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Thread: I Am The Cow, Destroyer Of Worlds (or: AT2018cow is one hell of a mysterious object)

  1. #31
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    The COW is BACK! Two new articles on AT2018cow, a very peculiar non-supernova no one has yet figured out.


    https://arxiv.org/abs/1810.10720

    An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients

    Raffaella Margutti, et al. (Submitted on 25 Oct 2018)

    We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity Lpk ∼4 10^44 erg/s exceeding those of superluminous supernovae (SNe), before declining as ∝t^−2. Initial spectra at ≲15 days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30,000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E≥10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with vsh ~0.1c with a dense environment (M˙ ∼ 10^−3 − 10^−4 M⊙ yr^−1 for vw = 1000 km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼10^50 − 10^51.5 erg over ∼10^3 − 10^5 s and resides within low-mass fast-moving material with equatorial-polar density asymmetry (M ej,fast ≲ 0.3 M⊙). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

    =====================

    https://arxiv.org/abs/1810.10880

    AT2018cow: a luminous millimeter transient

    Anna Y. Q. Ho, et al. (Submitted on 25 Oct 2018)

    We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics -- the high radio luminosity, the long-lived emission plateau at millimeter bands, and the sub-relativistic velocity -- have no precedent. A basic interpretation of the data suggests Ek ≳ 10^48 erg coupled to a fast but sub-relativistic (v≈0.13c) shock in a dense (ne ≈ 3 10^5 cm^−3) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UVOIR photons by energetic electrons within the forward shock. By Δt ≈ 20 days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  2. #32
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    Any guesses as to what the "cow" is? I've got nothing.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  3. #33
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    Revised paper on "The COW" from August. Not sure what was changed.


    https://arxiv.org/abs/1808.00969

    The Fast, Luminous Ultraviolet Transient AT2018cow: Extreme Supernova, or Disruption of a Star by an Intermediate-Mass Black Hole?

    Daniel A. Perley, et al. (Submitted on 2 Aug 2018 (v1), last revised 23 Nov 2018 (this version, v4))

    Wide-field optical surveys have begun to uncover large samples of fast (t_rise < 5d), luminous (M_peak < -18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT2018cow (ATLAS18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3-8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R<10^14 cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  4. #34
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    NEW PAPER with an amazing explanation of what caused the COW to appear: a neutron star spiraled into the core of a massive giant star, accreted material, and blew up. That's different!


    https://arxiv.org/abs/1811.11106

    Diversity of common envelope jets supernovae and the fast transient AT2018cow

    Noam Soker, Aldana Grichener, Avishai Gilkis (Submitted on 27 Nov 2018)

    We propose a common-envelope jets supernova (CEJSN) scenario for the fast-rising blue optical transient AT2018cow. In a CEJSN a neutron star (NS) spirals-in inside the extended envelope of a massive giant star and enters the core. The NS accretes mass from the core through an accretion disc and launches jets. These jets explode the core and the envelope. In the specific polar CEJSN scenario that we propose here the jets clear the polar regions of the giant star before the NS enters the core. The jets that the NS launches after it enters the core expand almost freely along the polar directions that contain a small amount of mass. This, we suggest, explains the fast rise to maximum and the fast ejecta observed at early times of the enigmatic transient AT2018cow. The slower later time ejecta is the more massive equatorial outflow. We roughly estimate the accretion phase onto the NS during the explosion phase to last for a time of about 1000 seconds during which the average mass accretion rate is about 0.0001 Mo/sec. We outline the possible diversity of CEJSNe by listing five other scenarios in addition to the polar CEJSN scenario.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  5. #35
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    FWIW: third revision.


    https://arxiv.org/abs/1810.10880

    AT2018cow: a luminous millimeter transient

    Anna Y. Q. Ho, et al. (Submitted on 25 Oct 2018 (v1), last revised 27 Nov 2018 (this version, v3))
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  6. #36
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    Was the COW a white-dwarf-white-dwarf collision?

    https://arxiv.org/abs/1812.07569

    FBOTs and AT2018cow following electron-capture collapse of merged white dwarfs

    Maxim Lyutikov, Silvia Toonen (Submitted on 18 Dec 2018)

    We suggest that fast-rising blue optical transients (FBOTs), and the brightest event of the class AT2018cow, result from electron-capture collapse following a merger of a massive ONeMg white dwarf (WD) with another WD. Two distinct evolutionary channels lead to the disruption of the less massive WD during the merger and formation of a shell burning non-degenerate star. During the shell burning stage a large fraction of the envelope is lost to the wind, while mass and angular momentum are added to the core. As a result, the electron-capture collapse occurs with a small envelope mass, after ∼10 2 −10 4 years. During the formation of a neutron star (NS) as little as ∼10 −2 M ⊙ of the material is ejected at the bounce-off with mildly relativistic velocities and total energy ∼ few 10 50 ergs. This ejecta becomes optically thin on time scales of days - this is the FBOT. During the collapse the NS is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed NS shocks against the ejecta, and later against the wind. The radiation-dominated forward shock produces the long-lasting optical afterglow, while the termination shock of the relativistic wind produces the high energy emission in a Pulsar Wind Nebulae-like manner. If the secondary WD was of the DA type - the most frequent - the wind will have hydrogen, of the order of 10 −4 M ⊙ : this explains appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, fast and light anisotropic ejecta producing bright optical peak, afterglow with high energy emission of similar luminosity to optical, hard X-ray and infra-red features, presence of dense wind environment, late powerful radio emission.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  7. #37
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    NO! WAIT! The COW was really a newborn magnetar! ... or something, who knows what

    https://arxiv.org/abs/1812.11673

    Multimessenger Implications of AT2018cow: High-Energy Cosmic Ray and Neutrino Emissions from Magnetar-Powered Super-Luminous Transients

    Ke Fang, Brian D. Metzger, Kohta Murase, Imre Bartos, Kumiko Kotera (Submitted on 31 Dec 2018)

    Newly-born, rapidly-spinning magnetars have been invoked as the power sources of super-luminous transients, including the class of "fast-luminous optical transients" (FBOTs). The extensive multi-wavelength analysis of AT2018cow, the first FBOT discovered in real time, is consistent with the magnetar scenario and offers an unprecedented opportunity to comprehend the nature of these sources and assess their broader implications. Using AT2018cow as a prototype, we investigate high-energy neutrino and cosmic ray production from FBOTs and the more general class of superluminous supernovae (SLSNe). By calculating the interaction of cosmic rays and the time-evolving radiation field and baryon background, we find that particles accelerated in the magnetar wind may escape the ejecta at ultrahigh energies (UHE). The predicted high-energy neutrino fluence from AT2018cow is below the sensitivity of the IceCube Observatory, and estimates of the cosmically-integrated neutrino flux from FBOTs are consistent with the extreme-high-energy upper limits posed by IceCube. High-energy γ rays exceeding GeV energies are obscured for the first months to years by thermal photons in the magnetar nebula, but are potentially observable at later times. Given also their potentially higher volumetric rate compared to other engine-powered transients (e.g. SLSNe and gamma-ray bursts), we conclude that FBOTs are favorable targets for current and next-generation multi-messenger observatories.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  8. #38
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    New paper: was it the formation of a compact object?


    http://cdsads.u-strasbg.fr/abs/2019AAS...23345609M

    An X-ray source shines through AT2018cow: the birth of a compact object

    Margutti, Raffaella
    American Astronomical Society, AAS Meeting #233, id.#456.09
    Publication Date: 01/2019

    I present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT) AT2018cow in the first 100 days of evolution. Over a rise-time of a few days, AT2018cow reached a luminosity of 4\x 10^44 erg/s larger than super-luminous SNe, and later declined as ~t^-2. Initial spectra at <= 15 days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching T~30000 K. Later spectra showed a persistent optically thick photosphere and the emergence of He and H emission features with v~4000 km/s with no evidence for cooling of the ejecta. Our broad-band monitoring revealed a hard X-ray spectral component at E~10 keV, in addition to luminous and highly variable soft X-rays, with properties that are unprecedented among astronomical transients. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with v~0.1c with a dense environment (mass-loss of M~10^-3-10^-4 Msun/yr). Taken together, these properties exclude traditional models of Nickel-powered transients. From our multi-wavelength analysis we conclude that AT2018cow harbored a central engine, possibly in the form of a compact object (magnetar or black-hole). With AT2018cow we might have witnessed for the first time, the formation of a compact object in real time.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  9. #39
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    Fifth revision, corrects several equations.


    https://arxiv.org/abs/1810.10880

    AT2018cow: a luminous millimeter transient

    Anna Y. Q. Ho (Caltech), et al. (Submitted on 25 Oct 2018 (v1), last revised 5 Jan 2019 (this version, v5))
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  10. #40
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    Is the "Cow" a newborn black hole?

    https://www.space.com/42973-mysterio...ole-birth.html

    Mysterious 'Cow' Blast in Space May Reveal Birth of a Black Hole
    By Mike Wall, Space.com Senior Writer | January 10, 2019 11:52pm ET

    Astronomers may have just witnessed the first few moments of a black hole's existence. A weirdly bright and brief blast dubbed "The Cow," which researchers first spotted last June, was likely generated by a newborn black hole or superdense stellar corpse called a neutron star, a new study reports.

    =============

    https://arxiv.org/abs/1810.10720

    An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients

    Raffaella Margutti, et al. (Submitted on 25 Oct 2018)

    We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity L pk ∼410 44 erg/s exceeding those of superluminous supernovae (SNe), before declining as ∝t −2 . Initial spectra at ≲15 days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at E≥10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with v sh ~0.1c with a dense environment (M ˙ ∼10 −3 −10 −4 M ⊙ yr −1 for v w =1000 km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼10 50 −10 51.5 erg over ∼10 3 −10 5 s and resides within low-mass fast-moving material with equatorial-polar density asymmetry (M ej,fast ≲0.3M ⊙ ). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

    ============

    http://growth.caltech.edu/news-at2018cow.html

    Why a global network of telescopes followed the “Cow”

    QUOTE: “This object is brighter than any supernova in millimeter waves,” said Ho, who presented at the AAS press panel along with Perley. “Our radio data revealed that a shock wave is traveling outward from the explosion at one-tenth the speed of light.”
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  11. #41
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    And yet another view of the Cow-centric conference held yesterday.


    https://phys.org/news/2019-01-holy-c...last-nasa.html

    Holy cow! Mysterious blast studied with NASA telescopes
    January 11, 2019, NASA

    A brief and unusual flash spotted in the night sky on June 16, 2018, puzzled astronomers and astrophysicists across the globe. The event—called AT2018cow and nicknamed "the Cow" after the coincidental final letters in its official name—is unlike any celestial outburst ever seen before, prompting multiple theories about its source.

    Over three days, the Cow produced a sudden explosion of light at least 10 times brighter than a typical supernova, and then it faded over the next few months. This unusual event occurred inside or near a star-forming galaxy known as CGCG 137-068, located about 200 million light-years away in the constellation Hercules. The Cow was first observed by the NASA-funded Asteroid Terrestrial-impact Last Alert System telescope in Hawaii.

    So exactly what is the Cow? Using data from multiple NASA missions, including the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array (NuSTAR), two groups are publishing papers that provide possible explanations for the Cow's origins. One paper argues that the Cow is a monster black hole shredding a passing star. The second paper hypothesizes that it is a supernova—a stellar explosion—that gave birth to a black hole or a neutron star.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

  12. #42
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    Newly revised paper on the topic, from last August. The argument continues.

    https://arxiv.org/abs/1808.08492

    Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?

    N. Paul M. Kuin, et al. (Submitted on 26 Aug 2018 (v1), last revised 10 Jan 2019 (this version, v2))

    The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint gamma-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 to one million solar mass Black Hole (BH) located in the outskirts of galaxy Z~137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    Mark Twain, Life on the Mississippi (1883)

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