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



Don Alexander
2018-Jun-26, 05:56 PM
Please bear with me... Before I write anything serious, I just have to post this:

https://mysteriousuniverse.org/2018/06/massive-explosion-of-unknown-origin-rips-through-neighboring-galaxy-at-incredible-speed/

The mind boggles, and the Woo woos.

Okay!! It seems there is no thread yet on AT2018cow. This is an Astronomical Transient (AT) discovered by the ATLAS survey on Mauna Kea (which has nothing to do with the Keck telescopes, and there is no "Keck observatory"...) - see http://www.astronomerstelegram.org/?read=11727. Opening this link should also list all other ATels posted.

The facts so far:

- The transient had a very rapid rise time. The probably tightest constraint is a non-detection (I estimate to 17th mag) by ASAS-SN 1.5 days before the discovery observation. It rose by more than three magnitudes over that time, which is quite unusual, at least for SNe that are this luminous.

- It is definitely associated with a face-on spiral galaxy with disturbed morphology at 60 Mpc (~ 200 Mly).

- The early emission which became bright so rapidly is NOT your typical SN emission but a very hot (about 25000 K) blackbody, excepting the narrow absorption lines from interstellar matter in the host and our Milky Way, the early spectra were featureless.

- Within some days, spectral features typical for a so-called broad-lined Type Ic SN (often associated with Gamma-Ray Burst [GRBs]) began to emerge, but the hot blackbody continues to dominate.

- The source has been detected brightly both in X-rays and in mm/sub-mm (and faintly in radio). This is quite atypical for SNe, and indicates a component which emits synchrotron emission, which usually implies matter accelerated to (ultra)relativistic velocities - as typically seen in GRBs. Except no one has detected an actual GRB from this event.

To summarize, this transient consists of three components. A (probably reasonably typical) broad-lined Type Ic SN, a relativistic synchrotron component (these two together form a "relativistic SN", a rare class of GRB-SN-like SNe without GRBs) - and then the very hot, extremely luminous, and seemingly hardly cooling blackbody emission, and THAT is the part which is unlike anything seen so far. There have been some GRB-SNe showing a similar (though less luminous) component at early times (SN 2006aj, for example) but this rose and decayed within about a day, and did not dominate the UV/optical emission as in this case.

Bright minds are probably already conjecturing models, but for now, the transient community is observing, observing, observing.

The SN, by the way, is not just luminous, but also bright in general, about 15th magnitude in the optical now, so amateurs with good scopes (20" or so) should be easily able to detect it.

antoniseb
2018-Jun-26, 07:49 PM
Thanks for this. I'll keep an eye out for more details. Looks interesting.

antoniseb
2018-Jun-27, 12:39 PM
I'm seeing more crackpot ideas on the internet claiming the AT2018COW as proof. Once again thanks Don Alexander for some facts before the nonsense arrived!

slang
2018-Jun-27, 04:15 PM
Doesn't post often, but when he does... No mentions of GW detections, are there none or possibly not published yet?

StupendousMan
2018-Jun-27, 05:42 PM
No mentions of GW detections, are there none or possibly not published yet?

LIGO is down for upgrades to its equipment (will start observing again in 2019), so it couldn't detect any GW associated with this event. VIRGO may have been active; their website doesn't provide any obvious status report, unfortunately. I certainly haven't heard any news from VIRGO on this event.

eburacum45
2018-Jun-28, 09:06 AM
The event—or object—is referred to as “The Cow,” which seems a distinctly boring name for a violent, galaxy destroying explosion of unknown origin. It was detected as far brighter than an average supernova, and moving at an anomalously fast speed. The team of astronomers working the ATLAS telescope at the Keck Observatory reported the event in the Astronomer’s Telegram, where it sparked international curiosity and where the automated naming and cataloging system of the Telegram listed the report as “AT2018cow.” This is why we shouldn’t let computers name things.

I am constantly expecting deep space astronomy to turn up something like this; evidence of star-killer warfare, or a failed attempt to rip a wormhole into the next brane?
Or just another rare event, like a collision between two supermassive stars?

Ken G
2018-Jun-30, 12:21 AM
The absence of gamma rays might not be so surprising-- they are typically beamed, perhaps moreso than the synchrotron emission and certainly moreso than the blackbody emission. Seems likely someone will suggest a fairly mundane model pretty soon! Though star killing certainly sounds more dramatic.

ngc3314
2018-Jun-30, 06:53 PM
I turned the telescope over to some colleagues for a couple of hours to look at it about 10 nights back.

"I haven't double-checked the guider parameters tonight"
"No prob, we'll use short exposures, it's really bright"
"Let's offset to check HOLY COW, IS THAT IT?"

(I contend that would be a better origin for the name than the ATel designation permutations, anyway).

Don Alexander
2018-Jul-09, 08:22 PM
Doesn't post often, but when he does... No mentions of GW detections, are there none or possibly not published yet?
Thanks for the honors. :)

As StupendousMan has stated, LIGO is down for upgrades. While I'm sure this source emitted gravitational waves upon explosion (as the is evidence it is some kin of core-collapse event), these were very likely undetectable at this distance (1.5 times the distance of GW 170817).

In recent days, more peculiarities have been posted:

- NuSTAR detected a hard X-ray spectral component which now seems to have disappeared, this could be a FOURTH emission component.

- The spectral features of the Ic SN have pretty much disappeared again! The spectrum as of now has gotten really weird, not explicable by a single blackbody + SN emission.

- Swift X-ray data have shown two bright flares which are not seen in the UV/optical data.

- The source seems to be decaying in some UV/opt bands but has come to a standstill in others. This goes even beyond the expected chromatic evolution of a cooling blackbody.

- multiple satellite missions now report... no detected GRB.

If anything, the Cow has become even more puzzling.


(I contend that would be a better origin for the name than the ATel designation permutations, anyway).
Actually, that's another error that is being propagated. The AT2018cow designation has been assigned by the TNS (Transient Name Server) at the time the discovery report was posted. It has nothing to do with ATel at all.

ngc3314
2018-Jul-09, 08:51 PM
To show how bright it is - here's an RGB (optical) composite from data at the Jacobus Kapteyn Telescope on La Palma early on July 6.

Don Alexander
2018-Jul-11, 07:08 PM
The Nordic Optical Telescope is now reporting the rise to prominence of a whole sequence of Helium emissions lines! It's rather unclear where these come from and whether they imply this is a Type Ib SN, not Type Ic...

So far, no Type Ib has ever been associated with relativistic emission.

Jean Tate
2018-Jul-17, 01:47 PM
Hot off the arXiv press: "The Cow: discovery of a luminous, hot and rapidly evolving transient", Prentice+ (2018) (https://arxiv.org/abs/1807.05965). Here's the abstract:


We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characeterized by a high peak luminosity (∼1.7×1044 erg s−1), rapidly evolving light curves (>5 mag rise in ∼3 days), hot blackbody spectra, peaking at ∼27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1−0.4 M⊙, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra of AT2018cow showed a number of shallow features overlying a blackbody continuum. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but the positions of shallow bumps in the spectra did not evolve, suggesting that they are produced in a shell or are potentially emission features. Using spectral modelling, we tentatively identify some features as being due to He I and He II and rule out that the features in the spectra are due to most elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data.

profloater
2018-Jul-17, 03:27 PM
fascinating , I note that the luminosity converts to 1.7 10^37 Watts while our sun puts out a bit less than 4 10^26 watts

Don Alexander
2018-Jul-18, 02:25 AM
And here is the next paper:

https://arxiv.org/abs/1807.06369

X-ray Swift observations of SN 2018cow

Supernova (SN) 2018cow is an optical transient detected in the galaxy CGCG 137-068. It has been classified as a SN due to various characteristics in its optical spectra. The transient is also a bright X-ray source. We present results of the analysis of ~62ks of X-ray observations taken with the Neil Gehrels Swift Observatory over 27 days. We found a variable behavior in the 0.3-10keV X-ray light curve of SN 2018cow, with variability timescales of days. The observed X-ray variability could be due to the interaction between the SN ejecta and a non-uniform circum-stellar medium, perhaps related to previous mass ejections from a luminous-blue-variable-like progenitor.

2ndClassCitizen
2018-Jul-18, 03:45 AM
a violent, galaxy destroying explosion of unknown origin. It was detected as far brighter than an average supernova,
Isn't it logical that the most energetic cosmological events (Quasars, 1b supernovae, nebulae, and gamma ray bursts) arise from the most energetic reactions between particles? Matter and antimatter annihilations?

Big bang predicted that all antimatter was destroyed 13 billion years ago. Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other. They also state that streams of antimatter positrons are created, and stream outward from the galactic center.

A precarious formation, a collision through the gc could upset this sensitive system, creating a cascade of annihilations.. Quasars are the galactic centers of galaxies.

Collisions by stars in arms of galaxies are rare (until they become old, full of dust, and arms begin to overlap).

When a matter star collides with streams of antimatter or another star composed of antimatter, one should expect a tremendous release of energy. This energy would quickly levels out when either matter or antimatter have been consumed leaving a remnant of the other.

tusenfem
2018-Jul-18, 07:00 AM
Isn't it logical that the most energetic cosmological events (Quasars, 1b supernovae, nebulae, and gamma ray bursts) arise from the most energetic reactions between particles? Matter and antimatter annihilations?

Big bang predicted that all antimatter was destroyed 13 billion years ago. Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other. They also state that streams of antimatter positrons are created, and stream outward from the galactic center.

A precarious formation, a collision through the gc could upset this sensitive system, creating a cascade of annihilations.. Quasars are the galactic centers of galaxies.

Collisions by stars in arms of galaxies are rare (until they become old, full of dust, and arms begin to overlap).

When a matter star collides with streams of antimatter or another star composed of antimatter, one should expect a tremendous release of energy. This energy would quickly levels out when either matter or antimatter have been consumed leaving a remnant of the other.


Dear 2ndClassCitizen
Welcome to CQ.
But be careful, you are treading here into "Against the Mainstream" waters. Please read the rules for this forum, which are linked in my signature.
Have fun here.

2ndClassCitizen
2018-Jul-20, 05:55 PM
Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.

slang
2018-Jul-20, 08:52 PM
Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.

You are violating rule 17 that prohibits taking threads off topic by arguing against moderator actions. Please don't do it again, and please take some time familiarizing yourself with this forum's rules. If you have a problem with a post, even a moderators, report it or use the feedback forum.

Don Alexander
2018-Jul-20, 10:53 PM
Everything i said i based on proven science, quotes of established theorists, and obbservations by NASA and Hubble. How is this against the "mainstream"?

The "against the mainstream" is code for "post your ridiculous ideas here" and there is a pre-assumption of incorrectness.
To quote you:

"Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other."

This is, yes, a "ridiculous idea."

There is clear evidence for sources of positrons in the Galaxy, but the actual amount of particles produced is vanishingly small compared to the matter mass present. There are no antimatter stars and there is especially no side of the galaxy that is made of antimatter.

Additionally, I see no relation to the actual topic of this thread.

Swift
2018-Jul-21, 01:51 PM
To quote you:

"Observations by NASA (Hubble) show that our galactic center is one half antimatter on one side and one half matter on the other."

This is, yes, a "ridiculous idea."

There is clear evidence for sources of positrons in the Galaxy, but the actual amount of particles produced is vanishingly small compared to the matter mass present. There are no antimatter stars and there is especially no side of the galaxy that is made of antimatter.

Additionally, I see no relation to the actual topic of this thread.
Given the lack of a relationship to the topic of this thread, and the two moderator warnings, please don't debate 2ndClassCitizen's ideas.

Don Alexander
2018-Jul-31, 10:14 PM
The Cow is still going strong in the radio and has now been pinpointed to micro-arcsecond accuracy by Very Long Baseline Interferometry:

http://www.astronomerstelegram.org/?read=11900

Roger E. Moore
2018-Aug-07, 12:38 AM
Mooooo-re on the Mad Cow of Space.


https://arxiv.org/abs/1807.06369

X-ray Swift observations of SN 2018cow

L.E. Rivera Sandoval, T.J. Maccarone, A. Corsi, P.J. Brown, D. Pooley, J.C. Wheeler
(Submitted on 17 Jul 2018 (v1), last revised 3 Aug 2018 (this version, v2))

Supernova (SN) 2018cow is an optical transient detected in the galaxy CGCG 137-068. It has been classified as a SN due to various characteristics in its optical spectra. The transient is also a bright X-ray source. We present results of the analysis of ~62ks of X-ray observations taken with the Neil Gehrels Swift Observatory over 27 days. We found a variable behavior in the 0.3-10keV X-ray light curve of SN 2018cow, with variability timescales of days. The observed X-ray variability could be due to the interaction between the SN ejecta and a non-uniform circum-stellar medium, perhaps related to previous mass ejections from a luminous-blue-variable-like progenitor.

Roger E. Moore
2018-Aug-07, 01:19 AM
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, Paolo A. Mazzali, Lin Yan, S. Bradley Cenko, Suvi Gezari, Kirsty Taggart, Nadia Blagorodnova, Christoffer Fremling, Brenna Mockler, Avinash Singh, Nozomu Tominaga, Masaomi Tanaka, Alan M. Watson, Tomás Ahumada, G. C. Anupama, Chris Ashall, Rosa L. Becerra, David Bersier, Varun Bhalerao, Joshua S. Bloom, Nathaniel R. Butler, Chris Copperwheat, Michael W. Coughlin, Kishalay De, Andrew J. Drake, Dmitry A. Duev, Sara Frederick, J. Jesús González, Ariel Goobar, Anna Y. Q. Ho, John Horst, Tiara Hung, Ryosuke Itoh, Mansi Kasliwal, Nobuyuki Kawai, Shrinivas R. Kulkarni, Brajesh Kumar, Harsh Kumar, Alexander S. Kutyrev, Tanazza Khanam, William H. Lee, Keiichi Maeda, Ashish Mahabal, Katsuhiro L. Murata, James D. Neill, Chow-Choong Ngeow, Bryan Penprase, Elena Pian, et al. (12 additional authors not shown)
(Submitted on 2 Aug 2018)

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 a detailed investigation of their properties until now. 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 behavior 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, AT 2018cow 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
2018-Aug-14, 07:38 PM
REVISED version...

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, Paolo A. Mazzali, Lin Yan, S. Bradley Cenko, Suvi Gezari, Kirsty Taggart, Nadia Blagorodnova, Christoffer Fremling, Brenna Mockler, Avinash Singh, Nozomu Tominaga, Masaomi Tanaka, Alan M. Watson, Tomás Ahumada, G. C. Anupama, Chris Ashall, Rosa L. Becerra, David Bersier, Varun Bhalerao, Joshua S. Bloom, Nathaniel R. Butler, Chris Copperwheat, Michael W. Coughlin, Kishalay De, Andrew J. Drake, Dmitry A. Duev, Sara Frederick, J. Jesús González, Ariel Goobar, Marianne Heida, Anna Y. Q. Ho, John Horst, Tiara Hung, Ryosuke Itoh, Jacob E. Jencson, Mansi M. Kasliwal, Nobuyuki Kawai, Shrinivas R. Kulkarni, Brajesh Kumar, Harsh Kumar, Alexander S. Kutyrev, Tanazza Khanam, William H. Lee, Keiichi Maeda, Ashish Mahabal, Katsuhiro L. Murata, James D. Neill, Chow-Choong Ngeow, et al. (15 additional authors not shown)
(Submitted on 2 Aug 2018 (v1), last revised 12 Aug 2018 (this version, v2))

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 a detailed investigation of their properties until now. 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 behavior 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, AT 2018cow 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
2018-Aug-28, 01:22 AM
NEW NEWS!

https://arxiv.org/abs/1808.08492

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

N. Paul M. Kuin, Kinwah Wu, Samantha Oates, Amy Lien, Sam Emery, Jamie Kennea, Massimiliano de Pasquale, Qin Han, Peter J. Brown, Aaron Tohuvavohu, Alice Breeveld, David N. Burrows, S. Bradley Cenko, Sergio Campana, Andrew Levan, Craig Markwardt, Julian P. Osborne, Mat J. Page, Kim L. Page, Boris Sbarufatti, Michael Siegel, Eleonora Troja
(Submitted on 26 Aug 2018)

The bright transient AT2018cow has been unlike any other known type of transient. Its large 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 γ-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 10^5-10^6 Msun Black Hole (BH) could provide an explanation to 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 subsequence 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.

slang
2018-Aug-29, 09:37 PM
Why do these last two papers have such different estimates for the BH mass in the TDE matches? The first suggests 10^4 Msun, the other 10^6 Msun (which is SMBH range). Is such a large black hole in a spiral arm exceptional?

Roger E. Moore
2018-Aug-30, 12:46 PM
THIRD revision....


https://arxiv.org/abs/1807.05965

The Cow: discovery of a luminous, hot and rapidly evolving transient

S. J. Prentice, K. Maguire, S. J. Smartt, M. R. Magee, P. Schady, S. Sim, T.-W. Chen, P. Clark, C. Colin, M. Fulton, O. McBrien, D. O`Neill, K. W. Smith, C. Ashall, K. C. Chambers, L. Denneau, H. A. Flewelling, A. Heinze, T. W.-S. Holoien, M. E. Huber, C. S. Kochanek, P. A. Mazzali, J. L. Prieto, A. Rest, B. J. Shappee, B. Stalder, K. Z. Stanek, M. D. Stritzinger, T. A. Thompson, J. L. Tonry
(Submitted on 16 Jul 2018 (v1), last revised 29 Aug 2018 (this version, v3))

We present the ATLAS discovery and initial analysis of the first 18 days of the unusual transient event, ATLAS18qqn/AT2018cow. It is characterized by a high peak luminosity (∼ 1.7 × 10 44 erg s −1), rapidly evolving light curves (> 5 mag rise to peak in ∼ 3.5 days), and hot blackbody spectra, peaking at ∼ 27000 K that are relatively featureless and unchanging over the first two weeks. The bolometric light curve cannot be powered by radioactive decay under realistic assumptions. The detection of high-energy emission may suggest a central engine as the powering source. Using a magnetar model, we estimated an ejected mass of 0.1−0.4 M sol, which lies between that of low-energy core-collapse events and the kilonova, AT2017gfo. The spectra cooled rapidly from 27000 to 15000 K in just over 2 weeks but remained smooth and featureless. Broad and shallow emission lines appear after about 20 days, and we tentatively identify them as He I although they would be redshifted from their rest wavelengths. We rule out that there are any features in the spectra due to intermediate mass elements up to and including the Fe-group. The presence of r-process elements cannot be ruled out. If these lines are due to He, then we suggest a low-mass star with residual He as a potential progenitor. Alternatively, models of magnetars formed in neutron-star mergers give plausible matches to the data.

Roger E. Moore
2018-Sep-25, 07:17 PM
Two notes previously unnoted. Tidal disruption event? Also, no gamma radiation.


https://arxiv.org/abs/1809.05048

Photospheric Radius Evolution of Homologous Explosions

Liang-Duan Liu, Bing Zhang, Ling-Jun Wang, Zi-Gao Dai (Submitted on 13 Sep 2018)

Recent wide-field surveys discovered new types of peculiar optical transients that showed diverse behaviors of the evolution of photospheric properties. We develop a general theory of homologous explosions with constant opacity, paying special attention on the evolution of the photospheric radius R ph. We find that regardless of the density distribution profile, R ph always increases early on and decreases at late times. This result does not depend on the radiation and cooling processes inside the ejecta. The general rising/falling behavior of R ph can be used to quickly diagnose whether the source originates from a supernova-like explosion. The shape of the R ph evolution curve depends on the density profile, so the observations may be used to directly diagnose the density profile as well as the temperature profile of the ejecta. All the well-monitored supernovae show such a R ph rising/falling behavior, which is consistent with our theory. The recently discovered peculiar transient AT2018cow showed a continuous decay of R ph, which is disfavored to be of a supernova-like explosion origin. Our result therefore supports the interpretation of this transient as a tidal disruption event.


http://www.astronomerstelegram.org/?read=11956
"The H.E.S.S. array of imaging atmospheric Cherenkov telescopes was used to carry out follow-up observations of AT2018cow (ATel #11727) between the 3rd and 5th July 2018, for a total of 2.4 hours. H.E.S.S. observed the region around the source position (RA, Dec) = (244.00, 22.27) from ATel #11727. Preliminary off-site calibration and analysis with the data taken by the CT5 telescope (CT5 mono), searching for a point-like gamma-ray source from the source target, revealed no significant detection."

Roger E. Moore
2018-Sep-25, 07:19 PM
...and/or it's a type Ib supernova, but a really weird one.

http://www.astronomerstelegram.org/?read=11836

Roger E. Moore
2018-Sep-25, 07:26 PM
Found the webpage for the object with the IAU Supernova Working Group.

https://wis-tns.weizmann.ac.il/object/2018cow

Roger E. Moore
2018-Oct-26, 01:41 AM
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
2018-Oct-26, 11:47 AM
Any guesses as to what the "cow" is? I've got nothing.

Roger E. Moore
2018-Nov-26, 01:53 AM
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
2018-Nov-28, 03:13 PM
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
2018-Nov-29, 02:12 PM
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
2019-Jan-03, 03:00 PM
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
2019-Jan-03, 04:21 PM
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
2019-Jan-07, 07:28 PM
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
2019-Jan-08, 02:02 PM
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
2019-Jan-11, 01:21 PM
Is the "Cow" a newborn black hole?

https://www.space.com/42973-mysterious-cow-space-explosion-black-hole-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
2019-Jan-11, 01:29 PM
And yet another view of the Cow-centric conference held yesterday.


https://phys.org/news/2019-01-holy-cow-mysterious-blast-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
2019-Jan-14, 04:17 PM
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
2019-Feb-28, 02:07 PM
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
2019-Mar-05, 02:36 AM
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
2019-Mar-06, 01:25 PM
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
2019-May-24, 12:45 PM
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
2019-Jun-10, 06:10 PM
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
2019-Jun-14, 02:32 PM
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.