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Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow

Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow
Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow
The brightest fast blue optical transients (FBOTs) are mysterious extragalactic explosions that may represent a new astrophysical phenomenon1. Their fast time to maximum brightness of less than a week, decline over several months, and atypical optical spectra and evolution are difficult to explain within the context of the core collapse of massive stars, which are powered by radioactive decay of 56Ni and evolve more slowly2,3. AT2018cow (at a redshift of 0.014) is an extreme FBOT in terms of rapid evolution and high luminosity4,5,6,7. Here we present evidence for a high-amplitude quasiperiodic oscillation of AT2018cow’s soft X-rays with a frequency of 224 Hz (at a 3.7σ significance level or false alarm probability of 0.02%) and fractional root-mean-squared amplitude of >30%. This signal is found in the average power density spectrum taken over the entire 60-day outburst and suggests a highly persistent signal that lasts for a billion cycles. The high frequency (rapid timescale) of 224 Hz (4.4 ms) argues for a compact object in AT2018cow, which could be a neutron star or black hole with a mass less than 850 solar masses. If the quasiperiodic oscillation is equivalent to the spin period of a neutron star, we can set limits on the star’s magnetic field strength. Our work highlights a new way of using high-time-resolution X-ray observations to study FBOTs.
2397-3366
Pasham, Dheeraj R.
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Ho, Wynn C.G.
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Alston, William
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Remillard, Ronald A.
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Ng, Mason
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Gendreau, Keith C.
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Altamirano, Diego
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Metzger, Brian D.
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Chakrabarty, Deepto
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Fabian, Andrew C.
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Miller, Jon
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Bult, Peter
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Arzoumanian, Zaven
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Steiner, James F.
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Strohmayer, Tod E.
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Tombesi, Francesco
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Homan, Jeroen
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Cackett, Edward M.
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Harding, Alice
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Pasham, Dheeraj R.
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Ho, Wynn C.G.
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Alston, William
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Remillard, Ronald A.
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Ng, Mason
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Gendreau, Keith C.
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Altamirano, Diego
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Metzger, Brian D.
eed8d6f0-7c5e-43d6-8336-010eb8178ab4
Chakrabarty, Deepto
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Fabian, Andrew C.
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Miller, Jon
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Bult, Peter
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Arzoumanian, Zaven
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Steiner, James F.
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Strohmayer, Tod E.
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Tombesi, Francesco
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Homan, Jeroen
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Cackett, Edward M.
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Harding, Alice
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Pasham, Dheeraj R., Ho, Wynn C.G., Alston, William, Remillard, Ronald A., Ng, Mason, Gendreau, Keith C., Altamirano, Diego, Metzger, Brian D., Chakrabarty, Deepto, Fabian, Andrew C., Miller, Jon, Bult, Peter, Arzoumanian, Zaven, Steiner, James F., Strohmayer, Tod E., Tombesi, Francesco, Homan, Jeroen, Cackett, Edward M. and Harding, Alice (2021) Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow. Nature Astronomy. (doi:10.1038/s41550-021-01524-8).

Record type: Article

Abstract

The brightest fast blue optical transients (FBOTs) are mysterious extragalactic explosions that may represent a new astrophysical phenomenon1. Their fast time to maximum brightness of less than a week, decline over several months, and atypical optical spectra and evolution are difficult to explain within the context of the core collapse of massive stars, which are powered by radioactive decay of 56Ni and evolve more slowly2,3. AT2018cow (at a redshift of 0.014) is an extreme FBOT in terms of rapid evolution and high luminosity4,5,6,7. Here we present evidence for a high-amplitude quasiperiodic oscillation of AT2018cow’s soft X-rays with a frequency of 224 Hz (at a 3.7σ significance level or false alarm probability of 0.02%) and fractional root-mean-squared amplitude of >30%. This signal is found in the average power density spectrum taken over the entire 60-day outburst and suggests a highly persistent signal that lasts for a billion cycles. The high frequency (rapid timescale) of 224 Hz (4.4 ms) argues for a compact object in AT2018cow, which could be a neutron star or black hole with a mass less than 850 solar masses. If the quasiperiodic oscillation is equivalent to the spin period of a neutron star, we can set limits on the star’s magnetic field strength. Our work highlights a new way of using high-time-resolution X-ray observations to study FBOTs.

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2112.04531 - Accepted Manuscript
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Accepted/In Press date: 20 September 2021
Published date: 13 December 2021
Additional Information: arXiv:2112.04531

Identifiers

Local EPrints ID: 454278
URI: http://eprints.soton.ac.uk/id/eprint/454278
ISSN: 2397-3366
PURE UUID: 646ba5b2-464f-4468-843a-2124088ee9fd
ORCID for Diego Altamirano: ORCID iD orcid.org/0000-0002-3422-0074

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Date deposited: 04 Feb 2022 17:45
Last modified: 13 Jun 2022 04:01

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Contributors

Author: Dheeraj R. Pasham
Author: Wynn C.G. Ho
Author: William Alston
Author: Ronald A. Remillard
Author: Mason Ng
Author: Keith C. Gendreau
Author: Brian D. Metzger
Author: Deepto Chakrabarty
Author: Andrew C. Fabian
Author: Jon Miller
Author: Peter Bult
Author: Zaven Arzoumanian
Author: James F. Steiner
Author: Tod E. Strohmayer
Author: Francesco Tombesi
Author: Jeroen Homan
Author: Edward M. Cackett
Author: Alice Harding

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