Calcium-rich supernova examined with x-rays for first time

Date:
August 5, 2020
Source:
W. M. Keck Observatory
Summary:
X-ray images give unprecedented view of extremely rare type of
supernova.

New information suggests that these supernovae start as compact
stars that lose mass at the end of life. Calcium-rich supernovae are
responsible for up to half the calcium in the entire universe. SN
2019ehk has the richest calcium emission of all known transients.



FULL STORY ========================================================================== Called "calcium-rich supernovae," these stellar explosions are so rare
that astrophysicists have struggled to find and subsequently study
them. The nature of these supernovae and their mechanism for creating
calcium, therefore, have remained elusive.


==========================================================================
Now a Northwestern University-led team has potentially uncovered the
true nature of these rare, mysterious events. For the first time ever,
the researchers examined a calcium-rich supernova, dubbed SN 2019ehk,
with X-ray imaging, providing an unprecedented glimpse into the star
during the last month of its life and ultimate explosion.

The study, which includes data from W. M. Keck Observatory on Maunakea
in Hawaii, is published in the August 5, 2020 issue of The Astrophysical Journal.

The new findings revealed that a calcium-rich supernova is a compact
star that sheds an outer layer of gas during the final stages of its
life. When the star explodes, its matter collides with the loose material
in that outer shell, emitting bright X-rays. The overall explosion causes intensely hot temperatures and high pressure, driving a chemical reaction
that produces calcium.

"These events are so few in number that we have never known what produced calcium-rich supernova," said lead author Wynn Jacobson-Galan, an NSF
Graduate Research Fellow at Northwestern University. "By observing
what this star did in its final month before it reached its critical, tumultuous end, we peered into a place previously unexplored, opening
new avenues of study within transient science." "Before this event,
we had indirect information about what calcium-rich supernovae might
or might not be," said senior author Raffaella Margutti, an assistant
professor of physics and astronomy at Northwestern University and a
member of CIERA (Center for Interdisciplinary Exploration and Research in Astrophysics). "Now, we can confidently rule out several possibilities."
'THE RICHEST OF THE RICH'


========================================================================== While all calcium comes from stars, calcium-rich supernovae pack the
most powerful punch. Typical stars create small amounts of calcium slowly through burning helium throughout their lives. Calcium-rich supernovae,
on the other hand, produce massive amounts of calcium within seconds.

"The explosion is trying to cool down," Margutti explained. "It wants
to give away its energy, and calcium emission is an efficient way to
do that." Using Keck Observatory's Low Resolution Imaging Spectrometer
(LRIS), the researchers discovered SN 2019ehk emitted the most calcium
ever observed in a singular astrophysical event.

"The beautiful Keck spectrum revealed it wasn't just calcium-rich,"
Margutti said. "It was the richest of the rich." 'A GLOBAL COLLABORATION
WAS IGNITED' Amateur astronomer Joel Shepherd first spotted the bright
burst in April 2019 while using his new telescope to view Messier 100
(M100), a spiral galaxy located 55 million light years from Earth. After
seeing a bright orange dot appear in the frame, he immediately reported
the discovery to the astronomical community.



==========================================================================
"As soon as the world knew that there was a potential supernova in M100,
a global collaboration was ignited," Jacobson-Galan said. "Every single
country with a prominent telescope turned to look at this object."
The worldwide follow-up operation moved so quickly, the supernova was
observed just 10 hours after exploding. Leading observatories such as
NASA's Swift Satellite, Lick Observatory, and Keck Observatory were among
the telescopes triggered to examine SN 2019ehk in optical wavelengths.

University of California Santa Barbara graduate student Daichi Hiramatsu
was the first to trigger Swift to study SN 2019ehk in the X-ray and ultraviolet.

The X-ray emission detected with Swift only lingered for five days before completely disappearing.

"In the world of transients, we have to discover things very, very
fast before they fade," Margutti said. "Initially, no one was looking
for X-rays. Daichi noticed something and alerted us to the strange
appearance of what looked like X-rays. We looked at the images and
realized something was there. It was much more luminous than anybody would
have ever thought. There were no preexisting theories that predicted calcium-rich transients would be so luminous in X-ray wavelengths."
UNCOVERING NEW CLUES SN 2019ehk's brief luminosity told another a story
about its nature. The Northwestern researchers believe the star shed
an outer layer of gas in its final days. When the star exploded, its
material collided with this outer layer to produce a bright, energetic
burst of X-rays.

"The luminosity tells us how much material the star shed and how close
that material was to the star," Jacobson-Galan said. "In this case, the
star lost a very small amount of material right before it exploded. That material was still nearby." Although the Hubble Space Telescope had
been observing M100 for the past 25 years, the powerful device never
registered the star -- which was experiencing its final evolution -- responsible for SN 2019ehk. The researchers used the Hubble images to
examine the supernova site before the explosion occurred and say this
is yet another clue to the star's true nature.

"It was likely a white dwarf or very low-mass massive star,"
Jacobson-Galan said. "Both of those would be very faint." "Without this explosion, you wouldn't know that anything was ever there," Margutti
added. "Not even Hubble could see it." The study was supported by the
National Science Foundation (award numbers DGE- 1842165, PHY-1748958
and AST-1909796.)

========================================================================== Story Source: Materials provided by W._M._Keck_Observatory. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Wynn V. Jacobson-Gala'n, Raffaella Margutti, Charles D. Kilpatrick,
Daichi Hiramatsu, Hagai Perets, David Khatami, Ryan J. Foley,
John Raymond, Sung-Chul Yoon, Alexey Bobrick, Yossef Zenati,
Llui's Galbany, Jennifer Andrews, Peter J. Brown, Re'gis Cartier,
Deanne L. Coppejans, Georgios Dimitriadis, Matthew Dobson,
Aprajita Hajela, D. Andrew Howell, Hanindyo Kuncarayakti, Danny
Milisavljevic, Mohammed Rahman, Ce'sar Rojas-Bravo, David J. Sand,
Joel Shepherd, Stephen J. Smartt, Holland Stacey, Michael Stroh,
Jonathan J. Swift, Giacomo Terreran, Jozsef Vinko, Xiaofeng Wang,
Joseph P. Anderson, Edward A. Baron, Edo Berger, Peter K.

Blanchard, Jamison Burke, David A. Coulter, Lindsay DeMarchi,
James M.

DerKacy, Christoffer Fremling, Sebastian Gomez, Mariusz Gromadzki,
Griffin Hosseinzadeh, Daniel Kasen, Levente Kriskovics, Curtis
McCully, Toma's E. Mu"ller-Bravo, Matt Nicholl, Andra's Ordasi,
Craig Pellegrino, Anthony L. Piro, Andra's Pa'l, Juanjuan Ren,
Armin Rest, R. Michael Rich, Hanna Sai, Krisztia'n Sa'rneczky, Ken
J. Shen, Philip Short, Matthew Siebert, Candice Stauffer, Ro'bert
Szaka'ts, Xinhan Zhang, Jujia Zhang, Kaicheng Zhang. SN 2019ehk:
A Double-Peaked Ca-rich Transient with Luminous X-ray Emission
and Shock-Ionized Spectral Features. The Astrophysical Journal,
2020 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200805102020.htm

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