Apep, pronounced /ˈɑːpɛp/, was named by a team of astronomers led by Joseph Callingham of ASTRON, who studied the system between 2016 and 2018 and published a scientific paper on their observations.[5][6] It was named after the mortal enemy of the sun godRa in Egyptian mythology, who was often illustrated as a giant serpent; their rivalry was described as "an apt allusion" to the appearance of the system and its stellar wind in infrared as "a star embattled within a dragon's coils".[5][7] In the XMM-Newton Serendipitous Source Catalogue (2XMM), a star catalog of X-ray sources observed by the XMM-Newton space telescope, the system is catalogued as 2XMM J160050.7–514245.[8] It is also known as WR 70-16.
Characteristics
Apep is a triple star system[6][9] containing a Wolf–Rayetbinary described as the "central engine", orbiting with a period of ~100 years,[10] and a third hot supergiant star described as the "northern companion", orbiting the central engine at a distance of ~1,700 astronomical units and a period of >10,000 years.[11] The binary at the centre of Apep is composed of two classical Wolf–Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes, making Apep the strongest case of a classical WR+WR binary system in the Milky Way.[3] Carbon-sequence Wolf–Rayet stars are often dust-making factories. A vast complex of stellar wind and cosmic dust surrounds the system,[7][9][12] resembling WR 104, another Wolf–Rayet star system producing a pinwheel nebula.[13] The wind, travelling at a velocity of 12 million km/h (7.5 million mph),[6][12] and dust travelling at 2 million km/h (1.2 million mph) at the edge of the system, suggest that at least one component of the central engine is rapidly rotating, so that its surface gravity is close to being balanced by its centrifugal force outwards.[9][14] This component produces faster stellar winds from its poles and slower winds from its equator, and the equatorial wind's interaction with the wind of its secondary produces the system's "pinwheel" shape.[15][16] Rapidly rotating Wolf–Rayet stars are theoretically capable of producing a gamma-ray burst during a supernova, and the system has been identified as a progenitor for a gamma-ray burst.[17] Apep is estimated to be at a distance of ~2.4 kiloparsecs,[18] or ~8,000 light-years,[10][19] from Earth, with a potential discrepancy of +0.2 and −0.5 kiloparsecs at its estimated visual extinction of 11.4.[18]
Apep is located in the constellation of Norma, at a right ascension of 16h 00m 50.5s and declination of −51° 42′ 45″,[1] The system can be resolved into two components, the "central engine" Wolf–Rayet binary, and the "northern companion" supergiant.[20] The total apparent magnitude of the system is 17.5, with the apparent magnitude of a resolved central engine and northern companion being 19.0 and 17.8 respectively.[21] Its infrared spectral energy distribution (SED) is unique, with brightness ranging from an apparent magnitude of 6.4 at 2.2 μm to −2.4 at 22 μm.[22] Surveys conducted with the European Southern Observatory (ESO)'s SINFONI instrument on the Very Large Telescope measured the apparent magnitude in the infrared J band for the central engine as 10.2±0.2, and for the northern companion as 9.6±0.2.[23] SINFONI also measured the apparent magnitude of the system in the K band as 6.9±0.2 for the central engine and 8.1±0.2 for C,[24] in the L band as 4.7±0.1 for the central engine and 7.3±0.1 for the northern companion,[24] and in the M band as 4.4±0.3 for the central engine and 7.0±0.2 for the northern companion.[24] SINFONI observations further detailed that the northern companion is possibly a conventional B1Ia+ high-luminosity star.[25] A and B show a typical spectrum from a WC7 star,[26] but with additional WN4 or WN5 star features theorised to be from one of the stars of the central engine; if confirmed, this would make Apep a rare binary system of WR stars.[27] An alternative hypothesis also based on SINFONI data proposes that the spectra could all be from an unusual transitional WN/WC star, and that the northern companion would then be a conventional OB star.[28] Combining the spectra of the WR stars EZ Canis Majoris and WR 90 would produce a spectrum almost identical to the one observed of the WR binary.
The system was the first gamma-ray burst progenitor candidate to be discovered in the Milky Way galaxy,[7] although it had not been known as such in early observations, such as those with the XMM-Newton and Chandraspace telescopes, where it had been identified simply as an X-ray source as early as August 2004.[29] Astronomer Joe Callingham first observed Apep during undergraduate studies at the University of Sydney with the Molonglo Observatory Synthesis Telescope,[12][30] and was noted as a potential colliding-wind binary, with a radio source as bright as Eta Carinae.[31] Callingham and Peter Tuthill, who led the discovery of WR 104 in 1998[32] and sought interest in Apep after observing its extreme infrared properties,[33][34] used the ESO's Very Large Telescope for observations in August 2016.[1][35] Further observations with the Anglo-Australian Telescope and the Australia Telescope Compact Array,[19] along with contributions from various international institutions,[b] led to the publication of a scientific paper in Nature Astronomy on 19 November 2018.[15] It concluded that the system was a Wolf–Rayet binary and a progenitor for a gamma-ray burst.[9][36] It had been previously assumed that such systems were only found in galaxies younger than the Milky Way.[16]
^A composite of two infrared images taken by the European Southern Observatory's Very Large Telescope on 13 August 2016 – one of the stars in the center of the system taken by the NACO instrument in a wavelength of 2.24 micrometres, and one of the surrounding dust and gas cloud taken by the VISIR instrument in a wavelength of 8.9 micrometers. The image measures 0.26 × 0.26 arcminutes across.[1][2]
^ abcCallingham et al. 2018, page 3, "Figure 1. VISIR 8.9 μm image of Apep taken on 2016 August 13, displaying the exotic dust pattern being sculpted by the system. The 2.24 μm NACO image of the region bounded by the blue box, of dimension 1.8" × 1.8", is shown in the upper right corner".
^ abCallingham et al. 2018, page 3, "we here adopt the moniker "Apep" after the sinuous form of this infrared plume [...] The serpent deity from Egyptian mythology; mortal enemy of sun god Ra. We think this is an apt allusion to the image which evokes a star embattled within a dragon's coils."
^ abcDvorsky, George (19 November 2018). "Stunning Pinwheel Nebula Is a Cosmic Cataclysm in the Making". Gizmodo. Archived from the original on 20 November 2018. Retrieved 20 November 2018. ...but to the researchers who recently investigated this enigmatic object, it's simply "Apep" [...] The speed of gas within the nebula was clocked at 12 million kilometers per hour [...] featuring a massive triple star system at its core—a binary pair and a lone star...
^ abcLetzter, Rafi (19 November 2018). "This Spinning, Snakelike Star System Might Blast Gamma Rays into the Milky Way When It Dies". Live Science. Archived from the original on 20 November 2018. Retrieved 20 November 2018. For the first time, astronomers have found a star system in our galaxy that could produce a gamma-ray burst [...] the researchers nicknamed it "Apep" after the Egyptian snake-deity of chaos. [...] The name works nicely for the system, which is surrounded by long, fiery pinwheels of matter cast out into space...
^ abcdCarpineti, Alfredo (19 November 2018). "This 'Cosmic Serpent' Is The First System Of Its Kind To Be Discovered In Our Galaxy". IFL Science!. Archived from the original on 20 November 2018. Retrieved 20 November 2018. Three stars are in this picture, although the two Wolf-Rayet stars look like a single one in the center [...] the winds are moving at 12 million kilometers (7.5 million miles) per hour. [...] The observations were possible thanks to the Very Large Telescope [...] the dust at the edge of the system is moving at the slower pace of 2 million kilometers (1.2 million miles) per hour.
^Plait 2018, "At 250 billion kilometers out from the bright star (about ten times the distance of Neptune from the Sun), it would take more than 10,000 years to circle it once..."
^ abcStrom, Marcus (20 November 2018). "Doomed star in Milky Way threatens rare gamma-ray burst". University of Sydney. Archived from the original on 20 November 2018. Retrieved 20 November 2018. ...the astronomers have measured the velocity of the stellar winds as fast as 12 million kilometres an hour, about 1 percent the speed of light. [...] We discovered this star as an outlier in a survey with a radio telescope operated by the University of Sydney.
^Plait 2018, "Sometimes, if they are in a tight binary, you get a pinwheel. The most famous example of that is WR 104..."
^Plait 2018, "The astronomers who observed it think that the primary (brighter) one is spinning extremely rapidly, so fast it's nearly at the breakup rate — in other words, spinning so fast that the gravity of the star at the surface is nearly balanced by the centrifugal force outwards."
^ abWeule, Genelle (20 November 2018). "Spectacular cosmic pinwheel is a 'ticking bomb' set to blast gamma rays across the Milky Way". ABC News Australia. Archived from the original on 20 November 2018. Retrieved 20 November 2018. Writing in the journal Nature Astronomy [...] the most violent star is creating stellar winds at two speeds — fast at the poles, slow at the equator [...] the beautiful pinwheel of blazing dust is created not by the fast polar winds, but by the turbulence that arises when the second star in the central engine passes through that first star's slow-moving equatorial wind.
^ abcDevitt, James (19 November 2018). "Scientists Discover New "Pinwheel" Star System". New York University. Archived from the original on 20 November 2018. Retrieved 20 November 2018. "It was not expected such a system would be found in our galaxy—only in younger galaxies much further away," [...] The discovery of the system [...] also included scientists from the Netherlands Institute for Radio Astronomy, the University of Sydney, the University of Edinburgh, the University of Sheffield, and the University of New South Wales. [...] is adorned with a dust "pinwheel"— whose strangely slow motion suggests current theories on star deaths may be incomplete.
^Clery, Daniel (20 November 2018). "Massive star system primed for intense explosion". Science. Archived from the original on 20 November 2018. Retrieved 20 November 2018. One of stars is an unusually massive sun known as a Wolf-Rayet star. When such stars run out of fuel, they collapse, causing a supernova explosion. Theorists believe that if the Wolf-Rayet star is also spinning fast, the explosion will produce intense jets of gamma rays out of either pole...
^ abCallingham et al. 2018, page 18–19, "If we use the visual extinction AV = 11.4 [...] we need a distance of d = 2.4+0.2 −0.5 to get realistic absolute magnitudes for the components. [...] Despite these uncertainties, all lines of evidence suggest that Apep is located [less-than around] 4.5 kpc, and likely around d ≈ 2.4 kpc."
^ abMannix, Liam (20 November 2018). "Super-powerful interstellar 'ticking time bomb' found not far from Earth". The Sydney Morning Herald. Archived from the original on 20 November 2018. Retrieved 20 November 2018. In a part of the Milky Way 8000-odd light-years away [...] The system was spotted by PhD student Dr Joe Callingham while he was sorting through data, and later confirmed using the Anglo-Australian Telescope at Coonabarabran in regional NSW.
^Callingham et al. 2018, page 2, "The 2.24μm NACO observation (Figure 1, inset) resolves Apep into a 0.739" ± 0.002" binary with a fainter companion to the North."
^Callingham et al. 2018, page 18, "...the known visual magnitude V = 17.5 for Apep (V = 17.8 for the OB supergiant that is the northern companion and V = 19.0 for the Central Engine)..."
^Callingham et al. 2018, page 18–19, "...was first noted as a high-luminosity outlier in our Galactic plane X-ray and radio survey, and revealed as an exceptional object on considering its infra-red spectral energy distribution (SED), where it brightens from an apparent magnitude of 6.4 at 2.2μm to −2.4 at 22μm, with both measurements on the Vega system."
^Callingham et al. 2018, page 14, "Apep was resolved by SINFONI [...] We summed the J-band data over the Central Engine and northern companion to derive the J-band magnitudes of 10.2±0.2 and 9.6±0.2, respectively."
^ abcCallingham et al. 2018, page 22, "Supplementary Information Table 2. Summary of the NACO observations of Apep. Separation refers to the angular separation between the Central Engine and northern companion, identified in the inset of Figure 1. The uncertainties reported are for 90% confidence."
^Callingham et al. 2018, page 21, "Despite this, we favour the northern companion being an B1 Ia+ supergiant but further observations, particularly optical spectra, are necessary to confirm this spectral type."
^Callingham et al. 2018, page 20, "...the spectrum of Apep shows stronger He II and weaker C IV line emission than is stereotypical for a WC7 star."
^Callingham et al. 2018, page 20, "The weakness in the J-band, where dust emission is negligible [...] points to the additional continuum from a companion star. The abnormal strength of the He II lines for a WC7 star suggests an early WN sub-type companion. The absence of N V and relative weakness of He I, and with comparison to WN spectra, implies the presence of a subtype WN4 or WN5 star. Double WR binaries are, however, rare, with very few known."
^Callingham et al. 2018, page 21, "An alternative spectral subtype classification to the WC7+WN4-5 model, that equally well describes the spectra shown in Figure 2, is that of a WR star in the brief transitory phase between WN and WC (WN/WC) with an unseen OB-type companion."
^Callingham et al. 2018, page 25, "Supplementary Information Table 3. Summary of the 0.2 and 10.0 keV observations of Apep. ObsID corresponds to the unique identification number assigned to each observation by the respective X-ray observatory.
^Callingham 2018, "The path that led to the discovery of Apep started with a relatively simple crossmatch between X-ray and radio surveys in the last year of my undergraduate study at the University of Sydney..."
^Callingham 2018, "Momentum was behind the idea that Apep was a new colliding-wind binary but the radio emission would make it the brightest radio colliding-wind binary discovered outside of the unique object Eta Carinae..."
^Callingham 2018, "This is where the imaging guru Peter Tuthill (University of Sydney) comes into the story as the extreme infrared properties of Apep particularly caught his attention. [...] It immediately brought to mind the so-called "Pinwheel Nebulae" that Peter had discovered 20 years ago, but this was larger and with more complicated structure than the clean Archimedean spiral observed in those systems."
^Callingham 2018, "We wrote a proposal to use a mid-infrared camera on the European Southern Observatory's Very Large Telescope (VLT) to image the source in the middle of my PhD..."
^Callingham et al. 2018, page 1, "Near-critical stellar rotation is known to drive such winds, suggesting this Wolf-Rayet system as a potential Galactic progenitor system to long-duration gamma-ray bursts."
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