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

[Roger E. Moore]

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.

[Roger E. Moore]

Any guesses as to what the "cow" is? I've got nothing.

[Roger E. Moore]

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.

[Roger E. Moore]

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.

[Roger E. Moore]

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))

[Roger E. Moore]

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.

[Roger E. Moore]

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.

[Roger E. Moore]

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.

[Roger E. Moore]

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))

[Roger E. Moore]

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 ∼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 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.”

[Roger E. Moore]

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.

[Roger E. Moore]

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.

[Roger E. Moore]

Slowly narrowing down the possibilities on what THE COW really was. Still a lot of options.

https://arxiv.org/abs/1902.10144

On the nature of the unusual transient AT 2018cow from HI observations of its host galaxy

Michał J. Michałowski et al. (Submitted on 26 Feb 2019)

Unusual stellar explosions represent an opportunity to learn about both stellar and galaxy evolution. Mapping the atomic gas in host galaxies of such transients can lead to an understanding of the conditions triggering them. We provide resolved atomic gas observations of the host galaxy, CGCG137-068, of the unusual, poorly-understood transient AT 2018cow searching for clues to understand its nature. We test whether it is consistent with a recent inflow of atomic gas from the intergalactic medium, as suggested for host galaxies of gamma-ray bursts (GRBs) and some supernovae (SNe). We observed the HI hyperfine structure line of the AT 2018cow host with the Giant Metrewave Radio Telescope. There is no atomic gas concentration near the position of AT 2018cow. The gas distribution is much more regular than those of GRB/SN hosts. The AT 2018cow host has an atomic gas mass lower by 0.24 dex than predicted from its star formation rate (SFR) and is at the lower edge of the galaxy main sequence. We detected two faint HI emitters 100-150 kpc away, but at low significance. In the continuum we detected the emission of AT 2018cow and of a star-forming region in the north-eastern part of the bar (away from AT 2018cow). This region hosts a third of the galaxy's SFR. The absence of atomic gas close to AT 2018cow, along with a normal SFR and regular HI velocity field, sets CGCG137-068 apart from GRB/SN hosts studied in HI. The environment of AT 2018cow therefore suggests that its progenitor may not have been a massive star. Our findings are consistent with an origin of the transient that does not require a connection between its progenitor and gas concentration or inflow: an exploding low-mass star, a tidal disruption event, a merger of white dwarfs, or a merger between a neutron star and a giant star.

[Roger E. Moore]

What does THE COW look like? Here's information on that.

https://arxiv.org/abs/1903.00477

HI 21cm mapping of the host galaxy of AT2018cow: a fast-evolving luminous transient within a ring of high column density gas

Sambit Roychowdhury, Maryam Arabsalmani, Nissim Kanekar (Submitted on 1 Mar 2019)

We report Giant Metrewave Radio Telescope (GMRT) HI 21cm imaging of CGCG 137-068, the host galaxy of the fast-evolving luminous transient (FELT) AT2018cow. This is the first study of the gas properties of a FELT host galaxy.

[Roger E. Moore]

More COW is good COW. Any COW is good COW.

https://arxiv.org/abs/1903.01535

Signatures of Circumstellar Interaction in the Unusual Transient AT2018cow

Ori D. Fox (STScI), Nathan Smith (Steward Observatory) (Submitted on 4 Mar 2019)

AT2018cow is a unique transient that stands out due to its relatively fast light-curve, high peak bolometric luminosity, and blue color. These properties distinguish it from typical radioactively powered core-collapse supernovae (SNe). Instead, the characteristics are more similar to a growing sample of Fast Blue Optical Transients (FBOTs). Mostly discovered at hundreds of Mpc, FBOT follow-up is usually limited to several photometry points and low signal-to-noise spectra. At only ~60 Mpc, AT2018cow offers an opportunity for detailed followup. Studies of this object published to date invoke a number of interpretations for AT2018cow, but none of these specifically consider the interacting Type Ibn SN subclass. We point out that while narrow lines do not dominate the spectrum of AT2018cow, as narrow Balmer lines typically do in SNe IIn, the narrow lines in AT2018cow may nevertheless be a mix of unresolved HII region emission and emission from slow, pre-shock CSM. We compare AT2018cow to interacting SNe Ibn and IIn and find a number of noteworthy similarities, including light-curve rise and fall times, peak magnitude, X-ray light-curves, and spectroscopic properties. In particular, the He I lines in AT2018cow closely resemble those in some examples of SNe Ibn or transitional SNe Ibn/IIn objects. We therefore explore the hypothesis that CSM interaction in a relatively H-poor system might have some merit in explaining observed properties of AT2018cow, and we go on to consider progenitor implications for AT2018cow, FBOTs, and SNe~Ibn.

[Roger E. Moore]

The COW is back in the news!

https://arxiv.org/abs/1905.09785

ALMA Polarimetry of AT2018cow

K.Y. Huang, J. Shimoda, Y. Urata, K. Toma, K. Yamaoka, K. Asada, H. Nagai, S. Takahashi, G. Petitpas, M. Tashiro (Submitted on 23 May 2019)

We present the first radio polarimetric observations of a fast-rising blue optical transient, AT2018cow. Two epochs of polarimetry with additional coincident photometry were performed with the Atacama Large Millimeter/submillimeter Array (ALMA). The overall photometric results based on simultaneous observations in the 100 and 230 GHz bands are consistent with the non-thermal radiation model reported by Ho et al. (2019) and indicate that the spectral peaks (∼110 GHz at the first epoch and ∼67 GHz at the second epoch) represent the synchrotron self-absorption frequency. The non-detection of linear polarization with < 0.15% in the 230 GHz band at the phase when the effect of synchrotron self-absorption was quite small in the band may be explained by internal Faraday depolarization with high circumburst density and strong magnetic field. This result supports the stellar explosion scenario rather than the tidal disruption model. The maximum energy of accelerating particles at the shocks of AT2018cow-like objects is also discussed.

[Roger E. Moore]

Revised version of older paper has come out, regarding THE COW's creation as a neutron star.

https://arxiv.org/abs/1812.07569

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

Maxim Lyutikov (Purdue University), Silvia Toonen (Astronomical Institute Anton Pannekoek) (Submitted on 18 Dec 2018 (v1), last revised 7 Jun 2019 (this version, v2))

We suggest that fast-rising blue optical transients (FBOTs) and the brightest event of the class AT2018cow result from an electron-capture collapse to a neutron star 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 the formation of a shell burning non-degenerate star incorporating the ONeMg core. 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 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 a time scale of days - this is the FBOT. During the collapse, the neutron star is spun up and magnetic field is amplified. The ensuing fast magnetically-dominated relativistic wind from the newly formed neutron star 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 manner similar to Pulsar Wind Nebulae. If the secondary WD was of the DA type, the wind will likely have ∼10 −4 M ⊙ of hydrogen; this explains the appearance of hydrogen late in the afterglow spectrum. The model explains many of the puzzling properties of FBOTs/AT2018cow: host galaxies, a fast and light anisotropic ejecta producing a bright optical peak, afterglow high energy emission of similar luminosity to the optical, and late infra-red features.

[Roger E. Moore]

News on the home galaxy of THE COW. (Could not get most of the Abstract to post properly, here's the core.)

https://arxiv.org/abs/1906.05446

ALMA observations of molecular gas in the host galaxy of AT2018cow

Kana Morokuma-Matsui, Tomoki Morokuma, Nozomu Tominaga, Bunyo Hatsukade, Masao Hayashi, Yoichi Tamura, Yuichi Matsuda, Kazuhito Motogi, Kotaro Niinuma, Masahiro Konishi (Submitted on 13 Jun 2019)

The ALMA data reveal that (1) CGCG 137-068 is a normal star-forming (SF) dwarf galaxy in terms of its molecular gas and star-formation properties and (2) AT2018cow is located between a CO peak and a blue star cluster. These properties suggest on-going star formation and favor the explosion of a massive star as the progenitor of AT2018cow. We also find that CGCG 137-068 has a solar or super-solar metallicity.

[Roger E. Moore]

AT2018cow: is it fully explained now? I'm going to wait and see.

https://ui.adsabs.harvard.edu/abs/20...1912M/abstract

The nearby luminous transient AT2018cow: a magnetar formed in a sub-relativistically expanding non-jetted explosion
Mohan, P.; An, T.; Yang, J.

The fast-rising blue optical transient AT2018cow indicated unusual early phase characteristics unlike relatively better studied explosive transients. Its afterglow may be produced by either a relativistically beamed (jetted) or intrinsically luminous (non-jetted) ejecta and carries observational signatures of the progenitor and environment. High resolution monitoring can distinguish between these scenarios and clarify the progenitor nature. We present very long baseline interferometry (VLBI) observations of AT2018cow at 5 GHz involving 21 radio telescopes from the European VLBI Network with five sessions spanning ~ 1 year. With an astrometric precision up to 25 micro-arcseconds per epoch, the rapidly fading compact mas scale source is found to be non-jetted with a proper motion of <= 0.15 mas/yr (0.14 c). This and a dense (number density ~ 10^4 - 10^5/cm^3) magnetized environment (magnetic field strength >= 0.84 G) are characteristic of a newly formed magnetar driven central engine, originating in the successful explosion of a low-mass star.

[Roger E. Moore]

New on The Cow: "AT2018cow may be either a tidal disruption event of a low-mass RSG [red super giant] by a black hole (BH), or a BH-forming failed supernova."

https://arxiv.org/abs/2003.05795

Wind-Driven Transients as A Unified Model for Peculiar Events AT2018cow and iPTF14hls
Kohki Uno, Keiichi Maeda
(Submitted on 12 Mar 2020)

We propose a wind-driven model for peculiar transients, and apply the model to AT2018cow and iPTF14hls. In the wind-driven model, we assume that a continuous outflow like a stellar wind is injected from a central system. While these transients have different observational properties, this unified model can explain their photometric properties which are not reproduced by a supernova-like instantaneous explosion. Furthermore, the model predicts characteristic spectral features and evolution, which are well in line with those of AT2018cow and iPTF14hls. Despite the different observational properties, the wind model shows that they have some common features; the large mass-loss rates (up to ∼20M⊙ yr−1 for AT2018cow and ∼30M⊙ yr−1 for iPTF14hls), the characteristic radii of ∼1013 cm for the launch of the wind, and the kinetic energies of ∼1051 erg. It would indicate that both may be related to events involving a red super giant (RSG), in which the RSG envelope is rapidly ejected by an event at a stellar core scale. On the other hand, the main differences are time scales and the total ejected mass. We then suggests that iPTF14hls may represent a dynamical common-envelope evolution induced by massive binary systems (∼50M⊙ each). AT2018cow may be either a tidal disruption event of a low-mass RSG by a black hole (BH), or a BH-forming failed supernova.

[Roger E. Moore]

Best paper title in ages. THE COW might have been a giant star Type II supernova.

https://arxiv.org/abs/2005.02412
A Blast from the Pasture: Studying the environment of AT 2018cow with MUSE
J. D. Lyman, L. Galbany, S. F. Sanchez, J. P. Anderson, H. Kuncarayakti
AT 2018cow was the nearest and best studied example of a new breed of extra-galactic, luminous and rapidly-evolving transient. Both the progenitor systems and explosion mechanisms of these rapid transients remain a mystery - the energetics, spectral signatures, and timescales make them challenging to interpret in established classes of supernovae and tidal disruption events. The rich, multi-wavelength data-set of AT 2018cow has still left several interpretations viable to explain the nature of this event. In this paper we analyse integral-field spectroscopic data of the host galaxy, CGCG 137-068, to compare environmental constraints with leading progenitor models. We find the explosion site of AT 2018cow to be very typical of core-collapse supernovae (known to form from stars with MZAMS ~8-25M), and infer a young stellar population age at the explosion site of few times 10Myr, at slightly sub-solar metallicity. When comparing to expectations for exotic intermediate-mass black hole (IMBH) tidal disruption events, we find no evidence for a potential host system of the IMBH. In particular, there are no abrupt changes in metallicity or kinematics in the vicinity of the explosion site, arguing against the presence of a distinct host system. The proximity of AT 2018cow to strong star-formation in the host galaxy makes us favour a massive stellar progenitor for this event.

[Roger E. Moore]

The COW was a Fast Blue Optical Transient, an unusual type of object seen only three times, according to this article.

https://www.cnn.com/2020/05/26/world...scn/index.html

[Roger E. Moore]

More on Fast Blue Optical Transients.

https://www.sciencenews.org/article/...-company-koala

[Roger E. Moore]

Annnnd one more view on what AT2018cow actually was.


https://arxiv.org/abs/2009.03852

Application of The Wind-Driven Model to A Sample of Tidal Disruption Events

Kohki Uno, Keiichi Maeda

An origin of the Optical/UV radiation from tidal disruption events (TDEs) has recently been discussed for different scenarios, but observational support is generally missing. In this Letter, we test applicability of the `Wind-Driven model' (Uno & Maeda 2020) to a sample of UV/Optical TDEs. With the model, we aim to derive the physical properties of the Optical/UV TDEs, such as mass-loss rates and characteristic radii. The model assumes optically thick continuous outflows like stellar winds, and one key question is how the wind-launched radius is connected to physical processes in TDEs. ... We also apply the same framework to a peculiar transient, AT2018cow. The model suggests that AT2018cow is likely a TDE induced by an intermediate-mass black hole (M-BH ∼10^4 M⊙).

[Roger E. Moore]

Nobody really knows what AT2018cow was, I guess. Too many unconnected guesses.

The Peculiar Transient AT2018cow: A Possible Origin of A Type Ibn/IIn Supernova

Danfeng Xiang, Xiaofeng Wang, Weili Lin, Jun Mo, Han Lin, Jamison Burke, Daichi Hiramatsu, Griffin Hosseinzadeh, D. Andrew Howell, Curtis McCully, Stefan Valenti, József Vinkó, J. Craig Wheeler, Shuhrat A. Ehgamberdiev, Davron Mirzaqulov, Attila Bódi, Zsófia Bognár, Borbála Cseh, Ottó Hanyecz, Bernadett Ignácz, Csilla Kalup, Réka Könyves-Tóth, Levente Kriskovics, András Ordasi, András Pál, Krisztián Sárneczky, Bálint Seli, Róbert Szakáts, T. Arranz-Heras, R. Benavides-Palencia, D. Cejudo-Martínez, P. De la Fuente-Fernández, A. Escartín-Pérez, F. García-De la Cuesta, J.L. González-Carballo, R. González-Farfán, F. Limón-Martínez, A. Mantero, R. Naves-Nogués, M. Morales-Aimar, V. R. Ruíz-Ruíz, F.C. Soldán-Alfaro, J. Valero-Pérez, F. Violat-Bordonau, Tianmeng Zhang, Jujia Zhang, Xue Li, Zhihao Chen, Hanna Sai, Wenxiong Li

We present our photometric and spectroscopic observations on the peculiar transient AT2018cow. The multi-band photometry covers from peak to ∼70 days and the spectroscopy ranges from 5 to ∼50 days. The rapid rise (tr≲2.9 days), high luminosity (MV,peak∼−20.8 mag) and fast decline after peak make AT2018cow stand out of any other optical transients. While we find that its light curves show high resemblance to those of type Ibn supernovae. Moreover, the spectral energy distribution remains high temperature of ∼14,000 K after ∼15 days since discovery. The spectra are featureless in the first 10 days, while some broad emission lines due to H, He, C and O emerge later, with velocity declining from ∼14,000 km s−1 to ∼3000 km s−1 at the end of our observations. Narrow and weak He I emission lines emerge in the spectra at t>20 days since discovery. These emission lines are reminiscent of the features seen in interacting supernovae like type Ibn and IIn subclasses. We fit the bolometric light curves with a model of circumstellar interaction (CSI) and radioactive decay (RD) of \Ni and find a good fit with ejecta mass Mej∼3.16 M⊙, circumstellar material mass MCSM∼0.04 M⊙, and ejected \Ni mass M56Ni∼0.23 M⊙. The CSM shell might be formed in an eruptive mass ejection of the progenitor star. Furthermore, host environment of AT2018cow implies connection of AT2018cow with massive stars. Combining observational properties and the light curve fitting results, we conclude that AT2018cow might be a peculiar interacting supernova originated from a massive star.

https://arxiv.org/abs/2101.08009