLSNe is also hydrogen-poor and abnormally luminous, but clearly powered by radioactivity from 56Ni. [6] [ verification needed ]

Increasing number of discoveries find that some SLSNe do not fit cleanly into these three classes, so further sub-classes or unique events have been described. Many or all SLSN-I show spectra without hydrogen or helium but have lightcurves comparable to conventional type Ic supernovae, and are now classed as SLSN-Ic. [7] PS1-10afx is an unusually red hydrogen-free SLSN with an extremely rapid rise to a near-record peak luminosity and an unusually rapid decline. [8] PS1-11ap is similar to a type Ic SLSN but has an unusually slow rise and decline. [7]

Astrophysical models

A wide variety of causes have been proposed to explain events that are an order of magnitude or more greater than standard supernovae. The collapsar and CSM (circumstellar material) models are generally accepted and a number of events are well-observed. Other models are still only tentatively accepted or remain entirely theoretical.

Collapsar model

Light curves compared to normal supernovae Comparative hypernova light curves.png
Light curves compared to normal supernovae

The collapsar model is a type of superluminous supernova that produces a gravitationally collapsed object, or black hole. The word "collapsar", short for "collapsed star", was formerly used to refer to the end product of stellar gravitational collapse, a stellar-mass black hole. The word is now sometimes used to refer to a specific model for the collapse of a fast-rotating star. When core collapse occurs in a star with a core at least around fifteen times the Sun's mass (M☉)—though chemical composition and rotational rate are also significant—the explosion energy is insufficient to expel the outer layers of the star, and it will collapse into a black hole without producing a visible supernova outburst.

A star with a core mass slightly below this level—in the range of 5–15 M☉—will undergo a supernova explosion, but so much of the ejected mass falls back onto the core remnant that it still collapses into a black hole. If such a star is rotating slowly, then it will produce a faint supernova, but if the star is rotating quickly enough, then the fallback to the black hole will produce relativistic jets. The energy that these jets transfer into the ejected shell renders the visible outburst substantially more luminous than a standard supernova. The jets also beam high energy particles and gamma rays directly outward and thereby produce x-ray or gamma-ray bursts; the jets can last for several seconds or longer and correspond to long-duration gamma-ray bursts, but they do not appear to explain short-duration gamma-ray bursts.

Stars with 5–15 M☉ cores have an approximate total mass of 25–90 M☉, assuming the star has not undergone significant mass loss. Such a star will still have a hydrogen envelope and will explode as a Type II supernova. Faint Type II supernovae have been observed, but no definite candidates for a Type II SLSN (except type IIn, which are not thought to be jet supernovae). Only the very lowest metallicity population III stars will reach this stage of their life with little mass loss. Other stars, including most of those visible to us, will have had most of their outer layers blown away by their high luminosity and become Wolf-Rayet stars. Some theories propose these will produce either Type Ib or Type Ic supernovae, but none of these events so far has been observed in nature. Many observed SLSNe are likely Type Ic. Those associated with gamma-ray bursts are almost always Type Ic, being very good candidates for having relativistic jets produced by fallback to a black hole. However, not all Type Ic SLSNe correspond to observed gamma-ray bursts but the events would only be visible if one of the jets were aimed towards us.

In recent years, much observational data on long-duration gamma-ray bursts have significantly increased our understanding of these events and made clear that the collapsar model produces explosions that differ only in detail from more or less ordinary supernovae and have energy ranges from approximately normal to around 100 times larger.

A good example of a collapsar SLSN is SN 1998bw, [9] which was associated with the gamma-ray burst GRB 980425. It is classified as a type Ic supernova due to its distinctive spectral properties in the radio spectrum, indicating the presence of relativistic matter.

Circumstellar material model

Almost all observed SLSNe have had spectra similar to either a type Ic or type IIn supernova. The type Ic SLSNe are thought to be produced by jets from fallback to a black hole, but type IIn SLSNe have significantly different light curves and are not associated with gamma-ray bursts. Type IIn supernovae are all embedded in a dense nebula probably expelled from the progenitor star itself, and this circumstellar material (CSM) is thought to be the cause of the extra luminosity. [10] When material expelled in an initial normal supernova explosion meets dense nebular material or dust close to the star, the shockwave converts kinetic energy efficiently into visible radiation. This effect greatly enhances these extended duration and extremely luminous supernovae, even though the initial explosive energy was the same as that of normal supernovae.

Although any supernova type could potentially produce Type IIn SLSNe, theoretical constraints on the surrounding CSM sizes and densities do suggest that it will almost always be produced from the central progenitor star itself immediately prior to the observed supernova event. Such stars are likely candidates of hypergiants or LBVs that appear to be undergoing substantial mass loss, due to Eddington instability, for example, SN2005gl. [11]

Pair-instability supernova

Another type of suspected SLSN is a pair-instability supernova, of which SN 2006gy [12] may possibly be the first observed example. This supernova event was observed in a galaxy about 238 million light years (73 megaparsecs) from Earth.

The theoretical basis for pair-instability collapse has been known for many decades [13] and was suggested as a dominant source of higher mass elements in the early universe as super-massive population III stars exploded. In a pair-instability supernova, the pair production effect causes a sudden pressure drop in the star's core, leading to a rapid partial collapse. Gravitational potential energy from the collapse causes runaway fusion of the core which entirely disrupts the star, leaving no remnant.

Models show that this phenomenon only happens in stars with extremely low metallicity and masses between about 130 and 260 times the Sun, making them extremely unlikely in the local universe. Although originally expected to produce SLSN explosions hundreds of times greater than a normal supernova, current models predict that they actually produce luminosities ranging from about the same as a normal core collapse supernova to perhaps 50 times brighter, although remaining bright for much longer. [14]

Magnetar energy release

Models of the creation and subsequent spin-down of a magnetar yield much higher luminosities than regular supernova [15] [16] events and match the observed properties [17] [18] of at least some SLSNe. In cases where pair-instability supernova may not be a good fit for explaining a SLSN, [19] a magnetar explanation is more plausible.

Other models

There are still models for SLSN explosions produced from binary systems, white dwarf or neutron stars in unusual arrangements or undergoing mergers, and some of these are proposed to account for some observed gamma-ray bursts.

See also

Related Research Articles

<span class="mw-page-title-main">Supernova</span> Explosion of a star at its end of life

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">SN 1987A</span> 1987 supernova event in the constellation Dorado

SN 1987A was a type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately 51.4 kiloparsecs from Earth and was the closest observed supernova since Kepler's Supernova in 1604. Light and neutrinos from the explosion reached Earth on February 23, 1987 and was designated "SN 1987A" as the first supernova discovered that year. Its brightness peaked in May of that year, with an apparent magnitude of about 3.

<span class="mw-page-title-main">Gamma-ray burst</span> Flashes of gamma rays from distant galaxies

In gamma-ray astronomy, gamma-ray bursts (GRBs) are immensely energetic explosions that have been observed in distant galaxies, described by NASA as "the most powerful class of explosions in the universe". They are the most energetic and luminous electromagnetic events since the Big Bang. Bursts can last from ten milliseconds to several hours. After an initial flash of gamma rays, a longer-lived "afterglow" is usually emitted at longer wavelengths.

<span class="mw-page-title-main">Magnetar</span> Type of neutron star with a strong magnetic field

A magnetar is a type of neutron star with an extremely powerful magnetic field (~109 to 1011 T, ~1013 to 1015 G). The magnetic-field decay powers the emission of high-energy electromagnetic radiation, particularly X-rays and gamma rays.

<span class="mw-page-title-main">Messier 74</span> Face-on spiral galaxy in the constellation Pisces

Messier 74 is a large spiral galaxy in the equatorial constellation Pisces. It is about 32 million light-years away from Earth. The galaxy contains two clearly defined spiral arms and is therefore used as an archetypal example of a grand design spiral galaxy. The galaxy's low surface brightness makes it the most difficult Messier object for amateur astronomers to observe. Its relatively large angular size and the galaxy's face-on orientation make it an ideal object for professional astronomers who want to study spiral arm structure and spiral density waves. It is estimated that M74 hosts about 100 billion stars.

<span class="mw-page-title-main">W49B</span> Supernova remnant nebula in the constellation Aquila

W49B is a nebula in Westerhout 49 (W49). The nebula is a supernova remnant, probably from a type Ib or Ic supernova that occurred around 1,000 years ago. It may have produced a gamma-ray burst and is thought to have left a black hole remnant.

<span class="mw-page-title-main">Type Ia supernova</span> Type of supernova in binary systems

A Type Ia supernova is a type of supernova that occurs in binary systems in which one of the stars is a white dwarf. The other star can be anything from a giant star to an even smaller white dwarf.

<span class="mw-page-title-main">Type Ib and Ic supernovae</span> Types of supernovae caused by a star collapsing

Type Ib and Type Ic supernovae are categories of supernovae that are caused by the stellar core collapse of massive stars. These stars have shed or been stripped of their outer envelope of hydrogen, and, when compared to the spectrum of Type Ia supernovae, they lack the absorption line of silicon. Compared to Type Ib, Type Ic supernovae are hypothesized to have lost more of their initial envelope, including most of their helium. The two types are usually referred to as stripped core-collapse supernovae.

<span class="mw-page-title-main">Type II supernova</span> Explosion of a star 8 to 45 times the mass of the Sun

A Type II supernova or SNII results from the rapid collapse and violent explosion of a massive star. A star must have at least eight times, but no more than 40 to 50 times, the mass of the Sun (M☉) to undergo this type of explosion. Type II supernovae are distinguished from other types of supernovae by the presence of hydrogen in their spectra. They are usually observed in the spiral arms of galaxies and in H II regions, but not in elliptical galaxies; those are generally composed of older, low-mass stars, with few of the young, very massive stars necessary to cause a supernova.

<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">Pair-instability supernova</span> Type of high-energy supernova in very large stars

A pair-instability supernova is a type of supernova predicted to occur when pair production, the production of free electrons and positrons in the collision between atomic nuclei and energetic gamma rays, temporarily reduces the internal radiation pressure supporting a supermassive star's core against gravitational collapse. This pressure drop leads to a partial collapse, which in turn causes greatly accelerated burning in a runaway thermonuclear explosion, resulting in the star being blown completely apart without leaving a stellar remnant behind.

<span class="mw-page-title-main">Gamma-ray burst progenitors</span> Types of celestial objects that can emit gamma-ray bursts

Gamma-ray burst progenitors are the types of celestial objects that can emit gamma-ray bursts (GRBs). GRBs show an extraordinary degree of diversity. They can last anywhere from a fraction of a second to many minutes. Bursts could have a single profile or oscillate wildly up and down in intensity, and their spectra are highly variable unlike other objects in space. The near complete lack of observational constraint led to a profusion of theories, including evaporating black holes, magnetic flares on white dwarfs, accretion of matter onto neutron stars, antimatter accretion, supernovae, hypernovae, and rapid extraction of rotational energy from supermassive black holes, among others.

<span class="mw-page-title-main">Supernova impostor</span> Stellar explosions that appear to be supernovae

Supernova impostors are stellar explosions that appear at first to be a supernova but do not destroy their progenitor stars. As such, they are a class of extra-powerful novae. They are also known as Type V supernovae, Eta Carinae analogs, and giant eruptions of luminous blue variables (LBV).

GRB 111209A is the longest lasting gamma-ray burst (GRB) detected by the Swift Gamma-Ray Burst Mission, observed on December 9, 2011. Its duration is longer than 7 hours, implying this event has a different kind of progenitor than normal long GRBs. It was first proposed that the progenitor of this event was a blue supergiant star with low metallicity. Later, it was also proposed that this event is the prototype of a new class of GRBs, ultra-long GRBs.

<span class="mw-page-title-main">ASASSN-15lh</span> 2015 hypernova event in the constellation Indus

ASASSN-15lh 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. 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. The emitted energy was exceeded by PS1-10adi.

A failed supernova is an astronomical event in time domain astronomy in which a star suddenly brightens as in the early stage of a supernova, but then does not increase to the massive flux of a supernova. They could be counted as a subcategory of supernova imposters. They have sometimes misleadingly been called unnovae.

<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 so much of gamma rays they usually 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.

iPTF14hls Supernova star

iPTF14hls is an unusual supernova star that erupted continuously for about 1,000 days beginning in September 2014 before becoming a remnant nebula. It had previously erupted in 1954. None of the theories nor proposed hypotheses fully explain all the aspects of the object.

<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.

Ken'ichi Nomoto is a Japanese astrophysicist and astronomer, known for his research on stellar evolution, supernovae, and the origin of heavy elements.

References

  1. ↑ MacFadyen (2001). "Supernovae, Jets, and Collapsars". The Astrophysical Journal. 550 (1): 410–425. arXiv: astro-ph/9910034 . Bibcode:2001ApJ...550..410M. doi:10.1086/319698. S2CID   1673646.
  2. ↑ Dado (2003). "The Supernova associated with GRB 030329". Astrophysical Journal. 594 (2): L89–92. arXiv: astro-ph/0304106 . Bibcode:2003ApJ...594L..89D. doi:10.1086/378624. S2CID   10668797.
  3. ↑ Krehl (2009). History of Shock Waves, Explosions, and Impact. Bibcode:2009hswe.book.....K.
  4. ↑ Heger (2003). "How Massive Stars End Their Life". Astrophysical Journal. 591 (1): 288–300. arXiv: astro-ph/0212469 . Bibcode:2003ApJ...591..288H. doi:10.1086/375341. S2CID   59065632.
  5. ↑ Quimby, R. M.; Kulkarni, S. R.; Kasliwal, M. M.; Gal-Yam, A.; Arcavi, I.; Sullivan, M.; Nugent, P.; Thomas, R.; Howell, D. A.; et al. (2011). "Hydrogen-poor superluminous stellar explosions". Nature. 474 (7352): 487–9. arXiv: 0910.0059 . Bibcode:2011Natur.474..487Q. doi:10.1038/nature10095. PMID   21654747. S2CID   4333823.
  6. ↑ Gal-Yam, Avishay (2012). "Luminous Supernovae". Science. 337 (6097): 927–32. arXiv: 1208.3217 . Bibcode:2012Sci...337..927G. doi:10.1126/science.1203601. PMID   22923572. S2CID   206533034.
  7. 1 2 McCrum, M.; Smartt, S. J.; Kotak, R.; Rest, A.; Jerkstrand, A.; Inserra, C.; Rodney, S. A.; Chen, T.- W.; Howell, D. A.; et al. (2013). "The superluminous supernova PS1-11ap: Bridging the gap between low and high redshift". Monthly Notices of the Royal Astronomical Society. 437 (1): 656–674. arXiv: 1310.4417 . Bibcode:2014MNRAS.437..656M. doi:10.1093/mnras/stt1923. S2CID   119224139.
  8. ↑ Chornock, R.; Berger, E.; Rest, A.; Milisavljevic, D.; Lunnan, R.; Foley, R. J.; Soderberg, A. M.; Smartt, S. J.; Burgasser, Adam J.; et al. (2013). "PS1-10afx at z = 1.388: Pan-STARRS1 Discovery of a New Type of Superluminous Supernova". The Astrophysical Journal. 767 (2): 162. arXiv: 1302.0009 . Bibcode:2013ApJ...767..162C. doi:10.1088/0004-637X/767/2/162. S2CID   35006667.
  9. ↑ Fujimoto, S. I.; Nishimura, N.; Hashimoto, M. A. (2008). "Nucleosynthesis in Magnetically Driven Jets from Collapsars". The Astrophysical Journal. 680 (2): 1350–1358. arXiv: 0804.0969 . Bibcode:2008ApJ...680.1350F. doi:10.1086/529416. S2CID   118559576.
  10. ↑ Smith, N.; Chornock, R.; Li, W.; Ganeshalingam, M.; Silverman, J. M.; Foley, R. J.; Filippenko, A. V.; Barth, A. J. (2008). "SN 2006tf: Precursor Eruptions and the Optically Thick Regime of Extremely Luminous Type IIn Supernovae". The Astrophysical Journal. 686 (1): 467–484. arXiv: 0804.0042 . Bibcode:2008ApJ...686..467S. doi:10.1086/591021. S2CID   16857223.
  11. ↑ Gal-Yam, A.; Leonard, D. C. (2009). "A Massive Hypergiant Star as the Progenitor of the Supernova SN 2005gl". Nature. 458 (7240): 865–867. Bibcode:2009Natur.458..865G. doi:10.1038/nature07934. PMID   19305392. S2CID   4392537.
  12. ↑ Smith, N.; Chornock, R.; Silverman, J. M.; Filippenko, A. V.; Foley, R. J. (2010). "Spectral Evolution of the Extraordinary Type IIn Supernova 2006gy". The Astrophysical Journal. 709 (2): 856–883. arXiv: 0906.2200 . Bibcode:2010ApJ...709..856S. doi:10.1088/0004-637X/709/2/856. S2CID   16959330.
  13. ↑ Fraley, G. S. (1968). "Supernovae Explosions Induced by Pair-Production Instability" (PDF). Astrophysics and Space Science. 2 (1): 96–114. Bibcode:1968Ap&SS...2...96F. doi:10.1007/BF00651498. S2CID   122104256.
  14. ↑ Kasen, D.; Woosley, S. E.; Heger, A. (2011). "Pair Instability Supernovae: Light Curves, Spectra, and Shock Breakout". The Astrophysical Journal. 734 (2): 102. arXiv: 1101.3336 . Bibcode:2011ApJ...734..102K. doi:10.1088/0004-637X/734/2/102. S2CID   118508934.
  15. ↑ Woosley, S.E. (August 2010). "Bright Supernovae From Magnetar Birth". Astrophysical Journal Letters. 719 (2): L204–L207. arXiv: 0911.0698 . Bibcode:2010ApJ...719L.204W. doi:10.1088/2041-8205/719/2/L204. S2CID   118564100.
  16. ↑ Kasen, Daniel; Bildsten, Lars (2010). "Supernova Light Curves Powered by Young Magnetars". Astrophysical Journal. 717 (1): 245–249. arXiv: 0911.0680 . Bibcode:2010ApJ...717..245K. doi:10.1088/0004-637X/717/1/245. S2CID   118630165.
  17. ↑ Inserra, C.; Smartt, S. J.; Jerkstrand, A.; Valenti, S.; Fraser, M.; Wright, D.; Smith, K.; Chen, T.-W.; Kotak, R.; et al. (June 2013). "Super Luminous Ic Supernovae: catching a magnetar by the tail". The Astrophysical Journal. 770 (2): 128. arXiv: 1304.3320 . Bibcode:2013ApJ...770..128I. doi:10.1088/0004-637X/770/2/128. S2CID   13122542.
  18. ↑ Howell, D. A.; Kasen, D.; Lidman, C.; Sullivan, M.; Conley, A.; Astier, P.; Balland, C.; Carlberg, R. G.; Fouchez, D.; et al. (October 2013). "Two superluminous supernovae from the early universe discovered by the Supernova Legacy Survey". Astrophysical Journal. 779 (2): 98. arXiv: 1310.0470 . Bibcode:2013ApJ...779...98H. doi:10.1088/0004-637X/779/2/98. S2CID   119119147.
  19. ↑ Nicholl, M.; Smartt, S. J.; Jerkstrand, A.; Inserra, C.; McCrum, M.; Kotak, R.; Fraser, M.; Wright, D.; Chen, T.-W.; et al. (October 2013). "Slowly fading super-luminous supernovae that are not pair-instability explosions". Nature. 502 (7471): 346–9. arXiv: 1310.4446 . Bibcode:2013Natur.502..346N. doi:10.1038/nature12569. PMID   24132291. S2CID   4472977.

Further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.